compression test hot or cold

[The red boat]

is it best to do a compression test when the motor is warmed up or cold?
I’m working in a Johnson RD-17

[reivertom]

I’ve always heard and done it cold without adding oil, etc. to get a base line of the wear. Of course, in a motor that has sit for years and is full of gunk, the PSI can come up after it is run a while and the rings loosen and get cleaned up a bit..

• This reply was modified 3 years, 6 months ago by reivertom.

[frankr]

First thing to do with a motor with problems is check the compression. How you gonna check it warm if it won’t run?

[Mumbles]

Testing warm will give a truer reading than testing cold as all the parts will be at their operating clearances. Testing cold works to as all you are trying to do is get an indication of what is going on inside.

1 user thanked author for this post.

The red boat

[billw]

The red boat wrote:

is it best to do a compression test when the motor is warmed up or cold?
I’m working in a Johnson RD-17

If your motor will run, it would be interesting to try it before and after it has warmed up. (I’d like to know what your results are!) I think you will find that on a two stroke, it is not going to make a huge difference. You should see a difference, just not huge. With today’s four strokes, you can really be fooled if it’s not done on a warm engine.

Long live American manufacturing!

[The red boat]

follow-up question:
do I open the throttle all the way and then test or does that matter?

I did a quick test cold last night and was getting 100 psi on one cylinder and between 95 and 100 on the other. My gauge is old and doesn’t hold the reading well so I had the wife watch the needle while I cranked the starter. she said the 100 psi reading was steady on one cylinder while the other cylinder fluttered the needle between 95 and 100.

not sure why the needle is steady on one and not the other.

[dave-bernard]

No the throttle position makes no difference on a 2 cycle.

[Author]

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