Thread: RR 3.9 Classic runs on gas but not petrol... Any ideas?

Originally Posted by Sitec

Hi all..

I've gone to fire up our RR Classic this morn, as there's a guy coming to look at it, and it'll run on gas, but won't fire up on petrol. Have a spark, power to the coil etc and know that side is all good because it runs on gas.. Any ideas? Cheers

delete that. 3.9

Most of the time it's a stuffed EFI fuel pump. Listen at the petrol filler for the whine of the pump running, normally they run even while on gas. VN Commodore pumps can be picked up almost anywhere for under $100 and do the same job as the original. However its worth checking all the connections and the fuse for the fuel pump as they are all prone to melting if a worn pump pulls too many amps.

Originally Posted by bee utey

Most of the time it's a stuffed EFI fuel pump. Listen at the petrol filler for the whine of the pump running, normally they run even while on gas. VN Commodore pumps can be picked up almost anywhere for under $100 and do the same job as the original. However its worth checking all the connections and the fuse for the fuel pump as they are all prone to melting if a worn pump pulls too many amps.

Fuel pump is running, fuses all 100% and very clean. It had a flat battery when I started, and needed jump starting.. Took 10 min with leads connected for it to put in a decent charge. When it wouldn't go on petrol, I thought I'd take a punt on gas, and it fired up (eventually). Ran at high idle for half hr before running out. When on gas, I switched several times to petrol and it died instantly. I've found a couple of the big spade fuses in the center console from the previous owner, but not found any place for them.. Is the ECU fused separately/surge protected?

Some gas system`s normally switch the injectors off to run on gas.
Id be checking to see if a wire is corroded or a crappy connection or come off something along that line seeing as you can hear the fuel pump .
Id be checking to see if the fuel is pumping even if you can hear it .
Remove or loosen a fuel line somewhere as the fuel just returns to the tank even on gas with some set ups

Originally Posted by Sitec

Fuel pump is running, fuses all 100% and very clean. It had a flat battery when I started, and needed jump starting.. Took 10 min with leads connected for it to put in a decent charge. When it wouldn't go on petrol, I thought I'd take a punt on gas, and it fired up (eventually). Ran at high idle for half hr before running out. When on gas, I switched several times to petrol and it died instantly. I've found a couple of the big spade fuses in the center console from the previous owner, but not found any place for them.. Is the ECU fused separately/surge protected?

With a D1 you know when the ECU isn't powered because the idle speed controller (stepper motor) no longer works. Normally you get a very high idle because that's the last thing the ECU does on shut down. If your LPG system is an Impco or similar they traditionally need power on to the two relays that cut off the injector negative side wires. Somewhere between the back of the engine and the ECU you should be able to spot where the ECU harness has been opened for injector cutting. Find the pair of relays that hang off the harness break and see if they power on with the switch on the dash set to petrol.

Also check for fuel pressure at the rail, sometimes the fuel pump gets replaced and a non submersible hose type is used instead of the OEM hose. The pump will run but there will be no pressure.