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Engine irregular at idle. Misfire, Low Compression, Vacuum leak?

6K views 4 replies 5 participants last post by  audinut!$ 
#1 ·
Within the last week or so my '90 740 started to shake strangely at idle. Its not uniform shaking and it should not be as I just had the engine mounts replaced earlier this year. I believe that it could be a misfire as the engine runs smoothly at speed. I wonder if after 136k the engine's compression has fallen a little to much. I really do not want to replace the rings. I really hope that it just needs new spark plugs. Guy at autozone said it could be a vacuum leak. Don't know much about the vacuum systems on cars. Any thoughts?

-Poritz
 
#2 ·
Re: Engine irregular at idle. Misfire, Low Compression, Vacuum leak? (Poritz)

Low compression generaly wont cause rough idle, more the opposite. Clogged fuel injectors, bad plugs, wires, and coil paks will do the trick. I just removed Bosch platinum plugs from my S40 because after 1200 miles it started to stumble and shake at idle. Put the old Volvo plugs back in, and all is well. Check the really simple stuff first, more often than not, the fix is cheaper than expected. How's the timing belt?
Doc http://********************/smile/emcocktl.gif
 
#4 ·
Re: Engine irregular at idle. Misfire, Low Compression, Vacuum leak? (Poritz)

You're worried about compression on a 136K mile motor??? Unless it has suffered some serious abuse, it should be in excellent shape.

A stumble at idle could be a number of things. Vacuum leak, bad/dirty/old spark plug, loose spark plug wire, worn or cracked cap or rotor, improperly set idle, gummed-up or worn-out IAC valve... those are the things that come to mind.
 
#5 ·
Re: Engine irregular at idle. Misfire, Low Compression, Vacuum leak? (11110000)

Quote, originally posted by 11110000 »
You're worried about compression on a 136K mile motor??? Unless it has suffered some serious abuse, it should be in excellent shape.


Speaking of serious abuse I just pulled a motor with 118k and 3 big holes in the block and peaking out one of them was a rod that looked like a pretzel.
 
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