rough idle

Brandon Rogers brogers at
Tue Mar 21 11:26:27 EST 2006

my motor seems to be idling a little rough these days - 

Idle speed is just over 750 rpm.  First I thought maybe it was a slight vacuum leak - haven't ruled it out yet - but the ECU has no stored codes.  I did pressure test the system not too long ago and found no leaks.  Also vaccum at cold idle was only about 9psi yesterday - usually it is closer to 11 -12.  Forgot to check again today.  FWIW temps are cold now - single digits and teens and my oil is old by my standards (but maybe not be some other people's, ahem) - M1 w/ about 7000 mi +/- (includes a 4500 mi highway roadtrip).  Warm idle vacuum is about 12 psi.  Was about 12-13+ psi vacuum  last I remember....

so lower vaccum at idle would indicate a small leak - I'll have to see if this was just due to extra cold..???  When I go to lunch I'll try to remember to look at idle vacuum at cold start.

So maybe there is a very small tear in a hose - any other ideas?  How much vacuum should there be at idle?  I know this is difficult to answer for many of you do to that awesome digital boost gauge...

Also - the motor does seem to be performing on par.

Any other thoughts?  Any input appreciated-



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