Hi everybody
My car is a 1992 300TE with 140kmiles. Since yesterday, all of a sudden, it showed the following behavior: Car runs fine at higher speed. When I slow down to a stop, the idle drops below 500RPM, the engine cuts off sometimes because of that. When I drive slowly in the parking garage my foot off the gas the idle jumps from 1200 back to 2-300, the engine cuts off occasionally and may not be restarted immediately. This seems to occur only with a hot engine. Starting the car is no problem (unless, as noted above, immediately after a cut off), hot or cold.
From reading earlier posts, the OVP, idling valve, the O2 sensor or air flow meter are the main suspects.
O2 sensor and OVP have been replaced four months ago,distributor cap, rotor, wires are fairly new too; so I suspect the idling valve? What would be the best systematic way to approach the problem?
Thanks, Bruno

Unless you are an ace mechanic, experienced with MB systems, and have all the proper equipment. I would take the car to a indy specializing in MB. When I had problems with my '92 M103, I watched, and listened to the shop owner as he was tuning the engine after replacing the EHA and, forgot the exact name, idle control valve. He went into detail explaining how the various components functioned. It ain't simple. I would never attempt it.


Steve

That's not very good advice for a DIY forum. But yeah, probably the idle control valve. Check your coil too.

Maybe....maybe not. What Softconsult was trying to point out is there's a LONG list of things that can cause this and to narrow it down quickly and w/o breaking the bank, one much have the proper tools and diagnostic skills.

There are a few diyers that fall into this category, but not many. Most will end up replacing more than was needed before nailing it.

Loss of idle speed control is usually a combination of leaking idle control valve hoses, dirty idle control valve (sticking) and/or a bad fuel pump relay or fuel pump.

If the stalling is ONLY when making fairly firm stops from speed, the idle control valve is sticking, remove it, fill it will carb spray, cover the nipples, shake for 15 sec or so, and dump the carb cleaner spray. Repeat until the stuff comes out clear (it will take a while).

If the idle control valve hoses are anthing less than perfectly pliable, replace them -- most likely they are rock hard and loose, allowing the dirt in that causes the idle control vavle to stick, and causing a serious vacuum leak as well.

If it bucks or stalls while driving, the fuel pump relay is bad, or you have a bad fuel pump(s).

Most likely it's bad idle control valve hoses and a sticking idle control vavle.

None of this is difficult except replacing the manifold side hose, it's a pure pain, takes two people usually (one to guide, the other to push).

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Through experience, I would also include electrical control of the ICV and/or FPR as a suspect.

Check the connections to the ICV and make sure they are good, open up the FPR and check the soldered connectors are OK, they quite often need resoldering. Then do the same with the ECU, it suffers from the same solder problem. That's where I finally tracked down and fixed my idle woes.

Mind you, I also had to replace cracked hoses, a sticking valve and all the usual suspects along the way. Every time I fixed one bit, it only improved part of the overall problem, which made it incredibly frustrating.

good luck

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1987 300e manual 250,000 km (sold)

I thank you all for your excellent advice, especially Peter! The problem is intermittent and exclusively at idle - the car runs fine at higher engine revs. So I first get at the idle control valve and its hoses. If that does not help, I check more stuff you suggested. Thanks, Bruno

PS. One fuel pump is new, I'll check the other one. My car has an integrated FPR (MAS unit), unfortunately more expensive than a "naked" FPR.

The #1 cause of intermittent loss of idle speed control is a dirty throttle valve switch or sticky throttle linkage. If the ECU does not receive and idle position signal from the TVS, there will be no idle speed control. The TVS is probably the most overlooked component on the system.

Trace the TVS pigtail to the connector at the top of the inlet manifold - disconnect it an repeatedly exercise the throttle and look for consistent contituity at idle. If sometimes it remains open, thoroughly clean the TVS and throttle valve shaft with something on the order of a spray bottle with mineral spirits, then shoot a little WD-40 in the same vicinity.

The TVS has three poles - ground, idle, and WOT. I think idle is pins one and two, which is ground, and 3-2 is WOT, but you will figure that out by testing.

