Fivehunnit

So, I've had a CLK500 (W209) for about 3 months, all was fine until about 4 weeks ago, I went for an overtake, no problem, then afterwards, when setting off from traffic lights and junctions, throttle response was non existent for a few seconds, then suddenly it would shoot forwards, it's also had a sort of irregular idle since a few weeks after I got it, but this got worse after I went WOT for the overtake. Switching off and back on again sorted it, no problems again (apart from the idle, which was sometimes ok, other times a bit shaky) until 2-3 weeks later, again upon WOT, this time, cough cough, all kinds of lights and noises from the dash board, "ESP faulty, visit workshop!", engine check light came on, no throttle response at all, car came to a stop and engine died. This time, upon turning the engine back on, the engine check light stayed on, throttle response was delayed by a split second every time without fail, sometimes it carried on right through until 3,000 rpm or so, other times it was just the split second cough and that's it, sometimes, coming to a stop at lights etc the car would shake, sometimes even shut off.
I had it plugged in at a local garage, which gave codes for "pedal value sensor (short circuit, path 1)" and "MAF sensor".
Having already changed the MAF sensor with a Bosch unit when the problem first happened, I thought it odd, I ordered a new pedal assembly from Mercedes (no part dealers stocked them), put that on, transpires there was no change at all, back to the drawing board.
A few days later, it's in an indy garage who plug it in and get the MAF code but no mention of the pedal sensor, they test the MAF, Potentiometer, throttle body, all found to be fine, they replace any possible wiring that may have been broken inside, no change, they say that the only possibility now, is that the ECU is defective and needs to be sent away to find out if it's fixable, or if I will need a new one, to the tune of £1,400 and 5 working days of delivery time, plus the labour for the diagnostics and work.
My question is, why would the ECU malfunction like that? Is it well known in the M113? I've struggled to find any post where someone had this problem with the W209, lots of older ones and other models caused by oil leaks up wires or ?HT leads shorting? if I get it fixed, or get a new one, is it likely that something caused it and will be ticking away ready to blow it again? Debating on whether I should sell it when I get it back, to avoid anything like this or worse happening on me with it again.
Thanks

Rudeney

Water. The ECU on that car sits in the front fuse box. If not properly sealed, water can get in a kill it and the front SAM.

Fivehunnit

Thanks, figures. Any preventative measures to stop it happening?

DanielFD

Well i wouldn't go change the ECU without first taking a look myself, is pretty obvious if you find water in there. Also your problem with the coughing and late acceleration really sounds like your MAF sensor is out of place. Had that happen with me and my car acted the exact same way your describing.

Fivehunnit

I've replaced the MAF a few weeks ago, and it's been tested by the indy garage, they say the ECU is the only thing it can be, it's been plugged into whatever the upgraded version of STAR is (forgot what they said it's called)

DanielFD

The thing is usually water damage in the ECU is much more serious, such as the engine fan always being on whatever key position the car is in and not being able to start the car at all, basiclly the only time you facing a problem is when you press down the accel pedal. That doesn't sound like a failed ECU but more like a failed throttle body or improperly installed MAF sensor. And the problem you are having, is not something common for sure, the M113 engine is solid. Come to think of it, maybe Rudney can chime in too, A HUGE leak in the intake manifold can cause symptoms similar to this, Did they check for air leaks? You dont always get codes for intake manifold leaks, sometimes different sensors read weird values because of it and the computer assumes the sensor itself is broken.

Fivehunnit

I don't know if they checked for air leaks, just put it in for the diagnostics and they said they traced it back to the ECU, I'm going to ring them today, will hopefully find out what's what with the ECU, hoping the place has had chance to look at it.
Maybe instead of water, it's a short somewhere in the ECU? Will keep this updated.

Fivehunnit

Not sure how accurate this is, and I know it's a different model to mine.

http://www.remmington.info/merc.html

"S-Class (220) S280 2.8 + S320 3.2 1998-04 Bosch ME 2.1 type. Suffer voltage output from ECU pin C22 to accelerator pedal position sensor, output voltage should be 5V"

Rudeney

To prevent water damage to the ECU, always make sure the front fuse box cover is properly closed and latched. Also, make sure the drains in the water collector below the windshield are clear of debris.

DanielFD

well you can check for air leaks yourself if your inclined, with the car on just run some water from a bottle (small amounts) on to where the manifold brings air into the cylinder, you should instantly see the engine behave radically if there is a leak. But you dont want water in the throttle body, if you dont feel comfortable doing it then dont.

Fivehunnit

Update, the garage rang today saying the ECU is back and the place they sent it were unable to get a proper diagnosis on it or something, the car has to be sent to mercedes themselves and a donor ECU put in to test, can see this being a very long and very expensive ordeal.

DanielFD

Best of luck then, hopefully the problem gets solved without breaking the bank. Keep us posted.

kb24forlife24

My car had the same problem that you had (while setting off from the light, hesitant acceleration and then shooting forward) but no check engine light. It's happened to me 2 times in the nearly 3 years I've had the car. If I get a light the next time it happens, I'll go from there, but for now, I guess I can't do anything.

Fivehunnit

An even bigger pain is the time it's away, it's been away for a week now, and if it needs a new ECU, that takes a week to get here allegedly.

Fivehunnit

Ok, update time.
Got a call from the garage today It's sorted, turns out that it was the MAF sensor after all, they put a new MAF in to test it ages ago and it was the same, they tried a different MAF today and it worked, so that means, the new MAF I put in and their new one they tried weeks ago were both no good.

Rudeney

Wow, that's a big deal - going from bad ECU to bad MAF sensor. I'm glad it's all sorted out.

Amir_AMG

i hope they didnt charge you for all the mis diagnosis

tw2

Glad it is sorted. ECU failure does occasionally happen but from what I understand it is very rare.

DanielFD

The Symptoms did sound most like a MAF sensor as i had said before, Large vacuum leaks can cause the same thing. Great to see its solved without messing around with the ECU!