SLK230 Mis-fire :( Thoughts please?

Apologies for the length of this mail!

My SLK 230 1999, 46k, intermittent use but regularly serviced and always reliable UNTIL after 5 weeks non-use and flat battery, restarted by mechanics jump leads and driven back to their (non Merc) garage for a service. My opinion after 2 secs of 'tappet rattle' was that it ran as smoothly as ever.
Collected car after oil/filter, coolant, brake fluid change service, their report 'slight mis-fire, suggest plug change'.

Slight misfire??! .. I could hardly get it to move back up the ramp to their work-shop. It continued to run lumpily throughout the warming up. They examined plugs whilst I watched, all were clean, and other than slight mis-colouring on the ceramic appeared OK. No surprise to me to be told later that plug change had not improved the running.

Now, after 'analysis' they propose renewing both ignition coil packs. Their analysis report sounded vague.

Are both ignition packs likely to fail without any prior warning or performance degradation?

Their opinion, when pressed, was that after driving it less than 1 mile from collection it started mis-firing. (I cannot believe that with the degree of mis-firing I found on collection, that they even offered the car back to me).

I fear they are incompetent. Should I cut my losses and move the car to a Merc expert before I find that changing the packs has not solved the problem.
All comments appreciated!

 

I get a lot of calls from other garages, mainly local ones, and was asked yesterday about an SLK230 with the same scenario (might be yours...).

I suggested an airmass meter fault as being likely, with possible oil contamination of the sensor.

Oddly the flat battery could be involved here. When the engine ECU looses power on these cars, it also looses any adaptive data it has stored. This can be why it runs 'OK' before the battery is disconnected, but poorly after. A very common scenario with the first M112 engined cars.

Obviously if you to get a second opinion, we would be happy to help.

 

Many thanks Alex, very helpful. From reading the forums I'd suspected MAF (I'm assuming that is the air mass meter you mention?) however, wondered how contamination of this occurred when the car regularly sat unused for several weeks and had always performed well after starting. However, it had never needed a jump start before, so given the ECU reset, rough running can be expected, but does it follow that MAF contamination will result?

I'd gladly bring the car to you, and although my home is Ipswich, the SLK is with me in Jersey at the moment where I spend half my time.

 

You could try disconnecting the MAF ------ the engine will use a stored default map- if running improves its probably the MAF. Coil or Coil extenders are a very common failure on these engines if the car runs at all then some of the plugs are working. each coil fires 2 plugs 1+4 and 2+3 Its normally recommended to replace both since if one goes the second is often not far behind. Worse scenario might be a damaged engine ECU IGNITION TRANSISTORS. Other possibility is that there was an accumulation of moisture in the coil plug area which only volatilised when the engine heated up and then condensed on the HT connections. So your garage suggestions of replacing the 2 coils is sound enough. The first thing I would do is replace the coil to plug extenders as they are relatively cheap and make sure everything in the HT area is bone dry.

 

You can also pull the camshaft magnet connector off, top front of engine and look for any engine oil in the connectors,,, if there is then get the blocker £30 from MB.

If the car runs better without the MAF, then either is contaminated or past its sell by date. Do not buy a Ebay version and any Bosch service center will have one at the best price

 

Thanks, Grober and TV, very much appreciated.

I'm hearing from other sources as well that jump starting (if it can be avoided) IS best avoided, due to risk of surge damage to the ECU. (Oooh, shudder). I'd not heard that before, although in my instance it couldn't be avoided since the SLK is an automatic.

Grober, the coil to plug extenders are 'just' the HT leads? All looked clean, dry and OK in there, but we have had rather a wet time, and the car sits outside, so that's certainly a good thought.

TV, I'd not heard of a camshaft magnet connector before? How might this cause a problem, is this the connection to the MAF?

I've also heard of an 'oxygen sensor' in the exhaust manifold being a problem. Anyway, work resumes on Monday. I'll update with (hopefully) progress, then.

 

Alex, I looked at your web .. impressive. Are you MB exclusively? I have a 350Z in Ipswich, but so far haven't been over impressed with Nissan service.

 

Yes, we are exclusively Mercedes, and regularly turn away offers of work on other makes - sorry.

 

With that set up there are 2 spark plug connections 1 a removeable extender under the coil to one plug [ see pic 1] and 2 a " flying lead" bit like a conventional HT plug lead to the second plug. Its the "extender " can can cause problems - cheap to renew.

see this thread http://www.benzworld.org/forums/r170-slk-class/1334720-spark-plug-removal_replace-slk230.html

 

Thanks Graeme

Apparently all's well after replacement of the coils. I've nor driven it as I'm away in the UK but my lady tells me it runs well.

Still somewhat surprised that a completely reliable car should fail on both coils after sitting idle for a few weeks, especially as that has been the usual routine for several years.

Thanks all, for the responses.

J

 

Glad you got it sorted. Chances are it's only one coil that failed but because both are subjected to thermal stress in that little "oven" on the cam cover-if one fails the other is often not far behind- So its good practice to renew both if they are both "getting on a bit"