SL500 R129 misfire at idle - what next?

[grober]

This lead to Mercedes changing the O2 Sensor and then the LH Control Unit at great cost. Despite this being a relatively fast job I was made to wait a couple of hours more as apparently they had trouble “dialing things in”. I was never happy with the car after that.

It's possible you did not get an exact equivalent to your old ECU but a "nearest equivalent" resulting in a slight sensor/ECU mismatch. Have you any documentation relating to the ECU change --part nos etc------what was involved in "dialing in" the new ECU? This may not be relevant but the history should be checked.

 

[w124coupe]

This lead to Mercedes changing the O2 Sensor and then the LH Control Unit at great cost. Despite this being a relatively fast job I was made to wait a couple of hours more as apparently they had trouble “dialing things in”. I was never happy with the car after that.

It's possible you did not get an exact equivalent to your old ECU but a "nearest equivalent" resulting in a slight sensor/ECU mismatch. Have you any documentation relating to the ECU change --part nos etc------what was involved in "dialing in" the new ECU? This may not be relevant but the history should be checked.

Might simply hve been trying to find someone in the Merc garage who knew how to code old cars! They probably had the car in emergency mode for a while until they twigged...

 

[MD5]

IIRC, even on a Star test, the coils weren't positively identified as being faulty, but it did show which cylinders were misfiring. Swapping the coils around caused different cylinders to misfire, which confirmed initial suspicions. Incidentally, when the coil first went, all the dashboard instrumentation fluttered.

 

[Nick-V]

The LH Unit change always bothered me...however, it is the right part number (I looked at both the unit itself and the invoice against the parts record for my car). Also, as far as I can tell (dealer conspiracies aside), it failed on emissions before and has been fine since (I kept all the test results). Further, the LH unit appears to do as it should...it is working in closed loop mode and shows an LTFT reading slightly other than 50% (can't remember) and emissions are ok so it seems to be listening and responding to O2 and MAS input.

In my less paranoid moments I also believe the dealer simply had a problem using STAR and passwords to dial the unit into my car (enter chassis number or whatever). For info, WIS implies (says) that you can swap LH Units for testing - Merc says you can't - there are a lot of contradictions about the coding and recoding of these units.

Of course the fluttering/pinking that started at that time could be loose exhaust bits or dying cat that started making noise when the O2 sensor was changed (same time).

Priority 1 is the idle misfire and I don't suspect the LH Unit for that. Priority 2 is the good old pinking/lh unit issue which is probably separate and may never be found!

 

[Nick-V]

Star cannot tell which cylinders are misfiring on my 1995 as it has OBD1 not OBD2. However, the Herman machine can (connects to all plug leads) but I am currently suspicious of the testing done. A retest now one fault as been corrected may well be wise - could be a coil.

 

[lynall]

As coils, caps, plugs and leads are consumable items i would replace them all, especially in light of the test saying all okay and you found a cracked rotor arm.



Lynall

 

[Nick-V]

Yep that test needs re-doing - I am resisting throwing money (I don't have) at items that may be ok...the items you listed a just a few of the things I'm been told to try !!!

Actually, plugs, caps, rotors (even the broken one!) are quite new. Coils, leads, plug caps are not...but neither are the ETA, MAS, O2 sensor etc...

Out of interest...Beru Rotor arms cost about £20 from Euro Car Parts and about £60 from Merc or an Indi charging Merc prices - same item exactly !

 

[Nick-V]

I visited the Indi this morning for a quick test and discussion. We didn't resolve much but I have some feedback and questions.

1) New info: idle speed is around 500rpm. This is low and cannot be adjusted. He considers it might be related to his continuing feeling that its running lean.

2) Question: where is the LTFT reading on the STAR menus ?

3 ) Question: The O2 Sensor (on-off ratio - can't remember the STAR terminology) fluctuated around 50% which is great. However, after a reset it fluctuated nearer to 40%. I GUESS we reset the LTFT setting which we cannot find) and after some trips this measurement will return to 50% and LTFT will not be neutral (50%)...why might LTFT not be neutral?

4) Question: using STAR we assumed the vehicle had LH1 rather than LH2 when we set up the program - is this correct and does it matter too much?

5) Question: using STAR the intake air temp read 31c - the actual air temp is about 7c - could this difference be that its breathing in warm around the engine when idling in the garage? Note I recently changed this sensor as the old one was suspected to be from the wrong SL500 - resistances were wrong.

6) Question: the exhaust analyser showed: CO=0.16, CO2=13.54, O2=2.3, C2H8=402, Lambda=1.14 although this was slowly reducing and I saw 1.074 so the cat may well be taking too long to do its magic...do these readings say anything much?

7) New info: at some point recently I removed the control modules to check wiring. Since then the ASR and ABS lights are on and did not go off after resetting any faults and turning the steering left and right?

8) Thoughts: pumping the brake pedal hard and fast causes the car to almost stall...he says one might well see a change in running but that is too much and indicates to him leanness again?

9) Plan: In a couple of weeks time he will do the Hermann test again to determine if 1-2 cylinders or if all cylinders. Previous results may have been confused by the damaged rotor.

10) Issue: despite changing the cam and crank sensors I am still getting a cam sensor stored error and a code 18 (not 17) crank magnet error. This is disappointing as the replacements were expensive and had little effect. I believe the code 18 can be thrown when the car revs over 5k in neutral (it hasn't to my knowledge) and is not a crank sensor problem. I also mentioned I had read about shimming the cam sensor but this was news to him?

11) Thoughts: He mentioned that a rotating valve might cause an occasional miss if sealing badly (expensive). He agreed cleaning the injectors may be worth a try (£300 or more) but he never really sees problems at 90k miles. He agrees (I think) that MAS and O2 sensors are irrelevant as it happens in open loop mode.

Not a good morning really...I was hoping to check the LTFT, reset the ASR/ABS and move forwards a bit...this is stumping us a bit !

 

[PeterBoyes]

I'd check the Spark Plugs are Bosch F8DC4 (Non-Resistive) 0.8-0.9mm Gap, as sometimes these are replaced incorrectly with FR8DC4 (Resistive) which causes a weak spark.

Cheers Pete

 

[BoristheBounce]

I'd check the Spark Plugs are Bosch F8DC4 (Non-Resistive) 0.8-0.9mm Gap, as sometimes these are replaced incorrectly with FR8DC4 (Resistive) which causes a weak spark.

Cheers Pete

Its 12 years ago since the last post; I think he's given up