Intermittent missing

I have a 2006 Mercedes-benz CLS 320 CDI automatic with the V6 OM642 engine. It has an intermittent problem and has been on Mercedes Star diagnostics a few times, but no fault codes have been found.

I had to charge the battery due to it being a little slow in turning over in the cold weather. From cold, as the temperature climbs above 45°C on the digital display and you rev the engine around 1000rpm, it will chug and miss. If you increase the revs to more than 1200rpm, it will disappear; bring the revs back down to between 1000-1200rpm and it will chug/miss.

Lately, there have also been clouds of blue exhaust smoke with a sweet smell. Driving it feels like a kangaroo at times, but all this disappears once it gets to its operating temperature of 85°C. Sometimes if you leave the car for an hour or so, then start the engine, it will chug/ miss between 1000-1200rpm till it gets up to operating temperature and then runs fine. On some days there are no problems at all. It drives OK, runs smoothly, has bags of power and pulls like a train.

Parts I’ve changed with genuine dealers replacements are: All six glow plugs plus control module Coolant temperature sensor Exhaust back pressure sensor Lambda sensor Inlet manifold swirl motor I had the Merc serviced recently by an independent Mercedes garage, where the guy test-drove my car and said he felt the chugging. He plugged in his Star diagnostics, but there were no fault codes. He said he had come across a similar problem on a 2008 S-class CDI, where they found the EGR cooler was blocked, but that he would have to strip the top end of the engine to get at it. Could an EGR cooler or EGR valve be causing my problems? Steve Yes, the EGR could well be the cause of your problem, and it wouldn’t necessarily flag any fault codes, so I would go with your mechanic’s recommendation. I had the EGR valve removed – the carbon build-up wasn’t that bad – and it was cleaned and checked to make sure the valve was sealing OK and operating properly when activated. The EGR cooler was also checked and cleaned, but the problem remains. The Mercedes mechanic doesn’t know what it could be.

The air intake induction pipe has two airflow sensors on the left and right – you have to disconnect the wiring on the airflow sensors to extract the intake induction pipe from the air filters and turbo and change the air filters in the housings. The right-hand air filter housing also has either an air pressure sensor or a MAP sensor, but you have to disconnect the wiring from that sensor to change the air filter. Is it possible for these sensors to cause the running problems but not store any fault codes? Steve If you haven’t already changed the battery then change it now. A borderline battery can cause all sorts of spurious codes, and if yours is the original then it is overdue for replacement. The

unidentified sensor you have found is the turbocharger boost pressure actuator. The MAF sensors should generate fault codes if they malfunction, but it is not unknown for them not to. They will influence the EGR operation and too much EGR flow will cause misfires. I replaced the battery a short while ago as the original was past its best and wasn’t holding charge for long.

I came across a few posts on a Mercedes forum where people had a similar problem due to the electronic boost control unit on the turbo, but this stores codes, puts on the EML and restricts performance. I had it replaced, but the problem is still there. Would a smoke test be worth doing on the induction system? Steve Yes, a smoke test is a very good next move. A lot of the causes for your problem will leave a fault code, but an air leak will not necessarily do this. Nor will a faulty MAF sensor or crank sensor. Did a smoke test at my local independent Mercedes garage and there were no leaks anywhere. The mechanic thinks that the swirl flaps are likely to be sticking at low revs when the engine is cold or warming up because a new swirl motor was put on just under two years ago. The mechanic says that unless the motor is faulty, it won’t show fault codes, but the whole inlet manifold will need to come off, including the turbo, to see the swirl flaps. Steve The swirl flaps are a fine tuning device and their malfunction will definitely not cause the chugging, kangarooing and smoking that you are experiencing. I’m really surprised that your garage considers them a possible cause. Have you checked the coolant temperature for correct output while warming up? Or you could change the sensor anyway and clean/ check the relevant wiring and connectors.

You don’t need MB to do these checks. You can get an OBD reader to link to a Smartphone or PC quite cheaply.