Thread: Dual Fuel Flapper V8 will not run properly on petrol

Hi,

This suggestion may not solve your problem but it only takes 5 min to see.

Get under the car in front of the rear diff and see where the return fuel line enters the fuel tank (about halfway up), so you know what this looks like, etc.

Find the fuel return line in the engine bay and blow air down it, ideally with the tank somewhat full.

You should be able to hear bubbles being blown in the tank.

If it seems blocked it will be most likely rust in the steel return line spigot, which can be cleared by disconnected the hose at the tank and poking wire through the spigot.

I'm not saying that's your prob, but that is what was wrong with a carb 350 correctly jetted that ran rich for no apparent reason.

DL

Hi 350RRC,

I've just run out and given this a go.

So you're saying when I blow into the return hose, the result shouldn't be almost complete resistance whilst I put as much force on as I can, and then having the back pressure forcibly shoot the fuel back out of the hose and into my eye when I pull my mouth away?

Further, this hard fuel line I've found where I got a big stick lodged the last time I drove the car before the rebuild shouldn't be pinched flat?

I think you're on to something with the return line...

IMG_20230704_193602350.jpg

Hi do you know how many times the head and block have been decked?
my previous Rangie 3.5 modded to 3.9 with holden 60 though pistons pinged like a dog on petrol until I fitted 30 thou composite head gaskets, I was then able to advance timing to 12 degrees btc and it ran good on lpg and petrol..I had no idea how many times it had been done before.
it also my help to know how many cc your head combustion chamber is on your heads compared to a known stock head…PLR are you able to help?😃

Have you checked the flap in the AFM? One LPG backfire, even with a blow-off system in place, will slam the thing shut under explosive force such that the pivot /hinge will be deformed and the flap will not move properly. If it is stuck partly open, the AFM will signal high airflow even at idle, the fuel will be metered accordingly and result in very rich mixture as you describe. I went through several of those AFMs before I saw the light- brings back bad memories!

Hi All, I've been working furiously on this both figuratively and literally all weekend.

Sadly replacing the crushed section of hard line made basically no difference to the running, but it was a good find to repair anyway.

Some updates:

Originally Posted by Bradtot

Hi do you know how many times the head and block have been decked?
my previous Rangie 3.5 modded to 3.9 with holden 60 though pistons pinged like a dog on petrol until I fitted 30 thou composite head gaskets, I was then able to advance timing to 12 degrees btc and it ran good on lpg and petrol..I had no idea how many times it had been done before.
it also my help to know how many cc your head combustion chamber is on your heads compared to a known stock head…PLR are you able to help?😃

Hi Bradtot, I have no idea what previous work has been done on the car, I picked it up with more than 350,000 on the clock. If I had to guess I would say the heads have been decked at least twice, once by me and the block decked probably once only by me. The engine had composite head gaskets on it which I don't think are correct for this year and there was other evidence of having had the heads off before. The engine number is still visible on the deck even after I had my work done so it can't have had too much taken off.

I've always wondered how to get the combustion chamber CC, thinking about it I suppose it could be done with the head sitting upside down on a bench and use a measured liquid to see how much fits?? Too late now as I'm not taking them off again if I can get away with it.

Originally Posted by POD

Have you checked the flap in the AFM? One LPG backfire, even with a blow-off system in place, will slam the thing shut under explosive force such that the pivot /hinge will be deformed and the flap will not move properly. If it is stuck partly open, the AFM will signal high airflow even at idle, the fuel will be metered accordingly and result in very rich mixture as you describe. I went through several of those AFMs before I saw the light- brings back bad memories!

I've been going through the magic GoMoG flapper guide I found at 87 RRC EFI Flapper 4cu trouble shooting wiring / ecu / AFM / regulator /, and also attached below for those who come later:

LUCAS_L_FUEL_INJECTION.pdf

The tests were made somewhat more difficult in a few spots due to what I would now describe as some absolute butchery of the engine management loom when the LPG was installed some 25 years ago.

