1982 380SL intermittent stumble at idle - PeachParts Mercedes-Benz Forum

timm9 · #1 08-18-2014, 05:45 PM

I have searched not only this site, and globally with no matching or similar results.

1982 380SL with 39,xxx original miles that is my ‘midlife crisis’ and is a garage Queen. I spent the last several years (when I have time) chasing down and correcting the ‘high idle’ issue but now find I have an intermittent ‘stumble’ at idle. You can start the car (cold or hot makes no difference) one time and the idle is perfect at 550 rpms with no stumble. Drive the car or shut it off and immediately restart and the stumble re-appears. The rpms do not drop, and the stumble is not quite bad enough to kill to engine. I’m at a loss to see any ‘rhyme or reason’ to the intermittent stumble.

Many components were replaced while rectifying (I’m sure I’m forgetting some) the high idle including O2 Sensor, Plus, wires, oil temp sender, filters, fuel pump, water temp sender, ICM, etc. I have also replaced most of not all vacuum lines and connectors. Additionally, I have ‘cleaned’ the idle control valve and see no improvement or correlation. I have also checked all injectors with no leaks. The car runs well with mileage on long runs (I have 5 meetings a year in Los Angeles at 6 hours each way) coming in between 22 and 25 mpg. Could this be caused by an intermittently failing Ignition Coil?

Any suggestions would be greatly appreciated.

meltedpanda · #2 08-19-2014, 01:05 PM

KJet - correct?

control pressures warm and cold within spec?

You say injectors don't leak, are they new, have you checked spray pattern?
Unequal flow or spray patterns could cause this issue

timm9 · #3 08-20-2014, 12:37 PM

Thank you for the response Melted Panda.

K Jet is correct. Additionally, during the 'high idle' fix all Injectors were removed, tested, and re-installed which was less than 1,500 miles ago.

timm9 · #4 08-20-2014, 12:37 PM

And control pressure was also checked at the time...

meltedpanda · #5 08-20-2014, 04:31 PM

Hmmm - have you checked adjustment on airflow sensor plate, does it move easily at the fulcrum point?

auxiliary air valve working correctly? Power to it , and opens up

Cold start valve - not leaking is it ?

If all this checks out - you could have an intake to engine block air leak - those seals can sometime be troublesome

all I can think of right now

timm9 · #6 08-20-2014, 07:25 PM

Thank you again Melted Panda.

The sensor plate, cold start, and auxiliary were double checked also during the high idle fix. Another suggestion I received was to check the 'rear axle ventilation valve(s)'. At first I thought they were joking, but after reviewing EPC there is indeed a left and right ventilation valve mounted on the differential at approximately $35.00 each. Have not had time to get it up in the air to look...

meltedpanda · #7 08-20-2014, 07:32 PM

Just call me Ron.

Indeed there are vents. I guess that could cause uneven fuel flow if blocked. Those are for the fuel tank. Good luck

sublettd · #8 09-28-2014, 09:50 PM

I've had a similar problem for years...rough idle at low rpm. Unable to get the idle RPM above 475 rpm.

Had a busted ring shortly after the rough idle first appeared. I wound up rebuilding the engine and much of that saga is recorded elsewhere in these forums. After putting the engine back together the problem still existed.

The idle on my engine (117.962) is suppose to be 650 +100/-50. The engine runs rich, causing fouling of the plugs, which also likely contributes to the rough idle. As a work around to get the idle up, my mechanic friend (35+ years of working on Mercedes) advanced the timing and also adjusted the linkages to get the idle up to 750 rpm--overriding the idle speed control unit (ISCU). While he managed to get the idle up, he did nothing to lean out the fuel mixture. Even tinkering with the FD adjustment doesn't lean it out. That is evidently controlled by the throttle and the ISCU.

I've chased the fuel distributor and warm-up regulator (both calibrated by Larry Fletcher over in Foley, AL), injectors, ISCU (black box behind the glove box), and many of the other things that have been mentioned above.

What my mechanic friend says the problem most likely is in the throttle valve that sits down in the front of the engine...where the vertical linkage disappears into the engine. The only way to get to the throttle valve is to pull the fuel distributor/throttle body (everything that sits in the "V" between the heads). This is a major undertaking and I'm not sure the pain is worth the gain. If you want to pay a mechanic to do it, you'll be hit for at least 16 hours of labor.

You can't get to the throttle valve because the throat of the black rubber diaphragm that is underneath the throttle body assembly fastens over the throttle valve. Cut into the diaphragm and you mess up the air flow. It has to be disassembled.

In my case, it seems that in order to get the engine to lean out and run smoothly I need to get the throttle valve issue taken care of so the ISCU will work as it's supposed to. Pulling the fuel system off is a job I have done twice, but it's not something I relish doing again. But I'm not going to pay a mechanic 16 hours labor to do it. So, I may.

I hope your problem is not the same as mine.