1995 w124 E280 lumpy idle Help Please

[killingtime88]

Hi there this car is driving me nuts.
Here is a little background on the problem and my work so far.
The car has been with me for 8 months now,when i first got it it was reluctant to start when hot anything over 70 degrees.this was traced to the four pin temp sensor near the thermostat housing. Problem solved I thought.
Last month took it for MOT at the local station everything fine apart from emissions 9% CO legal limit is 0.2% i believe so pretty bad.
Asked the diagnostics guy to connect up his reader and went back home for a cuppa.
10 mins later got a call from the garage saying you had better come round.
the diagnostics machine was going crazy showing faults all over the engine management system and the tech informed me that before he could make any sense of the readings i would need a new engine wiring loom as any fitted between 1991 and 1996 were rubbish,anyway being a little sceptical of this diagnoses i went home and started to investigate the wiring.I couldn't believe what i saw the insulation was dropping of the wires almost everywhere.
Mercedes wanted £587 + vat for a new one so i started to rewire my original loom one wire at a time.18 hours later and much muttering the loom was repaired and back on the car.
Took the car back to the MOT station this time the CO was down to 5% but still failing.
Back on the tester The MAF hot wire sensor was showing as bad no problem the garage changed this out as by this time I was quite bored of being under the big green bonnet.
Back to the MOT Passed emissions CO perfect HC within 4 PPM of failing bit a pass none the less.
Then the lumpy idle began when i am in drive the car bobs back and forth if i dont keep my foot firmly on the brake pedal it would at times kangeroo down the street.
Suspecting the throttle body as this seems to control the tickover i investigated its wiring, o my god it the same wire as the rest of the loom.
This has now been replaced right back to the circuit board in the throttle body,Anyway you might guess i still have the problem.
The only other device I can see is what i believe is called the idle damper with the two pin connector fitted to the inlet manifold.
Does any one now its true purpose i believe when operated the soleniod somehow vents to atmosphere and allows the vacuum part of the butterfly valve to move,i have sucked on the pipe and the flap does move as it should i have also tapped 12V onto the soleniod and it does appear to operate.
Could this part be causing my lumpy idle.
BTW once the wiring was redone and the MAF was replaced no engine fault codes are present.
Sorry for the war and peace
Regards Paul (electrical tech for subsea7)

 

[Richard Moakes]

Firstly, congratulations on rebuilding the wiring loom, you are a man after my own heart there. I am expecting to do this job on my fathers 1994 E320 at some point in the future.

I wonder if the oxygen sensor could be faulty and causing you to be close to the limits on emissions. I also believe there is a part between the coilpacks and the plugs which can also fail causing misfires like you describe. I remember reading that a faulty wiring loom can cause the ECU or coilpacks to be damaged, and wonder if you sustained some damage prior to rebuilding the loom?

Good luck with this, I am sure others will have different points of view on this. I can't believe how short sighted Mercedes were with this bio-degradable wiring loom they fitted in the nineties

 

[kth286]

I agree Lambda probe in exhaust needs changing if not been changed in first 100,000 miles. they do degrade on that generation car.

I wonder if MAF was a genuine replacement. Do you know? If not known, why not unplug it and see if that cures the hopping.

The main problem with wiring loom is where the coil feeds (within the thick rubber strapping under cover) short to each other (because insulation cracks) and then a coil packs up, and that means two cylinders down.

You would have had to replace those wires in the strapping - did you do that.?

You might have a coil down now - are you sure car is firing on all 6 cylinders ??????

Others have tried to repair their engine wiring looms and have ended up blowing the main engine ECU at a cost of close to £1,000 for part.

The existing loom has lasted 12/13 years . At £600 for a new loom, you will at least get another 12/13 years out of it, if not more, as it is supposed to be a better loom now.

Is a home made repair worth the risk, taking into account factors such as introducing additional resistances in patched up wires which the ECU may not like, and might cause poor drivability problems. It may be one of the ECU output driver feeds to a coil is already down along with a blown coil.

