High Idle - 300TE-24 W124 M104

[wellshj]

Hello again,

As per the recent thread on 02 sensors I measured the voltage on the unplugged black wire with a warm engine running. Unfortunately I don't have a duty cycle measurement facility on my voltmeter but the voltage seemed fixed at close to 0.9v. (The display is digital so responds quickly to changes)

Any tips appreciated!

 

[w124coupe]

Hello again,

As per the recent thread on 02 sensors I measured the voltage on the unplugged black wire with a warm engine running. Unfortunately I don't have a duty cycle measurement facility on my voltmeter but the voltage seemed fixed at close to 0.9v. (The display is digital so responds quickly to changes)

Any tips appreciated!

Reading steady rich like that means that either the O2 sensor is dead or the ECU is not in closed loop or the mixture is so rich that the ECU cannot adapt it.

You can test the O2 sensor (at least grossly) by inducing a vacuum leak (take one of the inlet manifold vac pipes off - PCV or brake servo as an example) and seeing if the reading drops as it should. If it stays steady, its dead (or badly fouled from the rich running).

There are a few reasons for open loop running but the first to check is to see if the KE control unit (N3) is powered - i.e. is the OVP relay providing it with a voltage? If it is, is the KE unit "pulsing" the EHA - i.e. trying to cycle it to send the mixture rich and lean to give the KAT the right diet of O2?

Rich running beyond the ECU's ability to lean it out could be due to a wrongly adjusted air flow sensor plate (i.e. the basic mixture position around which the EHA "tweaks" mixture). If the first two tests above don't show anything, this can be tried next.

 

[wellshj]

Thanks for your help w124 Coupe, I've not had much time for tinkering the last week or so but here's an update:

I took the car to a Bosch Service Centre ("BSC") for more diagnosis (it needed a tailpipe anyway). Before my visit I thought I'd see how it went with a leaner mixture so tweaked the mixture screw 1/4 turn CWW... big mistake! It ran beautifully but wouldn't restart despite me putting the screw back to where I thought it was before. Lesson 1: don't adjust this screw unless you have the right equipment hooked up and the engine running.

The BSC showed me the trick to restart it and set the mixture. Lesson 2: turn the screw a long way CWW and turn it gradually CW with the engine turning over until it starts then you can reset the mixture once the engine is running. I hope this helps another tinkerer as I've not seen it on any posts, some refer to holding the air plate down off the idle position but this didn't work for me (I eventually ended up with a flat battery...)

Back to the diagnosis, the car still has an erratic idle and nothing came up on the test equipment so she's going back in next week for tests on the individual components.

I might get time to do some more investigation tomorrow and will keep you updated.

 

[wellshj]

SOLVED at last!

I did some current tests on the EHA this morning and found a low (c.11ma) reading ignition on and motor off and high readings with the engine running at idle (2ma to 5ma depending on engine temperature). As an EHA is quite pricey I picked one up from the local breaker (they are common to many MB engines of the late '80s and early '90s so easy to find) and switched it for the old one. Instant improvement in running - no racing idle as the engine warmed up and easier starting. There was a slight fuel leak from the O ring seals that cleared up with tightening but I'll now splash out on a new EHA and get the mixture and pressure set by the BSC for a certain and complete cure.

I hope this thread points a few of the rest of you with similar problems in the right direction.

 

[wellshj]

Further Update...

Well the car was still not quite running right so I replaced the Lambda sensor and the fan temperature sensor (two pin one) as it was slightly out of spec. Some improvement again and more electric fan cutting in but still no complete fix. I then started from scratch again and spotted that the throttle position switch while working was worn so the throttle mechanism wasn't making very good contact with it in idle position. BINGO! I removed the switch, bent it a little so it operates more positively and now the idle speed is much more consistent.

 

[SokolS]

perfect job

 

[GlenSmall]

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

My mechanic left the idle control valve disconnected. Engine idles ok since then. Not sure of side effects