88 560SEL - Idle Stumble - PeachParts Mercedes-Benz Forum

Jigglybus · #1 04-30-2012, 05:36 PM

Hi,

My idle has a hard random stumble. It goes away above 1000 rpm, and purrs from that point up. It's worse when in gear as the idle is slightly lower.

I've changed plugs/rotor/distro/wire but no luck. Car has various vacuum problems (door locks, climate control), but as the idle is fairly low (700ish) I was hoping this isn't the problem.

Any ideas?

(ps.... you might notice I have a few new posts on here, sorry for the flood, I have quite a few gremlins... thanks for your help!)

JamesDean · #2 04-30-2012, 09:21 PM

I posted up in your other thread but I'll comment here as well. Check your engine for vacuum leaks. Common places are the big black plastic tubes under the air cleaner. They get hard and leak. Also check the vacuum line that runs over to the EZL/Ignition Module, my friends 560SEL had a break in that line and it caused a bit of hesitation/stumble.

The door lock system is vacuum operated but separate from the engine vacuum equipment. The climate control is not separate however. I would suggest getting a MityVac or other vacuum testing device and pull a vacuum on each connection to the switchover valve tree located on the passenger side of the center console behind the carpeted piece. Each of those connect to a vacuum pod that might be leaking. Its common for the recirculation door pods to go bad.

Both 420SEL and 560SEL idle about 550-600 at operating temperature in drive.

pmckechnie · #3 05-01-2012, 09:16 AM

Have some one smoke the engine to find vacuum leaks. To smoke an engine means to seal up the air intake, pcv system, etc then inject smoke into the vacuum system with a smoke machine and then look for smoke coming out where it shouldn't. Also, what kind of plugs did you use? Any thing other than NON-RESISTOR COPPER CORE PLUGS can cause the problem you have.
Paul

pwogaman · #4 05-01-2012, 02:48 PM

Make sure your EGR valve is not stuck open. That will give you just what you're describing (don't ask me how I know).

Jigglybus · #5 05-01-2012, 04:38 PM

Whats the easiest way to check the EGR valve?

pwogaman · #6 05-02-2012, 08:08 AM

I'm not sure how easy it is to see on a 560 versus a 300. On the 300 you can simply connect a hand vacuum pump to the EGR vacuum line and see if it operates through the side of the EGR. There is a pin that should move up and down. On mine it did not. I sprayed the hell out of it with WD40, to include inside the EGR vacuum actuator pod, and let it sit overnight. It worked loose by morning and had been fine ever since.

Someone else will have to chime in on where or how to best access the EGR on the 560 engine.