Disco 1 misfire,1995 3.9 V8

AFM came in the post yesterday not long after posting the above.
I removed the live battery cable from battery, (I’m told this clears any faults the ECU has stored) and fitted the AFM.
In fitted it to a warm engine, after starting back up it made no change whatsoever. After leaving it to cool for a few hours I went out (to pick up a take away, yummy!) and it started great and the drive was second to none, a HUGE difference.
Again, I left it a few hours to get cold and pooped (Edit: I meant popped!) out and fired it up, started and idled perfectly.

Hoping I get the same results today and this has fixed the ongoing problem.

Finger crossed then

discool said:

Finger crossed then

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Oh indeed! I’m that keen to try it I’m going shopping with the wife! Haha

That sounds encouraging. Did you test the old air flow meter to see if it was duff or just in need of adjustment? Testing is easy enough with a digital volt meter, you can check CO trim and air flow. Also, don't forget with a later catalyst engine like yours should be, once warm, the ecu ignores the air flow meter at idle and uses the O2 sensors to correct CO trim to maintain low emissions. You should check your CO trim value, IIRC should be 1.8v (which is rich) for a catalyst engine. You can check the O2 sensors with a volt meter but best is either a suitable ecu code reader or an oscilloscope. HTH

I did test the old one and voltages read as they should which was very odd, I followed info found on various forums for voltages, the aulr I think was one that was helpful.

After I’d cleaned the old one it made a little difference and was fine for a couple of days then it started poor idle from cold and cutting out again. When I cleaned it again it made it worse, that was when I replaced it. It’s been fine since, today’s trip was successful too, no issues whatsoever.

The O2 sensors I shall leave as they are at the moment but when/if they start causing an issue I’ll unplug them and change the tune resistor.

Ahh well, it was nice while it lasted!!

So, it cut out again this evening but with a difference..

This time I could ‘feel’ something wasn’t right, when coasting (its an auto, rolling along in ‘D’ with no revs) it wasn’t hanging on to the revs, revs would just drop to 1000rpm or slightly lower.
Every time I applied more throttle and looses the rpm would get lower and lower until I hit the brakes at a give way, it then threw the ‘check engine’ light, lost idle and stopped just as I was coming to a halt.

Turned ignition off and back on to start and it idles and drives fine again.

Any ideas folks? I’m running out of them now!

O.K. don't throw in the towel. This will be something stupid and probably easy to fix. The reason I say that is the engine itself will start and run suggesting everything is there - fuel, spark, bang.
The lack of idle I suspect is the clue and the check engine light is the key. Unfortunately, you're a bit far away so I can't pop over and plug in my ecu code reader or try my spare ecu. When the engine plays up is it only when it is hot? If so, O2 sensors are worth checking and also, maybe ignition amplifier.
If you search in the V8 engines section, try a searches for misfire and maybe stalling you should find enough information to check/test most of the efi and ignition system using a volt meter.

BTW nearly forgot to say, did you re-set the ecu after it popped the check engine light on? If you don't reset the ecu, it will run in limp mode which is very rich. This is O.K. if you drive everywhere with your foot to the floor, however, you'll probably maybe manage 6mpg and you'll kill your cats and O2 sensors.

classic kev said:

O.K. don't throw in the towel. This will be something stupid and probably easy to fix. The reason I say that is the engine itself will start and run suggesting everything is there - fuel, spark, bang.
The lack of idle I suspect is the clue and the check engine light is the key. Unfortunately, you're a bit far away so I can't pop over and plug in my ecu code reader or try my spare ecu. When the engine plays up is it only when it is hot? If so, O2 sensors are worth checking and also, maybe ignition amplifier.
If you search in the V8 engines section, try a searches for misfire and maybe stalling you should find enough information to check/test most of the efi and ignition system using a volt meter.

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Thanks for the reply.

I changed the ignition amplifier (Lucas) with no noticeable difference. Haven’t checked O2 sensors though, will try and find how to do that and have a look tomorrow.

Out of curiosity how far away are you? Would covering fuel costs and lunch at my local pub persuade you?! Haha.

Would be very handy to have it plugged in to give a definite diagnosis. My local garages don’t have the plug adapter to fit, or so they say. I have looked for fault readers but not really sure what I’m looking at and a lot seem like there no longer available.

