4.6 in Range Rover Classic - starting issue/ ecu chip upgrade - help please!

Hi All
I have a 94 RRC which was fitted some years ago with a JE Developments 4.6V8. It is still running the stock 3.9L 14CUX EFI system, although the ECU has been changed to a stock 4.2L version, presumably for improved fuelling characteristics. Stock exhaust with twin cats. Also runs on LPG (older type single mixer) It starts fine from stone cold every time, even if left for many weeks. The issue is after the engine has been running it then takes an awful lot of cranking to get it to fire up again on re-start. Leave it overnight and it starts on the button again. It doesn't seem to be solely temperature related as it can happen even if the original run was only fairly brief and certainly not enough to get it up to full operating temperature. If you just fill up with petrol and re-start it's fine, but if you park it up and do some shopping then it's really reluctant to re-start. The fuel temperature sensor has been replaced twice (genuine LR), as has the fuel pressure regulator. It has also had a brand new distributor and runs on Magnecor leads. I am getting to the stage where I don't know what else to change to cure this problem. The problem persists whether it's winter or summer, or whether it has been running on petrol or LPG. It has never actually not started but clearly it's not right that it only starts after an awful lot of cranking. I was wondering if a chip upgrade in the ECU might help, but this is pretty expensive if it doesn't do the trick and I don't want to just throw money at the problem. Any suggestions very welcome from all you guys that know this system far better than I do. Thanks in anticipation. Mike

Ignition amp or coil

Is your idle smooth?
Did you refit amp on to new distributor with heat sink grease smeared on?

It sounds very like a "classic" coil fault. Would be an easy fix too. Any chance you could borrow one for a short test?

As suggested, start with the easy stuff first, check the ignition system, have a good look at rotor arm and distributor cap then the plugs. If the ignition system is good then I would consider if the head gaskets are starting to go, check to see if the top hose and/or expansion tank are pressurising after a run and maybe do a compression test.

Another possibility is a failing O2 sensor(s). The 14CUX ecu will not apply lamda correction until the engine has warmed up and if the sensors are not working properly the ecu may be altering the fuelling based on faulty data.

Thanks all for your input. The replacement (new) distributor was fitted by a specialist when the car was with the PO. As such, I don't know for sure whether the ignition amp was changed or correctly fitted at that time but I will check. Parts are not expensive so a replacement won't hurt I guess and it's not a hard job by the looks of it.
I have already swapped out the coil with no improvement - sorry, should have said that!
I have also replaced all plugs, and the dizzy cap and rotor arm (genuine LR ones)
The O2 sensors have not been changed, so maybe worth a try. Can these be tested as they are pretty pricey just to change on spec?
The only other thing that hasn't been changed is the coolant temperature sensor (ETC8496) - might this cause these issues?
I have also changed the stepper motor and the fuel pressure regulator, so running out of options rather.
No sign of head gasket failure I am pleased to say.
The really annoying part is that once started it runs absolutely fine and as I say it always starts perfectly if left overnight.
Idle is a bit lumpy on LPG but is certainly no worse than any of my other RRCs have been when on petrol.
Thanks again for any more ideas?
Mike

O2 sensors can be tested, ideally you need an oscilloscope for this but a code reader should show how they are switching, that said if the sensors are properly faulty the efi warning light would (should) come on.

The coolant temp sensor on the other hand can be checked, it is just a thermistor. When cold the resistance is high, couple of megohms IIRC, and hot about 150 ohms so what you can do is measure the resistance with a multimeter and see what values you get or another method is warm the engine up then unplug the sensor and bridge the plug with a paperclip, this will bring the resistance right down so the ecu will think the engine is really hot. A bit crude but might help eliminate the sensor. HTH

Last time I had an amp fail it was when hot and had to be cooled a while before it would restart; progressively got worse. Passed mot, had to be pushed off rollers. Pushed it off petrol pumps too, took about half hour to fire again.

classic kev said:

bridge the plug with a paperclip,

Click to expand...

I'll put that in the memorybanks