Thread: Slight Miss 3.5 V8 Disco

Some years ago when I thought Top Gun leads were tops I had a similar problem, used to drive me nuts. I developed all the ignition mods you mentioned and found incremental improvements. The item that finally fixed it all was dicing the Top Gun leads and going to Bosch. Never had the problem since. Magnecor leads also have been recommended on this forum but I have no personal experience of them.

Have you given the MAF a clean?

try cleaning the MAF particularly the the wire you can see through the small hole on the side when MAF is removed from vehicle, there is a spray you can use, (MAF cleaner).
Also ensure the terminals on the MAF have a good connection with the plug.

Two more thing to drive you crazy but this time from the over fueling department....particularly if your only getting 20 or more litres per 100Klm, still have original Air Flow Meter and injectors but fuel rail pressure fuel pump/filter all OK...

.....MAF's on the way out (see MAF air flow signal test in the forum Good Oil) often tend to overfuel at those sort of revs (but OK elsewhere) and injectors dribble ( don't atomise) and or flow increases on account of wear under the injector disc seat with style of Lucas injector in RV8's (and that no amount of cleaning will fix). Backing off the go peddle as you have done can often reveal these two issues.

I would not be using Champion plugs either. Had too many dodgy ones over the years.

OK Blokes Thanks for the quick replies and advice.

I have removed the MAF, bought some MAF cleaner and gone through the GOOD OIL posts. After cleaning the MAF and the terminals I refitted it and checked the voltages. It came up at .41 to .46 volts and the idle trim was at 1 volt. I reset the idle trim to 1.4 volts, took it for a longish drive and the miss appeared again after about 15 min of driving.
Seems as though I might have to invest in a new MAF.
What seems to be the concensus on the Ebay ones from the UK?
What other options do I have apart from a genuine one which will cost almost as much as my car is worth?

Mal

Originally Posted by malrv1

1991 3.5 V8 Disco Manual. 240,00KMs
I am chasing a slight miss at about 2000rpm on light throttle. It seems to occur if I back off and then put my foot down again lightly. And only once the car has fully warmed up.
I know there have been threads on this before and I have waded through them and done most of the things recomended.

New distributor
New Bosch BM 024 ignition module
New Bosch HEC 715 coil
New Champion RN12yc spark plugs
New Topgun leads.

Timing is set at 9 degrees btc.

Each time I do something it seems to improve a little but the miss/stumble is still there.

I have tried disconecting the TPS with no apparent change. It idles beautifully and free revs to as much as you are game to go.

I have ordered a new coolant temp sensor but it has not yet arrived.

Anybody have any more ideas as where to go next.

THX
Mal

2 things you should check before buying a new MAF, check that the Vacuum advance unit is working and also check that IAC valve body is tight on the back of the plenum chamber, if these are all right I would have the Amp module on the dissy checked and is working properly, let us all know what the solution is, when you find the problem, Regards Frank.

Yes done all those things. The ignition module is a Bosch one. I have had the IAC out and cleaned it. The dizzy and vacuum advance are brand new. I replaced the dizzy as it was almost a cheap as just buying the vacuum advance unit for the old one.

Originally Posted by bee utey

Some years ago when I thought Top Gun leads were tops I had a similar problem, used to drive me nuts. I developed all the ignition mods you mentioned and found incremental improvements. The item that finally fixed it all was dicing the Top Gun leads and going to Bosch. Never had the problem since. Magnecor leads also have been recommended on this forum but I have no personal experience of them.

I'd be checking lead impedance too. I bought a set of "high performance" leads from Performance Ignition Services in Nunawading and while they are superb on LPG, they create enough RF noise to interfere with ULP injection or MAF signals (not isolated which yet). I can get away with running the highest impedance old coil-to-distributor lead I have in the parts box, but if I go back to the new performance lead it misses and carries on badly.