Compression testing while exhaust system is removed...

[unlvrebel]

Can this be done safely? I have researched the existing threads. Nothing specifically addressing this. Having the manifolds/risers/elbows obviously makes it easier to access the spark plugs for removal. However, the engine will not be warm. How critical is it to have the engine warmed up for this testing?

 

[tbirdvet]

You can do with cold engine. You want to compare the readings between cylinders with the lowest being within the % specified in the engine manual (assuming your highest is OK).

 

[CliffA]

You can do with cold engine. You want to compare the readings between cylinders with the lowest being within the % specified in the engine manual (assuming your highest is OK).

i don't know for sure, but the compression readings may actualy be a little lower when the test is done on a cold engine...i'm thinking that the pistons and rings may actually expand a little when warm which could make a better seal against the cylinder walls and would increase the compession reading...if the test results from a cold engine look OK it may be safe to assume the results from a test done while the engine is warm should be the same or possibly higher....

cliff

 

[rcknecht]

When I sold my 300, the surveyor did a cold compression test and the compress readings were well within the spec... I don't see how the removal of the exhaust would affect the readings, and it will be easier to get at the plugs... I gave the surveyor new plugs to put in so the guy I sold the boat to would know he had new plugs...

 

[Lazy Daze]

You could do it, but I wouldn't go through the hassle of removing the manifolds just to compression test. The plugs are accessible in your boat - you'll just have to do a good deal of it by feel (remove the hose on the bottom side of the manifold). But it can definitely be done - I've done it. You can do it cold and if all looks OK, you should be fine. If you find one of the cylinders out of spec, then do the check with the engine warm before condemning it.

 

[MasterFab]

The above statements are correct. If you wish to perform the test on a cold engine with no exhaust, you can do so. Yes, the readings will be a hair lower than with a warm engine, mainly for 2 reasons:

1 - The pistons will not be up to operating temperature. Many people are not aware that pistons are NOT machined into a cylindrical shape at the time of manufacturing. They are "barrel" shaped. That is, the tops of the pistons are a slightly smaller diameter than the body of the piston. They are machined this way because, as an engine runs, the tops of the pistons are subject to the most heating, and therefore expand the most. At operating temperatures, the piston assumes a cylindrical shape (hopefully!). As such, at low temperatures, the piston can "wobble" just a bit in the bores, compromising the ring seal, and lowering the compression readings.

2 - The rings also expand when heated, and the effective ring gap is reduced, thus reducing "blow-by" of compressed gases. When cold, a slight amount of compression is lost into the crankcase, again lowering your compression readings.

The bottom line is that you can conduct the test on a cold engine, if you are trying to determine the relative condition of the cylinders. If they all read the same, then you know the engine is experiencing balanced wear. If one or more readings are not within 10% (preferably 5%) of the highest reading, then you know these cylinders are experiencing abnormal wear.

If you are trying to determine the absolute condition of each cylinder, then the test should be conducted on a hot engine, to get the highest possible readings, and see if these readings meet design specifications. Hope this helps...

Dale

 

[unlvrebel]

Thanks folks. Good replies. I already have the exhaust pieces off, hence the question. I thought that since I have easy access and it is a relatively simple process, why not go through the exercise as I am waiting for my new manifolds, risers and elbows.