The V8 Owners Forum -

Hi all, I am having problems with plug fouling on a RV8.
It seems only to be a problem if the car is not used for a couple of months, normally no problem at all.
Have tried NGK bp6es and they were fine in Sept when car last used, this is the 2nd set as the first were fouled up in March after leaving it for 3 months.
The fouling appears to be soot and if brushed with acetone the plugs will fire on my test rig (will not fire until cleaned), have never reused them in the car though.
3.9 WITH 14CUX system no cats and 35dm8 distributor (GM hei based ignition system), changed distributor and hei module last time as well as leads and coil (Bosch)before realizing the plugs were fouled.
I have fitted a fuel pressure gauge in the supply line and it shows 35 psi on switching on ign, it holds pressure for about a minute when ign is turned off so I assume the injectors are not leaking. Rovergauge shows no faults and a tester I made shows injectors are firing.
Has anyone else had this problem?
On the test rig (made using the old hei,coil & distributor) the hei will make a spark on a plug that has had the electrode cut off and will jump a 20mm gap between ht lead and earth so it is a good unit.
Sorry for the long post.

Ensure the ignition system is up to par.

And equally important, ensure tuning is correct. If it's too rich, too often, it will foul even with hotter plugs.

You could try 5's, but the plugs are probably not the problem.

Have you checked you have the full 12v getting to the ignition system? Easy way to check is to connect a DVM set to volts to battery positive and coil positive. With the engine running, this will measure any voltage drop in the circuit directly under the normal load on that circuit. Should be no more than 0.5v And do the same with the ignition ground.

Full 12v as it is supplied by a dedicated relay,have checked connections,the test rig won't give a spark till the residue is wire brushed off.
The problem is after the car has been left for a month or two,no problem at all if used regular,if the mixture was wrong I would have this problem all the time.
If the injectors were leaking it would not hold pressure after pump is turned off,the residue on plugs is dry so not oil and there is no smoke on start up.
I have a spare set of injectors so will try them and see how it goes,i'm wondering if the spray pattern is bad due to clogging/carbon build up.
Will let you know my findings as I progress.

I have a similar problem in another vehicle - in my case the carb is running a bit rich and over a couple of days carbon builds up on the spark plugs with a commensurate reduction in performance until the engine will not run - there is no spark at the spark plugs so mimicking an ignition problem which I though the issue was at first. I replaced the entire ignition system but the problem persisted so looked at mixture.

Cleaning the plugs and the engine fires straight away and runs fine but again slowly deteriorates until the engine stops again.

In my case I rebuilt the carb and all I can assume was the accelerator pump was leaking fuel into the carb making it run rich.

So in your case all I can assume something similar in your engine but in your injection - either too much fuel per squirt or fuel is dribbling in from other injectors (I know you checked for this). However as has been suggested and I did in my engine, you need to ensure your ignition system is all good, no low 12v etc then look at the mixture issue.

Oh I also went one heat range higher to help burn off the carbon and went to long tip plugs to get the ignition point lower down in the combustion chamber - didn't fix the problem but did help.

Garry

Only thing I can suggest is to remove battery power from the CUX for 30 seconds and reconnect before starting, it resets the ECU and may help the problem.i know mine occasionally goes sloppy rich and this solves the problem every time. But if anything was wrong it should show on your diagnostic tool.

Still worth a try in my opinion

Ian

I have kiiled plugs with Mini engines. I favour NGK plugs because they are easily available almost everywhere. In most cases the engine has been in tuning stage and ran idling for some time. A goos example: I used to have a Cooper S wiht 970 engine and 8-port head with Amals. The head has small plug threads, C something in NGK plug designation. Anyway, it had 8´sin it and the engine idled nice while I was setting up the carbs. Next time I wanted to start it up: nothing. I replaced the whole ignition side but nothing. Since the plugs were brand new they could not be the problem. Well, since nothing helped I went and bought a new set of plugs, this time 7`s and it started instantly.

So, rich mixture and cold engine may kill the plugs. In my opinion even if you clean a wet plug it may not work again. I may actually have this issue with my RV8. I ran the engine after rebuild for a short time and next time with did not want to start and when it did it ran on 4 cylinders. I have now replaced the plugs but did not have time to check if it helped. I´ll report back...

Yep, runs on all cylinders now. Funny though, usually NGK 7´s are pretty tolerant but obviously they do not like rich mixtures either.

I had exactly the same problem with my 3.9 Disco. When laid up over winter starting it every few weeks for a few minutes just to let the motor run and get the oil pumping, it would sometimes foul a plug, usually noticed the next time I started it. I was using NGK BPR5ES plugs. Changed them to Iridiums but they were worse. Dry soot on the electrodes seemed to be the issue.

The AFM, TPS and coolant sensors are all fine and within the correct resistance parameters.

I have upgraded the dissy to new high output Bosch Scorcher (same as most historic racers use here) and MSD Blaster2 coil. I think the problem though is the 3.9 can run very rich on cold occasional start ups that only let the engine run for a short time, not long enough to get the engine properly warm enough to burn the deposits off the plugs. It doesn't seem to be a problem if I take the vehicle for a drive and it reaches operating temperature.

Will give the disconnect the battery for 30 seconds trick a go to see if it makes a difference.

I had a BMW which was notorious for fouling the plugs if you stopped the engine just after a cold start. Like moving the car out of the garage then stopping the engine while you locked up the garage. It had COP, so assume a pretty good spark. If this happened, new plugs were the simplest way to get it going again - cranking even at full throttle didn't get it going. Later versions of the same model seemed to have this sorted.

Try taking the MAF off and spraying the thermocouples with acetone or some other contact cleaner, you could also give the coolant temperature sensor a once over