Help diagnosing idle issue with M119
 

Hello,

Late last year I purchased a 1992 400e for my mother. Now the birthday present is kicking up a fuss and I need to fix it.

It's a lower mileage (140,000K) non ASR car. Originally purchased in Japan, imported by the original owner to Canada in the late 90's.

It has an intermittent idle/stalling issue.

Here's the key behavior.

1) after a warm restart it will often die when either putting it in gear or when trying to pull away from a stop. The idle seems low when this happens. If I keep a foot on the brake and maintain an RPM of 800 or better then it won't stall.

2) after a longer drive the idle will "hunt" dropping down to 400 and up as high as 900-1000

3) Occasionally it will only restart with the accelerator depressed.

4) other than the idle being low or hunting it does idle very smoothly which makes me doubt that it is an ignition problem

Is there a good testing procedure to isolate the real issue?

While the usual suspects seem obvious. (wiring harness, MAF, vacuum leak, dirty throttle body) I'm a bit clueless as to where the best place to start is.

The service manual points to replacing the MAF but I'd prefer to only do that if I am able to prove it is bad. Is there a reliable method for testing the MAF?

Thanks much in advance for any tips, clues or help you can provide.
Ian

Have wiring harnesses been replaced?

Have you read the codes?

Start here:

Check that the fuel filter is not blocked. Replace it if it looks very old. Don't be surprised if it's the filter that was installed at the factory.

Measure fuel pressure at the fuel rail.

Thanks for the replies.

James:
- I'm 99% certain it is the original wiring harness
- I have not read the codes yet. I'm mostly a diesel guy and at the moment I'm confused over which code reader to buy. Suggestions?

Harry: What pressure should I get at the fuel rail? Is there a standard test procedure for this? Could the fuel delivery cause problems with idle but still be fine at WOT? The car goes like stink!! :)

PS This is probably a total newb question, but how do I tell whether I have the LH-SFI or HFM-SFI?

Thanks.

Build the home brew tester at the very bottom of this link, pull the codes and use that site to determine what the codes mean:

K6JRF Auto Page

The Electronic Throttle Actuator (ETA) controls the Idle Sped Control, but if it's the original harness, I think most people here would agree that it is most likely in need of replacement. I would start with the harness after the cheaper stuff like the fuel filter, fuel pressure regulator, stop lamp switch and neutral safety switch.

Incidentally, I had a 1993 400E w/out ASR and I remember that for some strange reason my idle got lower and lower over a period of several years. So low that it occasionally stalled while moving from reverse to drive; but after changing the motor mounts the problem totally corrected itself. How the two were related is beyond me. Maybe it was just a coincidence.

I think all the 400E's were LH (hot wire).

Just some tidbits.....There is no wiring harness issue on 92 cars. The issue was 93-95.

While my 92500SL is CIS, I believe that 500E (same motor on the surface) is LH. Not sure about the 400, but stands to reason.

With any of the V8 MBs from this vintage, unless there is a gigantic, glaring problem, assume a code for "symptom x" indicates that about 2 dozen individual components are potentially at fault.

Basic troubleshooting - no matter if it's a lawn mower or a 400e, start with the most basic things first - as has been said - fuel filter is a good start. If you're going to replace parts based on what the codes say, unless you're wanting to undertake a $5k exercise, smallest part first. General stuff to any car.

What's your love for this car? If the ETA is original, then yes, it's something that fails and you should get a reman from beckmann technologies. I think they run around $800-$1200

Just like there are no "cheap" MBs (oxymoron of the century), beware of any MB that is a gift :D

I was "given" an 83 300D that was supposedly "in fine running shape," my first MB, and after I'd spent double what it was worth I finally pawned it off to a WVO nut else during the early biodiesel craze.

Not aware of the term HFI relating to MBs, however the cars of that area were CIS and moved to LH.

Try cleaning throtle body. and then relearning idle position with sds or shop hht,i chased a similiar problem and fixed just by cleaning t/b of carbon ,used carb spray and rag

Well, I was the one giving the gift. I can't expect my 66 year old mother to fix it herself. :eek: Hence my quest for some good advice on diagnostics.

It would be easier if it was my daily driver but as I have to almost wrestle her for the keys, my time to fix it is somewhat limited. (or I get stuck being her personal cab driver)

I read the service manual a bit more carefully and all the W124 with the M119 were LH.

Indeed they were, and in the 124s all had the wiring harness issue, including the 1992s. It seems unlikely that you have the original harness, check the part tag. In any case you should build the flash code reader and read the codes on the 38 pin connector. Hot wire MAFs are extremely durable, might need to be cleaned, very unlikely it needs to be replaced.

Thanks for the tips. Where would I look for the part tag? The car was very well maintained before but I didn't get all the service records. It would be a nice bonus if the harness had been replaced, but without any proof I assumed it hadn't been.

The part tag is on the harness near the source plug behind the black box in the right (passenger side) rear of the engine compartment. You should be able to follow it there from the injector wires off the fuel rail. Get a flashlight and a mirror. You're looking for an FD date, hopefully one that's after the production date of the car. The date is in the form day/month/year. In any case I wouldn't buy a new harness unless and until you actually see crumbling insulation, which you will see if it's original. If it's a replacement it should be good for the life of the car.

ETAs are a common failure item on ASR cars, not so much so on non-ASR cars. If it looks dirty clean it up with throttle body cleaner. You can clean the MAF wire with mass air flow sensor cleaner or electrical contact cleaner. If you do it on the car stuff a paper towel behind/below it, carefully.

To the original poster:

From the service manual CD:
Fuel pressure at idle should be 320 to 360 kPa (46 to 52 psi).