Engine idle rough S500 W140

[milservices]

Hi,

My s500 idles perfect in drive/reverse but rough in neutral or park. I've changed dist caps, plugs, rotors and air filters.
I've heard alot about the engine loom, could it be that?

I have this car for a while but feel it has to go. Too many niggling problems.

Many Thanks

Martin.

 

[jgevers]

Check the lambda sensor with a scope.

 

[milservices]

Rough idle S500

Hi,
Thanks for reply. I've had the Star computer check out the settings at my local stealers, cost £50.00 + Vat! They say no faults reported and all values are correct. They cannot understand why I have rough idle out of drive but not in drive. They think the engine loom may be at fault, but all the values are correct so how could it be that? I don't mind replacing the loom if it is that. They say it will cost £350.00 for the loom and about 5-6hrs to fit, that's £1000.00 in total!!
Do you know if the lambda sensor value is monitored by the diagnostic computer at the stealers?
Thanks for your help.

 

[stats007]

Did you change the HT leads? Idle problems can be anything from an idle control valve to injectors. In drive / reverse you have a partial load which would indicate a change in fueling / breathing / ignition.

 

[milservices]

Thanks,
Do you think it's worth changing the HT leads? I spoke to the S class mechanic for about half an hour and he could not put his finger or anything that could cause this problem. I asked about injectors but he said they are either good or fail totally and the engine would run rough regardless of drive conditions. He 'loaded' the air mass meter in and out of drive the values to his PC were perfect he said. This problem can't be unique, I was frustrated that an S class mechanic couldn't tell me what/where the problem is.
Thanks for your help.

 

[stats007]

I've read that the HT leads are problematic and changing them solved poor running - though this was usually in drive as well as neutral. A similar problem was solved on another forum by replacing the injector head assembly (see below). Have you looked through this web page?

THE PROBLEM HAS BEEN FIXED
It turned out to be the injector head assembly cost me £350 exchange for a reconditioned part plus shedloads of labour in fault finding going back over the OVP relay, idle control valve etc. Car runs like a dream now and hopefully will give improved mpg. Thanks to everyone for their contributions

 

[milservices]

Thanks for that.
I used the web site for my ticking cam shaft problem, solved that, was great.
I didn't think of this page for my current problem - what a dummy.
I'll take a look and see.
Martin.

 

[grober]

depends on the gear selector position

The system you have I am assuming is the combined electronic accelerator/tempomat cruise control unit. I would try to explain here but suffice to say that the unit monitors slightly different parameters when its in drive than when its in neutral. I have sent you a PM with a PDF file which I think explains it. It covers more than one engine make sure you just look at the 140 sections. Engine temperature seems to figure quite prominently so might be worth checking the sensor. It could be the dreaded wiring loom problem and the wiring inside the actuator unit has been known to go also. There is a site which described a similar system http://www.baxnet.com/merc/tadiags/ta-diags.htm
I will send the pdf when I find out how to attach it to a PM !
edit:-found I cant do this sorry

 

[grober]

I have had another look at the pdf file and tried to summarise.(I think this only applies if you have a fully electronic throttle)

In the idle state the system uses 2 hardwire switches to signal to the EAP/TCC/ISC- combined electronic accelerator pedal/tempomat cruise control/idle speed control unit. for now lets just concentrate on the ISC.

1.THE IDLE SPEED CONTACT SWITCH-located somewhere on the accelerator pedal? this tells the ISC that the car is physically in an idle condition-no throttle
2. THE SELECTOR LEVER POSITION SWITCH ( its the same starter lockout/reversing lamp switch normally found on the gearbox) lets the ISC know the car is not in gear D or R-- but park P or neutral N

now the electronics
there seems to be 2 main inputs effecting idle speed.

1. engine coolant temperature via the LH control unit

2.a diode matrix unit which adds extra idle speed above a certain rpm depending on extra consumer load -aircon, heated back screen, power steering etc- the unit "should "remember this extra load the next the next time the car recognises an idle state !!
BUT!!!!!
quoting from the manual " engine speed increase is not active when the selector lever is in P or N"


If switch 1. didnt work or was badly adjusted then that might cause a problem since the car wont recognise an idle condition SOMETHING SIMPLE WORTH CHECKING its probably a simple contact to ground.( correction-simple switch both contacts connected to the ISC unit)
If switch 2. didnt work then again the unit might be failing to recognise the car was in P or N and compensating for that.-- UNLIKELY probably.
Since the engine coolant temperature appears to be the other main variable might be worth checking the engine coolant temperature sensor is giving ACCURATE VALUES.
There are countless other feedback control loops involving the LH injection unit can bus EZL ignition unit etc which might also be at fault but impossible to test without the right diagnostic equipment.

Summarising the idle speed control unit does recognise if the car is in drive or P and N and uses different parameters to control the idle speed depending on which state it is in. It should also remember any extra loads when it idles again!
sorry if this is a bit rambling but does appear to contradict what the MB tech said. I am the first to admit these early systems are a nightmare to trouble shoot so feel free to ignore all this. All 3 units could each be replaced relatively cheaply if they were suspect.

 

[milservices]

Hi,
I've just returned from holiday so forgive me for not replying. I'll look at your detail and try a few things out.
Thanks
Martin.

 

[kps70]

I'm having similar problems with my w140 s500. It starts fine and when absolutely cold, tickover is fine. I'll go for a drive for a couple of miles and stop the engine for a few minutes then start it and the idle is really rough - it's often stalls when waiting at a junction and this problem can repeat for the next few minutes. The revs are searching - rising up and down.
When it's warmed up the idle is not too bad, maybe a bit lumpy, and you can notice the lumpiness when cruising on the motorway and on hard accelerating.
The other alarming thing is that the car often wants to fight against me and keep 'driving' when I'm in gear and moving at slow speeds - like the thottle is set too high. I don't know if this might be related or my weak right leg!
Any ideas/suggestions would be gratefully received. That pdf file sounds useful too.
Many thanks
Kieron
p.s. car is running on lpg (multipoint) but the symptoms seem the same on both fuels. This might provide a clue I guess?
pps I cannot believe the main stealers quote 5-6 hours to replace the loom. I should not take more than about an hour from a half competent mechanic with no prior knowledge of the system. Outrageous!