The part attached to the inlet manifold you talk about is the device that alters the effective lenght of the inlet track at mid revs and works in conjunction with the variable valve timing cam solenoid to enhance performance. You can feel the extra kick in the back just after 4000 revs when it all happens.

 

[killingtime88]

update

Hi yes i replaced all of the wiring to the coil packs it had been repaired once before (read completely bodged) how i repaired the loom was to cut the wires as close as i could to the connectors that were moulded such as the injectors and the 4 pin temp sensor and solder and sleeved with heatshrink to stop short circuits etc,most of the wiring went directly to the ecu plug the coils being the exception, they have what looked to be a permanent +Ve feed from the connector on the pasenger side flitch panel. the ecu then switches the -Ve wire to produce the spark , some of the plugs on the loom were easy to take apart such as the cam sensor ,temp gauge sensor and these are originally soldered anyway.
I dont believe i have a problem with ignition/missing as it went through the emisions test and all the plugs are an even colour and none are wet.Plus it goes like the wind on the open road its just the lumpy idle concerning me at the moment.
the MAF sensor is a genuine Bosch unit i thought the garage might have fitted the pattern part you can get for about £60 but they managed to get an original part for about £130 half the price the Dealer wants.
Thanks for explaining the purpose of the mystery item it doesn't seem
that it will be causing my present problem after all.
I have done some reading about the lambda sensor and it looks like they dont show up a fault on the reader unless they break but as they get old their response time slows down so that could be a good bet replacing that.
The next thing is where does its wiring come into the car i can see the wiring vanishing into the cockpit but cant see where its plug and socket is.

All wiring replaced has used tri rated single core which in my experience is pretty good and is rated at 105 degrees C.I have used 0.5mm squared which is rated at 12 A
The coil packs use thicker wire so i have run 3 seperate feeds one to each coil pack to compensate.
i appreciate what has been said about DIY fixes on the wiring loom but to be honest i only paid £1500 for the car 7 seater estate and the work i have done has saved almost that much already the loom £600 + VAT and the throttle sensor which are even more than that,It has really annoyed me that Merc spend so much time and effort on something like the throttle sensor it is like a swiss watch inside and then use this crappy wire to connect it all together.The throttle sensor was a real pig to get the original wires out of as it has a collar inserted and then its put on a press to retain it but there are only 8 wires to replace once your into it.
Thanks Dave and Richard for your quick replies

 

[killingtime88]

Stupid boy

I guess i should have done this a while back Thank you 124Coupe for providing the code reader/blinker circuit I am getting 9 flashes then 13 flashes reoccuring both faults relating to the Lambda sensor i will check the wiring first before jumping in feet first with a new sensor but from what i have read on the forums running for any length of time with a rich mixture wrecks then and it was quite rich for a while.
Still cant figure out why the garage reader didn't pick up these codes and how it passed its test?
i Still cant find where the lambda sensor wiring enters the cabin i have a manual for the car but thats about as good as a chocolate teapot
Thanks in advance

 

[killingtime88]

what a disapointment

Hi again just found what was causing faults 9 and 13 the connector which is under the drivers carpet was covered in verdigree a quick clean with a toothbrush and WD40 and the codes reset problem fixed.
Quick test drive confirmed not fixed still the same as before although in fact probably worse. I am wondering do i need to reset the adaptive memory if the car has been compensating some way for the faulty sensor readings .
I am just getting the one flash on the blinker now pins 1,8 and 16

 

[killingtime88]

Seems to be fixed

Hi all, reset the adaptive setting on the ecu this morning and it has been very close to perfect since.It has once or twice felt like it will start hunting again but so far so good.Will change the lambda sensor if i get any problems in the future.
I am going to pop round to the mot station to see if the emissions are any better now it seems to be fixed.
Smashing forum wouldn't have been able to fix it without you guys.
Regards Paul

 

[Richard Moakes]

Good news

Pleased you seem to have got to the root cause of the trouble.