The Ultimate Guide to Marine Engine Surveys

Welcome to our comprehensive guide to inboard & outboard engine surveys. This guide is for anyone planning to survey an engine, or hire someone else to do it. We'll answer the who, when, where, why, and what of engine surveys. If we did miss your question, please let us know by e-mailing us, so we can get you an answer, and add it to the guide!

Table of Contents

What is an Engine Survey?

‘Engine survey’ is a term used to describe an engine condition assessment (evaluation) in the marine industry. As with other types of marine survey, ‘engine surveys’ are usually performed during boat or ship sales, and may be required periodically for insurance purposes.

Formal surveys end with a written report, which describes the engine’s general condition, along with any specific problems that the surveyor identified, and what service should be performed on the engine in the near term. Some surveys may estimate the engine’s value and/or how much the service/repairs are likely to cost. Surveys vary in thoroughness/detail, according to how much the person commissioning the survey (by hiring the surveyor) wants to spend on it, but no survey will provide absolute guarantees, and most reports will contain a disclaimer of liability, describing the limitations of the assessment.

How Much Do Engine Surveys Cost?

Recreational marine engine surveysΒ are usually billed by the hour at the surveyor’s ‘shop rate’ (normal hourly billing rate), and any transportation costs and travel time are added to the bill. It is important to note that time spent writing a report will be billed in addition to time spent actually looking at the engine (and travelling to and from it). Any reputable surveyor will provide an estimate of the survey cost/price.

All of that being said, a quick recreational marine engine survey will usually be 1.5-3 hours of shop time, and a more thorough survey (possibly including a compression test) will usually be more like 3-6 hours of billable time (plus travel). Given a (2021 North American) shop rate of $100-$250/hour, you can expect anywhere from $200-$1500, depending on where you are, who you hire, and what you get done.

A commercial or industrial marine engine survey will take somewhere between a two and eight hours, and shop rates vary widely; depending on the make, model, location, and surveyor.

Is it Worth Doing an Engine Survey?

It’s almost always worth evaluating an engine’s condition before you take possession of it; even a free engine can be more trouble than it’s worth. That said, even the best survey has limitations, and a bad engine survey can be very misleading. So how should a buyer balance these benefits and drawbacks? The buyer has a few different options:
  • Commission a full survey by an expert. This is the most expensive option, and should be done if the engine is very valuable, or the buyer suspects that the engine may have some hard-to-find issues. A full survey will give the buyer peace-of-mind, and may provide some leverage in price negotiations.
  • Request a limited assessment by an expert. This option balances risk and cost, and may be done if the buyer believes that the engine is in good condition, but wants a ‘second opinion’. A skilled technician can pick-up on most (potentially costly or catastrophic) issues very quickly, and more thorough testing can be done if something suspicious comes up during the ‘quick assessment’.
  • Perform a DIY (do it yourself) check. This is a good option if the engine is relatively inexpensive, and the buyer is a skilled or resourceful mechanic who has some experience with similar engines before. If thisΒ checkΒ finds any issue, the buyer can hire a professional to perform a detailed engine survey.

Who Should Do the Survey?

The individual performing the survey should be someone familiar with the engine (design or type) to be checked. It is usually best to get someone who will be involved with servicing the engine in the future to do the survey, as they are likely to be more careful, because they will be held accountable for any mistakes or omissions. A skilled technician can often detect and diagnose issues from subtle symptoms, like valves clicking excessively, or unusual vibrations.

When and Where Should You Survey?

Most sellers will allow access for surveys after an offer has been made (and accepted), if the buyer has made it clear that surveys will be required (as a condition of the offer). The seller’s level of cooperation in surveys will usually depend on the price of the engine; higher-value transactions (sales) usually involve more thorough and inconvenient assessments.

The best place to do an engine assessment is a well-equipped workshop with an adequately-sized test tank. This provides the best access to the engine, along with allowing the use of any tool that the shop has. Though these are appealing benefits, shop-surveying an engine is usually impractical or impossible, because it is very inconvenient to move the engine around, and test tanks are often too small to allow for proper tests.

The ‘second-best’, and most-recommended place to do a survey is with the engine installed in a boat, tied up to a dock somewhere. Testing in/on a boat is always a bit awkward, but being on the water allows for testing with or without loads, and is generally most convenient.