The AFM actually tested up perfectly, which was unexpected but nice to have at least some wins.

What didn't test properly was the coolant sensor wiring, which despite me having a resistor bypassing the sensor was showing far too high resistance which was contributing to running rich. I managed to fix this and then moved on to...

The TPS which wasn't set properly, I managed to get it reading 325mV at closed throttle but it only reads about 3.7V at WOT rather than 4.5V which it's supposed to, which I imagine will be pushing towards a lean mixture at high load. It had a nice even rate of change through the range but just doesn't get high enough.

Also not testing properly were the injector circuits, of which cylinders 4 and 6 came up as open circuit! The connectors had disintegrated to the point where two weren't firing at all... which is why it was running so poorly. Both lean and rich, from a heap of sensors not working making the mixture rich but two cylinders not firing at all, or only firing on what was being scavenged.

On fixing the injector circuits and starting the car finally some progress, running much more evenly but still very rich.

Testing the injector circuits again now that they all had continuity, they were all reading about 40 Ohms resistance instead of 7-10, also reading about 11.4 volts instead of within 0.5 of battery which was 12.8 volts. This was doing my head in for ages as the resistance across the injectors was all testing perfectly at 2.4Ohm, and I had already fixed up the resistor pack so that each circuit was reading 6Ohm.. so WHY was this happening? I understand that too much resistance in the injector circuits also leads to running rich.

It turns out that the LPG set up had the power supply to the injectors running off the same circuit as the fuel pump, so that they cut when switched to LPG, where they should have a clean power supply straight from the main relay. Fuel pump runs, voltage drops in the circuit. I don't understand why it was set up this way, with two relays taking a signal from the main circuit to open the fuel pump voltage to the injectors, rather than taking the signal from the fuel pump to just open the main voltage to the injectors per the original circuit. Simple enough to rectify, by changing the wiring on the relay terminals. When I close my eyes to sleep I'm going to be seeing the cut and shut soldered and taped wires I found in the loom as part of the LPG install.

So, with that done and starting up it seemed... ok. Running evenly, stalling on idle, not hesitating so much when revved. Time for a drive to see how it's going on petrol.

So far so good, not there yet but good enough to drive away from home, pull over a couple of times to advance the timing a bit as the pinking had stopped, gets up to 100km/h okay, not feeling powerful but so much better than it was...

I try a full throttle pull at highway speed to see how it goes and it dies badly, foot off throttle and it's still dying badly... why? Suddenly terrible throttle response and losing speed. Drops down to 60km/h which is now top speed at full throttle on petrol. It worked for a good five minutes until I loaded it up and now it's worse than ever. Switch to LPG and go home.

Back to the drawing board, go through the entire test procedure again. Everything tests perfectly, other than the TPS which hasn't changed from not getting high enough voltage at WOT. Runs terribly below 2000rpm held on throttle. Extremely hesitant to rev again. I'm running out of hair to pull out. Walk away for a few hours, come back and fiddle with the LPG relays again, maybe it was running the fuel pump voltage for a reason? No change. Walk away for a few more hours, start up to think about how it's responding again and it's running much better. Good enough for a drive? Out on the street get about 100m from the house and it drops revs and loses all power again. At least a short trip back. Sitting in the driveway holding the revs on throttle again and it suddenly kicks right up and becomes responsive! Then drops back down and loses power. Now an intermittent electrical fault!!! This is some sort of progress at least? Sitting in driveway and goes back and forth between running ok and changing to dying badly.

Set up a trusty G-clamp on the butterfly to hold the revs and poking around at everything I've touched in the engine bay and I finally, finally discover... I can make it switch between running well and badly by jiggling the connection to the resistor block! Now that I've forced it to be in the right place... it drives on petrol!

What a ****ing journey. Sorry for the exposition but I needed to share what I've done to get to where I am now.




