3.9 V8 suddenly running off a cylinder.

sprintmaker · Oct 27, 2010

My 3.9 was running perfectly on friday and now after not using it for a few days has symptoms of being slightly off a cylinder.

Did the usual checking the plug leads ( new about 2k miles ago) and could see no tracking or sparks. Removed each lead individually to try to isolate which plug or lead but no joy.

Has the slight miss through complete rev range.

Checked plugs on RH bank today at lunchtime and they are all VERY black and sooty suggesting mixture too rich.

Any advice on where to check next.
Coil, leads and cap all replaced less than 2k ago and all seem ok.

Missfires from cold or warm, no coolant loss, pressurisation and oil is clean revs through complete range but with missfire as if a plug lead was not making a good contact.

Has not been overheated but has only been driven a few yards lately as on a sorn for the winter.

Coud it be the ignition amplifier? or what controls the mixture?

Not familiar with V8s as used to TDis any help gratefully accepted.

Mark

classic kev · Oct 28, 2010

Misfires are quite hard to diagnose but I suspect you might want to do the easy stuff first.

Check for condensation inside the dizzy cap. Clean inside the cap and the rotor arm see if that helps. A misfire may not be caused by the ignition system; you might have an injection misfire in which case it's going to take some time.

Do a search there is plenty to help you fix the problem. BTW are the battery and alternator in good shape?

eightinavee · Oct 28, 2010

You have made the classic mistake of starting it up,driving a few yards then turn it off before it got up to temp.It will be a fouled plug,which may not recover even with cleaning.Best to take out all the plugs,leave it overnight then crank it with the throttle wide open and the fuel pump relay removed.Do this for 20-30 secs then put the plugs and relay back in and try it.If it fires up on 8 keep it going for 15-20 mins to get it right up to temp.They really dont like being stated from cold then switched off after a short time.

sprintmaker · Oct 28, 2010

The dizzy cap is new and dry but am going to change it for a Lucas one anyway.

The metal contact at the plug end of one of the leads stayed on the plug when removing the lead so will get a set of Lucas leads as well. People recommend magnacore leads but I only drive it in summer months and only cover no more than 2k per year.
(NO RUST on bodywork at all and want to keep it in good order) so they would not be cost effective.

It has not been driven in about 4 weeks but has been started now and then so hopefully it is a fouled plug as you say Eightinavee.

Which plugs are best? NGK? There are new Denso's in it at the moment but I will change them.

Would the plugs soot up quickly if only being started and warmed up now and then?
I would expect some but these were black as your boot with dry soot!!

Battery is new and alternator is fine.

Thanks

eightinavee · Oct 28, 2010

NGK BPR6ES is all I ever use.You would be better not starting it at all than just running it for a few mins now and again - it needs to go for a blast,get it right up to temp.They do run very rich at start up,so black plugs are no suprise.

sprintmaker · Oct 28, 2010

Thanks again eightinavee,

Only had this V8 for 18 months but it did not do this last year. The security man at my work washed it for me and he may have left it ticking over when he did it as I store it in the work car park which is internal over the winter. 24 hour security and out of the elements for free.

Will go and get new plugs and see how it goes.

fett · Oct 28, 2010

sounds like a dream car store!

Go down there on a weekend, swap your leccy bits and give it a good 45 mins drive rounds and round yer car park when its empty, a few good blasts will help!

thats if you dont have it on the road at the moment

sprintmaker · Oct 28, 2010

It's only a small car park but there are 3 floors and the top floor is empty so I have it all to myself. Disco is on a SORN until March then I will bring it out again.

Go up to it at lunchtime and do odd jobs like curing sunroof leaks, fixing ABS fault etc.
(Did that last year)
Going to waxoil it this winter and restore the wheels back to ES spec. Doesn't really need anything else done!!

fett · Oct 28, 2010

apart form a good run, so do it!

sprintmaker · Nov 11, 2010

Eightinavee, you were absolutely correct, changed the plugs and the lead that the end came off and hey presto running perfectly.

Thanks a million old chap

3.9 V8 suddenly running off a cylinder.

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