The absolute worst place to do an engine survey is in/on a boat that’s ‘on the hard’ (on a trailer, drydock, or boatyard). It may seem convenient to get the engine survey done at the same time as a hull survey, for scheduling reasons, but this severely limits the engine test options.

What Tools are Used for Engine Surveys?

The most valuable tool for any engine condition assessment is the ‘mark-one eyeball’. A skilled mechanic (and we use that term with the utmost respect,) will learn a lot about an engine’s condition by watching and listening to it. That said, additional tools will help the surveyor collect more detailed and accurate information. So, how do you choose which tools to buy and bring out for the survey?

The main considerations you should take into account are the cost of each tool, the complexity of using them, and the potential utility they provide. If you’re taking a quick look at a cheap engine, it might be worth picking up a service manual, and getting to work. On the other hand, when you’re considering paying tens of thousands of dollars (or more), it’s probably worth having an arsenal of tools, to allow for lengthy and detailed examination and understanding of the engine’s state. Access to diagnostic equipment and knowledge of how to use it is also a good criteria for choosing who should perform the survey. Here’s a chart that summarizes the differences between some of the most useful tools:

Mk. 1 Eyeball-πŸ™‚πŸ™‚πŸ™‚πŸ™‚πŸ™‚πŸ”§πŸ”§πŸ”§πŸ”§πŸ”§
Engine Service Manual$πŸ™‚πŸ™‚πŸ™‚πŸ™‚πŸ™‚πŸ”§πŸ”§πŸ”§πŸ”§
Compression Tester$-$$πŸ˜πŸ˜πŸ”§πŸ”§πŸ”§πŸ”§
Ignition Tester$$$πŸ™‚πŸ™‚πŸ™‚πŸ™‚πŸ”§πŸ”§πŸ”§
Fuel Injection Tester$$$πŸ™‚πŸ™‚πŸ™‚πŸ™‚πŸ”§πŸ”§πŸ”§
Exhaust Gas Analyzer$$$$$πŸ˜πŸ”§
Thermal Camera$$$$πŸ™‚πŸ™‚πŸ™‚πŸ”§πŸ”§
Engine Oil Tester$πŸ™‚πŸ™‚πŸ™‚πŸ™‚πŸ”§

Engine Service Manual

The engine service manual (which is different from the owner’s manual) is a critical asset for understanding what to look out for, what each symptom might indicate, and how costly and difficult it will be to fix. A manufacturer-trained technician will be familiar with the issues common on engines in their portfolio, but even they will refer back to service manuals regularly. The troubleshooting section of the manual can be used as a sort-of checklist. If issues are found, the relevant sections of the service manual can give you a very good idea of how complicated and expensive they will be to fix, though a skilled mechanic will usually give an even better assessment, as there are many potential complications which the service manual doesn’t get into.


Most recent engines (produced since about ~2005) have an engine control unit (ECU), commonly known as an ‘engine computer’, which can interface with portable scan tools and output ‘fault codes’ describing the problems it has detected. These scan tools can quickly provide a technician with a wealth of data. So what are the draw-backs? There are no universal scan tools, and some are very expensive, so no technician has the ability to scan every engine, and in many cases, only official dealers have access to the full set of diagnostic codes. Even then, the scan tool is limited to problems that the engine can detect with its own sensors.


A hand-held or portable tachometer is useful for verifying the accuracy of the engine’s tachometer (if there is one), as well as for checking engine speed at idle and under load if the engine system doesn’t have a tachometer. GTC’s TA303 Universal Engine Tachometer is a popular choice for those working on diesel and spark engines with alternators. Our TA100 Smartach+ is a good choice for checking RPM and spark voltage for technician is exclusively working on outboards. Some surveyors prefer optical tachometers, though those can be difficult to use in cramped spaces. The least common type of tachometer that some (experienced) technicians still use is known as a ‘vibrating reed’ or ‘resonant reed’ tachometer, though these are now quite rare.

