Hello everyone, I'm a new SLK owner having bought an '02 SLK 320 last week and was thrilled because she was running like a top. Last night the car developed a peculiar 'stumbling' or bogging down when stepping on the gas from idle. The engine almost dies and then seems to catch then the car moves on. There is a sensation of lack of power then all of a sudden the power will come on and it will surge forward powerfully. This happens frequently, i.e. stumble, lack of power then surge then weak power then surge again etc. After a few hard runs the CEL came on and I headed home quickly.

As I live a couple of hundred kilometers from the one and only MB dealer in the country I was wondering if any of you gentlemen have any ideas on what is causing the problem before I head to the dealer for what I fear willl be a truly expensive experience.

BTW is there a catalytic converter just before the rear muffler? It seems that the previous owner had removed it because I see 2 sections of pipe welded to where I think the cat should be. There are 2 small canisters just after the exhaust manifold but nothing else but piping to the rear muffler.

 

Classic symptoms.

Likely MAF Mass Airflow Meter. See other posts.

Need to reset CEL and determine what code caused it to illuminate using appropriate scanner. Likely P0173 = MAF.

SLK320 has 4 cats. 2 on downpipes and 2 either side gearbox.
I believe ALL markets took the California emmissions M112 engine and dependent upon where the vehicle is being run the rear cats are somewhat unnecessary. Since my cats started rattling I run it with the original cases but the internals removed on the rear cats. Curedrattle! and nobody can tell that the cats were tampered with since I opened and rewelded along the weld line.Still meets UK/Euro emmission levels by a mile at the annual test and it didn't alter the (nice) exhaust note.

Stuart.

 

Thank you for your fast reply. Indeed I too suspect that it is the MAF at fault and I will have the car scanned asap. So in your case your car runs fine without the cat? What about the O2 sensors before and after the cat - are they not affected by removing it? I would suspect that removal will screw up the oxygen levels detected by the O2 sensor after the cat.

 

The O2 sensors are sited either side of the pre (first) catalyst.
The second O2 sensor doesn't alter the mixture. It just causes an error message (i.e. The first cat isn't working properly) and illuminates CEL (and possibly puts car into "Limp Home" IF the emmission (O2) levels are incorrect. Mixture adjustment is done by feedback from sensor 1.
There is no O2 check after the second cat so the engine doesn't know whether it is there or not.

Even WITHOUT the second cat my car still meet CURRENT UK NEW CAR regs since it has air injection etc which is fitted for Califorian which is MUCH more stringent.
UK USED car annual test limits are ridiculous lax.


Stuart.

 

Ah so the sensors are before and after the pre-cat. Now I understand why it makes no difference to the EMS if the 2nd cat is removed, many thanks Amigo. I did the same thing - gutted out the cat - in my old 190E 2.6 but it flunked emissions testing so I had to buy a replacement

BTW you are spot-on with your MAF diagnosis. The error code was P0100 MAF circuit. We cleared it and it hasn't come back yet. Car is running quite well again, even comparable to my C36 - at least until midrange. I'll be cleaning the MAF this weekend for a more permanent fix.

 

Cleaning doesn't always work!!

Stuart.

 

Here is an update for those who might have the same problems in the future:

1.Replaced the spark plugs with BOSCH FR8 DPP33 because the specified BOSCH F8 DPER are no longer available. This substitute part is ok as per MB but getting the plugs off is a pain! I had to remove the 2 rearmost coil packs to get to the plugs at the back

2.Cleaned the MAF by carefully disassembling it from its housing and spraying active component with electronic contact cleaner. I know that there is a MAF cleaner available but I don't have any so contact cleaner or even alcohol will do. The active part is the recessed metallic part at the very tip of the black plastic housing.

3. Replaced Air cleaner elements andfuel filter

4. Scanned then cleared the fault codes with my ElmScan ISO USB (I'm not related to them)

Result? No more stumbling or surging, and I think it purrs a bit smoother at idle

Cheers

 

Glad diagnosis was correct and that your car is now AOK.

As I say MAF cleaning does not always work and even when it does problem may return prematurely. The key to this issue is the LTFT data reading. Cleaning usually takes it below the threshold (set at about 30) but it doesn't usually take it back to "new" value.

As I understand it Bosch are not OE plugs. Beru is. However I'm sure Bosch will be fine so long as they are not three point tipped which can give problems.

Stuart.