Compression test on cylinders

kenewagner

Registered User
I am installing new colder plugs to prepare for another tuning session on the car. I borrowed a compression gauge and plan to check the compression since I have the plugs out anyway. What is the acceptable range for the cylinder compression on the 3.8?

Ken
 
You are looking more for how the compression varies between all cylinders more than absolute numbers. My Ford 89 T-Bird/Cougar shop manual says you are within specs if the lowest reading is no less than 75% of the highest reading. An example they give is if your cylinders range between 155 and 196 psi you are within spec.
 
don't bother with the compression test. it's useless. last year my 89 had no power at all. did a compression test 4 cylinders were 135 2 were 125. only took 3 cranks to build pressure on all cylinders. with those numbers i should be fine. anyway i pulled the motor and the 2 cylinders that had 125 had cracked top rings. so much for the compression test. i've read that a leakdown test is much better. thats my .02
 
don't bother with the compression test. it's useless. last year my 89 had no power at all. did a compression test 4 cylinders were 135 2 were 125. only took 3 cranks to build pressure on all cylinders. with those numbers i should be fine. anyway i pulled the motor and the 2 cylinders that had 125 had cracked top rings. so much for the compression test. i've read that a leakdown test is much better. thats my .02


Like I said you are looking for variations from the rest. So in your case the 2 cylinders that read lower readings than the rest had a problem.

You can also have your engine computer (EEC-IV) do a compression cylinder balance test for you after the KOER (key on engine running) test mode. The following describes it:

EEC-IV Cylinder Balance Testing Procedures
After you have performed the KOER test and corrected any problems you may now perform the EEC-IV Cylinder Balance test. This test checks all the cylinders in the engine for matched compression ratios. The Cylinder Balance test is designed to help isolate cylinders that have low compression ratios with respect to the others in the engine. This test is performed immediately after the KOER test has finished displaying codes.
To perform this test, the EEC-IV will bring the idle of the engine up to about 1,000 RPM. Once the idle has stabilized it will then shut off spark and fuel to one cylinder, during this time the decrease in RPM will be measured. The EEC-IV will then activate fuel and spark to the cylinder being tested, and will wait for several seconds until the idle stabilizes again. At this time it will repeat the process for the next cylinder, repeating until all have been tested. The results of the test will then output one or more of the following codes to your reader:
• 10 - Number 1 Cylinder Low
• 20 - Number 2 Cylinder Low
• 30 - Number 3 Cylinder Low
• 40 - Number 4 Cylinder Low
• 50 - Number 5 Cylinder Low
• 60 - Number 6 Cylinder Low
• 70 - Number 7 Cylinder Low
• 80 - Number 8 Cylinder Low
• 90 - Passed Cylinder Balance Test
 
Ken, I would not like seeing something as low as 120 psi or so. I would probably be looking for something around 150. I'm sure you know, but make sure all plugs are out for test. I've been told to hold TB open too, but never seen it make much difference.

For reference, I recently did a 4.3L Vortec Blazer with 95K and it was reading in the 180's. Then again, I did a 2.0L Jetta with 50K miles and was getting 160's...which seemed low, but not low enough to make much difference. I'm sure both motors are near 9.0:1
 
The other problem with running a compression test on these cars is I believe the engine is supposed to be warmed up to ensure proper readings. We tried doing this on Doug's car a few years ago when he was having problems. I burned the hell out of my hands and we still only got two cylinders done before I threw in the towel. It's bad enough changing the plugs on these when the engine is cold, try doing it when it's warm.
 
I will warm the car up before I start. Probably not operating temp. Pulled the plugs and run the test. Ill look for both varaince and PSI just so I know prior to the tune if there is a problem. Thanks all for the help

Ken
 
I've always done cold checks. Just knowing that psi may be slightly higher when warm.

Me too....trying to do it on a warm motor would be brutal.

How much compression you show will be influenced greatly by what cam your using. I think mine only makes around 135 psi (cold) and I've seen others with Coy Miller stage II motors making 150-155 psi.

David
 
I will warm the car up before I start. Probably not operating temp. Pulled the plugs and run the test. Ill look for both varaince and PSI just so I know prior to the tune if there is a problem. Thanks all for the help

Ken

You would need arms like ET to compression test these cars with a warmed up motor.
 
OK OK Im convinced:) Ill check it cold, you talked me into it.

The tuner called and I have the dyno reserved for the 20th at 5 PM. And Ill try it again

Ken
 
wait wait you can do it on a warm motor spray your exhaust manifolds with some cool hose water... thats what i did when i changed my plugs and my engine was still hot after 3 hours of sitting...:D
 
Well the stocker '94 motor I dropped in mine ran 175 ish on all six...of course the original bearings also looked like new, I think it had rather low miles.

Nick
 
wait wait you can do it on a warm motor spray your exhaust manifolds with some cool hose water... thats what i did when i changed my plugs and my engine was still hot after 3 hours of sitting...:D

Let me guess, you would pour hot water over the windshield if it was frozen over with ice.....
 
Well the stocker '94 motor I dropped in mine ran 175 ish on all six...of course the original bearings also looked like new, I think it had rather low miles.

Nick

That sounds really good. 94's had higher compression so that's why your #'s are higher.
 
Where do they get it? I just had the heads done from my '89, and will be doing hg's next week....BEFORE they blow-will I lose any compression?
Machinist took .010 off the decks to true them up.

Nick

That sounds really good. 94's had higher compression so that's why your #'s are higher.
 
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