Compression Tester

There are a few different compression tests, each of which provides a different type of equipment, and requires a different set of equipment. Compression testers vary in cost, with the
  • The most common compression testers are ‘cranking compression test kits’, which use a one-way valve and a gauge to measure the peak cylinder pressure during cranking. The same tester can also be used to perform the less-common ‘running compression test’. These kits are the least-expensive to buy, but may require specialized adapters, and quite a bit of labour.
  • Leak-down testers are less common than cranking compression testers, and also provide useful data on cylinder condition, but they are morecostly, as well as being more complicated to set up and use.
  • Relative compression analyzers are the quickest to set up and use, but they are the least reliable, and provide the least-detailed information. Relative compression testers vary in cost and complexity, but they are generally the most costly type of compression tester.

Ignition Testers

Ignition testers come in many shapes and sizes:
  • Some ‘spark checkers’ are simple capacitive pick-ups attached to lights that can detect sparks nearby. These can be useful for detecting ‘dead plugs’, but not much else.
  • ‘Spark gap testers’ are simple devices that allow users to ‘see the sparks’. Gap testers are connected in series between a high-voltage spark plug wire and the spark plug (which requires a bit of meddling), and create a spark (outside the cylinder) that the technician can see. The spark plug gap can usually be adjusted, both to see how long a gap the spark can jump, and whether the longer gap effects engine performance; neither of these tests is particularly meaningful.
  • ‘Engine analyzers’ are an old (now obsolete) tool that was used to observe the ignition system’s performance. The user would connect one lead to each spark plug wire on an engine, flip a few configuration switches and turn some knobs on the analyzer, then interpret the machine’s (sometimes confusing) outputs. These machines were very good for finding problems in Kettering-type (distributor ignition) engines, but have since been superseded by engine ignition analyzers.
  • Engine ignition analyzers are the newest and most comprehensive tools for spark ignition diagnostics. Our GTC505 Engine Ignition Analyzer quickly and easily detects engine RPM and spark parameters; it’s useful for checking for ignition system faults and misfires.

Fuel Injection Testers

Fuel injection system testers are generally more complicated and expensive than their spark-counterparts:
  • The most common style of ‘fuel injector tester’ consists of a (manual) hand-pump, known as a ‘pop-tester’, which can be used to test mechanical injectors (also known as fuel injection valves). These manual testers usually come with a pulse generator box; the hand pump is used to pressurize the fuel, and the pulse generator actuates the valve. These testers allow the user to check for opening and closing problems, and see each injector’s (low-pressure) spray pattern, but the testing is very time-consuming, as the user must remove each injector from the engine.
  • Larger, ‘fuel injector flow testers’ are used by larger shops, especially ones that specialize in diesel engines. These machines usually have 4-8 ‘spots’ for fuel injectors, with a fuel supply and electrical power/control cable for each, and a tube for measuring the injector’s flow rate. ‘Flow testers’ get their name from being able to compare the flow rates of all the injectors (from one engine) to each other side-by-side. Some people call these machines ‘fuel injector cleaners’, because you can also run cleaning fluid through the injectors, though this is discouraged by injector manufacturers. Flow testers are just as inconvenient to use as pop-testers, but they’re bulkier and more expensive.
  • The newest, quickest, and easiest-to-use diagnostic tool for fuel injectors is our GTC605 Fuel Injection Analyzer. The ‘605’ quickly determines an injector’s performance on the engine, avoiding time-consuming injector removal and re-installation.

Exhaust Gas Analyzer

Exhaust gas analyzers have fallen out of favor in recent years, but they are still quite useful in differentiating between intake, ignition, and fuel issues. They consist of a sensor that is placed in the engine’s exhaust, and a handheld or bench analyzer that translates the sensor’s output into gas composition information (usually percentages), and then displays the output. The biggest problem with using these tools in the marine context is that they’re very inconvenient, impractical, and fragile (they do not like water).


Borescopes (also known as endoscopes) are small cameras, usually on the end of a ‘gooseneck’ or cable, that can be inserted into small spaces such as spark plug holes, glow plug holes, intake manifolds, and exhaust manifolds, to examine the engine’s insides. Most shops and many mechanics now have borescopes, and they’re great for inspecting hard-to-get-to areas and parts, but they are somewhat invasive and time-consuming to use, which means that they may not be a practical option for a quick engine survey.

