SL400 misfire not a misfire....?

[TimC-SL]

Hi all
Just two weeks into owning my lovely SL400 and all's great apart from a glitch with what seems to be a misfire, but it's not....
In mixed driving speed and conditions the engine starts stuttering and shaking, no throttle response and EML comes on. Stop the car, turn it off and on within a minute, and all's good. Had a diagnostic check with a specialist in London last week, and it had recorded to have been over-revving at 4500 at idle, but it wasn't. They checked the coils, all OK. It had new spark plugs at 50k miles. They did a software update from Mercedes. Same problem has happened twice since. Any thoughts? ECU, fuelling.....
Thanks!

 

[TimC-SL]

Update from today, I took it back to the specialist today and the code this time was PO30022 - Multiple combustion misfires, all but cylinder 5. Today they checked the spark plugs (renewed 12k miles ago), coils, MAF, air filter, fuel pressure, all OK. They said take it to Mercedes as there's not much more they can do without inside knowledge of possible TB's, so I guess that's the outcome. Will post more once I know.....

 

[Uncle Benz]

I’d be looking for intake leaks. A smoke test of the intake system is where I’d be starting. It’s possible to CAREFULLY check for intake leaks with a can of spray brake cleaner. You’ll hear the engine note change if it draws the brake cleaner in where it shouldn’t.

 

[TimC-SL]

Thanks Uncle Benz.
Would that be for the air intake manifold gaskets?
I've just read a thread from JFW with the same issue a few years ago, seems he had those changed and in injector after visiting MB

 

[Srdl]

Which part of London are you from? We know some very good specialists who may be within reach.

 

[TimC-SL]

thanks srdl, I'm in south London, from Lewisham to Earlsfield for work/ home, do please pass on any details, especially anyone who can cure creaking door panels. I've been to Mercservices so far. Thanks!

 

[Srdl]

@alexanderfoti at AMF Auto may not be too far from you near Tonbridge.

Mercedes Servicing, Tuning, Remapping Tonbridge | AMF Automotive

Welcome to AMF Automotive Mercedes Specialists. For Mercedes Servicing, Tuning & Remapping Tonbridge come to us. We are Mercedes & AMG specialists in Kent.

www.amfmercedes.com

 

[AMGeed]

^ Good shout for Alex.

 

[Srdl]

I’d be looking for intake leaks. A smoke test of the intake system is where I’d be starting. It’s possible to CAREFULLY check for intake leaks with a can of spray brake cleaner. You’ll hear the engine note change if it draws the brake cleaner in where it shouldn’t.

Or, of course, @Uncle Benz himself who is somewhere down in West Sussex

 

[TimC-SL]

Thanks for those!
I’ve booked it with MB Bromley tomorrow for diagnostics. Keep you posted.

 

[TimC-SL]

Update from the diagnostic at MB today - they assessed that it was misfiring at 8-900rpm on test, and have suggested replacing all of the spark plugs and the coils on cylinders 1 & 2. If that sorts it, I'll be very happy. Apparently the turbo engines can eat spark plugs like they're going out of fashion, hence them (or some of them) only lasting 13k miles.....we shall see. I get it back on Tuesday. Once I've put some miles on it, I'll update again. Thanks for all the replies so far!

 

[Uncle Benz]

Vacuum leaks show themselves the most at idle or low engine speeds when the throttle is closed or very nearly closed. In this situation the intake manifold vacuum is very high, so any leaks will suck in a lot of unmetered air upsetting the air/fuel ratio. I hope their diagnosis sorts it, my fingers are crossed for you

 

[TimC-SL]

Cheers Uncle Benz. If it doesn't then that sounds like the next stop!

 

[Srdl]

Mine is just on 48k miles and plugs were routinely changed at 22k after 3 years. Next due in 12 months time (crosses fingers)

 

[TimC-SL]

interesting time frame srdl, cheers!
mine had them done 3.5 years and 13k miles ago, and would've been 3 years from new then. although 49k miles under its belt at that point....

 

[TimC-SL]

It's been a while, but here's the update on this one. Following the plug change, within only a few miles the misfire was back, and got progressively worse. Restarting seemed to reset it, but by New Year it became undriveable as it was happening intermittently on every journey, long or short.

Mercedes had said to bring it back when the EML light was on if it reoccurred, as they'd only be able to diagnose it when it was happening. I queried that surely any faults were kept on the ECU, but was told this wasn't the case once the ignition reset (which I found baffling, as my sister's VW can be read anytime unless the faults are cleared by a diagnostic tool....)

Anyway I waited for the time to be able to take it in with the light on, which duly arrived, but not til months later, by which time I was only running it at weekends to keep it ticking over, and always with the misfire.

It took MB two and a half weeks to diagnose the fault, and only after much head scratching and contact with Germany. Turned out to be a corroded earth connection, which was repaired by them for labour charge only. It's cured the issue, but after such an experience I've decided to sell it and move on. The emotional connection just wasn't there anymore.

I've posted this for general knowledge and for others who might encounter the same - seems mystery misfires can be a result of the simplest and cheapest of all the potential faults, worth checking if you're ever in the same circumstances.

Cheers to all of you with support from the beginning. Happy motoring!

 

[brandwooddixon]

It took MB two and a half weeks to diagnose the fault, and only after much head scratching and contact with Germany. Turned out to be a corroded earth connection, which was repaired by them for labour charge only. It's cured the issue, but after such an experience I've decided to sell it and move on. The emotional connection just wasn't there anymore.

As I read this post my first thought was a corroded engine earth strap. Just goes to show how some garages are just hung up on looking at fault codes.
Sorry that you had to suffer this and to be charged for fitting the strap when they had clearly fleeced you into replacing parts that didn't cure the issue is plain wrong.

 

[TimC-SL]

Thanks!
The outlay on the earth fix was nothing compared to the other 1200quid for plugs, diagnostics & fuel tank draining, then the extra for public transport and the capital loss of selling it. Lesson learnt and hope anyone else can benefit from this

 

[SL63 Mark]

I think the lesson is, follow forum advice and take it to someone who knows what they are doing. Clearly never a spark plug issue IMHO.

 

[DSK]

I don’t think it’s fully a garages fault either. I am sure even reputable places would be easily capable of doing similar, chasing fault codes due to the complexity of modern plastic cars but, it’s always the customer who foots the bill.


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