OM651 (Viano) Erratic revs on idle, only when cold

[MinionBob]

Hi all,

OM651 in a 2012 Viano. 75k miles. Pulls and runs great, hot or cold. No fault codes stored (on my ICarsoft)

Has developed a very slight erratic idle, but only when cold. Idles fine when warm.

By erratic I mean that the revs will change very slightly, for a moment every 3-6 seconds. Like the engine is being very, very gently blipped.

Any ideas or thoughts on where to start looking?

Thanks as always.

 

[Joemaxi531]

Mine does that too
Has done with seemingly no other adverse effects for a couple of years ?

 

[ajlsl600]

Sounds like the kit that controls cold fuelling can't make its mind up to me. Crud on sensor advising ecm on cold/warm temp wud be where I start.
There is always. "if it ain't broke don't fix it"? With this modern crzp even best of intents can backfire on you.....

 

[MinionBob]

I think I have it figured out: I have a new fuel quantity valve to fit but the weather has been so poor have been avoiding it! I will update when fitted.

You are correct about if it ain't broke.. But the bit that niggles me is that, assuming it is indeed this valve, if it fails completely it could end up in a non starter, and not worth the risk IMO as it would of course fail at the most inconvenient time.

Yes, there seem to be no other adverse affects, perhaps fuel economy when cold but not really obvious.

 

[Joemaxi531]

What’s the fuel quantity valve ?
Where is it located?
Interested to know how it goes

 

[Steve@Avantgarde]

Thermostat has 2 temp switches on it, one must be dodgy or the thermostat itself is goosed.

 

[MinionBob]

Just an update on this - now fixed.

It turned out to be, what Merc call the on the box a 'Flow Control Valve'. It seems however to go by the unofficial title of 'Quantity Control Valve' or 'Y94'. It is also, incorrectly in my view called a 'Quality Control Valve' on occasions, but not sure how a valve can control the quality of fuel?!?

Merc part number is A6510740084 and the new valve was slightly different to the 2013 original one.

A6510740084 is a valve that bolts to the fuel pump, NOT the fuel rail (this is Y74 i thnk?). I did not re-code A6510740084 but the Delphi blurb in the box did say to reset fuel trim. Made a LOT of burbling when being primed (cycle the key on off until no burbling heard). Fired up 1st time however.

A bit of a pig to fit on the viano as everything is in the way and bulkhead limits access. Get some new gaskets too while you are there for the EGR cooler (A6511420480 & A6511422480) and you will need a torx key (not socket - access is to tight) to unbolt A6510740084 from front of fuel pump.

You can clean the very, very sooty intake side of things while your at it - you will use a LOT of carb cleaner and make a massive mess!!

As noted above, very odd at there seems to be no detriment to the vehicle overall with a faulty QCV valve , just the occasional blipping of the revs on idle; drives the same too.