Thermal Camera

Thermal cameras (also known as infrared or IR cameras) have become relatively popular for troubleshooting engines, and they can be used to spot coolant problems and uneven cylinders (rich vs lean). Because of their relatively high cost and limited utility, and the requirement of getting a clear view of the engine, thermal cameras are most often used for detailed surveys of large (often commercial/industrial) inboard engines.

What Should be Checked in a Survey?

There’s no universal checklist that works for all engines, but there are a number of symptoms of common problems that apply to most marine engines. We go through the most common checks below, organized by how and when they can be done.

Visual Inspection

As Yogi Berra once said: “You can observe a lot by just watching.” An engine’s appearance is usually quite telling of how it has been used and maintained. The items below are some of the basic things that should be looked at, but an experienced mechanic will know many more things to look at and for.

  • Paint condition and corrosion (rust) are easy to inspect, and will tell you a lot about how the engine has been kept, unless the engine has been freshly-painted (in which case you may want to take out your borescope and look inside). A bit of rust is not the end of the world on a marine engine, and it often looks much worse than it is, but you may have to replace parts if they have severely deteriorated. It is normal to repair, rebuild, or replace most of the raw-water parts every 5-10 years, especially those exposed to both exhaust gasses and seawater; corrosion of parts which do not come into contact with raw water is generally more troubling and potentially more expensive.
  • Hoses should be checked for abrasion, cracking, kinking, deformation, and fitting, including corroded or missing hose clamps. Worn-out and badly-fitted hoses can cause major headaches later on, so any potential buyer should be aware of the these issues and the associated cost to replace the related parts.
  • Belts and (external) chains should be checked for wear, cracking, tension, and flexibility. Each type of belt (and there are many) has different failure/replacement criteria, which should be used in its inspection.
  • Gaskets, and filters should be examined for signs of failure where possible, though it is often difficult or costly to check these, as it may require (costly and risky) part removal and re-installation or replacement.
  • Fluid levels and conditions. Noticeably ‘new’ or ‘clean’ fluids may indicate that the seller performed an engine service just before the survey; this is often done to ‘cover-up’ long-term neglect. If the engine does have fresh fluids, they should be re-checked at the end of the survey, in case any leaks or issues have .
  • Electrical wires, cables, and connections should also be examined for corrosion and mechanical wear. The hot, wet environment around an engine can quickly corrode unprotected wires and terminals, especially where there is exposed copper. The survey should factor in the cost of replacing any non-ABYC-compliant electrical parts

Cranking & Startup Evaluation

Cold engine starting can expose many issues, and it is critical to pay attention to everything going on, and you should try to get a video of the startup if possible. It is important to check whether the engine is, in fact cold before starting it; many sellers will run an engine before a test or survey, because warm engines show fewer symptoms of problems. If the engine is cool, each of the following checks will provide useful information.

  • Instrument and warning light functionality (if applicable) should be checked before each and every startup. Broken sensors are a very serious matter, and indicative of a neglected engine. Extra caution must be taken in testing an engine with failing indicators.
  • Does the engine turn over, and if it does, how does it sound? Failing to turn over could indicate that the engine has suffered a catastrophic (complete) failure, and should be taken very seriously, but the cause is usually fairly easy and inexpensive to fix. The most common causes of an engine failing to turn over are:
    1. Dead batteries.
    2. Problems with the starter solenoid.
    3. Faulty starter motor
  • The starter motor should get moving very quickly after you switch
  • Time to start when cold choke and/or glow plug
  • Smoke emitted at startup may indicate substantial problems. A large cloud of smoke emitted from a diesel at startup is usually indicative of poor fuel combustion, often due to faulty injectors. Smoky startups can also indicate poor ignition, often due to weak sparks in gasoline engines, and failing glow plugs (if present) on diesels.

Idling & No-Load Testing

After the engine is started up, it should be left to idle for a few minutes while it heats up. When the thermostat opens, the engine is near operating temperature and the no-load behavior can be checked.

