Compression Test

[foreignracer]

I am just curious what typical compression readings are for a 98 non-PI GT. The readings I got were between 130 and 160. I did pull one plug that had a bit of oil on the threads and heavy carbon buildup on the plug. Another plug also had some carbon buildup but not as bad as the first. That bad plug was out of cylinder 5 and the other not so bad was out of cylinder 3. Any input would be great.

:Track:

 

[the98stang]

Your engine is slightly worn. Typically you want numbers that are within 10% of eachother. Cosistency is what you want. If you have oil on the spark plug threads, you are probably getting a little blow by. Poor a small cap full of oil down each cylinder and see if the compression numbers go up. If they do, there is something wrong with the rings or valves.

 

[cjones]

the98stang - if the numbers go up the rings are bad if the number stays the same the valves or headgasket is bad.

foreignracer - i'll assume you had all the spark plugs removed and the throttle at WOT when you did the test.

 

[the98stang]

the98stang - if the numbers go up the rings are bad if the number stays the same the valves or headgasket is bad.

foreignracer - i'll assume you had all the spark plugs removed and the throttle at WOT when you did the test.

I was wrong about the valves, sorry.

BTW- the test won't be greatly affected by the throttle plate. The test is more accurate on a warm engine with the plate open and plugs removed, but it won't affect the test that much.

 

[Dusstbuster]

When we did a compression test on mine all the cylinders were in the 150-160 range so that 130 might be a slight issue.

 

[foreignracer]

I pulled each plug individually and tested just cranking it, no throttle at all. I am supposed to pull all the plugs out at once and test them that way?

The only thing that I might have done differently is I might have turned the motor over longer on cylinders 1-4 vs. 5-8. 5-8 were the low ones. The number 5 plug did have a bunch of carbon deposited on the plug though.

 

[cjones]

pull all the plugs out and then do the test. remember to label the plug wires so they will be easier to reinstall.