1994 C280 M104 Engine Stall Idle Rough

[spedge]

Chaps

I have recently aquired a 1994 C280 I6 M104.

It has a problem:

Car starts fine, idles fine when cold.

When engine starts to warm it goes lumpy, rough and if driven surges.
It will rev freely and once above about 1000rpm is sweet but when accelerator pedal is released the revs drop and sometimes stall and sometimes runs lumpy again. It always starts first time at all temperatures, it just needs a little accelerator to keep it running when warming up.

Once the engine is warm it is beautiful, smooth, idles and revs freely.

Its only rough when warming up.

OVP is new type, bypass relay does work but I am curious to find out when it should kick in.

What are the sequence of events when the engine is cold to when it is warm, what switches in and out?

What other things could I try, Coils etc. can be ruled out as once warm it purs like a pussy cat.

Si.

 

[television]

Do a search on post by crammy and many of your faults are covered, pay attention to the vacuum pipes

http://www.detali.ru/cat/oem_mb2.as...GM=717.441&CT=M&cat=520&SID=14&SGR=123&SGN=06

 

[grober]

Sounds as if your rotary idle actuator is not working properly. This increases the idle speed while the engine warms up. You could alternatively have a faulty engine temperature sensor. Symptoms sound as if the mixture is slightly too rich during warm up?

 

[Mark300SL]

Have a long look at the wiring loom

I speak from experience here as someone who was 100% certain his wiring loom was not an issue.
I had checked and double checked it because i was aware of the problem, and yet all the time I was making things worse by unplugging individual components and unbeknown to me I was destroying the insulation beneath the sheathing.
Chatting to an MB foreman recently he attributes 90% of running issues on 93-95 M104 engined cars to the loom!

With the ignition off and battery disconnected, trim the insulation of the wiring leading to the coilpack connectors.
If you are unlucky it will look something like this - or in my case even worse!

See my thread here for my particular problems and my solution for them.
http://www.mbclub.co.uk/forums/showthread.php?t=51471


Of course it is also possible that your particular car has already had the wiring loom issue addressed and it wont be a problem for you. However I would urge all 104 owners to take a peek at the coilpack wiring to make sure they dont have an affected engine.

FYI the worst affected wires are those to the coilpacks, injectors and coolant temp senders that run very close to the engine. Any wires that were away from the engine were in my case ok, though I will still be replacing the entire loom.


Mark

 

[spedge]

Many thanks for the information,

Looking at the engine it resembles the second picture in the vacuum drawings. I have checked a lot of the hoses but will double check.

I heard the harnesses are bad but mine seem ok, disturbing thing about harnesses are that they can cause intermittent fauilures and mine is quite constant/repeatable.

Grober, where is the rotary idle actuator, how to test etc? SOunds like the problem as it's only as it is warming.

What is the difference between cold and warm, what is switched on/off, in or out?

Si

 

[grober]

Its item 17 here http://www.detali.ru/cat/oem_mb2.as...GM=717.440&CT=M&cat=520&SID=09&SGR=060&SGN=03
It effectively bypasses the engine throttle plate by allowing air to the engine even if the throttle is closed. Its a small cylindrical object with flexible rubber air pipes attached together with a control electrical connection to the ECU. As it rotates it shuts off /keeps open this air bypass with a rotating flap as the engine changes temperature. It can malfunction/jam but this can be due to several causes. the engine temperature sensor, the ecu control signal, low supply voltage, mechanical failure or jammed by dirt etc so is difficult to test without some knowledge of the INJECTION SYSTEM.

 

[Mark300SL]

I started with a wonky idle when warm too - surging from 500-1200 rpm IF it decided to start. Idle was so bad it would jump forward in gear.
I actually think Graemes answer is a lot more likely mine - but I will never again own an M104 engine without looking at the loom in depth - they are a real pain and quite expensive to repair once the stop working completely.

It costs absolutely nothing but time to check it properly - and it could save you a lot.


Mark

 

[spedge]

Well in between last post and this I have given the old girl a throttle body clean, dont think it had been done befor in it's 175,000 mile life.

Item 17, where in the engine bay is this?

Tickover seems a little different but still as it warm it was lumpy then purring like a kitten once warm.

I will check wiring tomorrow when light is better.

Many thanks guys

 

[LastMinute]

Item 17, where in the engine bay is this?

Underneath the air filter housing, towards the back of the intake manifold (behind the fuel distributor).

 

[spedge]

Are you sure that it is applicable to my model? I am sure its all electronic

 

[LastMinute]

Are you sure that it is applicable to my model? I am sure its all electronic

No, not sure...
I think you're right, though. The diagram is only applicable to older M104 variants (like mine!).
But that's where it would be, if you had one.

 

[spedge]

Does any body know if there is a description of operation for the intake system, starting etc.?

 

[crammy69]

If its the Bosch KE Jetronic try this:
http://rb-k.bosch.de/pool/resources/literatur/leseproben/u213e.pdf

Else search the Bosch Site:
http://www.bosch.com/content/language2/html/index.htm

Cheers

 

[LastMinute]

The Brooklands W124 book covers the M104.942 in the E280. Brooklands also have some other Mercedes-Benz books that may be of some use.

You can pay for access to the official WIS net, or obtain somewhat less legitimate versions of it on CD on eBay.

 

[Dieselman]

Is this the correct intake.?
It's for the Motronic system

http://www.detali.ru/cat/oem_mb2.as...GM=717.440&CT=M&cat=520&SID=07&SGR=040&SGN=03

If so the idle is controlled by the main throttle flap unit, I think.

 

[spedge]

Dieselman

Yes the 1st Picture is applicable.

Si

 

[LastMinute]

Is this the correct intake.?
It's for the Motronic system

I think so. (Well, almost. This is the link for the C280 - but it's a nearly identical page.)

 

[spedge]

I didnt get chance to check the internal wiring on the throttle control valve tonight, a few screws were a little rusted plus were security bolted so will have to get some security star bits, my normal ones wont fit.

 

[Mark300SL]

Dieselman

Yes the 1st Picture is applicable.

Si

Then it is the full HFM system that I have in the 190

In this case the idle/cruise and electronic throttle are all controlled by a single actuator

M104 problems that I know of from my own research

1. Coil pack plug extensions, some people in the know actually recommend changing them with every set of plugs as they are pretty fragile
2. Coilpacks themselves - I have just ordered a set of 3 from the US


3. Auxiliary air pump - now this could be pertinent.
The airpump exists to inject air into the exhaust stream when the engine is cold and therefore running a very rich mixture. after the air has been injected the much leaner mixture runs hotter and heats the cats up faster
No airpump could be electrical issues to the pump, pump drawing too much current etc etc.

Maybe it has the rich mixture/poor spark on cold start are the problems are connected to the coilpacks/extensions mentioned above.


Coolant temp sensor faulty causing overfuelling ?

If it is running a bit rich then cracking off the union to the brake servo hose will give a slight airleak and may redress the balance. At least its a free test

 

[grober]

Sorry been out to the pub till now.You may well have the later system which won't have the separate bypass idle actuator. Its always difficult diagnosing at a distance without the car in front of you.