Hi Barry.
Have you considered the fuel injectors?
I had a '97 SLK 230 (R170) for 12 years before. The only real problem I had with it was with the fuel injectors. Twice! Seven years apart.
The first time: Very rough idle and very powerless. Intense gasoline smell. One of the injectors must have failed in the open position.
The second time: Rough idle and slightly powerless, but not that bad. No smell. The car was drivable. The car dealership had a look at it and said it was the injectors this time too. But I traded in the car for a newer SLK (my current R171) instead of having it repaired, so I can not be 100% sure about the diagnose.
Your case sounds similar to my second time case with the exception that my problem didn't come and go. When it had failed it stayed that way.

 

moving to r170 general discussion

 

Hi, does your car have an EGR (exhaust gas recirculation valve) fitted? If so try blanking it off for a while. If it's sticking it will may be bleeding exhaust gases back into the inlet manifold when it should not be.

 

Since you were following cables and it started working, I would suspect an intermittent break (open circuit) in one of the wires supplying power to a coil or plug. I had this problem on an Audi and it took 3 years to track it down after the dealer threw thousands of dollars in parts at it. What you have sounds like an ignition problem.

 

As CarolinaBlueTec says, possibly a wiring problem.
Three things might change following a restart:
System reinitialisation; something got confused, and restarting cleared that.
Something cools down, and makes good; unlikely to happen regularly.
Wiring has a break, and teh shake of the engine starting up makes teh wiring good again.

Check wring both on the outgoing and earth (return) paths; put a meter between teh engine block and the negative terminal of the battery when it is misfiring, and if you have any voltage at all, there is your problem.

 

Thanks for your interest. I'll take this on board and investigate further. I'll remove the injectors to have them cleaned and tested. It's a possibility that this could be the culprit but I still think it's something else. Other clues from other members must be followed up as well. Cheers...

 

Have you looked into the CPS or the fuel filter as the culprit?

 

I wiggled every wire and loom with a decent force while the engine was running with no change. Someone had changed the O2 sensor without soldering but fixing this had no impact. Previously the engine had been idling for a long time during diagnosis and although not overheating, it suddenly corrected itself and we tried moving wiring etc but it stayed good for 30mins or so until a restart. I'd like to think it was a bad connection but not convinced yet. It may be something else.Looking for the EGR valve....if it has one. Might be able to find a pic. Cheers...

 

Thanks for the clues. Will check and report ASAP. Cheers...

 

Well I solved the problem with the rough idle....couldn't believe it was a faulty iridium plug! They had only done around 10,000km so I presumed they were still in good order. Also the fault was intermittent - not generally what I would expect from a spark plug. Last week two plugs were removed for inspection and some movement of wiring temporarily corrected the problem and I would have thought more of the wiring/coils rather than the plug.Anyway I re-fitted the original (standard?) BKUR6ET-10's and it's back to full smooth power.
So the question is should I have fitted the Denso IQ20 5302 Iridium's in the first place,is there something better, or was I just unlucky?
Thanks for all your thoughts....i will never presume again!

 

Glad you sorted it all out! I'd say you just got unlucky. I put Bosch spark plugs in mine and they perform same as the OEM NGKs!

 

Trying to find the info again....when I was researching info about issues with my car I came across thread about using platinum plugs and not the iridium. From what I recall they are not completely compatible and cause issues randomly. I do not remember if this was for the M111, m112, M113 engine or all of them. I have the M112 so I know it covered my engine so I changed my order from the iridium to the Bosch platinum. If I find the info again, I will post the thread. I am sure some of the seasoned members here will chime in if I am incorrect or they may be able to find the info quicker. Hope you do not have any new issues anytime soon.