3.9 V8efi - Missfiring at random?

Hello all,

I am having a strange, sort of intermittent problem with my 1992 Range Rover 3.9efi.

It seems to run rough when just running around town, idling or generally slow driving, but when you put your foot down it appears to clear up and run smooth. This happens around 8 times out of 10.

If you accelerate quickly and then ease off (say to 30mph) it runs smooth when eased off.

It's pretty hard to explain, but say if the engine is idling, and you rev it in neutral, the first time you pick the revs up it missfires, then the second time it will be smooth as silk.

I was just wondering if anyone else has had similar issues and could point me in the right direction, I am planning on servicing it over the next week or so, hopefully its just something simple like spark plugs?

Also not sure if it is related, but I have an issue with it using coolant, although it doesn't appear to be coming out of the exhaust and the coolant system does not over pressurise, so I don't think the head gasket is the issue. I will perform a compression test too this weekend but on listening to the engine being turned over with the king lead off the distributor it sounds about even on all cylinders.

Thanks in advance,

Michael.

Idle control air valve / stepper motor? Don't know if they have them that far back, but it's the sort of thing I'd look for first.

Peter

Thank you listerdiesel,

I was looking in to the RAVE guide for the classic and noted that on start up my idle appears to sit too high before the stepper motor brings the idle down to roughly 1,000rpm, I have set the the idle using the alan key on the throttle body to around 550rpm at warm idle which I believe is about what it should be. The missfiring has settled a lot but it still spits on one of the cylinders, think it needs a new set of spark plugs!

As it's using coolant as well I think I'd get a 'sniff' (hydro-carbon) test done by a garage that has the kit, although the results are not infallible. Are there any noises, ie tapping, from the engine or is the missing more pronounced when the engine is running cold from start-up?

mikkey180 said:

Thank you listerdiesel,

I was looking in to the RAVE guide for the classic and noted that on start up my idle appears to sit too high before the stepper motor brings the idle down to roughly 1,000rpm, I have set the the idle using the alan key on the throttle body to around 550rpm at warm idle which I believe is about what it should be. The missfiring has settled a lot but it still spits on one of the cylinders, think it needs a new set of spark plugs!

Click to expand...

The only time u need to adjust the idle is when setting the 'base idle' the EFI ECU then looks after the rest, mine is still as set at the factory, so leave alone once set. Idle RPM should be a bit higher than u state 650 to 725 iirc.. The correct info is in the manual.

Is the crank case breather clean? Also stick some injection cleaner in your fuel.

mikkey180 said:

Hello all,

I am having a strange, sort of intermittent problem with my 1992 Range Rover 3.9efi.

It seems to run rough when just running around town, idling or generally slow driving, but when you put your foot down it appears to clear up and run smooth. This happens around 8 times out of 10.

If you accelerate quickly and then ease off (say to 30mph) it runs smooth when eased off.

It's pretty hard to explain, but say if the engine is idling, and you rev it in neutral, the first time you pick the revs up it missfires, then the second time it will be smooth as silk.

I was just wondering if anyone else has had similar issues and could point me in the right direction, I am planning on servicing it over the next week or so, hopefully its just something simple like spark plugs?

Click to expand...

For an odd sounding misfire I think I would want to be looking at the air flow meter. If it isn't the air flow meter then it could be the ecu. However, before doing too much, try and find out if it is an injection misfire or an ignition misfire. It could be nothing more than a bad plug lead, but equally, the ignition amplifier may be on it's way out or the coil could be dying.

Ok everyone, did a compression test today and I am a little confused

Readings where:

1 = 140psi
2 = 150psi
3 = 150psi
4 = 140psi
5 = 150psi
6 = 150psi
7 = 150psi
8 = 150psi

But, one one of the cylinders I found that the spark plug was covered in some odd grimy like substance which was pretty much bridging the spark plug gap. which obviously isn't good but this cylinder read 150psi on the compression test, I tested it twice and it was consistent.

Unfortunately testing could not continue as one of the small coolant hoses which run behind the distributer has split.

The coolant system doesn't appear to over pressurise but does smell of exhaust fumes, and there is clearly something other than fuel and air getting in to the combustion chamber, but there is no blue smoke and not really any steam from the exhaust?

Is this looking more towards a cracked block than a blown head gasket due to the compression readings? Personally I have never seen this before, normally compression is down on the offending cylinder!

Thanks,

Michael.

Just a suggestion - I had this same problem and for ages I also thought it was fuel related. I even ditched the Efi and went to carbs it got so frustrating. Classic Kev might be on the right lines a I eventually changed the Dizzy and that did the trick. On looking at the old Dizzy two things had happened, first the rotor had been pulled up at some point with the center spindle which had damaged the AR springs underneath the plate and second, the little box on the side of the Dizzy (the Amplifier) seems to have been damaged at some point. Anyway, I haven't had any problems since. I'd recommend SimonBBC

Another suggestion, have you tried opening the bonnet and starting it up in the dark? Wondering if it could be losing current to the plugs due to dodgy HT leads?

This sounds similar to my own experience. Misfire on start up after having been sat for a while. Turned out a water gallery had cracked and was leaking coolant into cylinder 6. When left over night it caused plug fouling (corrosion) which took a little time to clear on start up. The gallery crack eventually opened up enough to empty the coolant into the cylinder and overheat the engine.

I would start with an ignition service, HT leads, Rotor Arm, Distributor cap, Condensor, and spark plugs. Do you have a picture of the fouled plug, does it look like any below? If it's like 3, 5, 6, or 7 you have oil contamination in the cylinder, probably from worn valve stem seals. V8's often suffer this as the earlier V8 stem seals were not much more than a rubber washer.