Maddening Running Issue

[willbrown]

Thank you! Will order.


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[willbrown]

It'll probably run with this squirted in the intakes if it's a fuelling problem.

https://amzn.eu/d/inJdQQO

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thanks rob. have added to the list of checks!

[willbrown]

Hello again - to update:

Got the pressure gauge and installed today. Ran pump and it was showing 12psi solid on first try and of course then she fired up without much trouble. See here:

https://youtu.be/YlsVygWm-Xc?si=y9tDeSbuKtyiHGPY

So after checking for any fuel leaks warmed it up and went round the block.

Now pressure has dropped to 9psi and you can see from this second video that it’s doing that weird hunting thing no matter what throttle input I use:

https://youtu.be/hAvmVciCwxo?si=5ybcAlgFoIT8DIrT

So what is the next logical step here? I am thinking it’s the check valve but your thoughts as always are appreciated.


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[willbrown]

Update - let the engine cool for 30 mins then ran fine at about 10psi


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[Lightweight_911]

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Fuel pressure for mfi needs to be ~ 14.5psi from memory ...

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[Bruce M]

The hunting is probably because the pressure isn’t enough to open the injectors, until the pump builds enough pressure then they open & pressure drops until they shut, then repeat.

[gav p]

The hunting is probably because the pressure isn’t enough to open the injectors, until the pump builds enough pressure then they open & pressure drops until they shut, then repeat.

Agreed. The question is therefore whether this is because of a dodgy pressure control valve or dodgy electric fuel pump. I’ll have a look through my notes from when I did this test a few years ago


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[willbrown]

Thanks Andy, from what I have read on Pelican it appears the consensus is that 11.8 PSI +/- 3 is spec so 8.8-14.8 is within spec so I would be at the bottom of that range but still should be ok. I haven’t checked flow yet so maybe that’s the best next step?

Or maybe just bite the bullet and get a new $$$ pump. Autoatlanta will do a rebuild for about $400 anyone have any experience of those pumps?


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[willbrown]

The hunting is probably because the pressure isn’t enough to open the injectors, until the pump builds enough pressure then they open & pressure drops until they shut, then repeat.

Agreed. The question is therefore whether this is because of a dodgy pressure control valve or dodgy electric fuel pump. I’ll have a look through my notes from when I did this test a few years ago


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Thanks both - that seems very logical.

GavP is there a way to test the control valve?


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[hot66]

official docs agree with Andy with 1bar ( 14psi)

[hot66]

That hunting is weird .. so that was at part throttle ? All 6 cylinders effected which does point to something pre the MFI pump if fuel related

[willbrown]

That hunting is weird .. so that was at part throttle ? All 6 cylinders effected which does point to something pre the MFI pump if fuel related

Thanks James. Yes, part, half, full, makes no odds.


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[RobFrost]

That hunting is weird .. so that was at part throttle ? All 6 cylinders effected which does point to something pre the MFI pump if fuel related

Thanks James. Yes, part, half, full, makes no odds.


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You should hook up a timing light during hunting and confirm there's a spark when the revs are falling.

Is there a smell of unburnt fuel?

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[willbrown]

That hunting is weird .. so that was at part throttle ? All 6 cylinders effected which does point to something pre the MFI pump if fuel related

Thanks James. Yes, part, half, full, makes no odds.


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You should hook up a timing light during hunting and confirm there's a spark when the revs are falling.

Is there a smell of unburnt fuel?

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Thanks Rob - I’ll add to the list. No smell of unburned fuel. I’m pretty convinced it’s not spark - all ignition parts new and have tested with a replacement CDI


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[RobFrost]

I asked re unburnt fuel partly in case the mfi timing is off. How about the plugs, what colour are they, and are they wet with unburnt fuel?

If it is fuel I can't be of much help as I don't knownthe mfi system well at all.

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