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Discussion Starter · #1 ·
Hi,

Well it's that time again...:rolleyes:

My 2001 4.0 SE runs absolutely fine on LPG, however on petrol...not so much...

What happens is that most times when I start it on petrol, it runs really really rough and stumbles untill the point it nearly dies.
When it does this, it is clearly not running on all cyllinders, however it doesn't seem to run rich as there's no black smoke, no smell of fuel.
Sometimes it'll clear, and it starts running on 8 cyllinders again at which point it runs and drives fine, untill it goes bad again.

As soon as I switch over to LPG it runs sweet as a nut, so I'm guessing it's not related to the ignition.
Having said that, I did fit two new coils, new sparkplugs and new leads.

I have also installed new cats and had all the injectors overhauled and tested afterwards, and they're all fine.
Oh, and it has a new MAF fitted as well.

A garage tested the pump, that flows enough fuel.

What could cause this problem? It is really doing my head in...
 

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LPG calibration may be out so it causes the long and short term fuel trims to wander off. Then when you run on petrol the trims have to alter to get it back into the correct fuelling. If you have an OBD reader, check the long and short term trims.

Or just never run on petrol......
 

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Most LPG ECU's piggyback of the petrol ECU and unless perfectly calibrated will cause the petrol trims to drift. Easy way to check is to reset the adaptive values and see if things improve. If they do, you need to have the LPG map altered to avoid the same problems coming back.
 

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Discussion Starter · #4 ·
Well, the prince of darkness seems to have taken to the old P38, because it has now decided to run fine on LPG but refusing to be switched to LPG....sometimes it will suddenly switch over tot LPG but then it switches back to petrol again, and so it runs like crap.

So, inspired by your answers I went to the LPG tech today.
There is a seperate ECU for the LPG system, which works fine, but the sensor that tells the ECU what rpm the engine is at, was not putting out any signal and hence it wouldn't switch to LPG.
A new one has been ordered.

As for the petrol side of things: the mixture is fine (not running lean or rich) but there are two units that tell the injectors to shut down when switching to LPG and those are most likely the culprit, as it won't tell the injectors to fire up most of the time. The unit's are sealed and cannot be opened to be inspected, so the have to be ordered new and soldered in place.
 

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From your description I'm trying to work out what sort of LPG system you have. Most don't have a separate sensor for rpm, just a connection to one of the ignition coils. Units to shut off the petrol injectors are normally referred to as emulators but a fault on them will usually result in not shutting off the petrol injectors when on LPG, so it will be dual fuelling and running very rich on one or more cylinders. However, these would normally be used with a single point system whereas a multipoint, which has additional LPG injectors as well as the petrol injectors, will have it incorporated in the ECU. Unless it is a Prins system which are a bit of an oddball.....
 

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I would consider a fuel pump. I had a '96 4.0, and what I found was that even though the pump was pushing enough fuel to run, it had worn enough that it would let the pressure off in the fuel system when it was parked. I saw similar issues with stumbling and rough idle for a few moments until the fuel system was fully pressurized again. New fuel pump cured that completely. Curiously, my mileage improved a little after that also. I think the new fuel pump wasn't working as hard because there was less fuel getting by the pump back into the tank.

So even though your fuel pump passes the mechanic's test, you might consider it!
 

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Discussion Starter · #8 ·
Fixed! Turns out all the problems were caused by the part that is responsible of supplying rpm signal to the ecu. Swapped and instantly all was well again! Thank you all for having a think about this AND pointing me in the right direction!
 

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If it helps, here's the Prins installation manual for when you get another problem......
 

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It's on my hard drive along with info and software for hundreds of other LPG systems. All my cars have run on LPG for at least the last 17 years.....
 
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