Land Rover Repair: fuel system, coolant temp, annoying problem

Question
Hi John, I have a 91 Classic 3.9L v8. I'm having problems with backfiring through my air intake.I was told i'm getting too much gas. I've troubleshooted "everything" from cat conv to stepper motor.It's done a little better, but it's still doing it. Now i'm told it's the fuel rail temp sensor for sure. What do you think? THANKS! P.S. it's also been tuned up.

Answer
Hi Frank,

I looked around and found a UK mechanic who had the same problem:  

<< I have a really annoying problem with a 3.9 EFI, i have a mis and backfire that i can't pinpoint. i have swapped all the electrical components with good items, have carried out all the Ohms/resistor tests which have showed no faults, all the ignition has been checked and double checked, the fuel system has been fully tested and showed no sign of any faults, infact everything seems ok.

The main problem is that it was not firing properly on no 7 & 8, ie remove the plug leads and no difference was noted, on inspection the cam seems ok, with full lift on the inlet and exhaust valves, but i noticed that there were 2 newer rockers on number 7? and different injectors on 7 & 8, they didn't have the normal green bands around them.

I moved the injectors on 7 & 8 to 1 & 2 and the fault moved! i then changed the injectors for the correct ones, refitted it all back together and i still have the problem, it seems to be from the right hand bank, i have timed up at 6 degrees BTDC and have full advance, i have changed the dizzy, coil, amplifier module, fuel pump, pressure regulator, stepper motor, coolant temp sensor, plugs, leads, cap, rotor,

I have done a compression test and all cylinders are between 140-150psi, The engine is spotless inside as it has been serviced regularly, the motor idles really well but when you hold the revs at a constant level between 1000 and 4000rpm the backfire is horrendous!

The CO meter is reading between 1.0 and 2.0, the butterfly is totally closed with no throttle, every pipe is clear of obstruction, the only thing that i have not checked is the heads/valves etc, and the only things that have not been changed are the 6 remaining injectors and the throttle potentiometer, so i am totally stumped at to what this could be. >>

Later, he wrote back and had this to say:

<<... I had a 4.2 kicking round the workshop, so i just swapped over the fuel rails complete with injectors and the fault has gone,, Yippee! >>

SO, I have either shed more light on the problem or I've flooded your eyes with more places to look.  Most 3.9 owners complained about the same problem (loss of power and misfiring) and it could be related to either poor carburation, burnt or sticking valve(s), injectors or the fuel rail.

I wish you the best of luck, Frank.

JohnMc

NB:  nice Rangie.