Cold running problems
I have an intermittent problem with a 2006 Mercedes-benz CLS 320 CDI automatic with the V6 OM642 engine. I have taken the vehicle to a few Mercedes independent garages, but the Mercedes Star diagnostics has not logged any fault codes.
The problem arises when the engine is running from cold. As the engine starts to warm up, the temperature climbs above 45°C and, when you rev the engine to around 1000rpm, it will chug and misfire. If you increase the revs to over 1200rpm, this will disappear, until the revs drop to between 1000-1200rpm. Even driving it feels like a kangaroo at times between 1000-1200rpm, but all this disappears once it gets to its operating temperature of 85°C. On some days, the car drives perfectly with bags of power.
This week, I had to charge the battery due to the car being a little slow in turning the engine over on cold mornings. The voltage was 9.6V and, after a charge, it rose to 11.46V. The alternator puts out 14.85V
when the engine is running. I think the battery is on the way out, as it is the original battery. After reconnecting the battery and starting the car, I had to reset the windows, sunroof, mirrors, heating system, etc.
The temperature was around 45°C when I revved the car around 1000rpm and it misfired badly, shaking the car, and a cloud of blue exhaust smoke came out with a sweet smell in the fumes. I held the revs for a while hoping the engine light would come on, but it didn’t. While this was happening I had all the auxiliaries on. Thinking the greater load on the alternator could be making the misfire worse, I turned off all the auxiliaries and some of the shaking eased, but the car was still missing and smoking, I increased the revs and it cleared up, and had gone altogether by the time the temperature reached 85°C.
The car has never thrown out thick blue smoke before. It's well serviced and the oil, oil filter, air filters, fuel filters and antifreeze were changed recently. Since I’ve owned the car I’ve replaced the glow plugs, control module, coolant temperature sensor, exhaust back pressure sensor, Lambda sensor and inlet manifold swirl motor, all with genuine Mercedes parts.
One mechanic said he had come across a similar problem on a 2008 Mercedes S-class 320 CDI, where they found the EGR cooler was blocked, but he said he would have to strip the top end of the engine to get at it. Would an EGR cooler or a EGR valve be causing my problems? Steven Mccreery
My first comment would be that if the battery is only showing 11.46V after being charged, then it is faulty and should be replaced immediately. The battery should deliver at least 12.5V after being charged; if this is not the case, the electronic systems of the vehicle cannot operate correctly.
Modern engine management systems are sensitive to low voltages and it may simply be the case that, until the engine has warmed up, the battery voltage is remaining too low to correctly operate the management systems. If, after replacing the battery, the fault remains, then the problem can be investigated further. I would first ask if the coolant level drops at all during normal use. The sweet smell from the exhaust you mention is normally associated with burning coolant and this may be connected to an EGR cooler problem, as your garage suggests. Or it may suggest a more serious head gasket problem is arising.
You have changed the coolant temperature sensor, which is one of the other suggestions I would have made, and so eliminated this as a problem. If you are not losing coolant, it is unlikely the EGR cooler or the head gasket is the problem. The next area to check is for any loss of oil in the sump. The blue smoke is normally associated with burning oil, although a variation in the colour of the exhaust smoke may indicate you have an injector problem.