Compression test

Terry Wilkerson · Jul 27, 2022

Will soon run compression test on my square. Plan on doing dry and wet test. Should I run the complete dry test on all 8 cyl. then the complete wet on all 8 cyl or run dry then wet test on each cylinder before moving on to the next cyl.

Bextreme04 · Jul 27, 2022

Terry Wilkerson said:
Will soon run compression test on my square. Plan on doing dry and wet test. Should I run the complete dry test on all 8 cyl. then the complete wet on all 8 cyl or run dry then wet test on each cylinder before moving on to the next cyl.

Shouldn't matter. I would do all of them dry and probably only worry about wet if they were all low or several of them are off by more than 5-10%. If they are all within 5-10% of each other and sufficiently high I wouldn't bother to go any further. Maybe just do one of them wet just to see in that case.

Why are doing a compression test? Don't forget to remove all plugs first and have the throttle full open before cranking.

85K304SPD · Jul 27, 2022

I would pull out all of the spark plugs and do a complete dry test first for a base line then the same with the wet test.

Terry Wilkerson · Jul 27, 2022

So pull all plugs out then do a dry test. I wound think you would be running the possibility of drawing something into the cyl,s.

Terry Wilkerson · Jul 27, 2022

The truck is down on power and sounds funny. Also had a small amount of coolant come out of #3 while rolling it over by had while setting vslves.

WFO · Jul 27, 2022

I only test wet when a cylinder is low, just to see if it's the rings causing the loss.

Bextreme04 · Jul 27, 2022

Terry Wilkerson said:
The truck is down on power and sounds funny. Also had a small amount of coolant come out of #3 while rolling it over by had while setting vslves.

If you have coolant in the cylinders, then yeah I would do a compression test. Yes you remove ALL plugs first, then go through and run the test on each cylinder with throttle wide open. You wont suck anything into the cylinder.

Having coolant in a cylinder I would also pull the radiator cap and see if you get bubbles showing up in the coolant while cranking, this would tell you that you have a cracked head or blown head gasket. If you have low compression on one side(or 1-2 cylinders on one side) then your next step is to pull the head on that side and have it pressure checked and decked. Then new headgasket(and maybe cylinder head) and you're back in business.

If you don't have low compression, but have coolant in the cylinder, then you want to look at a possible intake manifold leak/crack or still a possible cracked head.

73c20jim · Jul 27, 2022

What is compression for stock GM base motor?

85K304SPD · Jul 28, 2022

73c20jim said:
What is compression for stock GM base motor?

There is a lot of variables there, but I just put together one that has "stock" pistons and large chamber heads and it has 115 lbs. of cranking pressure.

Bextreme04 · Jul 28, 2022

You are actually reading dynamic compression with a compression tester, so it will depend on your altitude, static compression, camshaft, etc... Generally speaking a stock engine with stockish camshaft should be in the 135-185psi range. This will also depend on your gauge. Basically if they are over 115, you are probably fine as long as they are all within about 5-10% of each other. If one or more are significantly different than the rest, you have a problem.

Terry Wilkerson · Jul 28, 2022

Thank you to all. I feel confident about doing the test.

Catbox · Jul 28, 2022

Terry Wilkerson said:
Thank you to all. I feel confident about doing the test.

Now we are all going to be watching for your next post on this thread.
Make it count with photos and good words...

Terry Wilkerson · Jul 28, 2022

I'm working on another project car. I'm getting close to posting picks of the other project on other cars. I'm saying it may be a couple days or so before the compression test..
I will post the results of the test.

Terry Wilkerson · Aug 8, 2022

I did the compression test this morning. I warmed the engine up first. On first attempt to start it lock up. I knew #3 had water in it before. I pulled #3 and water ran out. I mean a lot. It started after #3 was cleared.
Readings are as follows.
1 - 150
3 - 135 water in this cyl.
5 - 155
7 - 140
2 - 160
4 - 80
6 - 85
8 - 150

fast 99 · Aug 8, 2022

Do wet compression on 4 and 6 [4 - 5 squirts of motor oil]. If it comes up near the others, needs complete rebuild. If they stay low might get by with heads. Water usually caused by head gasket or cracked head.

At the very least heads need to come off. However, if it were mine, I would get a long block.

Will add, a flat can or valve train issues can cause low compression because it may not allow proper air flow in and out. Either way appears a replacement motor is in your future. Don't toss good money after bad.

Compression test

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