Possible broken MAF

Hey all, just fitted the plenum back onto the v8 having had it off to clean it and rejig the throttle stop; it had fallen back in too far and the butterfly was jamming closed, and the thing is running like a heap of nonsense - I can barely get it to idle and it's smoking like a boss from overfueling. I've already drowned it once and annoyed the neighbours with odours of raw gasoline.

However, it runs fine without the MAF plugged in...fine is subjective; it's still not perfect but it runs reasonably well, if not slightly rich. I may be asking the obvious but is there a way to rule out the MAF being gubbed? I've swapped an awful lot on the old girl (sparks, leads, filters, oil, temp sensor) in a bid to solve a poor idle when warm issue; now it's running disastrously at any temperature.

Any advice would be most welcome.

when the maf is disconnected the ecu uses a default setting. i would suspect the maf is faulty but also is the butterfly set correct. it should be open a little when off throttle to allow air through. dont know the exact amount though.

First off try adjusting the MAF to see if there is any difference. Do a search to see how. IS it misfiring or just dodgy fuelling? I had to get the balance between the MAF and the BAse Idle

Thanks for the replies - the butterly is set correctly, I did this when the plenum was off so that's ok.

I suspect it's misfueling as there is helluva stink of petrol and clouds - literally - of the usual smoke when it's running. I actually flooded it trying to get it to run, it's pulling so much fuel in.

What I suspect is that the fuel/air mix screw on the maf may be broken, so I'm trying to source another MAF - any suggestions? I've tried the usual and i'm at a bit of a loss as to where to get one...I don't mind paying decent cash for it as if it fixes the issue, great and if it doesn't, then I can sell it on.

I'm based in Glasgow city ctr so if anyone out there has a flapper unit I could borrow to at least diagnose it, I would be greatly appreciative.

Thanks again for your help guys; I live half a mile from Taggarts JLR dealer and I AM NOT taking it anywhere near their £95 p/hour setup up there, so your help is invaluable.

sometimes the only way to be sure, is to substitute a suspect part out for a know working one. but from what you have said, MAF looks like the first port of call.
in the meantime though, you could do some checks, like make sure the electrical connector is good not damaged, an the pins are free of corrosion. also check that there are no oily deposits or dirt inside the MAF.

if you buy a spare, and its not that, and money is tight, you can always sell it on. or keep a spare for when you do need it.

Get yourself in to the V8 section and do a search. You can check/test your MAF with a multimeter. You'll need to check CO trim and air flow signal. You'll soon know if the MAF is dead as you won't be able to adjust CO trim and the air flow signal will be off IIRC.

If the MAF checks out O.K. then I'm afraid you could need a new ECU. Check MAF, substitution is the best way, then check ECU again by substitution with a known good one.

I would also suggest you check/set your ignition timing.

Thanks again for the replies with this - I have tried a donor MAF (an old XJ6 S3 unit - the correct Lucas Model, sadly with an incorrect diameter of exit chamber so it was only really useful for voltages) and i'm getting an identical reading using both MAFs - 1.6v. I'm assuming that turning the adjuster should change this? I'm pretty rubbish with electrics so some advice here would be super.

I have a different ECU to try also - hoping this will show some difference in results as up to now, I can't seem to get it to do anthing.

Yesterday, it started up (without the MAF) ran a bit grumbly for around 10 minutes then the revs dropped off, it stalled and it spat some coolant out from the rad, just above the left hand side fog light. I'm going to try out a different temp sensor this eve - I fear it may be something to do with that...

the temp senders are cheap and i redo them every so often for the sake of it.

At tickover it should be around 1-1.5v

Red/black = sensor ground
Blue/green = Air Flow Signal
Brown/Orange= Ignition Live
Blue/red = CO trim

With ignition on, but engine not running the Air Flow Signal should be about 0.2-0.7 Volts

from memory, but there you go.
Mine is a 3.9 Efi