1994 E320 high idle (previously over-revving) & surging

[freemach]

It all started about 3 weeks ago when the car had done a hot run, slowed down for a roundabout. When pulling onto the roundabout, the car revs started surging as though hunting. The ASR warning light also came on. Stopped the car and restarted and it all looked fine. Driving at 40mph, it started accelerating and then surging. Called it a day and was towed home by the AA. When the AA man arrived, when starting the car, it was starting right up to the 3,500rpm rev limiter in park.

Feeling this was a job for the specialists, the car went to the local dealer who have replaced the throttle body (£600+fitting!) and the OVP relay. The car now appears fine but when starting it, it revs up to and sometimes over 2000rpm (even when warm).

If you blip the throttle when starting the car, the idle can oscillate for 30 seconds hunting between 1700-1800rpm and the revs can take a while to drop from 1500rpm to 750rpm after this oscillation period. Any ideas, or advice of where to send the car if the local dealer can't solve the problem?

Putting the car on the diagnostic equipment showed no problems but showed that the throttle butterfly angle was remaining at 4.8 degrees during the oscillations and would remain at 2 degrees or so before dropping to the usual 0.7degrees (=750rpm). This is the only fault on the car (it drives perfectly now) - could this be a sensor? Or an Idle Control Valve? There was a stored fault of a problem with an O2 sensor (Clearly a recent problem) but this wasn't recurring on the diagnostic computer when the car was doing this.

 

[mjtray]

I could be wrong here, but the butterfly flap you refer to is in the throttle body isn't it?......bearing in mind that you have just had the throttle body replaced, could it be faulty?.....was it new or second hand?

I would take the car back to the dealer and get them to sort it pronto....it should not be doing this.

Good luck

 

[freemach]

The throttle body was brand new (£600 + labour!). The car has been in the dealership for 4 weeks and they've finally said they can't solve the problem without first replacing the ECU (and Mercedes wont provide a loan ECU to find out if that is the problem, so it's a £1000+ bill on top of this or accept it as it is).

Anyone have any suggestions?????

 

[mjtray]

I feel your pain!

When I had my E280 I had similar problems to those you mention above and my local dealer was telling me that I needed a new throttle body, as they claimed that some of the early throttle bodies on the M104 were defective from new.

Not convinced I did some searches on this site and others and then went through a process of elimination.

The first was the butterfly flap in the throttle body which collects crap on its outer edges, which then stops the flap from closing properly and causes the car to surge, especially when standing still with the car in gear.

Having cleaned this with carb cleaner it was much better but I still had a problem with the idle.

A Benz mechanic then mentioned that the a valve called the EGR valve (?) often sticks or gets gummed up with carbon and can cause idle problems. I replaced this valve, cost @ £60 and the idle again got a lot smoother....the valve is on the right inner wing as you look at the car and is located near the ABS gubbins. It has MOT printed on the top and has a plastic tube going into the top. Its easy to replace as it's just pushed onto a metal bracket.

By now my idle was back under control.....I should also mention that I had also replaced the OVP relay as you have done.

I hope this cures your problem.....I would check this out before you embark on replacement ECU's etc.

Good luck.

 

[freemach]

Thanks for the advice. The dealership have tried replacing this valve and apparently it makes no difference. The problem is a weird one as it only really manifests itself if you start the car with your foot anywhere near the throttle. If you don't touch the throttle it doesn't appear to do it.

Is there a switch which shows that your foot is on the pedal which might be sticking so the ECU doesn't drop the revs down? Just a thought...

It could just be gremlins related to the ASR system on the car (it was a very early car to have ASR fitted). Can anyone recommend a specialist around Cambridge/Bishops Stortford to investigate?

 

[mjtray]

There's an ECU for a 3.2 M104 engine on sale with Ebay with a starting price of £75.00

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=10398&item=2479514847&rd=1

 

[Smalley]

Your engine doesnt' really have a throttle body (like M103 engines).

Your engine has a throttle actuator. It is also called the EA / ISC / CC module (Electronic Accelerator, Idle Speed Control, Cruise Control).

Your problem sounds exactly like a faulty throttle actuator to me.

Have you replaced your mass air meter?

 

[freemach]

2 months after supposedly being fixed, this failed again with an identical fault ... the ASR light coming on and the engine losing power. When the ignition was turned off and then back on, the car was revving to 5000rpm or so against a rev limiter. Turn it off again and wait 10 mins and it was fine for another 10 mins before a repeat of the ASR light. Somehow I suspect the throttle actuator was never the original problem but maybe the ASR ECU or the main ECU. Any views?