  • The first thing that should be checked on any engine at idle is the amount of smoke coming out of it. Depending on the engine, smoking at idle may be a sign of anything from a poorly-tuned mix ratio, to overheating, or even a (potentially catastrophic) low compression ratio.
  • Another important data-point is the idle speed. The engine should maintain a steady idle at the speed recommended by the manufacturer. An engine idling ‘fast’ (above the recommended speed), may be ‘covering-up’ some low-speed instability, symptomatic of uneven cylinder performance. It is uncommon to find an engine idling ‘slow’, but this would usually be a symptom of a poorly maintained engine, which is still working fairly well.
  • The engine’s sound/noise can tell you a lot about how it’s doing, but only if you’re familiar with the model/design. You can often catch uneven cylinder power contributions, misfires, bad timing, and mis-adjusted valves.
  • Engine vibration should be observed throughout its operating range (RPM range), as excessive vibration can be a symptom of many different issues, each of which exhibits differently. Abnormal vibration at low speed is often indicative of uneven cylinder contributions, whereas abnormal vibration at high speed can indicate spark or fuel issues. If it happens throughout the range, the problem could be worn-out engine mounts.
  • A sluggish or ‘bogged-down’ response to changes in throttle/regulator position can indicate a variety of issues, usually having to do with the fuel system.
  • Smoky exhaust with no load is a very bad sign, except on diesel engines being revved-up. Unexpected smoke warrants further investigation (diagnostic troubleshooting).

Load Testing

If the engine is on a boat in the water, and securely tied-up to a strong dock, the engine can be put in gear and tested under (some) load. It’s usually not possible to run a marine engine up to full speed at the dock, because the engine would be overloaded, and the dock would be overstressed. Even with these limitations, in-water load testing can provide valuable information:
  • The first thing to note when putting the motor into gear is how it shifts. Different engines are prone to different problems, but the most common symptoms of trouble in a gearbox are ‘clunking’ during the shifts, and ‘grinding’ when in gear.
  • Next, the surveyor should look for vibrations or ‘rumbling’ at particular RPMs; this can indicate alignment issues,Β imbalances in the driveshaft, propeller problems, or even subtle problems in the engine itself.
  • Hesitation under load is the equivalent of sluggish unloaded response, and indicates a lack of power, often due to fuel issues, and sometimes because of a poorly-matched propeller.
  • As usual, smoke is a sign of trouble, except for when a diesel is accelerating, or at maximum power. More smoke under load than unloaded on a diesel is usually a sign of insufficient air or compression in one or more cylinders. A smoky gas engine may have airflow issues, or problems with fuel atomization (where the fuel is not being ‘misted’ into the air well).

Sea Trials

If the engine under examination is being sold as a part of a boat, you’re likely to do a sea trial. If there is a sea-trial, the engine should be monitored, as any issues are likely to show themselves (to individuals who are paying attention). Sea trials can be seen as an extension of load testing, with the benefit of being able to run up to full power level, and see how well the engine, transmission, and propeller are matched to the boat.
  • Excessive vibration is the most common symptom which you are likely to detect. This can be a sign of many things, from an imbalanced propeller shaft, to a fouled propeller, or even a power imbalance between cylinders (uneven power contribution).
  • Poor acceleration, particularly the amount of time to get on plane (‘out of the hole’) is very important. Engine torque and propeller sizing (diameter & pitch) are the most likely culprits if there’s an issue with this. A surveyor should check the engine RPM required to get the boat up, and use it to verify that it is (approximately) at the peak torque on the engine’s power curve.
  • Maximum speed is one of the most critical performance parameters, and an inability to get to it can mean that there is some issue with the vessel (fouled hull), propeller (incorrect size/pitch), or with the engine(s).
  • Power match (on boats with multiple engines) is very important to cruising efficiency, high-speed performance, and a wonderful sign of engine condition. If the boat pulls to one side at any given speed, the surveyor must investigate the cause. This could be caused by hull fouling (which is often asymmetrical), propeller corrosion, or internal engine issues.

