Mercedes CLK55 AMG dodgy behaviour

blocked exhaust/cat

Never thought of that, interesting. I suppose it's a possibility, I've often described it as being like the car can't breathe, but I never thought about the exhaust side. Are there any tests for this or is it a case of getting the car up on a ramp and dropping the relevant bits of the exhaust?

Also is this sort of fault general wear and tear, or could something more sinister cause it to occur? I don't want to replace expensive parts and then have the fault reoccur quickly.

TD gurgled happily, sounding much like they were saying:

Can be. A cat is, basically, just an exhaust box after all - with a very fine honeycomb "mesh" inside which the gases have to pass through.

a friend runs a garage that cares for many mercedes chauffeur cars and he sees it quite often. there is a pressure gauge that screws into the lambda sensor hole to check the pre cat pressure. they probably block because of extended idling, but it does seem that mercs are prone to it in any case. lack of servicing, poor quality oil, supermarket fuel, short runs, internal breakup caused by speed humps, all sorts of reasons they can block

once the engine management light has come on and it has gone to limp home mode then there will be fault codes stored.

Yes, MAF's are a common failure on merc's in general, and the V8's. They are almost impossible to test properly for good operation, so I would suggest you get a specialist who is familiar with Merc's to look at it, and who has the correct test gear. Luckily, the MAF's are not overly expensive. You must use a proper genuine one though.

Presumably it has had it's plugs examined, and replaced lately??

tim..

Well, the mechanic above calls himself a "specialist", and I have to say he's generally been good in the past. But he's clueless here. What's the correct test gear for a MAF when its at home? Oh and does it or does it not raise a fault code?

Hmm, missed that. Yes, it had new plugs at the last service. Assuming he actually changed them. *sigh*

In message , TD writes

Live stream data from the ECU if he has the correct gear, if he calls himself a specialist then he damn well should have.

I'd be considering taking the car out for a nice drive with a scan tool that's capable of recording the data stream attached and seeing if I could provoke a failure.

You could also use a 'scope but you need to know what you should see and if it only fails under load then he either needs an assistant who's clued up or a dyno (or engine tester if you can still get them) to load it up in the workshop.

I would expect it to raise a code but if it's a transient failure.

It may be as simple as a misfire.

My guess is the crank position sensor. When that fails it goes to a fixed ignition timing and relies on the cam position one. They can be intermittent so switching off and on can get it out of limp home until it loses the signal again. But should throw up a code.

The actual sensor itself is cheap - but may be fitted in an expensive custom housing and can't be replaced. Bit like ABS sensors - the actual chip costs a few pounds retail but is moulded into a special housing which then costs much more. Basically, a con.

them. So you can make yourself a small fortune reconditioning

You need to log it as you drive it down the road or on the rolling & compare it with the expected curve, until it's really dead it won't throw a code

Sadly not often possible as the sensor is moulded into the mounting. They could easily be engineered to allow sensor replacement - but no maker is going to do that when they make so much money out of spare parts.

Not normally no.

Tim..