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mechanical necromancer
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Discussion Starter #1
Ok the new troll is running badly on petrol but fine on lpg. The lpg system uses petrol to start and then switches over to lpg and viola! You're driving. However if you switch to petrol it starts ok but when you flick it in drive and try to move it you can't rev beyond 1800/2000 rpm before it starves out. If you keep the boot on it comes good at around 4500 rpm and starts to make power again but the whole time it's building to that there's nothing.

I've removed the power and left if off for an hour to reset the ecu and hosed the AFM down with CRC MAF cleaner and it was good there was solid power delivery all the way through the rev range. I switched back to LPG and everything was fine but then I flicked it back to fuel and the starving issue was back.

It seems like a fuel starving issue but could it be AFM, TPS, fuel pump or fuel pressure reg? If you plug into the OBD port can you get real time data from those sensors? The's no engine light on so it's not throwing any codes.

Given the AFM is $500 + the TPS is $280 ish and a fuel pump is $200 ish I'm not keen on replacing anything just for the experiment's sake. So brains trust has anyone gone through this issue?
 

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nissan
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Could be any one of the EFI related parts you mentioned (MAF, TPS, pump, reg, injectors) - first step would be to either get a hold of an ECUtalk to check out what the sensors values, or grab a copy of the factory service manual and check out sensor signals by backprobing the ECU with a multimeter. If you can't diagnose any troublesome sensors it's probably something which isn't diagnosable via the ECU i.e. blocked/faulty injectors, failing fuel pump, failing pressure regulator.

http://grosjean.pierre.free.fr/Y61 4,2/Service Manual/sm7e-1y61g1/ec.pdf

Try page EC-52 for the symptom/possible cause chart, and EC-58 for reference values.
 

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mechanical necromancer
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Discussion Starter #3
Cheers Scott, I will download that and look at getting my hands on a scan tool.
 

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mechanical necromancer
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Discussion Starter #4
Here's another question, what would happen to a MAF if it was getting covered in LPG? Looking at the system I've got it's feeding gas into the ducting about 300mm from where the MAF sits so at mid range revs when you back off the throttle wouldn't the gas fill the intake ducting back as far as the MAF/airfilter? Could the gas be leaving a chemical residue on the MAF and this is corrupting the values the ECU sees when fueling the injectors?
 

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There isn't really a reversion of flow in the intake, it's a reduction in flow speed when you back off. So I wouldn't be worried about LPG flowing backwards to the MAF. If you're ever about in the NW Suburbs of Melb send me a PM I'm happy to chuck the ECUtalk on it and have a look.
 
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Pass the Popcorn
Nissan Datsun Patrol
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I'd almost guarantee it's dried up fuel lines. Seen it may time on duel fuel cars when the car is run on gas 90% of the time. It used to happen on a duel fuel ford ute I had.

Definately not fun when it happens. Either run a full tank of petrol through the system or replace the fuel lines and all will be good again.

Depending on the age of the gas setup, most cars that start on petrol and then switch to gas need to run petrol for 5-10ks a few times a week.
 
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Not likely, most of the LPG installs on 4.5s I've seen, the fuel pump runs constantly anyway even when on LPG - the fuel pumps through the lines but the injectors are cut.
 
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You need to get a read on the sensors by using a gauge like Ecu talk. Mine was doing much the same. Found it to be a faulty Co2 sensor.


Sent from my iPhone using Tapatalk
 
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Fuel pump most likely. Alot of lpg installers dont fit new fuel pump to sub tank. They run the engine off the original sub tank pump which doesnt flow enough
 

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mechanical necromancer
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Discussion Starter #11
Scottmeister there's a relay to cut power to the fuel pump on my set up, it's connected to the lpg ecu under the bonnet. It lets the engine start on fuel then cuts so you can run on gas.

MQ Tray, I don't think it's dried up lines because it runs fine after the AFM get's cleaned.

freedomseka, It could be too small a fuel pump but it seems to run fine until you run it on gas.

I've cleaned the AFM again and it ran fine on fuel, flipped it to gas and back and bingo it was running retarded again. I really need to plug it into a scan tool and see what's going on electrically.

I did note something last time I cleaned the AFM, I cranked the engine and it started and ran with the AFM disconnected, it starved out and died when you tried to rev it though. The first time I did the clean I also disconnected the battery for 30+ minutes to reset the ecu, when I reconnected it the engine cranked and fired but wouldn't start. So it seems like the ecu is storing some values to allow the engine to run but when you reset it those stored values disappear.

I was wondering about adding an isolating switch to cut power to the gas system to limit the possibility of the lpg ecu causing some kind of interference with the GU engine ecu. But there only seems to be connections to the fuel cut relay and the TPS on the engine, there are connections to the gas converter, something that looks like an electrically controlled gas mixer that feeds into the inlet piping and the gas solenoid.
 

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mechanical necromancer
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Discussion Starter #12
Ok just a quick update. Ran dry on gas this morning on the way to work and flicked over to fuel and it was a pig, nursed it to work and tried an ecu reset and viola! It was 70% better straight away so now I'm thinking it's something electrical or data related, although I do think the injectors are off because there's still hesitation there.
 

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What about something simple like the fuel filter? I have a 4.5L GU 1999 model. Gas was put in after it was first purchased (not by me) I run one tank of gas then one of Petrol and so on....

I have a push button switch in mine, either starts on petrol or gas depending on whether the button is pushed in or not.

When you say "run dry on gas" do you let it run out completely? I have heard that running it out completely is no good for it. I just switch over when the gauge reads empty (still has another 70klms in it after that).

Regards Ed.
 

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4.5 Struggles on LPG

I had similar issues but OK on petrol but rugged & often cut out on LPG. Installer checked all the signal inputs & outputs to eventually find a faulty LPG ECU.
Also should have a balance line to overcome backfiring or it is set to run too lean.
 

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mechanical necromancer
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Discussion Starter #15
Hey all, well the petrol gremlin has been solved. I took it to the mechanic to get a suspected blown head gasket fixed and to find the petrol issue and it turns out it was a split air intake duct after the maf. It's been patched with tape for now but I'll be sourcing a replacement duct and then resetting the ecu and hopefully it's fixed.
 
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