Thread: Freaky 3.5EFI runs without ECU ???

Hoping you can help..

Whilst trying to fault-find a mates 88 rangie that refuses to start, we in-advertently started her up whilst the ECU was disconnected, and it runs and idles fine !!! It didn’t like throttle but it idled quite smoothly. We just let it idle away for at least 5mins and stop and started it a few times and it started fine and ran smooth ???

We had disconnected the ECU plug to check the water coolant sensor, the thermotimer, the extra air value and the injector resistances, all of which were fine. We also checked the fuel pressure via a guage in the cold start injector hose and that is also OK.

We thought it may have been a failed cold start injector so we isolated that but it made no difference.

Prior to this, we checked the compression which was excessive >> 200psi. So we took some plugs out and kicked her over only to see a torrent of fuel being thrown out. I also had a spare ECU which runs fine in my RR however that made no difference, although it did run OK for a few minutes before it seem to flood itself and die. From that point on it wouldn’t start again - back to square one.

Whilst we had the engine running with the ECU disconnected, we removed the leads from the even bank of cylinders (all 4) and it still ran as before. However if we removed the odd bank it wouldn’t start.

After about 30mins of test without the ECU, the engine no longer idled. It would start and run for about 1-2sec and then die. It did this many times - each time running briefly then dying. At this point it didn’t matter which bank of plug leads we removed - as long as one bank was connected it would run briefly. We then reconnected the ECU and it ran brilliantly - for about 5mins when it started flooding itself again. From that point the run would run and idle with the ECU once more.

We checked to see if the injector pins on the ECU plug were shorted, but they were all open circuit with the ignition on.

It seems like something is pulsing or holding the injectors open (ie earthing) independently from the ECU. Weird.

The oil level has also risen slightly and the oil is now thin and smells of fuel. I guess this is just fuel dribbling thru the ring gaps.

Any help greatly appreciated!

Well - It's always something simple...
When checking out the fuel regulator my mate discovered the vacuum hose was disconnected !!! So the regulator would have thought the throttle was wide open all the time, therefore increasing the fuel pressure to max. That together with worn/weeping injectors could explain the dribbling and running without ECU. The extra air from the disconnected hose would have allowed more air in behind the butterfly also. It seems to be running fine now. We are going to change the oil and take it for spin to be sure.

BigJon - if I may change the subject a little... Seems I'm following in your foot steps... I'm also preparing to replace my 3.5EFI with 3.9. I have obtained a second hand hotwire ECU, AFM, loom, injectors etc off a 3.9EFI, but I was actually thinking of keeping the 3.5 flapper EFI as I know it works. That, and it looks like it will take some time to work thru both wiring diagrams so I splice in the new system to the existing fuel pump ect. BigJon did you find it a big job? Would you do it again?

Thanks in adv,
Taz.