Chris Hobbs

Hi all,

This is my first post but I have used this site a lot forinformation on my issue with my 1999 SLK230 which has 100k miles on it. Myissue doesn’t seem to match any single post so I have started a new one andwould appreciate any help and have tried to provide as much background as I canto help others with similar issues or to help with any responses.

Issue – my car will stall when in slow moving traffic orwhen stationary in queuing traffic and once it stalled when travelling at about50mph. When it stalls it will not restart until the engine is cold. It will notcrank, all warning lights are on and I can hear the fan whirring. I turn thekey and nothing happens. I have read reasons ranging from the MAF, Crank &Cam Position Sensor and also the K40 Relay.

I bought the car in February this year and it ran great forabout 2 months. In April after queuing off a motorway junction it suddenly cutout as I tried to pull away from the lights. I pushed it to the side of theroad and waited for recovery which took 1.5 hours. I kept trying to start itduring this time with no joy. When the recovery vehicle arrived they towed meround the corner and couldn’t get it started. They tested all the fuses andlook inside the electrical housing box next to the battery where the K40 relayis located and found no issues. The reconnected the battery and then the carstarted fine. I drove it to a garage who looked at it and found no error codesand no problems with its running. However they found a breather hose from theair oil separator was cut and replaced it(more on that later, I suspect to prevent oily MAF issue)

I noticed in June the car started running rough, misfiringand losing performance so I put it into my local garage and again they couldfind nothing wrong with it. It was lacking power and misfiring under heavieracceleration and was especially rough in the morning when first driven. Afterthe garage couldn’t help I started visiting these forums. Searches led me tobelieve the MAF sensor was at fault so I cleaned this using one of thetutorials on here and the performance improved immensely and was better thanever. This lasted until August when the car started running rough again. Icleaned the MAF and it was back to performing great for about a week. I readthe source of this was oil from the air/oil separator getting into the MAF so Icleaned this and the hoses and replaced the oily air filter. It was then Iremembered about the hose being replaced but I didn’t know which one they hadreplaced. I have since read about putting an oil filter on the hose that goesfrom the A/O separator to the air filter box to stop this oily MAF issue whichI intend to do shortly.

In August the car cut out about 10 times during the monthand the performance was terrible. One time it cut out while driving after I hadmanaged to get it restarted. Again the garage couldn’t find any issues and itdidn’t stall when they drove it. I took it for a drive with the mechanic toreplicate the conditions under which it would stall (queuing, slow moving) andagain it behaved itself for a week with them.

The car would seem to stall when it was up to normal runningtemperature and it doesn’t then restart for about 2.5 hours until the enginewas cool. Usually it would then run ok (misfiring under heavier accelerationdue to MAF I suspect) for a little while and then stall again, usually at themost inappropriate times! It also sometimes wouldn’t start after I had drivenfrom work to the supermarket, popped in to get some bits and when I came out 15minutes later it wouldn’t start, no crank, just lights and the fan whirring.

I’m getting to the end of my tether with the car but am alsodetermined to sort the issues. When it runs well there’s no better car. I justdon’t want to throw more money at it than I have to.

Last night I made a bit of a discovery, when it stalled Iunplugged the MAF sensor to see if it would make it restart, but it wouldn’t.However when I re-plugged in the MAF after 10 minutes it started and ran till Igot home. This leads me to feel that the MAF has been at fault the whole time.I am going to put an oil filter on the A/O separator to air filter hose andblock up the hole in the air box and buy and fit a new Bosch MAF.

I’m wondering if anyone has any experience of the MAF beingthe root cause for the issues I have described? Most threads point to the CPS orK40. I have checked the K40 soldering and to the eye it all looks fine andthere are no burn marks as with other posters/pictures. Should I just bite thebullet and do all 3?!

Any help greatly received, thanks!

Alex.currie44

Did you get to the bottom of this?

Chris Hobbs

At first I replaced the MAF which improved the performance and the car didn’t stall for a while, after it stalled again I bought a new K40 relay unit and haven’t had any issues since so fingers crossed that’s fixed it.


I’m onto my next issue now, I snapped the key and it was theonly one I have plus the Funk Card. I’m trying to find someone who can cut anew key but it appears that Mercedes keys are somewhat specialist. My local MBdealer can do it for £200 but need to send it off the Germany to be made and itmay take 10 days!!

Dave33478

I'm in the process of troubleshooting the very same issue as you stated. I purchased a scanner but no fault codes captured. Im monitoring the MAF, idle RMP and TPS for drifting values as it gets hot but not knowing what it's supposed to be, I'm shooting in the dark.
MAF(lbs/s) 0.01 steady
RPM 730-760 flux
TPS 3.9 with spikes to 4.3
O2B1S1 bouncing between .1 and .9
O2B1S2 0.485 steady
Coolant tem 197F to 203F

it's frustrating because it only happens after it runs for a while. Stalls at stop light and will not turn over. All accessories and lights work. Reworked the K40 relay bd. No help.

Fredrik Nyström

Check the STFT and LTFT that can indicate a vaccum leak. If the car stalls at trafic lights and drives okay when above 2000RPM., most common issue is a vaccum leak.

Check after the throttle body and the small o-ring at the oil dip stick, and the throttle elbow andmake sure all clamps are tight and okay. And look at the small house going from the SC manifold that connects to the oil separator. Also check the houses from the PCV valve.

If STFT or LTFT is above 10% at idle you probably have a vaccum leak.¨

Good luck!

/Fredrik