I assume the throttle was wide open?
Al
Al
3.8l Compression
Hi,
The compressor for the A/C locked up and before I replace the system, I did a compression test on the engine. The first one I did was with about 4 - 5 revolutions of the crank and the second one was with 3 revolutions. Since I was doing it myself, I could not see when the needle hit the highest and had to assume the needle was pretty close.
1st round
Cyl 1 135
Cyl 2 141
Cyl 3 150
Cyl 4 125
Cyl 5 140
Cyl 6 140
2nd round
Cyl 1 120
Cyl 2 150
Cyl 3 135
Cyl 4 130
Cyl 5 142
Cyl 6 130
This is with all plugs out, cold engine, fuel off. I just want to know what the compression should be if new and what is acceptable in this engine before I say, enough - it's time for a new engine. I plan to re-run the tests with a remote starter so I can see the numbers as they rise and stop rising. Drop a couple tablespoons of oil in and see what happens too. So, before I spend $500 bucks on a new A/C system, are these compression numbers good enough? I couldn't find anything on the interenet about reasonable compression numbers for a 3.8l.
Thanks!
The compressor for the A/C locked up and before I replace the system, I did a compression test on the engine. The first one I did was with about 4 - 5 revolutions of the crank and the second one was with 3 revolutions. Since I was doing it myself, I could not see when the needle hit the highest and had to assume the needle was pretty close.
1st round
Cyl 1 135
Cyl 2 141
Cyl 3 150
Cyl 4 125
Cyl 5 140
Cyl 6 140
2nd round
Cyl 1 120
Cyl 2 150
Cyl 3 135
Cyl 4 130
Cyl 5 142
Cyl 6 130
This is with all plugs out, cold engine, fuel off. I just want to know what the compression should be if new and what is acceptable in this engine before I say, enough - it's time for a new engine. I plan to re-run the tests with a remote starter so I can see the numbers as they rise and stop rising. Drop a couple tablespoons of oil in and see what happens too. So, before I spend $500 bucks on a new A/C system, are these compression numbers good enough? I couldn't find anything on the interenet about reasonable compression numbers for a 3.8l.
Thanks!
1 - 10 of 10 Posts
There is no intake. You have to remove the intake to get to the cylinders on the left side.I assume the throttle was wide open?
Al
When doing a compression check, the actual numbers are far less important than consistency across the numbers. 120-130 is a little low, but it’s enough to get combustion, so I wouldn’t be too concerned about that. What does concern me is that there is significant variation not only between different cylinders, but between the same cylinders on the different tests. That leads me to believe the test was not done properly. The throttle needs to be held wide open the whole time it is cranking. Every cylinder should be cranked until the pressure stops rising. The car should ideally be on a battery charger, or at the very least make sure the battery is fully charged to eliminate variations in cranking speed. Given such differences between the numbers on the different tests, I would re-do it a 3rd time. If you don’t have a second person to do the test with, just crank it an extra long time to make sure you have gotten the highest possible number. Once that is done, look at the numbers and ideally you want no more than 10-15psi difference between the highest and lowest readings.
Correct, the tests are not done correctly, I know that. And as mentioned, I was doing this alone and not able to watch the meter. Also mentioned, my next test will be using the remote starter so I can watch the pressure rise in the meter. As for a throttle, there is no throttle! You have to remove the whole upper intake to access the spark plugs. What I did realize is that I had taped over the lower intake to keep anything from falling in and that could effect the pressure coming into the cylinder. I will remove the tape to expose the lower intake.When doing a compression check, the actual numbers are far less important than consistency across the numbers. 120-130 is a little low, but it’s enough to get combustion, so I wouldn’t be too concerned about that. What does concern me is that there is significant variation not only between different cylinders, but between the same cylinders on the different tests. That leads me to believe the test was not done properly. The throttle needs to be held wide open the whole time it is cranking. Every cylinder should be cranked until the pressure stops rising. The car should ideally be on a battery charger, or at the very least make sure the battery is fully charged to eliminate variations in cranking speed. Given such differences between the numbers on the different tests, I would re-do it a 3rd time. If you don’t have a second person to do the test with, just crank it an extra long time to make sure you have gotten the highest possible number. Once that is done, look at the numbers and ideally you want no more than 10-15psi difference between the highest and lowest readings.
Also, as mentioned, I need to do another test as these pressures are inconclusive and should be consistant. I am also aware of the 15% difference between the highest output recorded and the lowest.
Will update after I run this next set of tests.
You absolutely do not need to remove the upper intake to do spark plugs on a 3.8NA, that is why I commented on that, thinking maybe you meant you had removed the intake tube, since that part does have to come off to access the plugs. But regardless, if the ports were taped up, that is absolutely going to skew the results. Re-do the test with nothing restricting the intake or exhaust and see what numbers you get then.
Theoretically, the pressure maximum is 14.7psi x compression ratio. It will never go that high, tho.
You are correct, but actually, I was replacing the injectors with new ones, which requires the upper intake to be removed.You absolutely do not need to remove the upper intake to do spark plugs on a 3.8NA, .....
I ran the test again in the two lowest cylinders with the gauge high enough that I could see it through the front window while cranking. I also took the tape of the lower intake. I cranked the engine until the needled stopped moving on the two lowest cylinders, #1 and #4. #1 hit 142.5 and #4 hit 150. I think I am okay.
Got the A/C condenser replaced, radiator back in and filled back up and now I am waiting on my set of spark plugs so I can get those in and then finish installing the new compressor and the rest of the new A/C parts.
What I did find that was odd, and wondered why I did not catch this the last time I worked on the heads, but there was a large O ring in #4 in the fuel rail on top of the #4 injector that had an O ring already. Hmmmm...
Thank you T6Rocket, MadMikeyL and Grog6 for your input, it is much appreciated.
You want to use new o-rings, the old ones are dissolving by now. The new ones are green and blue, one for supply side, and one for the intake side. New ones, filters, and all can be found on ebay
Got myself covered on that. Got the full kit, compressor, full o ring set (more than i will ever need), A/C Receiver Drier / Accumulator, orifice tube, new suction and lines and will have a shop do the re-vac. Did this on my Town Car, but only replaced the compressor and accumulator. Thanks for the reminder note though, much appreciated.You want to use new o-rings, the old ones are dissolving by now. The new ones are green and blue, one for supply side, and one for the intake side. New ones, filters, and all can be found on ebay
1 - 10 of 10 Posts
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