Dodgy Tickover

[jerry1]

I,m driving home and a symbol comes up on my 240 c class 2000( new shape) nothing appeared to be wrong until I stopped at lights and its ticking over like an old tractor. However when I pull away its like nothing is wrong. How needy is a visit to my local stealership? Has an injector gone funny or is it more likely to be ignition.

 

[jberks]

We need to know what the symbol is. Normally if a sensor has failed, it will run smoothly enough when running but with no power, and idle will be like a 3 cylinder tractor. - at least it was with mine.

I have had plug leads break down too, and that affects idle but not very dramatically, just a bit lumpy. Normal running appeared unnafected.

We need to know a bit more - is it consistent etc?

 

[jerry1]

The symbol is "Engine diagnosis indicator lamp". This morning it fired up as normal with a smooth idle cold and warm and running was very smooth, but the Engine diagnosis indicator lamp remains lit. I would describe my idle yesterday as lumpy. Confused you will be I know I am!

 

[jon_harley]

Could be the air mass sensor going faulty - I had this go wrong with symptoms that sound like yours. Lumpy idle and the engine warning light on, then the next day idle sounded OK but the warning light still on, then the next day the warning light was gone too. It came back a few weeks later and I got it fixed under warranty.

According to the MB engineer I spoke to the lumpy idle results because with the air mass sensor wildly wrong, the fuel mix goes wrong in turn resulting in misfires.

It is very inadvisible to drive the car far with the engine warning light on! Some problems can result in overheating which could damage all sorts of systems long before the engine seizes up (I assume the LCD display would warn you as well if oil temp approches danger).

 

[jerry1]

Thanks for your input Jon, its a possibility however under normal driving it,s as sweet as a nut. On driving it this morning I didn,t put the engine under any pressure high revs etc and the idle was fine. MB have told me that as long as the warning is amber it,s ok to drive and it,s going in tomorrow anyway.

 

[jon_harley]

That does sound very much like my symptoms, the engine sounded fine over 1500 rpm, and the lumpy idle did go away again for several weeks. Then it came back worse than ever (it would stall in traffic queues) which is when I decided to get it looked at.

 

[jerry1]

The situation now is that MB diagnosed it as plug/s breaking down and so they were all (12) changed. The next oppurtunity I had to put the engine under pressure high revs etc the diagnostic lamp came alight again and here is the strange thing I stopped put some fuel in the car (it was just above a 1/4 restarted and the tickover was fine. When I leave work in about 5 hours I will again put it under pressure just to make sure i didn,t have some dodgy fuel. At the moment its going back to MB on monday.

 

[jberks]

I suspect it was the restart rather than the fuel that cured it but still you never know.

I guess dodgy plug leads will give the same indication as a plug breaking down. On mine, it was the corrosion in the supressor caps and that was at 3 1/2 years old and around 25k. MB failed to find the fault and after 3 attempts I gave up and handed it to a specialist who found it after 3 days of investigation! (he said he was embarressed but they tend to look for complex problems and miss the simple stuff and only charged me for 3 hours labour). Not had any problems since the leads were replaced but they are around £250 worth! Im still on the factory plugs and it doesn't miss a beat (5 yrs 55k).

Given MB charge a fortune to change plugs and yours were probably ok - I'd ask for a credit for at least part of the cost against the next bill. Be careful as they can go on indefinately replacing bits and charging you as they go with these issues. They'll probably tell you its the ECU next!

 

[jerry1]

Well they have had it for another day done a lot of exploring and stuff, at one point changed the fuel filter cause it was dirty. They now think its the number 6 injector. I realise sometimes it must be very difficult but is it just lack of expertise or what. The only very small consolation at the moment is they have loaned me a brand new A class for 24 hours.

 

[jerry1]

Well I still have a problem, the Dealer spoke with MB Technical about a loan ECU but they wouldn,t until all avenues had been explored. Which they now have and so a loan unit is winging its way to the dealer to see if it resolves the problem. Watch this space.

 

[jberks]

Jerry,
If it does turn out to be the ECU, they can be very expensive There are firms around that repair them for a lot less.
Just do an internet search.

 

[jerry1]

This loan ECU as I understand it has to be chipped to suit my car and if it doesn,t resolve my present problem it cannot be re-chipped for another car so its effectively redundant. Does anyone know if thats correct? Currently my car is running very well except for a miss fire on idle, the warning light has not re-appeared since the last reset; the ECU £730 + vat without fitting,( which I understand is 20 mins by a competent eng) seems a lot of money for a smooth idle!

 

[jberks]

As I understand it, there are some aspects of the ECUs are car specific, but its not true that this can't be reconfigured for another car, its just a programming issue and there are loads of people about that can do it, not just the dealers.

In any case, I presume that if it doesn't work, you can just re-install your old one and just pay 1hrs labour for the swap over.

A duff ECU can manifest itself in any number of ways, so it could well be that, but I do wonder if you have the same problem I had on my 240V6 with the corrosion in the supressor caps (symptoms - car runs fine but the idle is lumpy to a widly varying degree). For some reason, my dealer was incapable of diagnosing it. I wonder if yours is too?

They often seem to eager to replace expensive electronics when a minor mechnical fix is really needed. You wouldn't be the first (by a long way) to be sold an ECU you didn't need!

 

[jerry1]

I would like to rule out the corrosion in the suppressor caps and if I ask MB if they have checked that I would not believe them if they said they had. Is there a way I can check this myself bearing in mind I know the engine is at the front. I,m not that bad really but if you could describe the necessary I could have a go. I am of course assuming you know.

 

[jberks]

Ah - To be honest not really.

I have the old set in a plastic bag at home so I will have a look tonight and see if there is anything visually obvious about the duff ones and perhaps post a pic if I can find anything.

 

[Terminator]

Just a thought but did they check the lambda sensor as a fault with this could cause the symptons you are experiencing? A faulty lambda sensor could also lead to damage to your cat. so best to eliminate this as a potential fault.

 

[jerry1]

Before MB would agree to an ECU loan they asked the dealer to re-check all the obvious faults that would produce the symptons and they re-assured me that they had and a new ECU is coming from Germany for the trial. If my car was undriveable I would be seriously hacked off by now as it has taken 10 days so far for the delivery. Since my car was returned to me 10 days ago to wait 2 things have happened. 1. The diagnosis lamp has not come back on and 2, apart from a missfire on idle I can,t fault it. Why would I want to spend £1000 + to correct a missfire? When the garage ring me to say its arrived I,m not sure what I,m going to say. Any suggestions (sensible ones)