XR8 Fuel pump/regulator issues

pete r · 21-08-2021, 01:40 PM

G’day blokes, this is a follow on from the idle hunt issue I’ve been chasing…

It’s a 98 XR8 and in the Key On Engine Off position the fuel pump isn’t switching on. Dash lights on but no fuel pressure, Forscan shows the fuel pump is off.
However crank the engine and the fuel turns on and engine starts!

Another weirdo thing, I measured the fuel pressure with engine running and vacuum line connected to the regulator, the gauge showed 32psi. I gave it a rev and the pressure didn’t increase.

So I disconnected and plugged the vacuum line, started the car and the fuel pressure came up to 42psi. “Seems about right” I say to myself.

I connected the vacuum line back up, started the car and the fuel pressure measures 32psi, but now when I give it a rev the pressure flicks up to 42psi then back down again.

There is no fuel in the vacuum line so the regulator diaphragm seems ok.

First things first, does anybody have any idea why the pump wouldn’t be running in KOEO mode?

pete r · 21-08-2021, 01:49 PM

I wonder if there’s been an issue in the past and some one has run power to the pump by a different method

lumen8 · 21-08-2021, 03:58 PM

Kinda wierd that it works running and not KOEO. Could be a sticking pump as unlikely as it sounds, and when its running the extra voltage gets it going.

First thing to do i guess is see pump relay is getting its signal to supply the 12 volts in KOEO. Id start at the engine bay fuel pump relay because its easy to get to, check the relays low side obviously, i forget offhand if the ECU connects to earth or gives 12v on pins 85/86, it probably grounds, check the wiring diagrams.

If the low side of the relay is fine then you might want to check for 12v at the pump plug KOEO

Whether its KOEO or running, the fuel pump relay trigger circuit should be getting its signal from the ECU. I doubt somebody wouldve run power to it differently as they'd have to get quite tricky to signal it to only run when engine is running... I suppose its always possible someone mcgyvered it if the relay trigger signal from the ECU was playing up, just seems unlikely to me... if someone had done this it just seems more likely to me pump would still turn on when ignition is on, ie KOEO... it just wouldnt stop after a few seconds.

pete r · 21-08-2021, 08:44 PM

Quote:

Originally Posted by lumen8

Kinda wierd that it works running and not KOEO. Could be a sticking pump as unlikely as it sounds, and when its running the extra voltage gets it going.

First thing to do i guess is see pump relay is getting its signal to supply the 12 volts in KOEO. Id start at the engine bay fuel pump relay because its easy to get to, check the relays low side obviously, i forget offhand if the ECU connects to earth or gives 12v on pins 85/86, it probably grounds, check the wiring diagrams.

If the low side of the relay is fine then you might want to check for 12v at the pump plug KOEO

Whether its KOEO or running, the fuel pump relay trigger circuit should be getting its signal from the ECU. I doubt somebody wouldve run power to it differently as they'd have to get quite tricky to signal it to only run when engine is running... I suppose its always possible someone mcgyvered it if the relay trigger signal from the ECU was playing up, just seems unlikely to me... if someone had done this it just seems more likely to me pump would still turn on when ignition is on, ie KOEO... it just wouldnt stop after a few seconds.

12 volts is at the relay socket. It looks like the ECU is supposed to ground the coil to make the relay switch, but this ECU isn’t. Instead of measuring near zero ohms in KOEO the contact measures 185kohm.

I’m going to have to dig further, get the ECU out and back probe the wire to see if it’s grounding down there.

Bloody weird, it doesn’t explain why the relay switches when the engine is cranked.

The AU1 had Smartlock, did it disable the fuel pump?

lumen8 · 21-08-2021, 09:18 PM

Well when the engine is cranked im guessing the ECU must be supplying the ground, but doesnt want to with ignition on.

You can prove this by carefully popping the little plastic cover off the relay ( a spare relay is nice for doing this so you dont mess with yours ) and probing the relevant relay pin when the car is running... if its still not earthing and the car is running then that means your pump is getting 12v another way, modified wiring.

I assume if you get the ECU out you will check connectivity between the wire there and the relay pin also, make sure the ECU earth wire is indeed going all the way to the relay still. You will need a long piece of test wire

Smartlock does not affect fuel pump operation apparently, smartlock = the ECU cutting injectors, and the BCM cutting crank/starter motor. Have a read ..

https://www.tiperformance.com.au/kno...lock-problems/

pete r · 22-08-2021, 01:39 AM

Quote:

Originally Posted by lumen8

Well when the engine is cranked im guessing the ECU must be supplying the ground, but doesnt want to with ignition on.

You can prove this by carefully popping the little plastic cover off the relay ( a spare relay is nice for doing this so you dont mess with yours ) and probing the relevant relay pin when the car is running... if its still not earthing and the car is running then that means your pump is getting 12v another way, modified wiring.

I assume if you get the ECU out you will check connectivity between the wire there and the relay pin also, make sure the ECU earth wire is indeed going all the way to the relay still. You will need a long piece of test wire

Smartlock does not affect fuel pump operation apparently, smartlock = the ECU cutting injectors, and the BCM cutting crank/starter motor. Have a read ..

https://www.tiperformance.com.au/kno...lock-problems/

You champion, thanks mate. I’ll report back

11ford11 · 22-08-2021, 09:00 AM

Fuel pump only operates for about a second when you first turn ignition on and then stops until you crank engine

lumen8 · 22-08-2021, 09:28 AM

Quote:

Originally Posted by 11ford11

Fuel pump only operates for about a second when you first turn ignition on and then stops until you crank engine

Thats right, Ive assumed OP knows this and is getting no fuel pump operation at all.

Might be a bit more than a second but it wouldnt be much more than 2 seconds.

It is a safety feature built in, will kill the fuel pump if the ECU sees no crank position signal after x amount of seconds.

pete r · 22-08-2021, 01:08 PM

Quote:

Originally Posted by lumen8

Thats right, Ive assumed OP knows this and is getting no fuel pump operation at all.

Might be a bit more than a second but it wouldnt be much more than 2 seconds.

It is a safety feature built in, will kill the fuel pump if the ECU sees no crank position signal after x amount of seconds.

I’m getting no fuel pressure in KOEO mode so the pump can’t be firing up.
Just waiting for a bit of shade then I’ll go stab some wiring

11ford11 · 26-08-2021, 01:26 PM

Is fuel relay turning on
Is engine control relay turning on as it feeds power to fuel pump relay

pete r · 12-09-2021, 12:23 PM

G'day fellas, this is one of those things that makes you groan like Lurch...

I removed the ECU so I could get access to the loom to do some probing, then when I turned the ignition on I could hear the bloody fuel pump fire up (KOEO).

I refitted the ECU, the fuel pump still fires up KOEO.

Bloody thing. It definitely wasn't doing it before I pulled the ECU, the fuel pump is very easy to hear.
But anyway I took the opportunity to fit a Quarterhorse so I could work on the idle issues.

Like Grandad used to say "If its got tits or wheels its going to cause you grief"