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Misfiring TD5 Disco - fault codes?


Blackwolf

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I have a 2002 TD5 Disco which has recently developed a misfire and loss of power, esepcially when accelerating. Not having access to Testbook/Nanocom etc, my initial diagnosis was a faulty wastegate modulator. Changing it made no difference. I have checked the engine harness where it passes round the front of the engine and a/c compressor, and it is all OK.

I then checked for the oil-in-the-ECU connector fault - streaming with oil. I have cleaned all this out, replaced the injector harness with a new, fully sealed one, flushed out and dried the engine harness connectors, cleaned the ECU connector, checked inside the ECU gfor oil ingress (there was insignificant ingress). Monitoring over several days, there is no now visible trace of oil emerging, but the misfire is still there.

Yesterday, under moderate acceleration, the MIL (engine fault lamp) came on, and the car went into "idle only" mode. It reset OK after stop/restart, but seemed very rough. I drove to a local independent dealer who has T4 and asked him to read the faults off the ECU. He told me that there were three faults logged:-

1) "Driver demand 2 out of range"

2) "Faults detected with driver demand"

3) "Topside switch failed pre-injection"

Faults 1 and 2 I presume are telling me that the throttle position sensor is malfuncioning and should be replaced, but I have no idea what fault 3 is telling me!

If someone could confirm that 1 and 2 are TPS, and give me a clue about 3, I would be very grateful. I am hoping that 3 is the result of 1 and 2 and is nothing to worry about, since the only info I have seen suggests that it may be an internal fault with the ECU.

On a different subject, does anyone know anywhere that actually has a Nanocom in stock for sale at the moment? I have had one on order (from Bearmach) for over a month, and was told yesterday that due to supply problems it is unlikely that any new stock will come in before 15th May!

Thanks,

Blackwolf.

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Correct with 1 & 2 - "driver demand sensor" is engineer-speak for throttle pedal

Fault 3 means nothing logical (there is no such thing as a "topside switch"!) but it usually means that the ECU is knackered I am afraid... 95%+ likelihood this will be the problem.

My next step would be to fit a known good ECU and see if that cures it - I would have done what you have already done first.

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Check turbo wastegate has not seized if it has engine overboosts and goes into limp home mode.

Few drips of oil down shaft and lots of working back and forth worked on mine and several others i have read about.

Not sure why the oil works even though it must burn of but work it does.

Lynall

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Topside failure is the 2 big capasitors inside the ecu that fire the injectors,this is caused when the oil in the loom increses resistance and shorts them out.

That's interesting - I didn't know that. You learn something new every day :)

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  • 2 weeks later...

I have had exactly the same symptoms with my Disco 2. I replaced the injector loom as recommended by almost everyone, & cleaned out all plugs. I didn't expect the problem to clear immediately, as I knew there would still be some oil in the engine loom, but the problem got worse, even when the red ecu plug was spotless.

Reading engine ecu fault codes, the common factor especially when the MIL lamp was on, was various driver demand faults from driver demand2 fault, demand supply faults, demand inconsistency - all sorts!. These faults all returned, at different times, after driving only a short distance.

Playing it by the book, I changed the throttle pedal assy. No difference whatsoever. I then checked both throttle on an oscilloscope, across both pot. tracks with no errors at all.

By now I was stuck, until I spotted a diesel leak at the back of the cylinder head. On inspection, the leak was from the fuel pressure regulator. Luckily, a friend had a spare, but while fitting it was worht checking the gauze fuel filter fitted behind this unit. It was totally blocked up with what looked like paper fibres, my guess from the internals of the spin-on fuel filter. I cleaned this out, and re-assembled everything. All faults were cleared from the ecu, and didn't return, adn it now runs fine, probably better than ever.

My thoughts are that as the td5 ecu does not monitor fuel flow, or fuel pressure, it cannot detect issues of fuel starvation, it just notices that the engine rpm is not where it expects it to be for the throttle position selected and faults out. Throttle pedal faults will illuminate the MIL lamp, and drop to idle only. Disco 2 auto's dont move very quickly at idle, very annoying .....

Next problem - After all the missing & engine faults, eventually the abs/tc/hill descent fault lamps came on, and will not extinguish. After reading the workshop manual, this can be brought on by throttle position faults (communicated to the SLABS ecu from the engine ECU). And interestingly, this fault is considered a "permanent" fault, and hence will not clear on an ignition cycle, needs clearing by diagnostics computer - my own primitive diagnostics will not communicate with the SLABS ecu so it looks like a trip to a dealer ...

Hope this is of some use to someone else, as I spent a lot of time and effort trying to diagnose this fault - you dont always have to believe the computer, it will only give faults that it is programmed to give !!

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