Fuel Pump Relay(s) & Fuel Demand

Hi, New to the forum, need some help!
I have a '87 3.5 Efi Vogue with a fuelling problem. After bringing my first RR to its new home, a trip of about 80 miles with no trouble, it stubbornly refused to start the next day. The engine would crank and run for a short time, but then die. Everything pointed to fuel starvation and lo and behold the fuel pump had packed up. Having a hatch in the boot just for the occasion, the pump was rebuilt using spare parts and the wiring that caused the problem (inside the tank) swapped out. Now the pump is operating as it should.

This is where it gets bizzarre.

The engine will only run when I bridge the fuel pump relay (under the drivers seat), and 'force' the pump to work. When in situ the relay will click to request fuel only while the starter motor is cranking, and will switch the pump off once the engine is running. You can feel the relay click on, then back off. There is also a seperate fuel related relay under the bonnet near the air intake (fuel shut off relay?) but this appears to be working. I have checked all the relays for continuity and operation using a 12v supply, and they all perform well.

I'd be quite happy to wire the pump into the ignition, or have it on a seperate switch, but i'm worried that running the pump all the time might cause further problems. Or would any danger of 'overpressure' be dealt with by the return system and the fuel pressure regulator? I'm also wondering if there is something silly like a fuel cut off switch or an 'impact' sensor that might be to blame.

Any advice would be wonderful, as this has myself and several other people scratching our heads!

danceswithartics said:

Hi, New to the forum, need some help!
I have a '87 3.5 Efi Vogue with a fuelling problem. After bringing my first RR to its new home, a trip of about 80 miles with no trouble, it stubbornly refused to start the next day. The engine would crank and run for a short time, but then die. Everything pointed to fuel starvation and lo and behold the fuel pump had packed up. Having a hatch in the boot just for the occasion, the pump was rebuilt using spare parts and the wiring that caused the problem (inside the tank) swapped out. Now the pump is operating as it should.

This is where it gets bizzarre.

The engine will only run when I bridge the fuel pump relay (under the drivers seat), and 'force' the pump to work. When in situ the relay will click to request fuel only while the starter motor is cranking, and will switch the pump off once the engine is running. You can feel the relay click on, then back off. There is also a seperate fuel related relay under the bonnet near the air intake (fuel shut off relay?) but this appears to be working. I have checked all the relays for continuity and operation using a 12v supply, and they all perform well.

I'd be quite happy to wire the pump into the ignition, or have it on a seperate switch, but i'm worried that running the pump all the time might cause further problems. Or would any danger of 'overpressure' be dealt with by the return system and the fuel pressure regulator? I'm also wondering if there is something silly like a fuel cut off switch or an 'impact' sensor that might be to blame.

Any advice would be wonderful, as this has myself and several other people scratching our heads!

Click to expand...

Sounds like a duff relay or duff relay connector. Try checking the wiring at the relay housing.

There is a fuel cut off in that Range Rover it's in the engine bay on the side next to the coil, there's a relay to. Mine played up at this relay connector.

Charlotte

Hi Charlotte, thanks for the info. I can't find any faults with the relays or their connections i'm afraid. I cant find any kind of fuel cut off near the coil, can you describe what I should be looking for? I have a suspicious looking pair of wires coiled up near the top of the wing, perhaps this has been removed.

Is there any where on the web I can get a proper circuit diagram for the electrical system? The only one applicable to my model in the haynes manual I have is pretty vauge about these relays, and it would be nice to know if they are connected properly. The relay in the engine bay is directly wired to the -ve post on the coil, is this normal?

Hi, back again with an update.

Have sought professional advice and its an ecu fault, nothing to do with the relays or the pump. False alarm! Many thanks for the advice tho. Really interesting fault, will post more details when I have the time, as it may be useful for someone else.

Hi, i said i'd sit here and write this out, so here I go.

Fairly bizzarre symptoms caused by simultaneous failiure of not only the ECU, but the air mass meter as well. This one had the local specialists scratching their heads for a while.

Symptoms:
Fuel Pump and relays operating as normal.
Engine either will not start, or starts but runs for a very short time.
Large smell of fuel, check dipstick (!) look for signs of petrol in the oil.

From what I can gather, this is whats happened. The ecu controls the pulsing of the injectors from the fuel rails, in this case the ecu was not pulsing correctly and instead switches the injectors to permanently 'on'. This causes massive fuel build up in the heads, and as the mix is far to rich to combust. Continued attempts to start the engine compounds this, and forces fuel past the piston rings and into the sump. This produces unusual levels of pressure in the sump, making the starter labour (resulting in huge amounts of amps needed, we had the battery, a jump pack and a tow truck providing power!) and will possibly blow oil seals if left unchecked. Tell take sign, oil leak from main gearbox seal, but only when turning over.
The above is purely an ecu fault.

To compond this again, the air mass meter controls the fuel pump demand. It turns out that this particular mass meter has been tampered with (read wrecked). At tickover the demand is normal, and the fuel pump gets its full 12v quota or power. As soon as the revs rise, the flap in the mass meter reacts and drops this supply to 3v. This lowers the fuel pressure and starves the engine.

The end result of both these faults is an engine that will occasionally start when you manage to clear enough fuel to get a proper mix, then proceeds to starve itself as soon as you get above tickover. Only apparent solution is to replace ecu and mass meter. The fuel seepage/leak was so bad in this case that vapour got into the distributor cap and detonated.

According to my local specialist, they recon the mass meter failed first, probably overloading the ecu and scrambling its 'brains'. Moral or the story, dont touch a mass meter thats been opened and toyed with. I think I should have bought a carburettor model

I'm off to fit a freshly gained ECU, but I hope these ramblings may be of use to someone else.

Hi, Have you any idea which pins you jump to get the fuel relay to pass-through.

CHeers Ian