Idling problem on W124 E320

[Richard.Wilson]

I've just changed my trusty 230TE for a late '94 E320 Auto Estate, and coincidentally a friend has just bought a '93 E280 estate, both have the same problem....
There appears to be a glitch in the automatic idling. When driving at idle, as you do when reversing, no throttle, it surges up to 1000 rpm and back down, over and over. This makes it Kangaroo, worse than a learner in a Nova!!
Local dealer said most probably airflow sensor or the idle acctuator unit on the throttle body, but he cannot test these "old" models with his Mercedes diagnostic machine!! Swap the bits over with new until we find the culprit....£75 an hour ...I think not.
I'm going to a Bosch specialist, but wondered if this is a common problem?
Are there any other symptoms to look for. or tests I can carry out on the sensor or acctuator?
Thanks
Richard

 

[Richard.Wilson]

By the way,
I cleaned the EGR valve...no change
blanked off the EGR...no change
cleaned the throttle body and feed pipe...no change
No vacuum leaks I can see
What next?

 

[benzover]

it might be worth just taking off the pipe going to the inlet manifold from the purge control valve(normally close to abs unit with m.o.t. stamped on it)and blocking it off with bolt or suchlike and try it good luck[/b]

 

[porker]

Uneven Idle

After weeks of searching I eventually found the cause of uneven idling on my E280 W124. Have a look at the post for 'uneven idle'. It sounds as though you may have a similar problem judging by your comments.

 

[Richard.Wilson]

Thanks guys,
Having read a whole bunch of threads about this problem, I went back to the beginning, and really cleaned the throttle body and butterfly till they shine. BINGO!!!
I then went to a mate with the Bosch diagnostic kit, who checked it over and said it was fine, but I need a new air temp sensor, which is unrelated. He tells me that Carbon fouling is a very common cause of idle problems on many cars, especially german ones fitted with Bosch injections like Golfs etc. We are not alone.
I passed the info I had gathered to my mate who has the same problem on his E280, and it was the EGR valve (nice one Porker, cleaning it saves £73)

Interestingly, the local dealership said that the two most common parts replaced on these HFM systems are the Airflow sensor and the Idle control unit!!! but as they are unable to put them on the latest MB diagnostics, they change components until it goes away. I bet they clean the throttle body when they change the Idle control unit....that would have solved the problem on it's own in many cases.

There were a few things which now make sense:-

1. Don't try to check the airflow meter by disconnecting it, yes it will stop the surging, but only because the lack of signal from it has made the system go into "get-you-home" mode. The idling control is running with fixed settings so doesn't use the throttle acctuator. It will just record the error code to confuse you later (no date or time is recorded).

2. If the throttle body is jammed with carbon the butterfly will not open and close the correct amount or speed that was requested by the idle control. The control unit will over-compensate, and start to surge. The diagnostic box will normally detect this, and diagnose a faulty idle control unit.......£270 + VAT I believe. So make sure it is spotless before diagnosis.

3. When cleaning injection components on cars equipped with Catalytic converters, beware of using some "carb cleaner" sprays. These can burn up to produce gasses which destroy your Cat. I use the one made by Wurth and I'm assured by my Bosch mate it's OK. They should say on the can if they are OK.

4. If the throttle body is clean, but the diagnostics says it is faulty, I'd still be inclined to remove the whole unit (you would normally have cleaned it from the top without removal) and clean and lube all the linkages and pivots. If it's sticky and tight it will not respond correctly, same scenario. BTW my 230 had throttle problems a few years ago, and it was caked in dry grease and crap, cleaned it was perfect.

Hope that helps save you guys some money.
Richard

 

[Smalley]

Just for future reference, an idle fluctuation like you describe on M104 engines is only caused by one of three problems:

1. a vacum leak

2. a faulty Mass Air Meter

3. a faulty throttle acutator, aka EA / ISC / CC module (Electronic Accelerator, Idle Speed Control, Cruise Control). Throttle Acuators go bad for two reasons. The wirirng harnesses to the throttle actuators has insulation on the wiring that flakes off. This is a problem for 1993-1995 only. Throttle Actuators also have potentioameters that wear out over time.

If you have a fluctuating idle, the first thing to check is for a vacum leak as it could be simply a hose that has come disconnected or cracked. The fix is cheap.

A mass air meter costs about $250 (U.S. dollars), while a Throtte Acutator is about $1,400 U.S. dollars for the part. Ouch.