Just rebuilt (a donor) engine and installed it into my 1994 late (single belt) fuel injected 3.9 V8 Discovery One. Ran nicely, but the fuel rail/rear injectors (even with new o-rings) leaked fuel, so took off inlet system again and installed the original engine fuel rail with injectors. Leak stopped, started up straight away so drove to top of drive to bleed coolant system. Noticed a rough idle and rich exhaust gases, but thought little of it. Drove it a bit (15 mins) and seemed pretty gutless. Got it back in the garage, and was poking under the bonnet and realised I had left the MAF plug off. Ah. found reason for rough idle and richness? Well maybe, but when I pushed MAF plug on, the car stopped. Tried to start it, but revs to about 1500, then drops down significantly and stalls. Does this repeatedly. Tried revving to hold it but the same happens. Switched air flow meter from original engine and plugged it in, but the exact same thing happens (really high revs then stalls). Leave MAF plug off, and it idles OK as before, but lumpy and rich as expected. I have the following questions:

a) Can the ECU/ECM be `reset; or recalibrated to get it to read the MAF properly? It seems that the ignition system has compensated for no MAF readings for that short drive with the plug left off, but now is confused when the MAF plug is installed.
b) I have read that sometimes the wires on the plug can break. It seems very unlikely as it was just unplugged and replugged gently, but should I test each of the wires? I read somewhere 174 ohms is the factory setting (but think that may be a Disco 2?).
c) Do I need to just rev it, keep it going and drive it around for a bit for the ECM to reset/recalibrate?
d) Do I disconnect the battery for a while, and tap the leads together, or is that an old tale that is untrue, as per some of the threads? (I have also tried this anyway)

I read a lot of threads about people changing MAFs with no positive outcome, and also changing ECM/ECUs with also no solution being found.

Given that the only change was to switch the fuel rail over, and then reinstall the ramset and plenum (carefully sealing them), and that it runs OK without the MAF plug it, I am thinking it is something to do with the relationship between the MAF and the ECM?ECU

Appreciate anyone's thoughts on this

Thanks,

Mark.

 

Hi All,

Not sure of the policy about answering your own question, but I guess the good news is I found the solution.

Could hear an airleak, and thought it was the bottom o-rings on injector no.3. Bought new o-rings and reinstalled. Still air noise. Then realised it was coming out the bottom of the plenum chamber pre-warming plate bolts...as I had removed them! Doh!.

The previous owner bypassed the warmer as part of the LPG set-up, but left the plate on (I wonder why?...). In the process of rebuilding the engine and re-assembling, I thought I'd `clean up' the look and remove the plate. The bolt holes go right through the plenum chamber neck, causing a major issue with the air-fuel mixture, post MAF.

Cost me a day of stuffing around, but I now know how to take the fuel rail off (for the third time) and I'm getting much quicker...


Mark.

 

I am not aware of one against answering your own questions. Glad to hear that you got it sorted out, and nice of you to post with what worked for you. Could be that it might help someone else having the same issue