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I've read a number of different recommendations on how many times to cranks the engine during a compression test.

I've read those that say to crank until you reach a stable reading.

In the book, Engine Builder's Handbook, pg. 12, Monroe says:

Crank the engine over four times - no more and no less.

In the Mazda 626 Haynes manual, pg 2C-6, "General Engine Overhaul Procedures" it says:

Crank the engine over at least seven compression strokes...

So who's correct, and why?

  • I'd say it doesn't matter much as long as you keep the number constant between cylinders. Crank until the reading stops rising and you're good. Just don't go for so long that it discharges your battery, cranking speed is important as well. – I have no idea what I'm doing Nov 16 '15 at 10:42
  • You're going to get a lot of personal opinions here. I would recommend reading as much as you can and come to your own decision of what you are trying to accomplish. – HandyHowie Nov 16 '15 at 11:13
  • My gut feeling here is to follow the recommendations of the manufacturer. With so many engine design variations out there I think it will be hard to generalize that all engines will report stabilized pressures at the end of n cranks. – Zaid Nov 16 '15 at 19:48
  • @Zaid But many reputable people seem to make very specific recommendations. However, I'll for sure see if there is something specific in the WSM. – Robert S. Barnes Nov 16 '15 at 20:05
  • the number of strokes required depends in large part on the gage. A gage that seats right up against the port will come up more quickly, while one with a hose will take more strokes depending on the length of the hose. – agentp Oct 16 '17 at 11:09
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One pump; one pump only.

Which test we perform determines the procedure used. Three procedures are described and each test has a different purpose.

But keep in mind when the engine is running it only gets one pump per cycle.

Running compression test: A decent way to estimate volumetric efficiency (VE). This is not technically a compression test. Install a screw in gauge into on cylinder and run the engine at idle. Now read the max pressure for each compression stroke. “Burp” the pressure out of the gauge after each stroke. Average 5 readings. There are no published specs for this test. Compare cylinders to each other. Normal readings are 75 psi. The normal range is 50 to 100.

Cranking compression tests

Wide open throttle. Battery fully charged, ignition disabled. Fuel disabled.

First pump test: Same method as the running test, burp after each pump. The reading should be at least one half of the reading of the max compression test listed below. Depending on compression ratio and carbon deposit volume. I find this test gives a better indication of cylinder leakage due to worn cylinders and leaking valves because this value is the only one the cylinder actually sees.

Max compression test: The traditional test. I pump till max pressure is reached. I count pumps, the more it takes the more likely leaks are present. Others will have a number of pumps they prefer. Average reading for a normally aspirated gas engine is 180psi. 200 plus could mean excess carbon deposits. Low means leaks.

  • Are you saying to start the engine with the compression gauge installed in one of the cylinders, or am I misunderstanding? – HandyHowie Nov 17 '15 at 12:35
  • @HandyHowie Yes, I run the engine at idle with a gauge in one cylinder for a very short run since the catalyst could be damaged. It works fine but is a bit hard on the one way valve in the gauge. I use a pressure transducer for this teset now but that is beyond this particular question. – Fred Wilson Nov 17 '15 at 16:12

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