SL320 Cylinder 5+6 Misfire

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rajinder_1

Active Member
Joined
Jan 30, 2006
Messages
995
Location
Midlands
Car
01' Designo SL320 , 96' C36 AMG, 98' C43 AMG Estate 19' Focus & 96' ZX6R Ninja
Hi guys,
Have just been out in car and noticed the engine management light come on and it was real lumpy and poor respose to accelaration.....
I had a friend who ran diagnostics on the car and found that Cylinders 5+6 were misfiring on B. He changed the leads over and swapped the coil pack for a new one both with no joy.
We (well he) ran a test on the MAF and that was showing ok i think it read 16 which is ok on the M112 engine ( needs to be between 12-20 i think ).
He reccomended changing the plugs as this was the only thing left, now im not to sure as i would need to replace all of the plugs and according to the servoce these were only change around 1-2 yrs ago (from what i recall).
Has anyone had any problems like this with a similar fault does anyone know what could cause this?????

Regards,

Raj
 
As far as i'm aware you can't test a MAS. If it is faulty then it still doesn't show up in most cases.
 
KLP 92 said:
As far as i'm aware you can't test a MAS. If it is faulty then it still doesn't show up in most cases.

It will have an out of spec reading, usually low. The maf is a sensor that emits a voltage depending on air mass and flow. It has have specifications for different flow rates.
 
so does this mean that it isnt the mas sensor?????
 
Don't know if this will help or not, but when I changed the fuel filter on my C32 on restarting car misfired badly. Switched off, waited 5 or so mins and restarted - still the same. Diagnostic session said misfire on 2 cylinders. Fault code cleared and car has been fine since. Perhaps a fuel filter fault? Is your car 3 valve V6? this car has a fuel filter with 4 pipes on it. The one with no hose clip on it should not have any fuel in it when you pull it off. If it does you can get misfire problems apparently.
 
plugs??

whats the exact model/age of your car please?
Edit:-
Sorry didnt notice the M112 bit! So 12 plugs then? Sure they were all replaced last time?:eek: 5 and 6 cylinders the most difficult to replace? not familiar with your model layout:eek:
To eliminate the plugs it should only be necessary to replace the plugs on the suspect cylinders at first not all of them.
 
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rajinder_1 said:
so does this mean that it isnt the mas sensor?????

Well I can't see how the MAF could cause two cylinder to misfire but not the others.
You could try disconnecting the MAF but I think it's Ok.

This sounds like a weak spark, coil pack or wiring loom problem.
Oh I have just remembered....You probably don't want to read this bit.......the ECU has been known to short out if a coil pack dies which means a repair to the ECU and new coil pack.

Is the fault permanent on those two pots?

The coil pack and wiring can be tested without being connected to the ECU by shorting the -ve wire to ground and rubbing it across the surface to create a rapid make/break situation, with the +ve feeding 12v to the coil pack.

It could always be the wiring loom to the injectors if you're really lucky.
 
Hi we have tested the leads and the coil pack, the lead was swapped with cylinder 4 lead which was ok and a new coil pack was used but still the same response and still showed the same readings on STAR diagnostics?????
The fault occured today for the first time and has been permanant from then ie when i have stared it is still lumpy with rough idle.

Raj
 
guys,
Have been looking on the net for similar faults and found some guys in the states who who similar symptons, and put it down to water in the fuel? now i would normally think there cant be that much water or crap the would mess up the car surely, But apparantly if the tanks at the garage are emptyish there is supposed to be loads of goo and water at the bottom.
Now i got this problem after i stuck in £55 worth of BP petrol and recall a big fuel tanker parked up that was re filling the tanks or getting ready too and the pump did make a odd noise half way through the fill.....
Im not sure if im being para but could this maybe be a cause, i started it again today and the revs are ok just if i rev high it drops like its going to cut out and there is a weird smell from the exhaust as i went by back of the car????

any suggestions???
Raj
 
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I can't see how the fuel would affect only no 5 & 6 cylinders.
If the coil pack isn't working then it has to electrical. My guess is ECU.

The smell is the unburned fuel passing into the cat and burning off.
 
if it was the ecu wouldnt that have shown up on the star diagnostics as a fault????
 
When you cleared the fault codes did the Check Engine light come back on? IIRC a bank of cylinders will be shutdown from a failed MAF sensor, crankshaft / cam sensor or Lambda & O2 sensors on the V6. For peace of mind it may be worth going to a main dealer (Purslow?) for this.
 
Stats,
I didnt get the codes cleared as its going in today to my mechanic, It not a whole bank taht has shut down its just 5+6 B.
You think it will be a sensor then???
Will keep you all posted.
Raj
 
5 & 6b are a secondary pair - if you haven't had the fault codes cleared then I'd do this first and then see if another comes up whilst running.
 
thanks stats, will make sure they do that today, gave car to my mechanic this morning so they should have a look at this for me and let me know whats what hopefully.

Raj
 
Right not good news, been to my mechaninc, redone the diagnostics and now cylinder 4 is out so that 4,5 and 6 and 3 is on its last legs, this leads them to think its the cat breaking up!!!!!! not going to be cheap fix then !!!!

Any ideas or suggestions on replacement cats? do you go with dealer parts of aftermarket..... i have bought aftermarket for the c class but did notice a bit more noise and is raspy..... if anyone has replaced cats on SL could you please advise me on which ones you used

Regards

Raj
 
I don't know anyone that has replaced R129 cats with a brand other than MB. JJRoger did this recently I believe - they are not cheap - almost £1000 for a pair. Are you sure it's these and not the lambda and O2 sensors?
 
my mechanic is pointing his finger towards the cats they are going to check now on ramp with a temprature reader. it does feel like it possibly could be cats due to lack of power and all cylinders clogging up slowley....
im thinking about the mb cats too but its hardto justify double the price...

Raj
 
mmmm now after a long discuaaion with my mechanic and klp 92 they are doubting the cats..... and will be checking again.... i feel the same scenario coming along with what happened when i first got car and went into limp mode..... a long long story...
 
9 times out of 10 it would be ecu i get quite a few hfm and pms types all suffering with misfires they can be repaired as i say i do quite a lot of them please feel free to check my website and give me a call
 

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