MAF and misfire at idle

[johne]

I am still unable to track down the cause of my misfire at idle and as you pass through 2800-3000 RPM.

If I disconnect the MAF with the engine running at idle the misfire improves. I know that disconnecting the MAF in this way forces the ECU into limp-home mode with fixed timing but my question is, what does the fact the misfire is better in limp home mode tell me about the cause of my misfire?

My MAF checks out and meets the prj criteria of Group 000 2nd value under 25 - I get 23/24. Incidentally whatever that value represents it rises to 30 with the MAF disconnected.

No readings or fault codes out of the ordinary on VAGcom that I can see - except when you unplug the MAF obviously.

The car isn't nice to drive like this and yet passed its MOT Emissions tests easily with its best ever set of readings.

In an earlier thread I said that I've already tried replacing all ignition parts but no difference.

John

[mike74]

Maybe its running too lean below 3k rpm...dodgy lambda sensor? When was the fuel filter last changed too?

[S2central.net]

What values do you see at idle in Group 0 fields 8-9-10 ?

[johne]

Maybe its running too lean below 3k rpm...dodgy lambda sensor? When was the fuel filter last changed too?

Fuel filter is brand new along with drained and refilled fuel tank, I have a new lambda waiting to fit, just trying to not change too many things at once.

John

[johne]

What values do you see at idle in Group 0 fields 8-9-10 ?

At hot idle:

8 128 (range 126-131)
9 121
10 036 (range 35-37)

With MAF unplugged

8 drops to 096
Others unchanged.

Field 9 remains unchanged whatever.

John

[S2central.net]

I can't remember which but Group 000 field 8 or 9 should be moving about... should not be static on warm idle... I will have to look it up.

Meantime, check lambda value in Group 001 - that should modulate around 1.0 - it must NOT be static on one value in warm idle or cruise - with a scope on it you should it see it toggle between 0 and 1 Volt (in broad numbers) about 2-3 times a second. A DMM on that pin won't tell you much but it should see around 0.5V assuming 50% duty cycle from the lambda probe.

[S2central.net]

Also have a look at colour of the plugs, check for any loose one(s)... and also worth pulling off the injector connectors on idle to see if you can pin point a single cylinder (i.e. if misfire gets no worse as you unplug injectors 1 by 1 then you have your culprit for additional tests)

[UrS4boy]

One missing and hesitation diagnostic

Try going through this:

http://forums.quattroworld.com/s4s6/msgs/22611.phtml

[johne]

I can't remember which but Group 000 field 8 or 9 should be moving about... should not be static on warm idle... I will have to look it up.

Meantime, check lambda value in Group 001 - that should modulate around 1.0 - it must NOT be static on one value in warm idle or cruise - with a scope on it you should it see it toggle between 0 and 1 Volt (in broad numbers) about 2-3 times a second. A DMM on that pin won't tell you much but it should see around 0.5V assuming 50% duty cycle from the lambda probe.

Just checked and lambda is behaving exactly as you say it should both in Group 001 and looking at its output.

John

[johne]

Also have a look at colour of the plugs, check for any loose one(s)... and also worth pulling off the injector connectors on idle to see if you can pin point a single cylinder (i.e. if misfire gets no worse as you unplug injectors 1 by 1 then you have your culprit for additional tests)

Pulling the injectors makes no difference. Plugs and boots have been replaced with new. I've substituted new POS and coil packs but no change.

The problem, when it appeared back in July came on instantly 100 or so miles into a new tank of fuel.

John

[S2central.net]

Does the miss 'stick' with any particular injector when you unplug it ?

[S2central.net]

mmm - well if ignition is indeed all good, and the MAF is good, and lambda is good... it can only be fuel/injector related... but if its not sticking to one injector then it smacks of a potential electrical / wiring gremlin.

Have you checked the earth points on the IM and top/rear of the head are clean & tight ?

[johne]

Does the miss 'stick' with any particular injector when you unplug it ?

Pulling any of the injectors has an affect. No. 4 the least and pulling nos. 1 and 3 cause the ISV to make the most noise as it changes duty cycle to keep the RPM up.

John

[johne]

mmm - well if ignition is indeed all good, and the MAF is good, and lambda is good... it can only be fuel/injector related... but if its not sticking to one injector then it smacks of a potential electrical / wiring gremlin.

Have you checked the earth points on the IM and top/rear of the head are clean & tight ?

Just rechecked those earth points and they're good.

Engine is fine when cold, and drives better cold but at its worst when at operating temp.

Will try another set of new plugs and alternate the coil packs and POS units at the weekend just to be sure.

John

[newsh]

Clean ISV and clean MAF connector?

If disconnecting no4 injector has less effect, maybe that one is partly blocked?