mis-fire

hi all ive been using your forum for a while now grabbing help and hints and ive normlly find all the info i need.

how ever ive had a mis fire problem for a while now i originally noticed it with a slight dither when accelerating but now it doing it idling too you can here it in the exhaust too.

ive given it a full service inc plugs and leads, however when i changed the leads i checked for spark transfer and noticed and that one of them wasnt sparking so i changed the coil pack the mis fire is still there but the lead and plugs are now sparking but there aint no difference in engine noise when i remove the lead and replace it. so im beginning to think it a fuel problem but ive never had to go that deep into i was wondering if anyone has ideas or had a similar problem its a p38 4.0 v8 2001

there aint no fault codes or anything coming up


thanks in advance for any help

I've had exactly the same problem with mine, especially under load car almost stalls when you hit the pedal hard and it reaches 3000 rpm, have done leads, plugs & coil pack and no different, There doesn't seem to be any clear advice or solution apart from get it on a diagnostic with live data. Hopefully i'll find someone local to have a look at mine tomorrow and get it sorted once and for all. one other suspect mentioned on here is MAF meter but the DIY test solution is a bit confusing.

yeah id seen a few post about that but wasnt exactly sure.ill take mine to land rover tomorrow and see what they say. need to get it sorted mot up this week and i know it will fail on emissions

Do not worry about the misfire, sometimes a vehicle will pass the emissions test. If it's a definite heavy miss and you can put your hand nearly over the exhaust and feel it drawn back to the pipe then this will lead to a fail. With todays management systems a vaccum leak or manifold leak can cause misfires. You need to find a diagnostic fault chart and record when exactly the miss happens. Is it there when the engine is cold ? No then it could be a mixture problem. Do you have a O2 sensor on one bank playing up. If your test states its down to a O2 sensor then fit both from the same supplier. I had a V6 Audi and the O2 sensor on bank 2 was showing a fault code, so we changed just one and that lead to a problem with a mismatch with the other one. Ask the guys doing the test for a quote before carrying out any further work and a test report.
You are paying for the test and therefore need something more than a bill.
Leaks around the injectors can cause misses, I used a spray can near the manifold or a oil can with some 20/50 sae and put some around the injector hole. I noticed the oil was going into the manifold and replaced the seals on the injector, no more low end miss.

im gonna try and get it booked i tomorrow. ill update here once i have the report, im hoping its something simple

I would check knock sensors. One each bank.

well its been sorted the cat was blocked and the leads i had purchased were cheap and useless. had some new magnacore leads fitted and fuel values reset or something like that and a few taps on the cat and everything fines sailed through mot without any advisories happy days

Good job. I seriously misread the thread anyway, I thought you were complaining about pinking!

I have come to the conclusion that these so called "performance" ht leads at between £25 & £30 are useless i have had the same problems as you, checked my spark plugs earlier (new NGK's fitted last week) on refitting a couple of leads i did not get the positive click you usually get, on closer inspection i found that the metal contact inside the rubber boots were halfway round the so there is no way they could be making decent contact on the plug, out for Magnecors tomoz!