Wavering Idle

[Mohnish]

Hello,

Need your advice before I go ahead and purchase an expensive part:

W210 E220 CDI has a very unsteady idle at start up and then if the revs are held for about 20 seconds around 1000rpm the idle steadies out...

But then, intermittently, when holding steady in neutral or drive, the idle tends to waver (very very slightly, only about 50rpm) up and down, hardly noticeable (can live with it), but still present.

I have been informed by the local shop, after doing the diagnostics (which came through without any fault) that it is the "fuel pressure regulator" part no.: A6110780149 which fits onto the fuel rail.

Did a bit of R&D...Removed the same part (with same part number) from a C 220CDI and interchanged the parts...Wierd thing, both the engines were runnning steady!!! Even the C with my supposed bad fuel pressure regulator was running smooth!!! So is it the fuel pressure regulator? Was it just a co incidence that my E220 CDI ran smooth at that time, considering the problem is intermittent.

Any suggestions. I really don't want to purchase this part and still be faced with the idle problem.

It is similar to what would happen if there were air bubbles building up in the fuel lines...may be there are...any way to check that?

Also, if the engine is not started for a day or longer, at the initial start the problem is more pronounced. Hence my doubt of this being air leak related.

Thanks in advance.

 

[Mohnish]

Its been three weeks without reply, help me out here!!!

 

[Dieselman]

Try a strong dose of injector cleaner, it sounds like the ECU is performing cylinder balance possibly due to the fuel pressure fluctuating, or just injector imbalance.

 

[Mohnish]

Try a strong dose of injector cleaner, it sounds like the ECU is performing cylinder balance possibly due to the fuel pressure fluctuating, or just injector imbalance.

Thanks Dieselman for your response. A quick question however:

When you say "strong", I understand you mean to add more injector concentrate than reccomended on the bottle.

Could you suggest just how much an increase in the ratio would be permissable without any damage to the injectors / fuel system?

Also, you have not commented on the possibility of an air leak into the system. I hence presume, you do not suspect this to be an air leak. For my knowledge, I would be obliged if you could explain your reasoning for that.

Thanks in advance.

 

[Mohnish]

Update...

I added a product sold locally under the brand name System - D. Its reputed etc etc and is a diesel fuel additive which cleans the fuel system.

Suggested use is 10ml / 10 litres. I added 80ml to a 67 litre tank.

Quite amazingly, within 10 minutes of running, the wavering idle was gone!!! May be I shouldn't rejoice so soon, considering this is an intermittent problem, but I ran a total of about 3 hours in traffic covering about 50 kms. and the waver never showed.

However, on cold start (there was a stop in the above run of about 6 hours in between) the fluctuating idle was present as like earlier for the first 10 seconds or so and then gone. Am going to run through a couple of tanks (thats recommended by System - D) to get the full effect of the treatment.

 

[Mohnish]

Further Update...

Now, about another 200 kms later, the idling waver has not yet returned !!!

On cold starts, there is still a bit of waver, though it has reduced considerably. Will go in for another dose of cleaner at the next tank full.

Thanks Dieselman, I think you have saved me a futile expense and of course the associated stress.