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V8 Engine running problem...


simonr
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The engine will start and run for about a minute (hot or cold).

It will then start to stumble. If you increase the throttle, it will run fine for a second , stop firing for a similar period then, so long as it is still rotating fire & run for another second.

This goes on, getting worse for ten sec then it stops. You can re-start it almost immediatly (say 30 sec) and it does the same.

This happens of Petrol or LPG.

Ignition looks OK - plenty powerful spark.

The only thing I've noticed is that as it starts to stumble, the mixture goes quite rich - although this could easily be a symptom of not firing rather than the cause.

Also, it is much harder to start on either fuel than it was a week ago. Once running, air/fuel ratio looks OK - until the above!

Any thoughts? (other than helpful "buy a diesel" ones)

Cheers,

Si

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At first glance appears to be ignition system related. How do you generate your spark? Have you used a timinglight to observe whether the spark is present while engine is not firing?

The regularity of the problem suggests an amplifier with short-term overheating.

Are there any common parts to petrol/LPG control or paths? Just thinking about air supply as well.

jw

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The fact that it happens on petrol or gas tends to rule out fueling problems, unless it's a mechanical problem with the plenum or something like that.

The EFI is Megasquirt - which has been helpful, plug in laptop & see more of what it's up to. LPG has no electronics, it's just a venturi mixer.

I run electronic ignition - so no condenser. I'll try a different amp, just in case.

There appears to ba a spark right up until the last revolution of the engine. It could be that the timing is changing dramatically at the end I suppose.

Thanks for the suggestions - any more welcome!

Si

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The fact that it happens on petrol or gas tends to rule out fueling problems, unless it's a mechanical problem with the plenum or something like that.

The EFI is Megasquirt - which has been helpful, plug in laptop & see more of what it's up to. LPG has no electronics, it's just a venturi mixer.

I run electronic ignition - so no condenser. I'll try a different amp, just in case.

There appears to ba a spark right up until the last revolution of the engine. It could be that the timing is changing dramatically at the end I suppose.

Thanks for the suggestions - any more welcome!

Si

Whats the fuel pressure doing, pump or PRV, or does this mean it can't be due to GAS as well ?

Plugs ? another possibility

Nige

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Solved it - at least in part!

Nick (Rogue Vogue) said to me last night "Are you sure it's not just a coincidence that it behaves the same on gas & petrol"

I was sure at the time - but it turns out he was right. Well done Nick - have a star!

On Gas at least, the hose which supplies water from the cooling system was kinked. That meant that the vaporiser was freezing and the flow of gas stopping. It was taking a minute or so to freeze, but de-frosting almost immediatly.

It only manifested itself this week because the temperature has dropped.

No idea what's happening on petrol yet - but it's something different!

Si

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All clever people have an achillies heel, your's Simon appears to be that you miss the obvious ;)

That's why he once helped to rewire my car so I could jump start it through the cigar lighter before we noticed the gear selector was still in drive! :D

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That's why he once helped to rewire my car so I could jump start it through the cigar lighter before we noticed the gear selector was still in drive! :D

Wait a moment Mr Electrical Guru Hirst! I didn't notice you in the pub jumping up & down saying "I bet it's the.......". Or anyone else for that matter - only Nick suggesting it might be a coincidence, which could be read as check the obvious I suppose!

At least I put my hands up to it! Thanks to all of you for your kind words of support! ;)

Si

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hehehe brilliant fault. reminds me of when i worked in a volvo dealer. demo car died like it was out of fuel, a day later after playing with the computers etc for ages someone took the fuel pump out the tank to find it had no gogo juice. the level sender was stuck on a wire so it showed half a tank

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