Intermittent idle problem

[coker]

Well hi there - I've just signed up and this is the first forum I've been on!
I have a 1996 E320, straight six, which has recently developed an idle problem. Originally it happened once the engine had warmed up and at idle the engine would rev between 600 - 900rpm all by itself.It was a real problem in traffic and especially in reverse.The car was due a service, during which a faulty HT lead was found and replaced, fingers crossed that was the problem! It wasn't and the garage took the car back and cleaned around the throttle body - which did make a difference. However the problem is still there, but not all the time, it can run perfectly OK one minute and then change back to the hunting. I had a local Merc specialist put it on their diagnostic machine only to find no fault showing. They had seen the problem when I first made contact with them, they suspected that there may have been some air leak. I contacted them after to explain the problem still existed and they said the first thing they would do was to replace the MAF, but at £300 + I'm not convinced that is the fault - any ideas or are there any "Specialists" local to me in Orpington, Kent.

 

[television]

Hello and welcome to the forum.

Most idle hunting faults are vacuum leaks, normally caused be a poor fit or split pipe in the vacuum system.

You have max vacuum at idle, the engine sucks in air via the leak and speeds up, the vacuum is goes down as the engine speeds up, and so the revs drop down again, and so on.

The leak could be on the inlet manifold

On cars with a MAF they will run with it unplugged to prove this point in some cases and they only cost £147 from Bosch.

Is the car a 124 or 210 please

 

[coker]

Idle problem

Wow that was a quick response!
The model is a W210 and i'll try running it with the MAF disconnected to maybe rule that out.
Thanks again for your response.

 

[coker]

Idle problem

Well I've been driving the car and waited for it to play up in idle. It started to race so I turned off the engine and disconnected the MAF. Hey presto no problem - what is the purpose of the MAF? Drove for a few miles then connected the MAF back again. However it appears to be better now with the MAF connected again and I feel a bit like I did years ago when the children were not very well - took them to the Doctors and they feel better!
Does the above prove that the MAF may be at fault?
I'll carry on driving and report back in a day or two.

 

[television]

Yes this could well be the MAF then, do not buy an Ebay version Bosch service center is the cheapest

 

[coker]

Erratic idle

Ah! Curses!!
Drove the car today and it started to play up when idle. I disconnected the MAF but it was still running badly on idle.So I reconnected the MAF and now it's running perfectly. Could there be a dodgy connection?

 

[television]

This could still be the MAF,,my V70 behaved in the same way

If it come back again, it should be the MAF

 

[5907]

This is interesting; my E280 seems to have a similar problem, particualrly in reverse. It revs by itself even without any pressure on the accelerator, and surges against the brakes. Doesn't seem to do it in neutral as much.

I might have a look at the MAF too.

 

[television]

This is interesting; my E280 seems to have a similar problem, particualrly in reverse. It revs by itself even without any pressure on the accelerator, and surges against the brakes. Doesn't seem to do it in neutral as much.

I might have a look at the MAF too.

On many cars the idle is increased when in D or R

 

[5907]

On many cars the idle is increased when in D or R

It doesn't just increase though; it surges quite powerfully against the brakes.

This is my second Automatic W124, and my last one didn't do this. It's not really a big problem, it doesn't always do it and usually goes away when you put it into N then back to R.

Of course, it may be related to the intermittent poor throttle response I sometimes experience. Other than that, the engine runs perfectly.

 

[coker]

Well thought I had it cured at the weekend.
A garage had it in and recalibrated the throttle body, all appeared to be OK on Saturday,
but come Monday it's gone back to it's old ways.
Sorry I'm sounding like a stuck record here - could it be the MAF?

 

[mtinlin]

I did have a similar issue with a 190 2.3 16v a while back. It had an erratic idle but being a manual it wasn't such an issue. Anyway, having been told it would need vast amounts of cash spening on it by a local garage, I replaced the idle control valve with a used part and cured the problem. 'Not sure if your fuel injection system is similar (this was a 1988 car with mechanical fuel injection) but if you can locate the part and give it a good clean with carb cleaner you might cure the problem for very little effort.

 

[television]

Well thought I had it cured at the weekend.
A garage had it in and recalibrated the throttle body, all appeared to be OK on Saturday,
but come Monday it's gone back to it's old ways.
Sorry I'm sounding like a stuck record here - could it be the MAF?

I think that you must bite the bullet and get a new MAF, that is one of the only things that plays around, ie on a re start.

 

[coker]

Erratic idle

Well i bit the bullet and fitted a new Bosch MAF - problem is still there!

 

[roofless]

it could be the regeneration valve as it's a intermittent fault try blocking the vac pipe at the engine side the see if it is any better 26 in the link http://www.detali.ru/cat/oem_mb2.as...509722.605&CT=M&cat=520&SID=14&SGR=170&SGN=11

 

[stewartw]

Hello and welcome to the forum.

Most idle hunting faults are vacuum leaks, normally caused be a poor fit or split pipe in the vacuum system.

You have max vacuum at idle, the engine sucks in air via the leak and speeds up, the vacuum is goes down as the engine speeds up, and so the revs drop down again, and so on.

The leak could be on the inlet manifold

On cars with a MAF they will run with it unplugged to prove this point in some cases and they only cost £147 from Bosch.

Is the car a 124 or 210 please

Sorry to jump in with my problem, but i'm having similer problems and what you have just explained above sounds like my engine. When you mention

"You have max vacuum at idle, the engine sucks in air via the leak and speeds up, the vacuum is goes down as the engine speeds up, and so the revs drop down again, and so on."

This happens to my A150 as I build the revs it just comes back down again unitl my foot is halfway down to the floor then it starts to bounce between 2000rpm & 2500rpm like its hunting. Will keep you informed of progress.

 

[coker]

Thanks Roofless - that appears to be the problem! I have a friendly Mercedes mechanic who disconnected the pipe. He said nothing would happen overnight, but could take a week or two. Problem sorted - he is going to recalibrate things when he has a chance - i'll report back.
Thanks very much for your help to you and others who have made suggestions - trust this might help others!