Compression Test Question???

[a1nowell]

What are the proper proceedures to get a correct reading on a compression test. Would these proceedures be the same for all 4 stroke engine?

I have a M/C 470, 3.7L engine.

Thanks in advance,

Larry

 

[Oshkosh1]

Re: Compression Test Question???

Warm it up...remove ALL of the plugs, ground the coil wire(I use the carb stud), open the throttle all the way, crank the engine over about 10 times and watch how quickly the pressure builds. Use either a remote starter, have someone key it for you, or position the gauge so you can watch it from the helm.

 

[a1nowell]

Re: Compression Test Question???

Thanks Oshkosh.

Larry

 

[Don S]

Re: Compression Test Question???

I just posted this on another thread, you might as well have a read of it too.

View attachment 97-25 Compression Test.pdf

 

[Oshkosh1]

Re: Compression Test Question???

As the engine runs...it warms. As it warms things expand and seal better. As "things" expand and seal, they create higher compression. Virtually every other manual I've used on other engines(including GM) has stated to bring the engine up to "operating temperature" prior to checking it. Seems that this would give a better and truer snapshot of what the running compression really is.

On the other hand...if you're just needing to get a reading to establish equality, and find any glaring anomalies in the cylinders themselves...I can see how it wouldn't really make a difference as they're all equally "cold", therefore the "playing field" is leveled regardless of overall temperature.

I suppose a cold test also makes it easier to remove the plugs and lessens the chances of stripping a plug thread with the tester as the head is still hot.