1996 W124 E220 Estate auto - rough/hunting idle - last stop gearbox?

[riccardo]

Hi,

Our 1996 E220 W124 auto (last of the W124s!) has got a pretty rough idle - if in drive or neutral when at a standstill, the revs fluctuate between 400-700rpm in a cyclic manner - roughly a second or so to go from 400 to 700 and back to 400.

However, it makes slow speed manouvering slightly tricky, as in reverse or drive, it is a bit jerky due to the fluctuating revs.

We only got the car a few months ago, and the oxygen sensor had been newly replaced by the previous owner. We took the car to a garage today, and he noticed the new O2 sensor.

So, he cleaned out the throttle body, the butterfly valves, and all pipes associated with this area. No difference.

He then ordered a new MAF from a motor factor - there was a choice of two - one was £60 and one £150, so he went with the £60 one. It made no difference....but the box had already been opened so he figured it might have been a used one....so he got the factor to return with the £150 one, wait around for 5 mins while he tried it, and then returned it to the factor. It made no difference.

So...tried all 3 usual suspects : throttle body cleaning, O2 sensor, MAF. No joy.

He suspects it might be the gearbox as electronically controlled, but he doesn't have the diagnostic equipment for W124....only W202 onwards. So, suggested taking it to Merc dealr for fault code on gearbox if any.

Also, change a bit rough from 1st to 2nd to 3rd when cold, but fine after a few mins.....prob needs new gearbox oil & filter....so will try that too.

Also....when coming to a halt sometimes, the revs stay a bit high momentarily, causing the car to jerk to a stop....it's almost like the engine is being fed too much fuel momentarily on stopping....it also once stalled once at a stop.

Anyone any ideas? Could it be gearbox?

 

[Dieselman]

This sounds like an inlet manifold air leak. Have the vacuum checked and all the vacuum lines.
Try sealing off all the tappings into the inlet manifold and see if it fixes it.

 

[riccardo]

Is that expensive?

PS It's a petrol engine if that makes any difference (doubt it does)

What exactly are 'tappings'? I'm a software engineer, not a mechanical one. Sorry

 

[Dieselman]

Not unless you want it to be..

The vacuum lines will unplug in about 1 minute and connecting a vacuum guage will take a bit less time, sooooo....

 

[Dieselman]

PS It's a petrol engine if that makes any difference (doubt it does)

Actually it makes a huge difference. Diesels don't run with vacuum in the manifold because they have no throttle flaps to throttle the flow of air.

I already sussed it was a petrol.

 

[riccardo]

Actually it makes a huge difference. Diesels don't run with vacuum in the manifold because they have no throttle flaps to throttle the flow of air.

I already sussed it was a petrol.

Hehe...ok, thought you might have, but just wanted to check. I don't know much about diesels.

 

[Dieselman]

What exactly are 'tappings'? I'm a software engineer, not a mechanical one. Sorry

Holes with spigots for the pipes to push onto.

 

[riccardo]

OK...may pop down to the mechanic again to see if he can test it quickly. He did all the diagnosis today for free, which was very good of him.

 

[riccardo]

..and presumably it will be cheap to repair if it's just a leaky pipe, but if a crack in inlet manifold (is this part of the main engine casing?) then will be ££££££££££££?

 

[Dieselman]

..and presumably it will be cheap to repair if it's just a leaky pipe, but if a crack in inlet manifold (is this part of the main engine casing?) then will be ££££££££££££?

It won't be cracked, and it's a seperate item anyway. More likely would be leaky vac pipe or leaky gasket.

 

[Ian B Walker]

Idle valve comes to mind. They have a habit of pulsing fuel when they get old.

 

[LastMinute]

if a crack in inlet manifold (is this part of the main engine casing?)

No. The inlet manifold bolts on to the cylinder head, which bolts on to the crankcase.

then will be ££££££££££££?

Not necessarily. It might just need a new gasket - £4 plus 1-2 hours' labour.

 

[riccardo]

Great stuff. Cheers.
If you have time, can you briefly explain what the pipes into the inlet manifold do and where they come from? By blocking them up to test it, what is that proving? Air can't stay in the manifold? What will I expect the engine to do if I block them up when idling?

 

[riccardo]

OK...idle valve - how much are they? Difficult to fit?

As you can imagine, it's one of these things that could be anything, hence diagnosis could take a lot of labour

 

[Ian B Walker]

OK...idle valve - how much are they? Difficult to fit?

As you can imagine, it's one of these things that could be anything, hence diagnosis could take a lot of labour

Don't know what they cost but one from a breakers should be cost effective.

To fit is an easy job. One or two vac hoses , 2 bolts and an electrical socket.

 

[Dieselman]

OK...idle valve - how much are they? Difficult to fit?

As you can imagine, it's one of these things that could be anything, hence diagnosis could take a lot of labour

Disconnecting the electrical plug should be a satisfatory test.

 

[LastMinute]

can you briefly explain what the pipes into the inlet manifold do

Provide a vacuum to components that need it. The vacuum is created by the action of air being drawn through the manifold into the cylinders.

and where they come from?

Brake servo, automatic gearbox, fuel injection, ignition, ...

By blocking them up to test it, what is that proving? Air can't stay in the manifold? What will I expect the engine to do if I block them up when idling?

If the engine continues to idle erratically with all the vacuum lines blocked, the problem is probably with the manifold or gasket. If it idles fine, the problem is with a vacuum line or connected component.

 

[riccardo]

Cheers LastMinute.
Your explanation is making sense.

If the vacuum was poor, would it not affect all these other component's operation too? ie, brake servo etc?

 

[riccardo]

Disconnecting the electrical plug should be a satisfatory test.

Again, sorry for my mechanical naivety here, but if you unplug the electrical supply to the idle valve, what would this prove and what would I be looking for?

 

[Dieselman]

The idle would probably dive to the lower setting but wouldn't cycle up and down.
This is just a thought and not fact as I haven't tried it.