SL320 Idling problem

[Blucher]

Hi
Hope someone can help me.
Late 1997 SL320 I6. Mileage approx 31k.

Over the past couple of weeks car has had rough idle and when D or R selected revs drop
and she cuts out.
Can take 2 or 3 turns to start.
Only seems to happen when engine is warm.
Above idle she pulls well with no problems.

I have also noted that when driving and I take my foot off the accelerator, the mpg gauge doesn't go to max
but stays about 30 mpg and revs fluctuate between c750 - 1000.
So I'm thinking the mixture is too rich.

I note from the service history that the car had some rough running in the past....
In 2016 the MAF sensor and O2 sensor were replaced on 2 separate occasions.
I've had the car 3 years without any issues.
Have cleaned MAF sensor with electrical contact spray but no improvement.
Tried disconnecting MAF but problem still there.
Checked purge valve which seems fine.
Car has had new plugs and air filter in last 1000 miles.

If I got a 38 pin adapter and obd2 reader do you think this would show anything useful ?

Any help appreciated.
I hope SLs can't catch coronavirus or can they ???

 

[SL300-24]

Could be the OVP relay maybe?

 

[Dodipops]

Air leak? Coil pack? A diag might show something

 

[grober]

On these older straight 6 M104 engines quite a lot of engine functions use vacuum actuated/electrically controlled valves. You may have a vacuum leak problem somewhere. One example would be the variable flap inlet manifold which alters the effective length of the inlet manifold with RPM but there are other engine functions also.
m104 engine power issue, vacuum leak, switchover valve verification
M104 Resonance Flap

Another problem that can occur with these engines is with the cylinderhead gasket which can show up by examining the spark plugs appearance, looking for oil scum in the coolant reservoir or white emulsion streaks of white in the black oil and confirmed by a cylinder compression test. Then there are problems of HT breakdown on plug leads, coils or extenders or their low tension electrical wiring which can get brittle- do your problems date back to your plug change--wrong plugs fitted?---- JUST A FEW POSSIBILE CAUSES???

 

[zipdip]

Well the above is about the most comprehensive list of all the things that could be going wrong with your car,not bad for a 5am post Graeme.

 

[amazighman]

Best way to check for vaccum leaks is to get the intake smoke tested....I did the same with my E39 BMW and found several leaks that I couldn't spot.
Had the smoke test done for 25 quid ,. So will worth it instead of throwing money it

 

[amazighman]

Best way to check for vaccum leaks is to get the intake smoke tested....I did the same with my E39 BMW and found several leaks that I couldn't spot.
Had the smoke test done for 25 quid ,. So will worth it instead of throwing money it

 

[BlackC55]

Have a listen to see if you hear any hissing that indicates a vacuum leak. Do your headlamps go up and down?

 

[grober]

VACUUM SYSTEM 129 MERCEDES SL-KLASSE [Car] [CHASSIS] (EUROPA)
HYDRAULIC AND VACUUM SYSTEM MERCEDES SL-KLASSE [Car] [CHASSIS] (EUROPA)

 

[Blucher]

Hi All
Thanks very much for your replies.

Today I let the car idle up to 80c.
Revs began to surge.
I checked exhaust and there was a fair bit of grey smoke.

I'm now thinking faulty trans modulator valve...
might be sucking trans fluid into cylinders.

I know someone who used to work with auto boxes...
will call him tomorrow.

I'll give an update.

 

[BlackC55]

The the vac pipes that go to the gearbox and see if there is any ATF in there.

Are you sure its not steam?

 

[Blucher]

So decided to take it in to my local garage.
They did a pressurised smoke check of air and vacuum system - no faults detected.
Diag check showed P0100 maf sensor fault - values out of spec.
They said would be £138 for a new sensor which I said was too expensive as it had been replaced in 2016 for £58.
I sourced my own from Germany and fitted it yesterday.
I let the car get up to temp on my driveway and it showed no improvement.
Do I need to take it for a good run so the ecu knows there is a new sensor or is this just a myth ?

 

[grober]

Could it be an engine wiring/ connector fault- high supply/signal line resistance leading to erroneus "out of range" Maf readings? the engine loom wiring is 20+ years old ?

 

[SL300-24]

I don't think 1997 cars had the biodegradable harness issue

 

[grober]

I don't think 1997 cars had the biodegradable harness issue

Agreed makes it less likely but even the pukka wiring insulation is open to embrittlement if exposed to engine heat for long enough Remember it must be among the last of the i6s ??

 

[BlackC55]

£138 for a proper air mass sensor is too cheap. The £58 one will be a cheap copy of a genuine one. Put a proper air mass sensor on there from Bosch or genuine. Don't skimp on this part, its very important. Live data needs to be checked, this can be tricky unless you know what you are looking at. If I can help, let me know, I specialise in 129's. Ive owned about 40 of them and do them daily.

 

[Yuri84]

Hi. I agree that cheaper versions of maf sensors are no good.
I think that you had fault code for maf in ecu memory because you have disconnected and ran car like it. It would trigger fault code,so there might be nothing wrong with maf. If fault was cleared then car needs to ran for a bit and checked again.

 

[Blucher]

Car seems back to normal now after a 20 minute drive !
Rev counter showing steady idle c 700rpm when up to temp and no more exhaust smoke.
I think the maf was definitely faulty as garage said code reappeared when they ran engine after they had cleared it.
The faulty ( ￡58 ) maf fitted in 2016 was a Bosch one - looks genuine with Merc and Bosch part numbers embossed on it but always the chance it isn't.
Strangely, my garage said they get more defective mafs on lower mileage cars !
Will have to up my annual mileage from 1000...

Thanks everyone for your help and suggestions.