Engine dies, wont restart

I've had my 3.9 off the road for a few bodywork repairs. Everything was fine beforehand but on trying to get her started something very stange is happening...

If I connect up the battery and turn on the ignition I get a hum from the fuel pump for a few secords (so far so good) but when cranking there's no power from the main fuel relay and it won't start.

If I disconnect the ECU (while the battery is connected, but with the ignition off) and then reconnect I get feed to the pump and it start up fine.

After a short while - anything from 10 seconds to a minitue or so - it will cut out (still got power to the fuel relay at this point). When I try to restart it I'm back to square one - it primes the pump but doesn't energise it while cranking and I have to disconnect/reconnect the ECU to get it running.

I've done all the checks listed in the Haynes manual and everything checks out (feed to the ECU, sensor reading etc.). Also checked the ignition and getting a good clean trigger from the ignition module and a nice fat spark.

Any ideas because I'm stumped!!

you could try a replacement ecu?
have you checked the feed to the pump when you are cranking it?

Sorry,but I find your post confusing - can you confirm that the engine ECU is still providing an earth path to term 85 of the fuel pump relay,when the engine dies out.If it is,is there still battery voltage on the white/purple wire that goes to the inertia switch then on to the pump.Connecting a 12v lead light to the connector at the pump is a good idea as it tells you what is getting to the pump.Please give more info of actual checks you have done - it can help to write down a flow diagram and mark on it what needs to be done and what you have actually done.
Good luck.

Sorry for the confusion - here goes with what I've checked so far using the Haynes manual as a guide (with the engine stopped and ECU plug in/out and ignition on/off... whatever the manual stated for each test).

Checked all fuses, cleaned up the earths in the engine bay.
12v to and from the the main relay is OK.
12v to the fuel pump relay is OK, fuel cut off switch is OK.
Resistance for both injector banks is fine.
Air flow meter, throttle pot. air valve stepper motor all give correct resistance/voltage readings (air valve was mucky so I've now cleaned that out with a little carb cleaner).
Resistance for the autobox inhibitor switch is fine, as is the tune resistor (no cats).

Spark from coil is bright blue and jumps about 1 inch to earth - getting a good spark at the plugs too.

Multimeter shows 12v from the fuel pump relay (for a few seconds) when the ignition is first turned on (and you can hear the pump) but after that... nothing (even when cranking). It's as if the ECU isn't recognising that the engine is turning or just doesn't want to start the pump.

If I unplug the ECU and then reconnect it I get the usual 12v from the fuel pump relay when the ignition is switched on, but this time I also get 12v when I crank the engine. Engine starts and runs immediately - perfect idle, revs up fine. After a while (sometimes 10 seconds, sometimes a few minutes) it just dies. Still got 12v from the pump relay for a couple of seconds after it dies, then the relay clicks off.

What's really puzzling me is that unplugging the ECU provides an instant 'fix' and I can start it again (for a short while). But if I disconnect the battery it doesn't work - I still need to unplug & reconnect the ECU to get it running.

Guessing it may be a bad ECU but don't want to spend a forture on a new one only to find it's something else. Can't give you any error codes as mine doesn't have a readout - just the diagnostic plug.


Hope that makes it clearer - let me know if you need any other info (or if there's anything else I should be checking).