Mechanics

COMPRESSION TEST

1999 Saturn SC2 • 150,000 miles

I have a 1999 SC2 that is idling rough. It does burn a little oil too. I did a compression test. I had never done one before. For the dry test, all 4 were 195 to 210. When I did the wet test the pressure increased significantly, 270 to 300. I did not expect that much of an increase. According to what I have read, a significant increase in pressure means the piston rings are worn. I am not sure that I trust the results of the test. Could I have added too much oil?

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RandyMI
November 21, 2010.




Don't really think you have a ring problem specs are 185-205 psi.
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Mmprince4000
Nov 21, 2010.
Thanks for the reply. If the rings are ok, what should I check next? Should I do a leak down test? I have never done one, but it does not look very hard.

Tiny
RandyMI
Nov 21, 2010.
If your compression is good then doing a leak down test is a waste of time the only time you would do a leak down test is when you had a low compression issue. So does the engine only run rough at idle?Also do you have any codes?

Saturntech9
Nov 22, 2010.
The engine runs rough at idle and seems to lack power. I do not notice it running rough when driving 30 MPH or above. I took it to Autozone and had them hook up the scanner. The guy said that cylinder 1 was misfiring. I did not actually see the code. I have checked the plug wires by doing a continuity check. The wire for cylinder 1 was just over 6,000 ohms. The rest of the wires checked ok too. I also recently replaced the plugs. I checked the resistance on the fuel injector, it was 12 ohms, which is in specs according to the Haynes manual. I also listened to the fuel injector with the engine running by using a long screw driver like a stethoscope, and I could hear it clicking very fast. Would trying a spark tester make sense next? Thanks again.

Tiny
RandyMI
Nov 25, 2010.
Doing a spark test would only help if the number one was a dead hole meaning it never ran. At idle you can determine that by taking off one spark plug wire at a time and see if the rpm's drop down to let you know if that cylinder is running or not. As far as the wire's go they they could test good and still be bad when put under a load. How are the plug's what kind are they?

Saturntech9
Nov 28, 2010.
I did as you suggested and there was no noticeable difference in rpm when I took off the plug wire from number one. I did put new plugs in when I first started. The plugs are AC. They were about $2 each. There was a more expensive plug, about $8 each, but I went with the less expensive ones.

Tiny
RandyMI
Dec 3, 2010.
As long as you don't have platnuim plug's in your engine you will be ok. Have you checked fuel pressure?Back pressure?Also have you made sure your plug wire's are routed correctly?

Saturntech9
Dec 3, 2010.
I pulled the plug last night, just to double check the gap and I noticed that the plug was wet with gas. When I went to check the gap, I noticed that the porcelain around the tip slipped down. I replaced the plug again and I think number one is firing now. I think it is good. Thanks for all of the advice.

Tiny
RandyMI
Dec 4, 2010.
Glad to hear you got it figured out that's what we like to hear.

Saturntech9
Dec 5, 2010.

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