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Old 04-20-2007, 12:25 PM
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Join Date: Oct 2006
Posts: 7
'92 300D (W124) n/asp idle speed problem

I would appreciate some advice please!

Car is a 1992 UK 300D W124 (straight 6, non turbo) in daily use, with 160,000 on the clock, good oil pressure and no more or less rattly than any other 300D. Generally it runs well.

I bought the car four-ish years ago at 116,1000 (and let me say it's the best car I have ever owned). Anyway, with the transmission in park, if I revved the engine to, say, 3000 rpm and then let the revs drop, they would fall to 500 and then quickly come back up to 650. It was possible (but difficult) to stall the car by, for example, letting the revs drop with the power steering on full lock. I had some suspension work carried out by Mercedes-Benz at this time and asked them to raise the idle speed slightly, as I felt that this was indicative that it should be higher. They said that there didn't appear to be any way to do this. This seemed odd to me. They also did not appear to be very knowledgeable about this car though...

Around a year ago (yes, I know!) the idle speed started to fluctuate. It would drop from 650 rpm to 500 rpm, and then jump up suddenly to 650/700, before settling. I think it happened when it was cold and it certainly happened when the engine was hot. (An interesting experience with auto transmission when sitting in traffic!) A few minutes later, and it would do it again. It seemed to do it more when the oil was towards the lower end of the dipstick and less when I topped it up, although this may have been my imagination. When it jumped back up, it was far faster than I could possibly make the revs rise by pressing the accelerator, it was almost an instant jump. It did this for a couple of months.

Then it stopped doing it. The revs have dropped permanently to 500 rpm (and dies on idle when the engine is cold). I was busy (or lazy) and certainly didn't have money to have it investigated, so I became an expert at driving it with two feet and it hasn't bothered be in the slightest for the past year/10,000 miles. However, I would really like to get to the root of the problem and I would appreciate any suggestions as to diagnosis/repair.

Very many thanks in advance.


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Old 04-20-2007, 04:38 PM
dieselbeagel's Avatar
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Join Date: Nov 2003
Location: California
Posts: 334
Have u (or has the previous owner) changed the OVP since u owned the car?

It is a square top, about 3-4 inches long, has a couple of fuses on top. The old one may have only one fuse.

It sits behind the battery, shielded by a plastic guard. Remove the plastic guard behind the battery and u will see a bunch of components back there.

If u r not sure which component it is, go look at one at the dealer, or better still buy one and u will know which to replace.

Mercedes has updated this component a few times after the original design. The dealer should have the latest. I would not go to anyone else as they be trying to offload the old inventory.

Hope this solves ur problem.
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Old 04-20-2007, 05:59 PM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,558
So I'm guessing it won't pass MOT running as is...

If the car is at all like U.S. model Merc diesels from that era, it sounds as if the electronic idle speed control has died. The diesel injection pump is a mechanical affair, so the car will run fine. But the idle will be too low.

There are a bunch of possible causes, so you'll likely need to get a local diagnosis rather than one via the Internet. The previous pointer to the OVP is not the worst guess, but if it were correct then your ABS warning light would also be illuminated.

- JimY
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Old 04-20-2007, 06:16 PM
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Join Date: Jan 2003
Location: Matthews, NC
Posts: 1,356
I don't know if it applies to the 300D but I have been working on a 92 300E that the OVP relay was bad but I never got an antilock brake error. I know the abs worked. I kept telling myself that the problem I had was not the OVP relay because the abs worked. The problem I had was extended crank time when the engine was cold. The OVP relay WAS bad and NO ABS lignt.
84 500 SEL (307,xxx miles)
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Old 04-23-2007, 04:16 PM
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Join Date: Oct 2006
Posts: 7
Thanks for the ideas guys.

ABS light is not on.

I have not changed the OVP unit. Anyone bought one recently and know how much they cost?

Cranking time when cold is under 1 second before engine fires (well, the coldest it ever gets here is -5 ish).

It passed the MOT last year with the eratic idle, so I suppose it should pass now. There doesn't appear to be any smoke. I may take it to Diesco (diesel injection specialists). Mercedes dealer doesn't seem to know anything about this engine. Could a dirty fuel filter cause this also?

I know it's a non-turbo, but I'm pretty sure that it should be a bit more powerful than it is too.

Many thanks,

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Old 04-27-2007, 01:12 PM
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Join Date: Oct 2006
Posts: 7
Located it...

I went to the dealership to buy some filters today, so I sneaked a look at the OVP unit (57 pounds, not too bad). I then looked at mine, and established that the fuse in the top was not blown. Does anyone have any ideas as to how I test the function of the ovp unit?

Something I did notice though, was that the ovp that the chap in the dealership showed me had 2 fuses in the top, mine has just one. Sounds like the updated design that dieselbeagal mentioned.

How to start fault diagnosis?

Thanks all!

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