Sudden Drop In Idle Speed

[Helios59]

Hi all,

So I just noticed that my warm engine idle speed has dropped from about 900 to about 775 rpm. Came out of nowhere. It's a 1986 w/Motronic, no mods.

Oil pressure and throttle response are unchanged - both are fine. What could have caused this?

I changed from 15W-40 to Motul 10W-40 oil but that was 900 miles ago, and the idle drop just manifested itself today. Nice weather - in the high 70s.

Thanks in advance.

[steely]

Hi Ray - one potential cause would be a vacuum leak, so check the vacuum lines to make sure they're connected, and their condition to make sure they're ok. If they're original, they are suspect. Even check the oil cap to make sure it's on. You can search on low idle or vacuum leaks. Let us know how you make out.

[JJ 911SC]

Originally Posted by steely

... Even check the oil cap to make sure it's on...

+1.

If its on correctly, remove it and if the RPMs do not drop further down, you definately got a vacuum leak.

[Joe Bob]

Yeah the cap....my first choice. If it's tight and doesn't drop further even if you block the filler with your hand ...there IS a leak....just not there at the cap.

[Helios59]

Y'all were on the money...I didn't have the oil filler cap on correctly.

All is well now.

If only they ALL were that easy!