Thinking about it I haven’t changed the coil condenser, couldn’t find one for a disco. I do have a new Lucas condesor for a 2.25 series engine though. Are they the same?
Could the condensor cause the idle/cut out issue?

V8demon said:

Thinking about it I haven’t changed the coil condenser, couldn’t find one for a disco. I do have a new Lucas condesor for a 2.25 series engine though. Are they the same?
Could the condensor cause the idle/cut out issue?

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Same or very similar, just pull the connector and leave it disconnect and see, it’s not required unless u have a cheap head unit that picks up interference.

Is the fuel and air filters on your list of items changed?.

discool said:

Same or very similar, just pull the connector and leave it disconnect and see, it’s not required unless u have a cheap head unit that picks up interference.

Is the fuel and air filters on your list of items changed?.

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Ohh right, I’ll be honest, I didn’t know what the condensor did!
New fuel filter and air filter isn’t that old but I have tried without it too.

classic kev said:

BTW nearly forgot to say, did you re-set the ecu after it popped the check engine light on? If you don't reset the ecu, it will run in limp mode which is very rich. This is O.K. if you drive everywhere with your foot to the floor, however, you'll probably maybe manage 6mpg and you'll kill your cats and O2 sensors.

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When I fitted the AFM I disconnected the battery for a few minutes and reconnected after the AFM was installed.

Would that reset the ecu or is there a certain procedure?

Yes that would reset the EFI ECU.

discool said:

Yes that would reset the EFI ECU.

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Just looking for the O2 sensor test procedure, can’t seem to find it though.
Looking in the V8 section....

Edit: Found the O2 test procedures

Hi folks, haven’t had chance to test O2 sensors yet but I have given the issue some serious thought and have some info that may help you to help me.

Not long after owning this disco I had a rattle which sounded like gearbox, it turned out to be the right side cat. Leads me to believe that O2 sensor could be dodgy, yes/no?

Also, now the temperature has dropped when driving the cutting out issue has got a lot less however, if I start the engine and leave it to warm up it will cut out two or three times.

Still has a poor idle, not terrible but still not right.

I’m hoping to get the sensors tested this weekend.

Update, was driving along today when all of a sudden it misfired badly for about 2 minutes, almost like I’d gone through a huge puddle and got the dizzy wet, instant smell of petrol at the same time.
I dropped it into ‘N’ (neutral) and it revved freely, no misfire, back into ‘D’ and the misfiring returned. I put my foot to the floor and it misfired all the way up to 4000rpm then cleared, backed off the accelerator and misfiring returned. Then just like flicking a switch it continued to drive as normal with no misfire.
Goes without saying there was a huge power loss when misfiring, I’m guessing but I’d say I was down on 3 to 4 cylinders.

Any obvious answers?

A few years ago when I first bought my Range Rover I used to get a similar problem. I had the ecu tested, I had the air flow meter tested, apparently both were fine! I looked at fuel pressure regulator, distributor, ignition timing and anything I could imagine. Every test and check I did showed no faults. This went on for nearly two years, nearly gave up a few times believe me. Then I asked somebody I knew who was a bit handy with Rover V8's. He came back with ecu faulty, not reading the air flow meter. I bought a second hand ecu and have never looked back (and that was 10+ years ago!).

My point here is that you probably need to get some sort of diagnostic done on it although you may need to check your efi loom has the diagnostic connector plug as I understand not all Discos have one. It might also be worthwhile getting a good used 14CUX ecu and swap out the ecu. You might also consider getting a code reader, I use an Ecumate, it is simple to use and really handy. Others use Rovergauge, it's a downloadable piece of software but you'll need to make a plug.

Lastly, what you describe could be related to O2 sensors and/or cats. If the O2 sensors are failing it is likely your mixture will go rich but you can't really see unless you have a reader that can show what the ecu is doing with fuel trim.

HTH

BTW, nearly forgot, interesting that at high revs no misfire. This points to lambda control problems because at higher engine speeds and definitely at wot the ecu ignores the O2 sensors.

Funnily enough My father mentioned today about finding another ECU and trying it, I may just do that.

I tried looking for an ecumate but from what I gather the guy who made them has no stopped or retired?

I’m determined to get to the bottom of this issue, it’s certainly taught me a lot that’s for sure!

I have power to the O2 sensors and checked both of them. The one on the right, which was the cat that broke up appears to be knackered! It was also black, almost sooty. I have ordered a replacement in the hopes that will cure the issue.