Pre-ignition on hot start

[Norman Willcox]

My 1.9  2006 Diesel Octavia 2 makes a terrible shuddering banging failure to start when the engine is hot; starts beautifully when cold. Any one know what can cause this? It is evidently pre-ignition, but why?

[KenONeill]

On 23/08/2019 at 20:39, Norman Willcox said:

It is evidently pre-ignition

Well, the only way you can get preignition on a diesel is if the injection timing is too far advanced. You need VCDS to determine what the injection timing actually is.

[J.R.]

Sounds like when the DMF failed on my MK1.

 

If it is that then diagnose and sort it ASAP before it shakes the pressure plate drive straps to pieces and carves the bellhousing.

[Norman Willcox]

I have Ross VCDS, but I have very limited ability with it. I have used it for diagnosis of airbag faults and sorted those, but I have never interfered with the software.  Can the injection be delayed during hot starting?  How to do that?  I assume that the timing and the positioning of the toothed belt is correct since the engine performance seems fine. ( I have had a new second hand engine fitted after a failure of the DMF damaged both.

[KenONeill]

On 23/08/2019 at 21:20, Norman Willcox said:

I have never interfered with the software.

Nor has anyone asked you to. I did suggest that you should check the fuel injection timing, but' that's just reading values.

[Norman Willcox]

Ouch! JR You think that the DMF is about to fail again? It certainly must be being bothered with the horrible banging about. But that is not likely to be the cause of the pre-ignition - just a result?

[J.R.]

Its a little intimidating at first Norman I agree but practice makes perfect, you should not be looking to adjust anything until you know that its out of limits.

 

Scan for faults then check go into measuring blocks under the engine control module, look at the torsion value.

[Norman Willcox]

I have no idea how to check the injection timing and I have no figures to judge whether it is correct or not. If I were able to establish that the timing was wrong - then what to do?

[J.R.]

You do not know that it is pre-ignition at this stage.

 

Will a diesel even fire early enough to kick back if the injection timing is advanced, combustion occurs when the cylinder pressure is high enough to cause self ignition unlike a petrol engine where the spark dictates when the bang will happen.

[J.R.]

The torsion value is the variation from the correct camshaft and injection timing, I assume yours is a PD engine.

[Norman Willcox]

OK I shall run VCDS tomorrow.  No faults have appeared on the car screen. I suspect that VCDS may not show any either - but I shall see.  When I turn the ignition switch, when hot, there is a very nasty banging vibration which gives the impression that the engine fired backwards, and then fails to start. It usually can be started the second time but that too is not as smooth as silk.   Engine code is BJB.  What torsion value should I expect?

[KenONeill]

On 23/08/2019 at 21:32, J.R. said:

Will a diesel even fire early enough to kick back

That's a classic definition of a "backfire" in a petrol engine, and not all pre-ignition is that early in the stroke.

[Norman Willcox]

A garage mechanic has suggested that an injector has started to fail. Is that a possible cause?

[KenONeill]

On 24/08/2019 at 11:00, Norman Willcox said:

A garage mechanic has suggested that an injector has started to fail. Is that a possible cause?

It could be, but at the price of those injectors I'd want diagnostic data rather than "It might be this guv' " before spending out.

[Norman Willcox]

So did an auto scan with Ross VCDS - no faults shown other that a permanent radio one and dodgy windscreen wiper steering-wheel switch.

I don't know whether or how I can investigate further for problems of injector timing/faults with VCDS.

 

[KenONeill]

On 24/08/2019 at 23:23, Norman Willcox said:

how I can investigate further for problems of injector timing

OK, I'm not a VCDS guru, but I think you want to log a parameter called something like "injector duty cycle", which will be range 0 to 100.