Mis-fire

Hi,

I have a strange mis-fire with my 99 P38 4.6 only on cylinder 3and 4, I have changed plugs, leads, coil pack, swapped the injectors around to see if that moved the fault, done a compression test for the hell of it! tried most things.
My garage seem to be sure its a porous block issue but it doesn't lose water so that cant be it can it?
Test book has logged rich and lean faults in the two cylinders which is odd, rough running on all cylinders but thats intermittent, and to top it all it only seems to do it when under load, i.e accelerating from a junction or even climbing a slight gradient on the motorway at 70+!
Its got me baffled anyone got any ideas?

run it till its hot and then unplug maf and give it a run.

sounds like classic plug leads breking under load, but hou have changed them. Swap lambdas over see what happens to the readings. Mebbe lazy lambda?

Check crank sensor for damage and the rinb for the tabs see if they are all stright.

Also clean cam sensor of any swarf,

J the Range, Thanks, but the model with the Banana manifolds won't run without the MAF plugged in I've tried that.

Gav Briggs, I will try those mate and reply over the weekend, but what do you mean by the rinb?

Its been great this week gloating about having a big 4x4 in the snow, when all I usually get all year is jibes about how useless it is, 'an expensive toy' since I don't take it off road!! Fools they don't know what they're missing eh!!

Cheers all.

mine did 2000 4.0 thor engine, you have to start it with maf pluged in run till hot then unplug, if poss get some body to keep it running while you unplug.

J the Range, Well I am nothing but persistent, but I tried to run her hot without the MAF but sorry she cuts out, wife with foot on throttle as well!
Had a look under crank/cam sensors on Rave and it has come up with a number of faults that look promising, My garage are going to reset the ECU to factory settings and see if tha sorts the issue and then hopefully the sensors will register the faults, if not its a new cam sensor and so on..............

Bosch Motronic ECU systems should run without the ECU plugged in as it is a common test for the MAF as used by the VW Audi boys when they suspect a dodgy MAF which also uses the Motronic systme of the same era as the Thor engined P38.

by rinb i meant ring, the ring with the tabs on which is riveted to the fly wheel!

I have a feeling that this is something simple. You seem to have covered the basics, but its time for some more detailed information on what is going on. Some good advice I recieved when I had a similar miss-fire (thanks Eight) was to put a couple of extension wires onto the O2 sensors so you can monitor them under miss-fire conditions. This is easy enough, connect a wire to the black wire from each sensor and run it up into the car. Its nice to have an ocilloscope to watch the wave form but if you have a multimeter this will do. Expect to see them cycling from 0v to .8 every second or so under normal idling or steady driving, go upto .8v under heavy load (rich) and down to 0v on overrun. This should give you an idea of whats going on. If you have a laptop consider buying one of the cheap USB OBD interfaces so you can capture the data and view at your leisure. This should also give you the ability to read the output from the MAF so you can rule that out. I have somewhere a set of readings from a known good one at certain rev ranges so you can compare, I will dig them out and post them up.
Dont overlook connections. Check them all, not only for security but corrosion. As mentioned by Gav, check the condition of the crank position stator ring, again its nice if you can put a scope on the sensor to check the pulses. Lastly, dont forget the ECU. I went through all the above trying to diagnose a miss-fire under light load, only to find the culprit was corrosion in the ECU plug in the area of the crank sensor input. Might be worth starting there. I spent a good hour with an emery board on the pins and used an oxy/acetylene nozzle cleaner on the socket. Fixed.

Found them - thanks again to Eight in a V...

Idle 680rpm - 22Kg
1200rpm - 32.5Kg
2000rpm - 55Kg
3000rpm - 92Kg.

Can you tell its a slow day at work, this has got me thinking.

You are getting rich and lean faults on that one bank. My theory is that when you are getting the missfire the O2 sensor is picking up the extra fuel, logging a rich fault and then trimming the fuel to try and fix it. This leads to lean running once the miss-fire clears, leading to a lean fault being logged... just some food for thought.

You are not getting the same fault on both banks so it must be something unique to that bank, injectors, plugs leads, cylinders? Did you use good quality leads ? correct plug gap ? Have you checked for traking on the leads/plugs and is everything spotless on the leads ?

Thanks to all for the replies, I have had a little progress. I spoke to a gent at P38spares.co.uk (he had a cheaper cam sensor) after a long look at the Rave manual post Gav briggs reply. He advised I get the ECU re booted to factory settings BEFORE replacing any sensors as the ECU could be stuck in a loop, much like the home computer freezing, well that's a cheap option so off I went yesterday and had it done. Rough as a badgers arse for the first few miles (to be expected apparently) then almost fine, no misfires until 3250rpm (via rev counter), really smooth gear changes etc great. Obviously I have a misfire but not until high revs which narrows the fault down I hope! Now I will get the computer plugged in again and see what the fault codes are and see where that takes me, so please wait out. Many thanks for all responses as Gav's so far has already saved me a few quid!

Thanks to Land Ranger services for your support!

Update 2
Well the RR has had its computer read today, lean and rich faults were logged along with mis fires on all cylinders. It ran upto 3500rpm (warmer today!) before the mis fire so as you have all been saying it must be the MAF! P38spares was right when he said the ECU was getting confused with the amount of faults and needed resetting to sort it out, now it runs much better and has given some helpful info out. New MAF ordered and will be fitted on Wed night, lets hope it is right eh!!
In advance Thanks to you all for your help and advice!! (lets hope its not premature!)