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RPM erratic when idling - 1996 E220

32K views 21 replies 7 participants last post by  Zamanumer 
#1 ·
The last few weeks i am facing this problem, does not seem to be getting worse but the RPM moves between 750 and 800 when idling.Have changed spark plugs and engine mounts recently. Could it be clogged nozzles in the fuel injection system?
 
#2 ·
There are many reason for erratic idling, mostly one of hardest thing to diagnose on HFM engines. Most people live with it, running behind it might prove costly.

Possible Causes( could be one or multiple of these)

- Vacuum leak
- Purge circuit leak
- Bad IAT sensor
- Bad HFM sensor
- Erratic throttle controller.
- Bad wiring harness
- Bad fuel circuit pump, filter, regulator, injector.

Diagnose, start one by one and check results after each fix.

- Vacuum leak, Follow all the vacuum lines that run into intake manifold and check for leaks, some might be closed with rubber stopper, check if rubber is brittle. Check if break booster pipe valve looks weak (Brake pedal feels hard sometimes with engine running).
- Purge Valve, Check if purge valve is not always open, for testing you can just block the thorttle body input.
- IAT, Found at last bend on intake pipe, just remove and clean it with any contact cleaner.
- HFM, Search for info on cleaning this, but it is temporary solution. Best way is to swap a know working unit and see.
- Throttle, cleaning might help.
- Fuel circuit, just clean or replace the filter. Bad Pump, regulator, injector would have other symptoms like hot/cold start.
 
#3 ·
Have checked for vacuum leak have changed the fuel filter recently.What is an IAt and HFM sensor what do they stand for and where can I find them? Also where is the throttle controller? and when you say circuit leak, how is this checked? The idling is still rough.
 
#4 ·
1) IAT is intake air temperature sensor
2) HFM or Mass Air Sensor can be found after air filter in intake circuit.
3) Throttle Controller is connected to intake manifold.
4) By circuit i mean pipe/hose lines, checking those lines for leak.

I have modified a image from net, see below, should give you an idea.

Also you can read error codes form ping 8, error codes should give some starting point. see http://www.benzworld.org/forums/w124-e-ce-d-td-class/1229198-code-reader-2.html
 

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#5 ·
So what happened guys ??
I have the exact same engine in my new 124, after driving a diesel engine for 08 years in a 124 I have a petrol engine now, im sure its simpler then a diesel engine but im just not familiar with it YET.
I am having idle speed issues as well, I couldn't find a knob/switch/adjuster to adjust the idle speed, been reading forums for two days and ive figured theres no such thing in this engine.. currently my issue is the engine rpm being too low when AC is switched on and occasionally dying when i put it in D after being stopped at a signal

Can anyone please also highlight the IDLE SPEED CONTROL VALVE in the same picture ??
Is there any way i can get a detailed diagram of this particular engine so i know what is what when reading the forums...
 
#6 · (Edited)
On W124 E220, HFM controls the IDLE automatically by activating the purge valve and ISA. Normally it is the clogged purge valve caused by the blown charcoal powder from the upstream canister. Other causes to rough idling could be many...such as OVP (with/without low voltage), binding clutch of AC compressor, belt tensioner and its tensioner absorber. You might check the DTCs to narrow down the cause as well. Besides, if you have an older ISA by VDO, the cables inside will be biodegradable, thus causing internal fault of the ISA control.

http://www.benz124.com/forum/viewtopic.php?f=44&t=63386
 
#7 ·
i couldn't find the HFM sensor in mine either :/
Its W124 E200, 1995 111.940 Engine
Same engine as the picture.. IAT is there and I will go clean it today
Ive changed the accelerator cable (actually just the housing part and soldered back the lock) and it has helped majorly, changing the belt tensioner shock has made the engine less noisy (it was rattling) and surely made the engine smooth
engine got into a rhythm between 500 rpm and 1500 last night, it was warm but hasn't done it so far today... its very steady on 700 and running beautiful
my only problem right now is when I turn the AC on, it goes under 500 and dies cause of low rpm randomly...
 
#9 ·
i couldn't find the HFM sensor in mine either :/
If you do not have HFM sensor like in the picture above, you have M111 engine with PMS engine management (i.e no Air mass sensor). The PMS system is believed to be more reliable than HFM.

