Weird compression test results
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Weird compression test results
My '01 Ram has been getting more and more sluggish recently and has seemed to be doing some strange shifting (seems to want to upshift at weird times). I also heard what I believe to be EXTREME pinging under heavy engine load this week while driving up an incline at hightway speed. So, I decided to start troubleshooting today. I really feared a head gasket issue, because the last time I did a compression test, I had a couple of cylinders that read significantly lower than the others. However, I have never found any evidence of coolant in the oil, or exhaust gas in the coolant. I do go through coolant like crazy, but I am almost certain that is 100% related to the significant coolant leak I have at the thermostat housing, which I haven't been able to resolve yet. That is another story. I have also been burning 1-2 quarts of oil between oil changes, and my exhaust is pretty smoky.
So today, I figured I'd start with replacing the spark plugs and doing another compression test while I had them out. If my records are correct, the current plugs only had about 20,000 miles on them and were installed about 3.5 years ago. The current plugs were the OEM Champion copper cores, but after doing some reading here, I decided I'd try the Autolite copper plugs this time. I was expecting to find a couple of plugs that looked like they had been drenched in uncombusted fuel in cylinders 7 and 8 (which had the low compression test results last time). However, all of the plugs I pulled looked pretty normal, except the gap on every single one was WAY out of spec (between 0.05 and 0.07). I know the copper electrodes vaporize much faster than the fancy plugs, but with only ~20,000 miles, I didn't expect to have so much change in the gaps, particularly since I had these plugs out once since I installed them when I repaired my plenum (for the second time
), and I re-gapped them then.
Once I got all of the plugs out, I did a compression test. The first trial, I just did one check of every cylinder. Then, I repeated the check with each cylinder, doing two trials on each before moving on to the next. My results were as follows:
Cylinder #1: 160/101/118
Cylinder #2: 85/114/130
Cylinder #3: 120/134/132
Cylinder #4: 96/123/121
Cylinder #5: 95/107/124
Cylinder #6: 94/122/129
Cylinder #7: 106/138/142
Cylinder #8: 81/140/137
Huh? How is it possible to have such wild variations in the same cylinder? If they can be trusted, I'm thrilled with the final readings (my truck has 190k miles on it - original engine). But, CAN they be trusted? Each test was performed with with wide open throttle. I disconnected the ignition coil, but the fuel injectors were still connected and firing, so the only thing I can figure is that the raw fuel spraying into each cylinder cleaned up some deposits so that by the third trial, the piston rings were sealing better than during the first trial. Does that make any sense? I guess the other possibility is that my pressure gauge is a piece of junk.
So, my question is two-fold:
1) Is it normal to have such change in the spark plug gaps in 20,000 miles?
2) Assuming the test gauge is not the culprit, what could cause my extreme variation in compression readings?
For what it's worth, the truck does seem to drive better with the new spark plugs (no surprise there, considering the gaps were so large on the old plug), but it still seems to want to upshift at weird times. I am not convinced I have found all of the issues yet (of course, on a 16 year old truck with 190k miles, I will likely never find ALL of the issues...).
So today, I figured I'd start with replacing the spark plugs and doing another compression test while I had them out. If my records are correct, the current plugs only had about 20,000 miles on them and were installed about 3.5 years ago. The current plugs were the OEM Champion copper cores, but after doing some reading here, I decided I'd try the Autolite copper plugs this time. I was expecting to find a couple of plugs that looked like they had been drenched in uncombusted fuel in cylinders 7 and 8 (which had the low compression test results last time). However, all of the plugs I pulled looked pretty normal, except the gap on every single one was WAY out of spec (between 0.05 and 0.07). I know the copper electrodes vaporize much faster than the fancy plugs, but with only ~20,000 miles, I didn't expect to have so much change in the gaps, particularly since I had these plugs out once since I installed them when I repaired my plenum (for the second time
), and I re-gapped them then.Once I got all of the plugs out, I did a compression test. The first trial, I just did one check of every cylinder. Then, I repeated the check with each cylinder, doing two trials on each before moving on to the next. My results were as follows:
Cylinder #1: 160/101/118
Cylinder #2: 85/114/130
Cylinder #3: 120/134/132
Cylinder #4: 96/123/121
Cylinder #5: 95/107/124
Cylinder #6: 94/122/129
Cylinder #7: 106/138/142
Cylinder #8: 81/140/137
Huh? How is it possible to have such wild variations in the same cylinder? If they can be trusted, I'm thrilled with the final readings (my truck has 190k miles on it - original engine). But, CAN they be trusted? Each test was performed with with wide open throttle. I disconnected the ignition coil, but the fuel injectors were still connected and firing, so the only thing I can figure is that the raw fuel spraying into each cylinder cleaned up some deposits so that by the third trial, the piston rings were sealing better than during the first trial. Does that make any sense? I guess the other possibility is that my pressure gauge is a piece of junk.
So, my question is two-fold:
1) Is it normal to have such change in the spark plug gaps in 20,000 miles?
2) Assuming the test gauge is not the culprit, what could cause my extreme variation in compression readings?
For what it's worth, the truck does seem to drive better with the new spark plugs (no surprise there, considering the gaps were so large on the old plug), but it still seems to want to upshift at weird times. I am not convinced I have found all of the issues yet (of course, on a 16 year old truck with 190k miles, I will likely never find ALL of the issues...).
