Another Rough Idle Mystery - PeachParts Mercedes-Benz Forum

mbboy · #1 07-09-2009, 03:09 PM

1981 380sl. Car runs spectacularly after top end rebuild, but hiccups at idle. However, if AC is on, idles smoothly. Electrical components are all in good shape. Suspect vacuum leak, but why not with AC on? Any ideas?

ctaylor738 · #2 07-10-2009, 09:46 AM

AC "on" signals the idle relay to increase the RPM at idle. So you may have an injector or port in the FD that isn't quite cutting it at normal idle.

mbboy · #3 07-13-2009, 07:32 AM

Quote:

Originally Posted by ctaylor738

AC "on" signals the idle relay to increase the RPM at idle. So you may have an injector or port in the FD that isn't quite cutting it at normal idle.

Great deduction, Chuck, though the fix was even easier than you suggested. I enriched the mixture, and turned the idle up from around 600 to around 700. Now she purrs like a kitten with the AC on or off.

Thanks for the suggestion.

ctaylor738 · #4 07-13-2009, 08:09 AM

Glad it worked out, but you might want to dig a little deeper.

The mixture on these cars usually doesn't need to be adjusted unless there is a problem or change somewhere else. You may still have a marginal injector, a slight leak in an injector seal, or a problem with the FD. By richening the mixture, you may have added enough extra fuel to mask the symptoms, but now you may be too rich on the other cylinders.

Did you use a meter to set the mixture, or did you just tweak it?

mbboy · #5 07-13-2009, 04:02 PM

Quote:

Originally Posted by ctaylor738

Glad it worked out, but you might want to dig a little deeper.

The mixture on these cars usually doesn't need to be adjusted unless there is a problem or change somewhere else. You may still have a marginal injector, a slight leak in an injector seal, or a problem with the FD. By richening the mixture, you may have added enough extra fuel to mask the symptoms, but now you may be too rich on the other cylinders.

Did you use a meter to set the mixture, or did you just tweak it?

I just tweaked it, but as long as it's running as good as it is, I can't see looking a gift horse in the mouth. OTOH, if the problem returns, I guess I'll have to.

rowdie · #6 07-13-2009, 09:36 PM

As Chuck said, your idle should increase with the AC on. You still have a problem. If you choose to ignore it, that is up to you.

As to your other thread. The PS pump is a closed system with no enigine oil going to it.