If the TVS idle signal is misbeaving, it would also be a good idea to disassemble the throttle linkage for a thorough cleaning and lubrication.

Duke

Great advice, thank you all! I will follow your suggestions (including electrical aspects of TVS, ICV and FPR) and post results. Have a nice day, Bruno

I stand by my position. People interested in Do-It-Yourself auto repair come to the party with a wide variety of skill levels. Repairing your car may be a hobby. It could also be a financial necessity. I have seen posts on various forums that are absolutely scary. In other words, the questions indicate little to no knowledge of things mechanical. Some people want to skip the tough part and just ask questions on the internet. When I learned , what little I know, it was a matter of reading manuals and figuring it out through trial and error. But way back then (60's) things were much more straightforward. Today you could make a mistake that could literally cost thousands of dollars.

The diagnostic steps are the tricky part. Throwing parts at a problem is not a smart thing to do. If a person doesn't have the knowledge, then it is a time intensive exercise to gain that knowledge. I think this is particularly true of electrical/electronic skills. Often a person doesn't have the correct diagnostic tools. Spending a bunch of money acquiring tools to do a job that might occur every 100K miles makes little sense to me. Here's a for instance.

I wanted to go through the front end on my '92. I elected to do the easy stuff myself. All tie rods, idler arm bushing, steering shock, shock absorbers. Then I took the car to my indy for ball joints, rear differential bushings, pinion seal, etc. I knew from a couple of forums that the pinion seal is tricky. Mess it up and may be buying a differential. It's still D.I.Y., but with decisions to farm out what I couldn't do or simply didn't want to do myself. The huge advantage that an independent brings to the party is seeing the same things over and over again. An individual with a couple of cars typically has a much smaller set of experiences.

Want further proof? Get to know a mechanic. Ask them how many times they are called upon to fix what a Do-It-Yourselfer screwed up.

Just my opinion.

Steve

Steve, I guess the point is to know one's limits. I doublecheck information I get online with my WIS CD manual since I want to understand what I do (I will take some time to digest the information I got here too). Unfortunately I had bad experience with both dealerships, independents, tire shops etc. Doing my own repairs, I learn a lot, save (big!) money and will have a nicely running car.
Just one bad indy experience for illustration: my car cut off in the middle of the run. I had it towed to a recommended independent specialized in MB and BMW. It turned out to be the rotor, distributor and OVP. Those are standard items to check in such conditions as I learned later from this forum. I got a bill of 850$ of which 4.5 hrs were for diagnosis only. I have more examples of this kind (I may attract bad luck, who knows), bottomline is I prefer to do my own repairs whenever possible.
Bruno

Yes, a key ingredient is finding someone you can trust that is at least somewhat reasonable. I found one in my area. Very honest. Charges $40 bucks an hour. I told him he was too cheap.

Knowledge is power. The internet and forums like this one provide armament when listening to what mechanics allege. It can take them a while to figure out
that you really do know something.

One guy claimed that I needed a valve job based only on sparkplug appearance. I told him that, in my opinion , the plugs looked normal. I told him a leakdown test was the way to diagnose valves. It's now about 70K down the road with no valve problems.

I still don't really trust any one relative to car repair. I always go behind an very that tire pressures are correct, that oil level is correct etc etc. Have found some major oops.

Steve

Hi all
I (actually we, I always get generous help from another forum member) have cleaned the ICV and replaced its vacuum hoses. The hoses were due, the ICV was relatively clean. We also replaced the air mass sensor potentiometer. Doing so, I realized that all the vacuum lines are very brittle and I will replace them as well. In addtion, we installed a new air mass sensor potentiometer, the old one was worn.
However the stalling condition did not get cured. In a next step, I will therefore get at the throttle control switch (is this part also called microswitch?) and linkage. I will also replace the coolant temp and intake air sensors on this occasion.
I will keep you posted, have a nice day, Bruno

The TVS is on the end of the throttle valve shaft opposite the throttle stop. The "microswitch" is on the linkage.

Duke

Hi all
I checked the throttle valve linkage, cleaned and lubed it. That seemed to help - the problem has not returned so far. Thanks, Duke2.6!
Bruno

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1992 300TE 160 kmiles