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Compression drops 40-PSI after 500 miles - normal? [Mitsubishi/Dodge Eclipse-Talon `94, 1.8 lt.]

I had the head professional done to the point of sparkling clean. I apply 20w-50 synthetic oils to the cylinder walls and install the head, torque in three steps to 72 lb using genuine gasket. I check the compression and they read 190, 190, 190, 200-psi. It runs great. After 500 miles, I retorque the head bolts precisely to 72 lb.

I remove the 500-miles old spark plugs and found so much carbon deposits but white on the ground electrodes. I check the compression and they read 150, 150, 150, 160-psi. It still runs great. Does anyone have suggestions why I could've lost 40-psi? Are something here you would've done differently?

Thanks

Reply to
Tibur Waltson
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When you checked the compression the oil would still be on the pistons and rings giving a falsely high compression reading! Any oil laying atop the pistons reduces combustion chamber volume and would also bump up compression temporarily! What do they call for? Are they new rings or

100,000 miles? Now the used r> I had the head professional done to the point of sparkling clean.
Reply to
Dodgem440

you didn't say anything about honing the cylinder and re-ringing the pistons... your initial compression numbers were arrived at with the cylinder coated with oil. If you were to squirt oil down the plug hole now and retest I would expect your numbers to rise back to the 190~200 numbers that you first saw.

150 psi is not bad, the service limit is 131psi.... The only thing that catches my eye here is the 10 psi jump on #4 cylinder. is the piston top covered in carbon? 14psi in the max difference in compression per the factory manual.

Reply to
simpleton

squirt some oil in the hole and try it again. if it goes up its ring related

Reply to
robs440

Sounds like the oil from the install was giving you a false reading..What are the factory spec's on the compression? The 150ish does not seem terrible to me...?? I've found that you have to be suspicious of gauges too....And don't forget to hold the throttle open as you test so you get airflow......

Reply to
Outlander

And run the compression test at the same temp as the previous one...

Reply to
Stephen Bigelow

To all responses:

Yes, the rings are 100,000 miles old. I didn't hone the cylinder and re-ring the pistons...

No, the throttle weren't open on the second compression test. :- (

No, the compression test immediately after oil coating the walls and after putting the head on, not ever running the engine.

No, the test was done cold on the first attempt and warm on the second. :-(

Uhmm... I'm not sure what the shop manual call for in psi. Someone mentioned the service limit is 131psi. +,- 14.

Excellent idea. Will try it correctly this time.

The 10 psi jump on #4 cylinder is mysterious as the the jump occurred at the initial test which there are no carbon deposits of any kind (I clean out all the carbon deposits with a lot of carb cleaner before installation.) I coat the walls with oil because they were bone dry from the carb cleaner.

Someone mention I have to be suspicious of gauges. I'm suspicious of this $45 gauge because the needle would jump to 210-psi, for example, then a couple seconds later rest and hold steady at

200. Maybe wrong method. Anyway, I doubt my method is correct because I've been genuinely advised of a few mistakes already.

Tibur

Reply to
Tibur Waltson

"Uhmm... I'm not sure what the shop manual call for in psi. Someone mentioned the service limit is 131psi. +,- 14."

that was me...131 is the lower service limit after which the motor needs rings and a hone. the 14 psi figure is the maximum differential between cylinders not the tollerance range.

"Someone mention I have to be suspicious of gauges."

snap-on is pretty much the benchmark standard. the biggest difference in gauges is the plug adapter portion that goes into the plug hole. there is a variation from brand to brand as to how much volume the adapter displaces in the combustion chamber which changes the peak psi reading on the gauge.

retest, on a warm motor with the throttle open and i bet you end up very healthy numbers. good luck.

Reply to
simpleton

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