I have a 2001 SLK 320 with 44,000 miles, under hard acceleration the car misses between 3000 and 4500 rpm, under easy accelleration the car moves right up through the rpm's with no problem.

The CEL finally came on and gave the codes; P0300 (Multicylinder misfire), P0301 P0302 P0303 (misfire cylinder 1,2,3) and P0173 Fuel Trim Malfunction (Bank 2).

I changed Fuel and air filters, Replaced the plugs and swaped the coil packs from bank 1 to bank 2 and am still having the problem the same problem and codes.

Any Ideas would be greatly Appriciated

Thank You

 

Simple!
Mass Air Flow sensor out of range.
Replace MAF. Straightforward DIY job.
Will need security torx bit IF you just change just the sensor. MB supply sensor and housing together.
Simple way is buy a new MB unit but expensive.
DONT buy cheap NON MB copies of the sensor unless you want to gamble. Non OE sensors SHOULD BE OK but lots of people seem to experience problems.
I get my sensors off eBay but ensure they are OE Bosch (=MB) sensors. They have both MB and Bosch part numbers on them.
Ensure you get correct sensor / unit. There are sensors that look identical for different engines and also petrol/diesel.
Can try cleaning but 50/50% chance of success.
Stuart.

 

Thank you

I was leaning towards the MAF but wanted to hear that from someone else before I spent the Money.

Thank you

 

Don't replace the MAF, too expensive and unncessary most of the time. The fuel trim error is probably the MAF but it can also be a vacuum leak. If it's the MAF, take it off, remove the insert and spray the heck out of it and the sensor wire with CRC MAF Sensor Cleaners (available at many auto supply stores). Don't touch the wire as you may break it or contaminate it with your skin oils. Also, misfire codes can be fuel injector related, specially if you are getting only on certain cylinders, they maybe clogged or weak. A general misfire code can also be fuel pressure related, put a fuel pressure gauge on the fuel rail and check, should probably be about 50 psi, much more or less is a problem. Start with the MAF, $6 for a can of cleaner is cheap enough and it should be cleaned as part of maintenance, not just when you get a code.

 

Thanks

I will try cleaning it first.

 

I agree that it could be a manifold leak and I omitted to mention this. However this isn't very common unless the engine manifolding has been disassembled for some reason.

I suggested cleaning. Incidentally the wire you see is not the sensor. That is a temperature sensing thermistor. The thin film sensor is hidden.
I agree that the MAF is expensive and I agree should only be changed after ALL other possibilities have been eliminated.
However MAFS begin to give problems at this mileage.

The MAF of course controls both banks but the misfire is only on one bank.

This supports the leak theory but my own 112 gave the single bank code when the Fuel Trim setting went out of range.

OK. So what is required is for the OBD to be scanned to determine the LTFT (Long Term Fuel Trim) and STFT (Short Term) settings. This will confirm whether things are going out of range rich or weak and to what extent etc.

I still believe you will find it is the MAF.

Stuart

 

i 95% belevie its maff sensor