1996 W124 E220 Estate auto - rough/hunting idle - last stop gearbox?
LastMinute
Active Member
I just hope it's factually accurate.
It sounds plausible, though, don't you think? I hope somebody will be quick to correct me if I'm wrong.Brakes: probably not so much that you'd notice.
Automatic gearbox: might shift a bit early or late.
Other stuff: might make the engine run erratically. Q.E.D.
bolide
MB Enthusiast
I think the clue here might be in the cycling. If it's regular that implies there's a feedback system with a time constant. That would suggest electronic control to me and therefore would suggest an ECU-related fault. So the ECU's getting a signal to change the idle setting, and kicking off a cycle where the revs rise, the ECU detects the rise, drops the fuelling, the revs drop, the ECU gets a signal that the revs are low, and on ad infinitum
If you can't monitor the putput of the ECU, monitor the inputs. Are the sginals into the ECU changing over time?
As Dieselman says, if you unplug a sensor / unit you remove it from the loop
I'd do that or test with another ECU
Nick Froome
www.w124.co.uk
If you can't monitor the putput of the ECU, monitor the inputs. Are the sginals into the ECU changing over time?
As Dieselman says, if you unplug a sensor / unit you remove it from the loop
I'd do that or test with another ECU
Nick Froome
www.w124.co.uk
OK...phoned the mechanic again. His opinion was that the idle was fine in neutral, and engine only seemed to surge when in gear.
He had checked the vacuum on the inlet manifold and all ok.
He hadn't checked the idle control valve as reckons expensive part....
..but suspects gearbox given only happens in gear. Need to see if there's a diagnostic code there - I fancy rigging up the simple LED solution to read it myself.
He had checked the vacuum on the inlet manifold and all ok.
He hadn't checked the idle control valve as reckons expensive part....
..but suspects gearbox given only happens in gear. Need to see if there's a diagnostic code there - I fancy rigging up the simple LED solution to read it myself.
grober
MB Master
CHECK YOUR TEMPERATURE SENSOR Is OK
Heres a diagram of how a close loop idle speed control works. Its for an older KE jetronic system but principle is the same. Newer systems employ a crankshaft sensor to sense engine rotation rather than the distributor but otherwide the same I think. As far as your problem is concerned one of the variables to be input to this control is engine temperature, so make sure your engine temperature sensor is OK. This is a different sensor from the coolant sensor which provides the temperature gauge readout.
Heres a diagram of how a close loop idle speed control works. Its for an older KE jetronic system but principle is the same. Newer systems employ a crankshaft sensor to sense engine rotation rather than the distributor but otherwide the same I think. As far as your problem is concerned one of the variables to be input to this control is engine temperature, so make sure your engine temperature sensor is OK. This is a different sensor from the coolant sensor which provides the temperature gauge readout.
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Further update - the diagnostic code on the engine shows a fault with the "air mass meter" according to a new garage. I assume "air mass meter" = MAF?
They suggested a new "air mass meter" - which is £175. But previous garage already tried a £50 MAF, and a £150 MAF...with no joy (thankfully got them returned to motor factor).
So - where does this leave us. If the ECU says problem with MAF but replacing MAF does not help, then could it be a problem with :
- ECU
- sensor cable between ECU and MAF
Seems logical to me??? Feasible?
They suggested a new "air mass meter" - which is £175. But previous garage already tried a £50 MAF, and a £150 MAF...with no joy (thankfully got them returned to motor factor).
So - where does this leave us. If the ECU says problem with MAF but replacing MAF does not help, then could it be a problem with :
- ECU
- sensor cable between ECU and MAF
Seems logical to me??? Feasible?
It really is driving us nuts. In some respects, you pay mechanics for diagnosis, and then you get a call saying "It might be the air mass meter, and that's £175, but to be honest, that might not fix it - what do you want us to do?". Are they not the experts? 
Frustrating
Frustrating
grober
MB Master
Guess it could be a problem with the cable to the Maff. Cars of that age are known to have engine wiring loom problems--insulation goes brittle resulting in short circuits. Mercedes clip the Maff wiring directly to the cam cover-why I dont know----- but that means its more liable to get "cooked"
LastMinute
Active Member
Yes! Ian Walker has a story about that...
Cut back the cable sleeving at the MAF sensor and inspect the wires inside. If the insulation is damaged (cracked, brittle, missing
, etc.), get that section of the loom replaced IMMEDIATELY - it's a fire hazard.
LastMinute
Active Member
It may have been just another episode in the saga of misfortunes Ian has had with his E220 coupé recently, but here's what happened:
Ian replaced his MAF sensor and when he started the engine the wiring loom (close to the sensor) caught fire. This must have been caused by a short circuit in that section of the loom, but the fault only became apparent after the wiring had been disturbed to fit the new sensor. On inspection the insulation was found to be brittle and cracked, and a lot had disintegrated completely.
Ian's engine had done a lot of miles before that, and the insulation breakdown will have started a long time ago. What I'm getting at is that your car is not necessarily about to burst into flames. Sorry if I alarmed you.
Ian replaced his MAF sensor and when he started the engine the wiring loom (close to the sensor) caught fire. This must have been caused by a short circuit in that section of the loom, but the fault only became apparent after the wiring had been disturbed to fit the new sensor. On inspection the insulation was found to be brittle and cracked, and a lot had disintegrated completely.
Ian's engine had done a lot of miles before that, and the insulation breakdown will have started a long time ago. What I'm getting at is that your car is not necessarily about to burst into flames. Sorry if I alarmed you.
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Ian B Walker
MB Enthusiast
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I also have had the ECU bootstrapped at Mb. When they were doing this they suggested a new Maf. I had already tried another maf as I thought it may be that but it wasn't (then). Long story short. Erratic idle, new maf, new wiring, problem solved. Note, this may not be so in your case.
Hi Riccardo.
I am very interested in knowing how your problem is coming along - is it fixed perhaps?
I'm interested because I have exactly the same problem! I have a 1993 w124 E280 sportline. I have replaced the o2 sensor and cleaned the MAF but the problem remains. I have been reading alot of MB forums on this problem and from that it seems that most often it is either a dirty throttle body or a bad Wiring harness / wiring loom (basically degraded insulation on some wires, can be anywhere but most likely near the throttle body or the plugs).
So today I am going to try to clean my throttle body, I hope that works ! I have had this problem for months now and it is simply driving me mad
Best of luck with yours, any news would be appreciated - what more has been tried and did something work..
/svetor
I am very interested in knowing how your problem is coming along - is it fixed perhaps?
I'm interested because I have exactly the same problem! I have a 1993 w124 E280 sportline. I have replaced the o2 sensor and cleaned the MAF but the problem remains. I have been reading alot of MB forums on this problem and from that it seems that most often it is either a dirty throttle body or a bad Wiring harness / wiring loom (basically degraded insulation on some wires, can be anywhere but most likely near the throttle body or the plugs).
So today I am going to try to clean my throttle body, I hope that works ! I have had this problem for months now and it is simply driving me mad
Best of luck with yours, any news would be appreciated - what more has been tried and did something work..
/svetor
kth286
MB Enthusiast
All
Do not forget the engine coolant temp sensor that feeds the signal to the main engine ECU.
Inexpensive and an easy fit but a critical item and they do go out of spec.
The signal from it is processed by the ECU to control both fueling and ignition functions.
Do not forget the engine coolant temp sensor that feeds the signal to the main engine ECU.
Inexpensive and an easy fit but a critical item and they do go out of spec.
The signal from it is processed by the ECU to control both fueling and ignition functions.
Hello,
No, no further forward. The car was back in a garage yesterday for more diagnosis, but after 2 days, they still haven't even started on it even though it was booked in for yesterday & today!! I got all sorts of excuses about mechanics being on holiday "last week", and mechanics taking people to the airport, hence not having the time to look at it. Obviously I took exception to this as these are not my problems, and the car is being taken away from them tmmrw....time for another garage.
I am tempted to find another wiring loom and try that, as seems to be a point of failure...while keeping my mind on all the other suggestions. Frustrating, as a small problem overall, but REALLY annoying!!!
No, no further forward. The car was back in a garage yesterday for more diagnosis, but after 2 days, they still haven't even started on it even though it was booked in for yesterday & today!! I got all sorts of excuses about mechanics being on holiday "last week", and mechanics taking people to the airport, hence not having the time to look at it. Obviously I took exception to this as these are not my problems, and the car is being taken away from them tmmrw....time for another garage.
I am tempted to find another wiring loom and try that, as seems to be a point of failure...while keeping my mind on all the other suggestions. Frustrating, as a small problem overall, but REALLY annoying!!!
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A really common problem on these is the throttle flap becoming faulty internally. A second hand one is normally really cheap and worth a try.
Also I have had a similar fault on a c-class with the same engine and after a few weeks of head scratching the engine control unit was the culprit.....
good luck with the search and let us all know............
Also I have had a similar fault on a c-class with the same engine and after a few weeks of head scratching the engine control unit was the culprit.....
good luck with the search and let us all know............
