Help diagnosing idle issue with M119

[1992_400e]

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

[James Redman]

Have wiring harnesses been replaced?

Have you read the codes?

[HarryM]

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.

[1992_400e]

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!!

[emerydc8]

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).

[Will_w202]

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

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.

[nulu]

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

[1992_400e]

Originally Posted by Will_w202

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

Well, I was the one giving the gift. I can't expect my 66 year old mother to fix it herself. 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)

[Will_w202]

Originally Posted by 1992_400e

Well, I was the one giving the gift. I can't expect my 66 year old mother to fix it herself. 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)

lol, I know, I was just givin you a hard time

[Will_w202]

Originally Posted by gerryvz

This is absolutely incorrect. Wiring harnesses WERE an issue on 1992 model year V-8 cars (500E and 400E).

The 400E/E420 used Bosch LH injection, NOT HFM (which was used on the 6-cylinder M104 cars starting in model year 1993; 1992 M103 300Es used Bosch KE-Jet).

ETAs are a fairly common failure (due in large part to the same issues with their internal wiring as the upper wiring harnesses have) but again you need to do some things before you start throwing parts at the problem.

Don't bother changing out fuel filters until you do the following:

a) replace the wiring harness, particularly if you don't know for sure that it has been previously changed. That is a baseline that you MUST do.

b) Pull the codes using the home-made code reader as mentioned previously.

You may gain some benefit from the forum at ********* Galactic HQ as it shares the same engine, and there are a few 400E owners/experts on that forum (as well as a fairly new sub-forum that is specific to the 400E/E420).

Don't throw money and parts at the problem until you replace the upper harness, and pull the codes (in that order). The codes will point toward the real issues. The reason I say to replace the harness first is that it is tied into the proper operation of the engine; and a faulty harness has also been known to fry one or more of the car's computers. Not saying that it will happen, but it certainly has.

Cheers,
Gerry

Really! Learn something new every day. I've seen the opposite posted here several times. Why have I been told otherwise? And why does my 92SL not have the harness issues? What does the harness have to do with LH? Did they use a biodegradable harness on one assembly line and not on another? Interesting. I've always heard it was the 93-95 cars that had the issue.

[gerryvz]

It depends on the model - not all 1992s had the issue as this year was when MB was phasing in changes across numerous model lines, switching from M103 to M104 engines in the w124 sedans and wagons, and so forth. But very definitely 1992 model year V-8 cars using LH injection had the harness issues. I don't believe that KE-Jetronic injection V-8 or inline-6 based cars (which your 1992 500SL would have been, if I remember correctly) had the harness problems. I had a fairly late-production 1992 300TE wagon (M103, KE-Jet) and it didn't have the problem.

[deanyel]

Originally Posted by Will_w202

Really! Learn something new every day. I've seen the opposite posted here several times. Why have I been told otherwise? And why does my 92SL not have the harness issues? What does the harness have to do with LH? Did they use a biodegradable harness on one assembly line and not on another? Interesting. I've always heard it was the 93-95 cars that had the issue.

You really missed it on both ends - there's compelling evidence that the problem went into 1996 on the HFMs. The internet can be a great tool for repeating misinformation.

[1992_400e]

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

Thanks.

[Will_w202]

Originally Posted by 1992_400e

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

Thanks.

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

[1992_400e]

Originally Posted by Will_w202

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

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