Strange Idle - Here's an interesting puzzle for any enthusiasts

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Joined
Nov 1, 2018
Messages
31
Location
Wimbledon London
Car
Mercedes W124 E280
I have a W124 M104 E280 I think a .942. It doesn't have cruise control. I have gone through many fixes since acquiring the car and as I go, more little niggles come up. Here is an odd one. After many diagnoses, it has a new wiring harness and the ECU was repaired by one of the recommended repairers on these forums and also replaced its injectors(which two were faulty due to originally a bad harness that must have shorted the system). I have read tons and ofcourse most of these 124's had the degrading harness issues that caused these things. The car is running really great bar this one last issue. The car idles fine but when sometimes near idle and dependant on how warm the car is, when reversing or inching forward and braking if you touch the accelerator slightly and back off, the car does a weird bouncy idle/surge thing. You have to hold the brake until it settles. Then when it is doing that, when you accelerate for a split second as you touch the pedal, the revs dip just slightly and then it accelerates normally and it's fine. I pulled the whole intake system apart to check for leaks or pipe cracks today and nothing seems an issue and in doing so I came across the little button/switch on the throttle body, which makes a tiny click when pressed. This is only engaged when the little spring is on, so fully at idle position. The switch works fine and this I found, when idling correctly as soon as you depress the accelerator when this switch switches off, that's when the idle dips ever so slightly. So it's on the action of that 0 gas position sensor/switch disengaging that the idle dips/ If you hold the pedal so as to disengage it but not enough to increase the revs then the revs you can feel slightly battle to idle, a very slight grumble. As the switch engages all good again. I have new BOSCH MAF on there and more interestingly, I also have a crappy no name brand one and when this product is on the car this above totally disappears and in fact the car has slightly more power(only just). However as the car reaches hotter temps..that MAF totally screws things and it will not go. So the BOSH product is the one I trust of course. I've tried this with two BOSCH correct MAFs, same issue. Logically, it shouldn't be a vacuum leak if the no-name MAF eliminates this VS the proper BOSCH ones but of course I could be wrong but as above, cannot find anything leaking. This all seems to be the ECU not being able to get the fuel /air mixture right and as it tries to adapt the above issue occurs. There is also a 'rich' exhaust smell , well at least I think it is. However besides this the car is a great, drives like a big old 6cyl should. Can anyone give any suggestions short of replacing maybe the throttle body or possibly a ECU that the repairers didn't help? Much appreciated.
 
The does look like a vacuum leak of some kind. Have you checked the pipes that go to the gearbox at the gearbox end? Is the O2 sensor old?
 
Is the accelerator linkage and cable adjusted properly. I think that the switch you're talking about is a idle microswitch.

I don't know much about petrol engines, but my diesel does something similar.
 
Is the accelerator linkage and cable adjusted properly. I think that the switch you're talking about is a idle microswitch.

I don't know much about petrol engines, but my diesel does something similar.
The accelerator cable is adjusted correctly. Yes I think its the microswitch but it is supposed to engage at idle, which is it!
 
The does look like a vacuum leak of some kind. Have you checked the pipes that go to the gearbox at the gearbox end? Is the O2 sensor old?
It certainly behaves like a vacuum leak yes . If say I take the fuel tank vapor hose off to essentially create a big leak then I get bad bouncing idle, to be expected. I haven't checked the pipes at the gearbox end but what I did was took off the vacuum pipe right from the intake, I think it's a grey one and tried to see if I can just draw air from it to see if something on some other end after the splits is allowing air in but it seemed fine. Seems to hold pressure but maybe I should get a device to double check that. I am doing an oil change this coming week so I can get under there to look, do you know where what side I can find them (unless obvious) and how many?
 
The does look like a vacuum leak of some kind. Have you checked the pipes that go to the gearbox at the gearbox end? Is the O2 sensor old?
And yes sorry, O2 sensor was replaced a year ago but you never know what a mechanic does, I could maybe just replace that too. On the vacuum lines, both the gear shift hold on cold start to warm up the catalytic converter and the S/E feature work 100%, I assume that's what the vacuum lines are for?
 
The gearbox needs vacuum to operate smoothly.
 

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