Missfire 960
 

Hi all. My 1993 960 380k miles has started to misfire. The engine will fire up and clearly very lumpy. I have checked the ignition side; both the two ignition amplifiers on the sides of the inlet unit are OK, checked by swapping them over; all 6 coil units are fine and produce lots of spark when removed; the wires going to the coils running along the top of the head, whilst not perfect but do not short anywhere; all 6 plugs are nearly new and checked by substitution and are OK. I can't see or detect any inlet or vacuum hose leaks...the latter hoses are recently new. Just done a cylinder pressure test and I get 150 psi on no. 1,3,4,6 cylinders, 0 psi on no.2 and about 30psi and no.5.
Unless any of you can suggest anything else to check, I think I have to take the head off and see whats up....I suspect either valves or something with the hydraulic tappets.
I use only fully synthetic oil (for at least 10 years)
Any views please?
Thanks.

Many of the cases i have seen with this symptom have been the gasket broken between inlet manifold and head . If you spray some WD 40 or similar around that area of each port see if the rpm and engine tone changes which would prove the point ... That does not account for the poor compressions , It seems odd that 2 cylinder not next to each other have a similar issue .. Have you not used the car for a long time ? In which case let it run and warm up to see the the hydraulic cam followers sort themselves out ..

Thanks for that Clan, the car is in regular use every day and have just driven back from Malta on what I thought was 5 cylinders but after the compression test, may be only 4!
With the poor or zero cylinder pressures I suspect the head has to come off. Fortunately I have a spare reconditioned head!
But I appreciate your comments.

I suspect you're right about the hydraulic tappets Garth but before decapitating the engine, try adding 0.5L of Carlube ATF-U to the engine oil and drive as normal, at least until the next oil/filter change. At that point, substitute 1L of ATF-U for engine oil then top up with your normal engine oil.

You should start to notice some sort of difference after a few hundred miles though, improved economy, smoother running, lack of clatter on start up etc.

You may still have a failed hydraulic tappet or a burned valve but IMHO worth trying the simple fix first. :thumbs_up:

Fuel injectors may be loose or seals gone on the effected cylinders, check these first as you will lose compression if they are.

Correct me if i'm wrong here, but i didn't think the white block 3.0 was direct injection?

On a diesel defective injector seals will cause loss of compression but on a petrol (except GDi) a defective injector seal will either cause a nice big fuel leak from the fuel rail or allow air into the inlet manifold, causing lean running on the cylinder it serves.

Yes Dave, you are correct, the 6304 is not direct injection but injects into the inlet track and then into the cylinder via the valves.
On my engine I suspect the No.2 and No.5 exhaust valves to be the problem as there is no indication of any feedback into the inlet track or into the radiator.
Thanks.

Give the ATF-U in the engine oil a try then Garth, if nothing else it will help free up the hydraulic tappets and clean the internals and revive the oil seals a bit. It may even cure the problem in which case it will save you an expensive strip-down, HG set etc.

Hi Dave, thanks for your guidance and I have regularly read your advice on the use of ATF; I don't have Carlube but regular Comma ATF although its not fully synthetic. I'll see how it goes and run it for a week or so. There is nothing to loose and should it save the head strip down, that's a bonus. When I first got the car in about 2001 and 170k on the clock it used to regularly have trouble with the hydraulic valve sticking in the cold and that's why I changed to fully synth lube oil which cured the problem. I'm not convinced though that the various brands of fully synth lube available all work as well in this respect.
Thanks
Garth.

While mineral ATF will work Garth, it takes a lot longer. Whichever one you use (you could just go and buy a litre of ATF-U), once it's in there, leave it to do its work for quite a while. On my Jeep that had noisy tappets, (and usually only ran on 4 out of 6 pots), it took about 1000 miles to free them up properly.
It's possible that because you're using synthetic oil in the engine that the mineral ATF will work a bit quicker. However i've not tried that so i can't be sure.

Good luck with it and hopefully it saves you a stripdown job. :thumbs_up: