High Idle - 300TE-24 W124 M104

[wellshj]

The time has come to call in the experts, so here is my little weekend teaser for you all...

I've read many posts on this and other forums and spoken to two mechanics trying to solve a problem with my 300TE-24 and would appreciate some new ideas.

The Car:
- 1990 300TE-24, M104.980 (3.0 litre, KE-Jetronic with MAS unit, non-biodegradable loom)
- Airbags, LSD, ABS and cruise but no ASR (so mechanical throttle)
- c.200k Miles/315k KMS
- Good service history, mostly at main dealers

The Symptoms:
- Slightly slow cold start leads to even, correct warm up cycle. After 30 secs or so the idle rises to 1,500 or so and then continues to increase as the engine warms reaching c.2,000 if left to its own devices.
- Restart at operating temperature brings on correct, even idle (c.700)
- Restart at less than full operating temperature brings on correct, even idle but it will creep up as the engine warms further
- Unplugging and replugging idle control valve leads to correct, even idle
- Fuel consumption slightly high, possible slightly rich mixture (no analyser but from smell and slightly excessive amounts of light exhaust smoke), oil consumption normal, power seems good.
- Basically the idle control valve seems to be more active then it should with a warm engine.

What I've Tried so Far:
- Idle control valve looks reasonably new and seems to open and close well (revs instantly drop once it is unplugged).
- Voltage OK to idle control valve
- New genuine MB thermo-temperature switch/sender fitted
- No leaks found around inlet manifold (hoses look good and all joints and hoses sprayed with brake cleaner, no change in idle).
- New genuine MB injector seals fitted (all three parts: O-ring, seal and holder)
- Cold start valve tested, functions correctly at start up and shuts off with no leaks
- New genuine MB camshaft sensor fitted
- Air meter system cleaned and seems to move well
- Throttle mechanism cleaned and seems to move OK, cruise works fine
- MAS unit was replaced with new genuine MB part in 2004
- OVP replaced at some point, not tested but no ABS light etc to indicate problems
- At this mileage the car must have had a head gasket done in the past, no signs of usual oil leak at rear or oil in water.

Remaining Suspects:
- Perhaps the ECU, I've not been able to find a correct good one locally to test yet. Does anyone have experience of this ECU failing, perhaps with symptoms? The mechanics say they rarely see failure of this unit (NB later M104s are another type).

Thanks in advance, while I can work around this problem it would be nice to have her running perfectly...

Harvey

PS apologies for the long post, I hope that the detail is relevant

 

[Ian B Walker]

Have you changed the engine temp sensor? This also controls the injection fueling on a cold engine

 

[wellshj]

Do you mean the two pin one? If so then no, early on in my investigations I checked the resistance on it at various temperature readings and they came out a little low could this indicate it going out of spec? An interesting idea, I thought the four pin thermo-temp switch/sender was the important one for the injection system so replaced that.

Thats not too expensive one to try either, I'll put it on the suspect list thanks!

 

[crammy69]

Have you looked at the Idle air valve / rubber hses around it. Bit of a soand so to get out, but ok if you got small hands.
LINK

Also have you looked at the EHA (I think ive a spare one in the tool box from when I had issues with my old 300-24v) I think a diagnostic would give it a mention if its faulty.

 

[wellshj]

Yes, I've checked the hoses around the idle valve, they look in good shape (probably replaced as look new, like the valve itself) and spraying brake cleaner around them didn't alter the idle. i have similarly tested all the various seals and joins around the inlet manifold.

Thanks for the suggestion.

 

[kth286]

does it have cat & lambda sensor ?

if so, then change sensor for correct fueling, if not been done.

 

[wellshj]

It had a cat when new (since removed as not required for emissions) so still has a lambda sensor. How would I know that it is faulty? I though a wavering idle was more typical of dodgy lambda sensor? (NB I have little experience of these as I usually run cars with carbs...!)

OK, I've done a bit more research and can see some tips on the sensor at the following post, i'll add these to my to do list. Thanks for the advice.
http://www.mbclub.co.uk/forums/engine/69205-w124-m104-code-11-o2-sensor.html

 

[w124coupe]

I'm away from documents etc but have you examined the fuel pressure regulator?

Remove vac pipe and make sure no fuel in it.

Suck on the pipe (while connected to the regulator) and make sure it holds vacuum.

 

[wellshj]

Am with the car at the moment so was able to perform test on fuel pressure regulator. The pipe appears vacuum tight with no fuel in it (lucky really as it would have ended up in my mouth!). Thanks for the idea.

 

[w124coupe]

I assume this car has a purge valve for tank fumes?

Little box on passenger side wing under bonnet with "MOT" on the top.

Usually fails open and causes weak mixture issues but, if you have rich running, it could supply extra air and speed the fast idle.

ECU triggers it a short while after after warm up so the timing might be right....

Try plugging the pipe from the "MOT" side to the inlet manifold at the purge valve end and see if it changes the symptoms?

 

[w124coupe]

Not sure without looking if this car has just the purge valve or a thermo valve as well!

 

[tippandy]

hi there have similar problems . in a thread i started a few weeks back i was advised that all the 104 engines suffer from the loom problems. there is also a solved thread by dave lewis referring to o2 sensor which may be of help to you.

 

[tippandy]

ignore the second part of that post ,you have already seen it, sorry

 

[wellshj]

I assume this car has a purge valve for tank fumes?

I'm not sure, I have just had a good look around the engine bay for the offending item but cannot find it. See attached photo of LH engine bay between EZL unit and hydraulic reservoir. If anyone can spot it then please shout. ...Unless my further research has correctly identified it as the "Regeneration valve", item 53/1 here:
http://mb.auto.pl/wis/w124/CD01/Engine/103/47-8201.pdf

I also note that there is an empty white plastic grommet inside the LH wing behind the hydraulic reservoir - this could be the one that normally carries the fuel tank breather pipe into the engine bay?

Second photo shows another box with vacuum leads in the front right hand wing behind the washer reservoir. Could this be it?

NB the car is left hand drive

 

[wellshj]

Also have you looked at the EHA (I think ive a spare one in the tool box from when I had issues with my old 300-24v) I think a diagnostic would give it a mention if its faulty.

From reading other threads on EHAs (including the ones describing your original issues) it sounds as though diagnosing this will have to wait until I despair and head to a proper garage, will bear it in mind though so thanks for an extra "suspect item"!

 

[wellshj]

Update on the Lambda test - steady high reading indicating rich running due to other reason. The search continues...

 

[Ade B]

be interested to see the outcome of this, I have an intermittent high idle (1500rpm) presently I can reset it with restarting which cures it 75% of the time.

Was advised by my local mech that it could need a new throttle actuator (cruise control is intermittent too) but they recommended going to a bosch specialist.

Ade

 

[wellshj]

Based on your input so far I've done a bit more online research which brings up two suggestions. Both are mentioned in this thread from PeachParts:
'80 300se start,stall,start,stall,start,stall - Page 2 - PeachParts Mercedes ShopForum

1. The potentiometer for the air flow meter is a common failure on KE-Jetronic systems, especially at the idle position. The part is quite cheap when available, which seems only intermittent since 2007.
(NB also good description of R&R here: SG-Motorsports Air Flow Meter Potentiometer Replacement)

2. That the EHA can be tested with a simple resistance test, it should show close to 19 ohms.

Anyone got experience of either of these points?

 

[wellshj]

Update on the EHA and potentiometer tests:
The potentiometer readings were very good, nice smooth change in resistance as the air plate was depressed with expected tail off at the end. This therefore seems out of the frame.

However I didn't get a reading anywhere close to the 19.5 ohms reading across the terminals on the EHA, has anyone else successfully diagnosed one with this test? It is rather an expensive part to replace as a test...

 

[w124coupe]

Update on the Lambda test - steady high reading indicating rich running due to other reason. The search continues...

How did you measure this? Do you mean the on/off ratio? If so, what was the reading?

Duty cycle at 100% is a "fault code" saying the KE unit is not powered - this can be because of a faulty OVP relay and leaves the car in emergency running mode with no EHA control performed....(just mechnical mixture control based on airflow). Usually this means it runs quite well, but rich.