1991 318is Rough Idle...

[Irishguy]

Hey Folks,

Here's the issue:

It only has 78,000 original miles, I am the second owner.  I just bought it a couple of weeks ago, and it is an amazing car.  Well maintained and always garaged.  It is pristine.  The car runs great accelerates smoothly and has great power.  However when it idles, it just idles a little rough.  

I have already made sure that it has fresh gas and I have run some good Lucas fuel injector cleaner through it for a couple of tanks.  I have also ran the car pretty good, some redline shifts, etc...  Because I got the impression from the husband of the lady that owned it that she just sort of puttered around in it.  It may have some carbon so I am also considering doing the whole Seafoam upper intake cleaning process on it too..  I have done it to a couple of my Miatas with good results.

Next, I am going to take a look at the throttle body and see if there is any gunk in there that may be keeping the butterfly partially open.  If it looks dirty then I will clean it with a toothbrush and carb & choke cleaner.

If that doesn't work I am going to clean the ICV and see if that helps.

If that doesn't work I am going to take a can of ether and spray around the intake and vacuum hoses while the car idles and see if it idles up which will tell me if there is a vacuum leak.  If that is the case then I am goin go go ahead and do the heater plate delete and clean up the old vacuum hose mess with new lines.

Does this sound like a good game plan?  Are there any other "known issues" that I should be looking into regarding this problem?

Thanks in advance for any help.

[DesktopDave]

Your plans are exactly what I'd do.  The ICV and vac hoses are always my first suspects. Clean the crank position sensor as well...and check the AFM door for smooth operation.  I'd see how old the plugs were too.  Hopefully it's just the little things.

I'm also of the opinion that this is a pretty rough-running engine normally...especially compared to the very smooth Mazda 1.6/1.8 DOHCs.  BMW put very expensive engine mounts in the car to damp down vibration, along with a very heavy flywheel.

The heater plate/hose mod is a great idea too.

[Choking Hazard]

Yep.  Check the ICV, although they usually don't gunk up on low mileage cars.  Also check the plugs...