So... now it runs on petrol well enough that I would trust it on a journey, and know which wires to jiggle if it does fail. Huge win. But - it's still disappointingly down on power, vmax about 120km/h on petrol and worse than on LPG. It's timed up properly now that it's running on both fuels, at about 8BTDC.

Somebody tell me it's the TPS not getting to 4.5 volts making it run lean under load, and the problem will go away if I sort that out? If so I could possibly even just ignore it until the big eventual megasquirt day, just knowing that I can at least switch to petrol to get me to the next LPG fill up if I have to.

Somebody tell me it's the TPS not getting to 4.5 volts making it run lean under load, and the problem will go away if I sort that out? If so I could possibly even just ignore it until the big eventual megasquirt day, just knowing that I can at least switch to petrol to get me to the next LPG fill up if I have to.

No I won't tell you that because it isn't true.
The TPS is like a volume control , variable resistor which tells the ECU how much fuel to inject. It really acts like an accelerator pump in a carby by giving demand calculation before the flapper is open enough to give the air mass reading.
However The effect will just be that you do not get WOT as the primary source of air mass calculation is the flapper and a temperature sensor.
Funny I ran a 14CUX for a couple of months without the TPS connected as the wire had separated and the only reason I knew was that the override injection cut did not work at 1500RPM.
Regards PhilipA
PS AFAIK your flapper will have a
supplementary injector run by a thermo time switch. If the Thermo time switch is faulty the supplementary injector will keep injecting.
Funny I fitted a Federal injection to a 77 RRC and a Thor manifold to a 14CUX but have not done anything with the flapper which was between the Federal and 14CUX.

Originally Posted by btbtbtbt

Set up a trusty G-clamp on the butterfly to hold the revs and poking around at everything I've touched in the engine bay and I finally, finally discover... I can make it switch between running well and badly by jiggling the connection to the resistor block! Now that I've forced it to be in the right place... it drives on petrol!

What a ****ing journey. Sorry for the exposition but I needed to share what I've done to get to where I am now.




So... now it runs on petrol well enough that I would trust it on a journey, and know which wires to jiggle if it does fail. Huge win. But - it's still disappointingly down on power, vmax about 120km/h on petrol and worse than on LPG. It's timed up properly now that it's running on both fuels, at about 8BTDC.

Somebody tell me it's the TPS not getting to 4.5 volts making it run lean under load, and the problem will go away if I sort that out? If so I could possibly even just ignore it until the big eventual megasquirt day, just knowing that I can at least switch to petrol to get me to the next LPG fill up if I have to.

Over the years, I have recommended to so many people that have had odd running and starting issues with the flapper EFI in RRCs (since I had to diagnose and quite literally do my own roadside repair which exposed this very issue) is that the first place to start is the wiring and plug on the resistor pack, including moving it around while the engine is running as there is usually some defect to be found in either the terminals of the connector or the pins on the resistor pack. This is due to how the resistor pack is positioned from factory and how the wiring harness is routed to the resistor pack in typical British ingenuity fashion. And from my experience, both the flapper and hotwire systems are extremely sensitive to the parameters/readings of the TPS, which is unfortunate because there is no new OE ones, real genuine NOS ones are exceptionally rare and eye-wateringly expensive, all of the aftermarket ones that I have encountered have been defective and the only conversion kit that I can remember seeing is offered by Tornado UK at considerable cost, there may have also been another that converted to a Ford TPS.


Anyway, glad to know that you got it running 'well enough' again. I would definitely be resolving the TPS issue before proceeding and if it is the real Lucas original TPS, doing whatever you can to save it. Alternatively, you can also use a TPS from a hotwire system but you will probably need to modify the plastic ears of where the TPS mounts so that it is adjustable, as the original flapper TPSs were. Reason being is that the hotwire EFI TPS has no adjustment in position/rotation as the EFI could "self-learn" throttle use and compensate the TPS within reason.