Your A/C or idling issue looks related to throttle body, try cleaning it. If you still have issue after cleaning check for echo junk wiring like below

 
#10 ·
Before touching any suspect harness, check the build date on the harness and throttle body.

Harness build date is located behind battery splash shield (look for F.D. xx.xx.xx) and throttle body build date located on sticker shown in above pic. Look for xxMxx, where the first 2 digits signify build year.

In both cases, hopefully you have build dates of 1998 or later. Otherwise, the eco junk wiring bug is at play.
 
#11 ·
I did clean that IAT and I don't think it made any difference, engine is already as smooth as one can be. I think my harness is fine.

When I turn on the ac I believe there should be a valve or switch that increases the idle speed. It's not functioning, so once the AC load is added rpm goes down. I've checked all vacuum pipes and they all seem to be connected perfectly without leakage.

I did spray contact cleaner in the air intake a few weeks earlier and it definitely improved the smoothness and engine response, perhaps I need to clean the throttle body thoroughly. Any pictures I could get ? Any diagrams that would help me in taking it off and then cleaning.

Anything that can help me know what not to do while trying to take it off :)
 
#13 ·
okay so I took down the throttle body and cleaned it as much as possible. It helped but the trouble maker in my case was my accelerator cable. The plastic coating on it was worn out and it had some friction which wouldn't let the lever go back 100% and that's what was causing the issue, replaced with a brand new cable and all my trouble seem so far away :p

My fuel pump was choked. I am guessing it had never been replaced. Replaced it with a new one and that helped a lot with the initial pickup.

while we are at it, I also figured that my accelerator pedal was out of shape, the angle on it wasn't right so it was hard and wasn't pressing fully. Opened it up, fixed it and installed it back.
 

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#14 ·
okay so we fixed the problem earlier and a friends W124 with M111 engine was having issues so we swapped throttle body, now that ive put my throttle body back my starting has messed up... i have to press the accelerator to start it and on cold engine it just stalls.. grrrr

what should i do ?? i believe the connector is fine but alot of things were opened up so we have definitely messed up somewhere..

Another thing i have been searching for hours and could not find color coding for vacuum lines...

I have a few lines that i dont understand what they are for
one is black with green strip and another one is black with red stripe

also some of the pipes coming to the engine are looped back into other hoses... should they all be connected to the engine ??

4 pipes are looped back, 2x2
i tried sucking on them and two of them hold vacuum but two dont... does this mean they are leaking somewhere ??

MY VIN number is WDB1240192C270953
if i could get a picture or someones engine showing where every hose is connecting in the engine that would be great help
 
#15 ·
There should be a vacuum diagram on the front radiator/headlight frame when you open the hood.

Does your car have a secondary air injector/pump? That should turn on and keep engine at 1k rpm on cold starts for a minute or so. Sometimes the vacuum tubes and rubber connectors leak on these. I would just replace them all. Check and replace if needed the vacuum tubes from your intake manifold as well.


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#18 ·
I have checked all vacuum lines and connectors, none of them are leaking
couldnt find a diagram in my car, any possibility if someone else could post them
is there a chart which could tell me what color vacuum line goes to which part ??

Attaching a picture or a part, is this the additional vacuum part ?? if not then what is this ??
 

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#19 ·
I believe that valve is either the carbon canister purge valve, or the vacuum controller for the transmission upshift retard (keeps car in lower gear when its cold so it warms up the cat faster). reason I say 'either' as they moved those valves around a fair bit. sometimes they are behind the brake vacuum booster.
 
#20 ·
my wiring harness is gone basically... earlier I found some wires that had the insulation worn off so we simply taped them and it worked fine for two years. NOW ive figured out that the connector for throttle body is all short circuites on the wiring harness side... I will try and open up the wiring to see the exact part number but is there a way I could use my VIN number to find exactly what the part number would be for my wiring harness ???

MY Vin# WDB1240192C270953
 
#21 ·
Changed the wiring harness, changed the throttle body, changed the ignition control module since my last post in this thread :grin
back to the above pictures, this small valve.... I have this in my W124 300CE as well and again I am not sure where should this be connected to get vacuum..

isn't there a way to find out what is the use of BLACK vacuum line with RED STRIPE ??
any possibility I could find a picture that could show me which vacuum line should go where... too many of them and they are all connected together through home made t-connections... I am sure some should be separate from others
 
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