85 2.8 compression.

[baconops89]

I recently did a compression test on my engine prior to replacing valve stem seals and my readings were all around 120psi. Is this normal for the 2.8?

 

[adsm08]

That's towards to low end, but not too bad for an engine that old. 150+ is optimum, 90 is the minimum needed to run.

 

[RonD]

Yes, ^^^

Compression test is mainly done to determine if one or two cylinders may have ring or valve issues.
You would test all 6 cylinders at one time, then remove the highest and lowest cylinders, add up the other 4 and divide by 4, that will give you the "average" compression on that day with that equipment.
No cylinder, including the highest and lowest should be more than 10% away from the average number.
So if your average number was 120 then no cylinder should be 12(10%) less or more than 120, 108 to 132 would be acceptable range

Because of variables in the testing the actual numbers can vary widely.
The gauge itself can read high or low
The amount of compression generated is directly related to the speed of the crankshaft, so battery condition and having all spark plugs removed matters.
Having throttle propped open matters, better air flow into cylinder
Warm engine will always read higher, but IMO that doesn't matter for the purpose of the test.

18.3 is the magic number that has always served me well over the years.
2.8l engine has compression ratio of 8.7:1
8.7 x 18.3 = 159.21
So expected compression number on new 2.8l engine should be about 160psi at sea level.
With 200k miles on the engine, I would expect about 145-150, with all spark plugs removed and good battery.

 

[baconops89]

Oh well when I did it I had all the spark plugs in except the one with the compression tester on it obviously and I didn't have the throttle open. So if I was to do it again and pull all the plugs and open the throttle do you figure I'd get higher readings?

 

[RonD]

Oh well when I did it I had all the spark plugs in except the one with the compression tester on it obviously and I didn't have the throttle open. So if I was to do it again and pull all the plugs and open the throttle do you figure I'd get higher readings?

Yes, higher, crank can't get spinning very fast with 3 compression strokes per revolution.
Metal rings around metal pistons sliding against metal cylinder walls and metal valves sitting on metal seats do not seal very well, and they are not suppose to seal very well, just temporarily hold pressure.
The slower the piston moves up the cylinder wall the more air will leak out past rings and thru valve seats.

 

[RonD]

Oh well when I did it I had all the spark plugs in except the one with the compression tester on it obviously and I didn't have the throttle open. So if I was to do it again and pull all the plugs and open the throttle do you figure I'd get higher readings?

Yes, higher, crank can't get spinning very fast with 3 compression strokes per revolution.
Metal rings around metal pistons sliding against metal cylinder walls and metal valves sitting on metal seats do not seal very well, and they are not suppose to seal very well, just temporarily hold pressure.
The slower the piston moves up the cylinder the more air will leak out past rings and thru valve seats.

 

[baconops89]

Ok. Thanks for the info! For now I'm just gonna leave it alone. Seems to run good and all of my readings before were well within 10%.