- This topic has 5 replies, 4 voices, and was last updated 10 years, 3 months ago by
.
-
Topic
-
I’ve just bought the OTC compression test kit to try and help narrow down my intermittent misfire. The supplied instructions say to warm the engine up to normal operating temp before removing the spark plugs, but I’ve always been warned not to remove or install plugs unless the engine was cold, in order to avoid seizing problems due to expansion/contraction.
What to do? It’s a Jeep Liberty 3.7L, so an aluminium head.
-
Replies
-
Pulling the plugs with the engine warm will not hurt the engine. Just be careful not to break or cross thread the plugs.
The compression numbers on a cold engine compared to a warm engine can vary drastically. The only time I do a compression test on a cold motor is a crank no start condition.
Look for around 170+ psi on all 6 cylinders a good healthy motor will vary about 5% between cylinders 10% is acceptable variance anything more typically indicates mechanical probblems
[quote=”SigSaurP228″ post=117559]Pulling the plugs with the engine warm will not hurt the engine[/quote]
So why is it so often recommended not to change spark plugs unless the engine is cold? I’m understandably keen not to damage the threads in the cylinder head 🙂
Crack the plugs loose when the engine is cold. Then snug them back just enough so that the engine can idle until warm. I agree, a warm engine test is the way to go. Also, the throttle plate needs to be wide open when doing the test.
[quote=”SigSaurP228″ post=117559]Pulling the plugs with the engine warm will not hurt the engine. Just be careful not to break or cross thread the plugs.
The compression numbers on a cold engine compared to a warm engine can vary drastically. The only time I do a compression test on a cold motor is a crank no start condition.
Look for around 170+ psi on all 6 cylinders a good healthy motor will vary about 5% between cylinders 10% is acceptable variance anything more typically indicates mechanical probblems[/quote]
Because it is easier to cross thread or break the spark plugs when the engine is hot. But compression numbers are much more reliable with a warm engine. I actually meant to say that earlier break the spark plugs loose with the engine cold, reinstall with anti seize and then warm the engine up, remove then run compression test.
If it is an intermittent misfire I would tend to believe more of an electrical problem than compression problem. I would check spark on all 6 cylinders first then go on from there.
- You must be logged in to reply to this topic.