I've just finished fitting a new head gasket and cylinder head to my wife's Mk1 Fabia 1.4 Mpi and I've noticed it has a slight and irregular misfire (wife has now told me it has always been there!) and a inconsistent idle RPM.

So, out with VCDS to try and diagnose the problem or at least gain more information.

Of course I forgot that it is pre-CAN BUS so very limited!

This is what I've come up with so if anyone can advise on the specifics of this information, or can advise how to gain more appropriate information, then I'd be very grateful...

The car idle speed flits between 768 & 800 RPM once warm;

Ignition timing ranges from 4.5 - 7.5 BTDC generally (but occasionally 3.8 - 8.3) but no relationship to the idle RPM;

This was much wider 3.3 - 10.7 before I aligned the throttle (at least I think I did);

These figures are at a load of 3.5%;

Ignition timing is stable when engine under load (e.g. at 1500/2000/etc RPM).

It's had new oil and coolant today, engine cooling fans come on, but the engine feels a little erratic at idle and a slight misfire can be heard from the exhaust (not consistent or regular and not that noticeable unless listening for it).

Any thoughts anyone?

Is there something else I should be looking for in VCDS to help diagnose or modify the situation?

I do have VCDS scans as well as CSV files of the data collected from the vehicle when warm and at various engine speeds (including idle).

Fuel filter and spark plugs about to be changed to see if they might be the cause of the misfire.

Then.....

Injectors? Fuel pump? Timing?

I've no idea, so would appreciate any help I can get please.

Can you see the knock sensors on that engine in the measuring blocks ?

I take it no codes thrown to indicate it is detecting a missfire on any of the cylinders ?

Plugs is a good starting point, how is the spark done on that motor, individual coilpacks ? If so are they directly fired from the ECU or is there an intermediate driver like on my old 99 1.8T that upped the ECU voltage to that required on the coilpacks ? They had an issue where the contacts needed a clean from time to time to stop intermittent misfires.

Paul

I'll try to answer those questions - but you lost me after the second one...

I'll take a look for the knock sensors in VCDS to see if they are there. If they are, what am I looking for?

No fault codes to indicate a misfire - It's just evident when listening to the exhaust (it's only a little, irregular one).

Plugs changed today for new Bosch ones - it still seems to be there, but I've not had a chance for a proper listen as yet. (Also had new fuel filter, oil filter, new oil, new coolant, new cylinder head, head gasket and all other gaskets, and a new coolant temp. sender. Oh, and tyres and alignment!)

Coilpacks??? No idea, but the info you have given me gives me something to go and look into. Thanks.

Coilpacks are what steps the 12v or ecu voltage signal up to the high voltage required to create a spark across the spark plug gap. Some have individual ones (usually bolted or push fitted directly onto the top of the spark plug) and some have a block of coilpacks in one device..

If you scroll through the data blocks using vcds in the Engine module, you may see a block for the knock sensor for each cylinder, you can monitor with the engine idling and see if one particular cylinder is causing the misfire and take it from there.

I have seen some posts suggesting that a misfire at idle is a characteristic of that engine.

Paul

In that case, this has a coil pack that pushes on top of the spark plugs and links them all together.

I'll have a look at the engine module with VCDS tomorrow, but I can't recall seeing one of the measuring blocks having anything to do with a knock sensor.

I'm assuming you gapped the plugs correctly.

I'd look at coils or ht leads next. Might be worth a clean out with redex (cylinder method) before spending more money if you have no fault codes.