Compression test
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Tuned
Joined: Aug 2015
Posts: 396
Likes: 2
Compression test
Ran a compression test on my 95 4.9l with 192K on it and they are as follows...
1-160
2-160
3-140
4-160
5-155
6-160
These readings are probably +/- 5 points each as I was doing it myself and was just glancing at the reading as I was cranking it. This was a dry test. I actually did #3 twice,the first time I got a 130-135.. I can't seem to find anything online with how much compression it should have. How does these numbers look to someone who knows?
1-160
2-160
3-140
4-160
5-155
6-160
These readings are probably +/- 5 points each as I was doing it myself and was just glancing at the reading as I was cranking it. This was a dry test. I actually did #3 twice,the first time I got a 130-135.. I can't seem to find anything online with how much compression it should have. How does these numbers look to someone who knows?
Postmaster
Joined: Nov 2011
Posts: 3,533
Likes: 17
From: SW Florida
Those numbers are about what I'd expect to see. After you get the results of a compression test the first thing you want to look at is the difference between the lowest and highest value, this shouldn't exceed about 10%. You have about a 14% difference there with #3 being 140 and several others being 160, but if it's running good I wouldn't worry bout it because the cylinders on either side of it (#2 and #4) both posted good values so it's almost certainly not problem with the head. Might just have a sticky ring, try some Seafoam thru a vacuum line.
The values themselves are fairly useful to determine overall wear... 150-160 psi is still a brand new engine to me so you're in good shape there. But really as long as you have around 120+ compression just about any gas engine will run fine. What's more important is that they're all about the same value.
EDIT: Welcome to FTE by the way!
The values themselves are fairly useful to determine overall wear... 150-160 psi is still a brand new engine to me so you're in good shape there. But really as long as you have around 120+ compression just about any gas engine will run fine. What's more important is that they're all about the same value.
EDIT: Welcome to FTE by the way!
Postmaster
Joined: Nov 2011
Posts: 3,533
Likes: 17
From: SW Florida
As for cranking it four times... I never count revs. Too much room for error... instead I just hold the starter in til the gauge stops climbing. Remember, you ain't trying to find how many strokes it takes to get to a certain value, but rather how much pressure that cylinder can generate.
If you want a better way to determine wear on an individual cylinder, get a leakdown tester instead. Personally I don't even own one... between the vacuum gauge and compression tester I'm pretty happy.
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