DaFox

Hi guys.

Have a non-starting problem and after trawling the forum I can't find any solutions. Would really appreciate any pointers here

She's a 1997 XJ 4.0 - The very last of the pre-facelift models but seems to have some engine components (TB, ECU, no ceramic fuel pump resistor) of post-facelift.

Initially, She would start, run for about 3 seconds then stall.
I hooked up a spark tester and a noid light and could see that there was no issue with ignition, but the injector pulse was cutting out and causing the stall.

Have replaced the ECU, distributor, crank sensor, tried running without the TPS and MAP sensor but no change.

Now after disconnecting all the injectors and then reconnecting I'm not getting any pulse at all.

I'm completely stumped.
Only other thing I can think now is to test the pin-outs on the ECU to eliminate any fault in the injector harness... Anyone else have any other suggestions?

SeriousOffroad

97 was the first year of the new body style.

Have you thoroughly inspected the injector harness for cracked/melted,missing insulation?
Inspect and clean all of your chassis grounds including the PCM grounds.

DaFox

It's a UKDM car. Has old style body (door bump strips/guards, front window quarterlights, etc) and interior, but new body style engine and management. I think it must have been an end of line. I also have a 94 and 2 x 98 XJs that I can call on for component swaps.

Injector harness looks fine as far as I can trace without striping back the factory trunking.

I will add the grounds to the checklist.

Thanks SeriousOffroad

DaFox

Just to add:

It's not throwing up any codes other than 12 and 55, so no help there.

SeriousOffroad

Keep us updated.

ehall

check for voltage on the injector harness, you'll need an FSM to do anything more

SeriousOffroad

For the OP and those that don't know...

With the KEY ON the injectors receive a constant 12+Vdc signal. the PCM completes the Ground Circuit. Once the Ground Circuit is complete the injector opens allowing fuel to flow.

The PCM determines how long the injector needs to be open and opens the ground path accordingly to close the injector.

If there is not any voltage at the injector, work backwards to find the source.

This thread might help.
https://www.cherokeeforum.com/f2/not...or-pcm-188783/

DaFox

That's awesome!

Have printed out the injector wiring diagram and pin-outs.
Will check for feed from ASD and negative pulse from PCM.

Much obliged good sir

DaFox

UPDATE:

No issues with supply from ASD - its all good as it should be.
The -ive pulse from the ECU is the problem. It shuts off around 3 seconds after the engine starts running.
I discovered a dead short in the harness between injector 1 and 4 which turned out to be a quick-connector that some complete muppet had fitted inside the multiplug at the ECU. It's now removed but I'm still getting the same problem only now she isn't misfiring for those 3 second. She actually sounds really good!

Am I right in thinking that the injector timing signal is fed to the ECU - solely by the cam sensor in the distributor?

CCKen

Correct.. Sorta, but it needs the pulse from the Crankshaft Position Sensor (CPS) as well.


The PCM will shut down the injector system within 2-3 seconds if it does not receive a crankshaft rotation signal from the CPS.

DaFox

That does point a finger of blame at the Crank sensor, even though I have already swapped it out once. Could point to a break/short in the harness, might be worth checking.

The only concern I have is that the ignition seems to be firing fine.
Doesn't the crank sensor determine the spark timing? I'm fairly sure that when my Crank sensor failed a couple of years ago -I had no spark at all...?

CCKen

The camshaft position sensor tells the PCM exactly which cylinder is in place to inject fuel and to trigger its spark. The CPS tells the PCM which set of two cylinders are in-line for fuel and ignition.

BlueRidgeMark

I think a bit of clarification is needed here. The PCM closes the ground path to fire the injectors.

When an electrical path is open, no current flows. When it is closed, current can flow.

DaFox

Thanks man. I got that.

DaFox

Update:

Signal from crank and cam is getting to the PCM but it's still shutting down the injectors 3 seconds after the engine gets going. If I disconnect the ignition coil, the injectors pulse until the battery goes dead so can anyone suggest what might be causing the injectors to shut down like that?