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Why Hold the Throttle Open During a Compression Test?

Almost every compression testing guide tells you to hold the engine’s throttle wide open during a compression test, but most won’t tell you why it may (or may not) matter.

On gasoline engines, the throttle is a valve, which reduces the amount of air going into the cylinder. Closing the throttle reduces the amount for air sucked into the cylinder during the intake stroke. Having less air in the cylinder means that there is lower pressure at bottom dead center (BDC), so that when the piston moves ‘up’ and compresses the air, the maximum (or peak) pressure is lower than it could have been. This lower pressure means that the readings will be misleading. If you encounter an engine with low compression readings on all the cylinders, you should consider that there may be something obstructing the airflow to all cylinders.

What About Throttle-by-Wire?

Engines with throttle-by-wire (also known as an electronic throttle) may not allow you to open the throttle during cranking. These engines can refuse to start while the throttle is not at the idle position. This is usually because the engine control unit or powertrain control module (ECM or PCM) detects the throttle sensor’s output, and ‘diagnoses’ it as a sensor fault.

Many engines without throttle-by-wire will give you trouble codes if you hold down the accelerator during cranking. This is because the ECM or PCM will detect the throttle’s position, and ‘diagnose’ a sensor fault. In this case, you can either complete the compression test with the accelerator in idle position, or reset the fault code after completing the test.

What About Diesels?

Most diesel engines do not restrict airflow into the cylinders, they control power and speed by using a regulator to adjust the amount of fuel. On these engines, the ‘throttle’ is connected to the engine’s regulator, and will not affect compression readings.

Modern automotive and heavy truck diesel engines use a valve to control engine airflow, and control their exhaust emissions. These throttle valves are open under most operating conditions, and usually won’t affect compression tests.

Does it Really Matter?

Many people do compression tests with closed throttle valves, and get decent results. This is because the throttle body usually does not completely block airflow into the engine. There is usually a small ‘bypass’ passage around the edge of the butterfly valve, or an idle air control valve (IACV), designed to supply enough air for the engine to idle (with no load). Idle speed is usually 5-10 times faster than cranking speed, so the passage or valve usually allows enough airflow to get a compression reading within a few percent of the wide-open-throttle (WOT) value.

The Physics Explanation (without much math)

I mentioned that the throttle’s idle bypass (or IACV) usually allows enough airflow to give you reasonable compression readings, and the physics support it!

If you imagine that the bypass is similar to an orifice (hole), the air velocity is proportional to the square root of the pressure difference from one side to the other. We already know that the cylinder can get suck in a significant amount of air with the throttle ‘closed’ while at idle, enough to maintain speed. Since the engine cranking speed is usually more than five times slower than idle speed, the cylinder can collect much more air during cranking than at idle. Because the amount of air rushing into the cylinder decreases as the cylinder fills (due to the reduction in the (pressure difference), cylinder pressure while cranking will be always less than five times the idle cylinder pressure.

It is impossible to come up with a hard number for how close the closed-throttle cranking pressure will be to the wide-open-throttle (WOT) cranking pressure because of the differences in idle bypasses, cranking speeds, and idle speeds between engines. That said, the closed throttle cranking pressure will be much closer to the WOT cranking pressure than the cylinder pressure while idling; usually a few percent from WOT cranking pressure.

That’s All!

If you’re looking for a clamp-meter to test starter motor current consumption, check out our CM600 0.1-600 Amp AC/DC Current Clamp Meter.

If you have any ideas for topics we should cover in future blog posts, please send us an e-mail.

Long Term Fuel Trim and Short Trim Fuel Trim

What is Fuel Trim?

Fuel trims are what engine control system use to compensate for all problems relating to air-fuel ratios (known as λ or lambda,) and combustion.

The Electronic Control Module (ECM) or Powertrain Control Module (PCM) uses a few sensors to determine how much air is flowing into the engine. It then uses the airflow data, along with a target air-fuel ratio (λ or lambda) to calculate how much fuel it should inject. The ECM or PCM then uses oxygen sensor readings to find out what the actual air-fuel ratio was, and ‘trims’ the fuel quantities based on this ‘real’ air-fuel ratio.

If the PCM/ECM detects that the calculated quantity of fuel needed is too much or too little (with the oxygen sensor), the PCM/ECM will add or subtract fuel to ‘trim’ the quantity of fuel and optimize the combustion. The difference between the calculated and the actual quantity of fuel being delivered to the cylinders is what is called “fuel trim”. Fuel trim is reported in two different forms: long-term fuel trim (LTFT) and short-term fuel trim (STFT).

fuel trim flowchart

Fuel trim is related to a variety of sensors and variables.

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Testing Oxygen Sensors

There are a few different tests for oxygen sensors (also known as lambda sensors), some of which can be run without dedicated tools. The most effective tests tend to be done under normal operating conditions, on a sensor installed on an engine system, though there are some tests which can be done off-vehicle. You can test oxygen sensors with the following tools:

Caution: Be sure to follow the oxygen sensor manufacturer’s precautions when testing, as well as the tool manufacturer’s directions, and read the vehicle (or other system) service manual before doing any test. Oxygen sensors get very hot when in use, be careful!

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How to Test Glow Plugs

Glow plugs are installed on many diesel engines to help with cold starts. They usually fail because of corrosion, overheating, mechanical damage, or metal fatigue, and their failure can cause a variety of problems. The easiest way to test a glow plug is by using a clamp-meter, though digital multimeters can also do the job, and glow-plug testers also work.

We will cover the following topics in this post:

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How to Use a Battery Tester

Most battery testers have two clips (or test leads), you first attach these to the battery’s terminals (connectors), then you press a button (or switch), and a few seconds later, the tester displays or prints a result indicating the battery’s condition. The details of you use a battery tester depends on what kind of battery you have, the electrical system it is connected to, and which tester you plan to use. Some battery testers require disconnecting the battery from the vehicle, while others have different limitations. Most battery testers only work on one, or a few different types of batteries. We describe how to use five different battery testers below:

  1. Carbon Pile Battery Testers
  2. Hand-Held Battery Load Testers
  3. Battery Hydrometry Testers
  4. Battery Conductance Testers
  5. Multimeters

Caution: Be careful to follow the battery manufacturer’s precautions when testing, as well as the tester manufacturer’s directions, and read the vehicle (or other system) manual before doing any test, or even disconnecting the battery.

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