: Compression test results 98 Seville

03-28-05, 12:27 PM
I performed a compression test on my 98 STS (65000 miles) over the weekend. The throttle was NOT open during testing. I cranked it about 5-6 times for each cylinder.

Cyl 1st stroke/Final stroke
1 100/160+
2 120/160+
3 120/160+
4 130/160+
5 120/160+
6 110/160+
7 90/150
8 120/160+

I also replaced the plugs, AC/Delco 41-950, gapped at .050. The ones that came out were pretty clean, no oil, just a little white ashy material. The gaps were all over the place, from .055 to .062. It is idling much better now.

Based on my limited knowledge, I don't think I've detected any problems. Experts, please comment.

KD in NH

03-28-05, 12:38 PM
I don't like #7 personally. I'd try a Top-Engine cleaning to eliminate carbon as a possibility but you "could" be looking at a potential problem in that cylinder.

03-28-05, 12:47 PM
When you mention "idling much better", was you engine a little rough during idle before? A little rough meaning that when at a stop light there is just a little vibration while the engine is idling.

This is how my 99 STS idles most of the time. Now at 75000 miles, I would consider cleaning and regapping my plugs, or replacing them with new ones if I knew I could minimize/eliminate this.

03-28-05, 12:50 PM
WOT the engine a few times. In 50mph second gear WOT untill 75-80 mph then let off gas and let vehicle coast back to 50mph. Do this a few times. Sometimes helps to burn off carbon buildup and excercise the piston rings. This procedure has been outlined many times in here. I do this a couple times a month. Makes a difference. May boost the #7 compression reading also.

03-28-05, 01:10 PM
If the throttle wasn't open during testing the results are questionable at best.

03-28-05, 04:13 PM
Thanks for the quick replies

tstach: Yes, it was idling a little rough, minor vibration at startup and at stoplights. The new plugs seem to have helped, definitely went from minor to what I'd consider minimal or no vibration.

dkoz: what does opening the throttle do differently? I'm going to double check #7 (and the rest of them) in warmer weather, and I'll be sure to open it. Can I just press down on the pedal instead of blocking it open?

The white ashy look of the plugs is normal, right? Could the gap have increased by lots of WOT?

Thanks all
KD in NH

03-28-05, 04:23 PM
When the throttle is closed it restricts air flow into the cylinder so there is reduced air in the cylinder to compress which gives a low reading. Just hold the throtle open 1/2 or better. This will affect the first stroke more than later ones. Your plug gaps just show normal wear and were probably good for many more miles.

03-28-05, 04:36 PM
Thanks dk, seems if the readings were low then I'm in pretty good overall shape. I'll still redo soon to confirm.

03-28-05, 09:44 PM
I think you are perfectly fine. Drive it. Do an occasional/frequent WOT accel to redline in the lower gears to clean the carbon out and keep the rings exercised.

When doing a compression check like that always block the throttle open. With the idle air control operational there is always some inconsistency in the readings due to the IAC allowing more air bypass at one time than another...effectively opening the throttle different amounts and causing a change in the pumping efficiency of the engine while you are cranking. Opening the throttle will negate this effect.

In any case, the numbers are plenty high and fine for comparison. It needs to be aired out frequently and driven now.