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

 

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 rings will be letting a little compression
by, pretty normal!
Dodgem

 

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.

 

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

 

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......

 

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

 

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

 

"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.