Running on 3 cylinders, help!

[walkinginperu]

OK so my scirocco is now running on 3 cylinders. Number 3 won't fire at all at idle, but seems to be running on all four when it's revved or while driving. I've changed plugs, leads, dizzy cap, rotor arm, and turned the engine over with spark plug from number 3 lying on side of head - definitely sparked. But if i remove the ht lead from plug while it's idling it makes no difference to engine speed and therefore no. 3 isn't firing. I'm confused. If 1,2 and 4 are firing they must be getting fuel,so i presume 3 must also be getting some as I have a single weber carb. There seems to be spark on no. 3 too, so i'm very confused. If anyone has had something similar and found what was wrong, or has any ideas i'd be very grateful!
Garage had it last week and said compression test showed 120psi on each cylinder if that helps.

[scalainj]

I had something very similar years ago - after many weeks of trying everything an old friend said - change the oil pump

I did and it cured it

Andy

[MikeH]

If No 3 has a spark, but doesn't appear to be firing in any worthwhile way, then you've either got no fuel, or no compression. I can't see how you're not getting fuel to No.3 either, and the garage did a compression test. It's possible that they got it wrong, but I'd hope not.



If you take No.3 plug after running the engine, is it wet with fuel?
If you swap the plugs around (I know you just replaced them, but it's worth a try) does the fault move?
Is there maybe a buildup of gunk on the thread that's stopping the plug from earthing to the head (seems highly unlikely to me)
If you turn the engine over on the starter, with No.3 plug removed, can you see a cloud of fuel vapour being pushed out of the plug hole?

EDIT: 120 PSI seems low to me, but my memory isn't 100% on the subject. Have you checked this with the Haynes.

[walkinginperu]

If No 3 has a spark, but doesn't appear to be firing in any worthwhile way, then you've either got no fuel, or no compression. I can't see how you're not getting fuel to No.3 either, and the garage did a compression test. It's possible that they got it wrong, but I'd hope not.



If you take No.3 plug after running the engine, is it wet with fuel?
If you swap the plugs around (I know you just replaced them, but it's worth a try) does the fault move?
Is there maybe a buildup of gunk on the thread that's stopping the plug from earthing to the head (seems highly unlikely to me)
If you turn the engine over on the starter, with No.3 plug removed, can you see a cloud of fuel vapour being pushed out of the plug hole?

EDIT: 120 PSI seems low to me, but my memory isn't 100% on the subject. Have you checked this with the Haynes.

Hi Mike,
I've just swapped plugs and leads from 3 to 4, and no. 3 is still not firing at idle. Because of the fact that it runs on all 4 at high engine speeds, could it maybe be a stuck/worn tappet? They're hydraulic, so at idle they'd get low oil pressure and at higher speeds they'd still open if sticky because of the higher pressure? Does that make sense?

[Jay4424]

Head gasket?!? ...water in number three cylinder.

[walkinginperu]

Head gasket?!? ...water in number three cylinder.

Could be, but no sign of mayo under the rocker cover, had 120psi on all four cylinders and isn't using any oil or water. Don't know how else I would diagnose it, and don't really want to crack the head bolts if idon't have to. Any other way i can tell if it's definitely h/g?

[Brunty]

So what does no. 3 plug look like after it's been missing?

[walkinginperu]

So what does no. 3 plug look like after it's been missing?

Thanks for all the help and suggestions guys.
Haven't checked plugs after swappping them round to check if that was the problem, and it's now dark. I'll check it out tonight if i can find a torch!

[dozzum]

I borrowed one of these from one i my dads work mates. Dont know where he got it from but doesnt look too expensive. and he said its really good.

[Brunty]

I borrowed one of these from one i my dads work mates. Dont know where he got it from but doesnt look too expensive. and he said its really good.

A torch?

[tinkerbel]

Check it running in the dark, it could be arking somwhere, and in the dark you will be able to see it. good luck.

[big nick]

I borrowed one of these from one i my dads work mates. Dont know where he got it from but doesnt look too expensive. and he said its really good.

A torch?

thats what I thought?!

[walkinginperu]

Check it running in the dark, it could be arking somwhere, and in the dark you will be able to see it. good luck.

Couldn't find a torch last night so couldn't check the plugs. No arcing in the dark, however.

[Jay4424]

Head gasket?!? ...water in number three cylinder.

Could be, but no sign of mayo under the rocker cover, had 120psi on all four cylinders and isn't using any oil or water. Don't know how else I would diagnose it, and don't really want to crack the head bolts if idon't have to. Any other way i can tell if it's definitely h/g?

as I said... water... when you remove number three spark after running (on idel, when you think its only on three cylinders), is it wet... is it wet with fuel (should be) or water (shouldn't be!).

[Nate]

did they do wet and dry compression tests? have you put a vacuum guage on it? the diagnostic power of the vacuum guage is much ignored!

i'm thinking this may be a valve problem...