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Engine shuts down after sharp bump?


DiscoClax
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OK, firstly, it's a D1 3.9 auto V8 (with dizzy) 8.1 comp, no cats no oxys. It's dual fuel, running a 42ltr petrol tank in the RHR wing with factory fuel pump and sender. Twin LPG tanks where the petrol tank used to be. Its a 14CUX EFI system with Lucas hot-wire (all engine and injection is factory standard, as built).

The problem:- If I'm running on petrol and hit a sharp bump, like an aggressive speed hump/pot-hole or while off-road, the engine will just die or it'll stammer and cough with no power at a low very rough idle irrespective of throttle position. Generally recovers in a few seconds or after an ignition cycle. Has never done it when running on LPG, so I'd assume it's not an ignition related issue (it's an older "dumb" carby-type gas system with no interface to the EFI computer, so the fuel and ignition systems are stand-alone). I've replaced the fuel pump with a known good unit*, checked fuel/electrical connections at the rear end, ensured lines are clean, filters replaced, etc. No change.

* Nothing seemed wrong with the old unit when I pulled it, but there was a fair amount of rust and crud in the swirlpot and stuck to the filter of the old pump. All squeaky clean now.

I'm thinking the EFI computer is copping a jolt and somehow dropping out. Ba$tard of a thing to get at, so I don't really want to pull the bugger out unless really necessary. However I have heard that they are problematic? I'm looking to convert to vapour injection, so I've got to get the EFI working properly first.

As an addendum, it's never really run totally properly on petrol. Starts OK, but has gone through random phases when it over-fuels badly, currently has a hole on tip-in and sometimes loses power and misses if held at WOT high revs for a while like when overtaking on a hill). Is sluggish and doughy when cold and feels like its overfuelling (like an oversize cab). Quality 98RON fuel used exclusively and tank, lines and filter all cleaned/replaced recently. Runs fine on LPG with none of these issues.

Economy on LPG is around the 20-22ltrs/100kms (~13mpg) mark - 90% freeway, and about 18-20 ltrs/100kms (~15mph) on petrol for same usage, which seems on the high-side to me, but it's generally pretty consistent. Note that my vac adv isn't working at the moment either (split diaphragm) - refer wanted items in the classified section :P

Sorry this is so long, but it (hopefully) reduces the number of question and answer iterations required.

I'd welcome others thoughts and feedback (excluding converting to diseasal...) :ph34r:

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You don't mention cleaning the float bowls...

Never had it on a car but my old Triumph Bonneville used to exhibit the symptoms that you mentioned, ie: It would lose power and almost die, but then come back after a period of very rough (would stall if I didn't keep blipping) running or would also come back after a 'rest' of a couple of minutes. This was generally caused by running at high revs or by sudden jolts. The problem was manky water in the float-bowls getting disturbed (in the case of high revs - by the vibration. It was a Triumph ;) ) and being sucked up into the main jet emulsion tube.

I did have a slightly similar thing on a car, although in this case it was not triggered by bumps and although it could be brought on by high revs, generally it was a random occurrence. In this case it was the crank case breather producing gobbets of emulsion and spitting them straight into the carb mouth. This would cause sever loss of power (although it would tick-over fine) and would clear after maybe a minute.

Good luck with it...

TwoSheds

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Has it got an inertia switch (fuel cut-off)?

Could be either on the bulkhead or inside the car on the A' pillar trim.

Could be weak, causing momentary loss of power to the fuel pump.

Now why didn't I think of that...? That'd make a lot of sense. I'll try bridging it out and belt it over a few bumps to see if that sorts it. It's just behind the washer bottle on mine, so at least it's easy to get at. Cheers :)

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You don't mention cleaning the float bowls...

Never had it on a car but my old Triumph Bonneville used to exhibit the symptoms that you mentioned, ie: It would lose power and almost die, but then come back after a period of very rough (would stall if I didn't keep blipping) running or would also come back after a 'rest' of a couple of minutes. This was generally caused by running at high revs or by sudden jolts. The problem was manky water in the float-bowls getting disturbed (in the case of high revs - by the vibration. It was a Triumph ;) ) and being sucked up into the main jet emulsion tube.

I did have a slightly similar thing on a car, although in this case it was not triggered by bumps and although it could be brought on by high revs, generally it was a random occurrence. In this case it was the crank case breather producing gobbets of emulsion and spitting them straight into the carb mouth. This would cause sever loss of power (although it would tick-over fine) and would clear after maybe a minute.

Good luck with it...

TwoSheds

I remember carbies... Had three of 'em on my last car (175 CD-2S Stromby's to be precise). Bugger to balance them all and keep them all tuned and maintained but so worth it when they were just right. :D

Good suggestion with the crank breather, TwoSheds. I'll check the amount of pull-over. Not sure why it'd do it more on petrol than gas, but stranger things have happened. Probably running richer on petrol, maybe enough to cause the poor running when you add a bit of oil. :unsure:

Or it might be the rail pressure regulator, or the injectors, or the computer, or the wiring.... Ah, bugger it... Maybe I should just drop a Holley on it... :lol:

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I remember carbies... Had three of 'em on my last car (175 CD-2S Stromby's to be precise). Bugger to balance them all and keep them all tuned and maintained but so worth it when they were just right. :D

Ooops - sorry! For some reason I got it into my head that you had carbs... Another senior moment I guess.... :o

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Ooops - sorry! For some reason I got it into my head that you had carbs... Another senior moment I guess.... :o

Hmmmm, sometimes I wish I did have carbs... :P

Scary how much of what you mentioned still had some validity. It's still burning hydrocarbons at the right time in the right quantities in the right order...

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If it doesn't run properly on petrol but is fine on LPG then the problem is not going to be in the ignition system but is likely to be in the petrol delivery control system.

Have you thoroughly checked that all connections are in good order and none of them are loose? try wiggling them whilst the engine is running on petrol to see if anything odd happens.

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