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.

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What's a Throttle?

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.
cross-section of a butterfly valve
Simplified Cross-Section View of a Butterfly Valve

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 (older) 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!

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