Compression Testing

Compression testing is often held up as the best way to detect major issues in an engine. This is because compression issues usually cost more to fix than fuel or (spark) ignition issues. Compression testing should always be done on a warm engine, because engines work best when warm, and parts often fit loosely when cold. Each of the three most popular compression tests is quite different, and can detect certain issues, here are the basics:
  • Cranking compression tests are the most common for surveys. These involve attaching a pressure sensor to one cylinder in place of a spark plug, glow plug, or in-cylinder fuel injector, disabling the fuel supply, and cranking the engine for about five seconds. Each cylinder is usually tested separately. This test is useful for detecting significant leaks between the piston and cylinder wall (blow-by), and issues with the valves sealing (such as corrosion, wear, or carbon deposits).
  • Relative power contribution tests are imprecise, but often very informative. Power contribution won’t tell you much about the details of an engine’s state, but it’s very quick and easy to do, and it lets you know if there’s something very wrong with any of the cylinders.
  • Leak-down tests are unusual in surveys, but still fairly common to do. The leak-down test involves pressurizing a cylinder and seeing how quickly the air escapes. This is very useful for detecting excessive or unusual cylinder wear, and minor valve issues.
  • The least common test is the ‘running compression test’. This is similar to the cranking compression test, but it is done on a running engine. This test is tricky and risky, but can detect some issues that occur at higher speeds and pressures.
You should note that even the best compression test can miss some critical (but uncommon) issues, such as corrosion of the cylinder walls, or fatigued (cranking) parts.

Detailed Diagnostics

Here is where you get to use all the fancy tools! Spark testers, fuel injector testers, and borescopes can be used to find the ‘root causes’ of any symptoms which have been detected in earlier tests. These ‘precision tools’ can also be used to verify the performance of critical components, even if there are no obvious signs of trouble.
  • Spark testers, engine ignition analyzers, and engine analyzers can help you pin down the cause of misfires or ‘dead cylinders’ in gasoline (or propane and natural gas) engines. Ignition issues can be very difficult to diagnose, but they’re generally inexpensive to fix once you know what you have to do.
  • Fuel injection testers and analyzers can help the surveyor pinpoint some of the most common issues in diesel engines. Combined with some knowledge about the engine, this tool can help differentiate between fouled injectors, a bad pump, and clogged fuel filters, which are recurrent issues with all sorts of diesels.
  • If a compression tester detected a problem, the technician should follow-up with a borescope inspection of the affected cylinder, to determine the root cause of the issue. This check can tell you whether there’s a damaged cylinder wall (which is expensive to fix), or just a cracked head gasket or misadjusted valve (which is much cheaper to address).
Some people try to test engine components without the proper tools; we recommend against this, as it risks damaging the system, and injuring the individual. Here are some examples of what not to do (especially when it’s not your engine):
  • Disconnect spark plug leads and leave them dangling on the engine, to see what color the sparks are. This can damage the ignition system (which is not designed for such long sparks. This can also cause sparks to jump to a vulnerable part of the engine and destroy it, or else electrocute you. Even if this test does not appear to damage anything, it will put raw (un-combusted) fuel through the exhaust system, and wash the oil off the cylinder walls.
  • Take (diesel) fuel injectors out of their cylinders to see the spray patterns, and check for ‘dirty’ injectors. The injector may appear to spray well through short bursts in uncompressed air, but that doesn’t mean it will do well in the cylinder. Second, high-pressure fuel is extremely dangerous, so it can hurt the individual doing the test, or damage the engine bay/area.
  • Disconnect sensors to see whether the engine responds. This usually causes an engine to go into ‘limp mode’, which is a fuel-rich setting that causes increased wear. Depending on the engine, re-connecting the sensor can be costly, complicated, and time-consuming.

Results & Report

A formal engine survey will result in a report, which usually contains the following:
  • The surveyor’s name, contact information, and accreditations, certifications, or qualifications.
  • A disclaimer describing what was not checked, and what is or is not guaranteed by the surveyor.
  • One or more pictures of the engine (usually).
  • A description of the engine, including model, serial number(s), accessories, and performance data.
  • A list of issues observed by the surveyor, including corrosion, wear, and age-related issues.
  • The results of any tests, including idle and full speed measurements if checked, along with cylinder pressures if a compression test was done.
  • A to-do list describing what service should be performed on the engine to render it serviceable. This could be anything from a simple fluid change, to belt replacements, or a more extensive rebuild.
The prospective buyer should carefully review the report, and contact the surveyor if they have any questions. It is also a good idea to get a quote for the required service, so that you understand how long it will take and how much it will cost to prepare it for your use.

That's All!

Thanks for reading our guide, we hope you found it useful! Please send suggestions for future posts, and improvements to this one to

We plan to post some engine survey checklists in the near future, please keep an eye on our blog for them.