sc not running on one cylinder

[gordoncoll]

we have been working for a week plus on my 911 sc euro spec 204bhp
here is what we know

the car is running well on 5 cylinders
cylinder 6 is not firing at all.

the cylinder has:

good compression
good leakdown and therefore is sealing well (rings etc)
all valves are adjusted correctly
valve springs are fine

fuel system

new injectors all producing good flow rate
new fuel pump
fuel pressures are all within correct range
fresh fuel

all vacuum hoses are correct and tight mostly new. good vacuum being produced measures at dizzy connection.

ignition electrical:

leads good
both visual plug test for spark and continuity pass
new coil
newly recon alternator
dizzy cap good
rotor arm good

cdi working correctlyl

run the car on just 5 cylinders with the no 6 injector blanked off and we have low CO and low hydrocarbons, reconnct the injector and we have high high high hydrocarbons and popping in the exhaust. so we can assume fuel is going via the intake valve through the barell and out the exhaust valve

now either the car is defying the laws of physics as it has all the elements needed to run properly but just isnt. or we have missed something, probably something very obvious.

can anyone shed any light on this????? What I am saying here is "help!!!!!!" as this is driving us round the bend.

we are in dorset uk by the way.

[JP911]

Bad valve guide(s) and/or broken head stud(s)? A compression and leakdown will not always reveal failing valve guides.

[pete3799]

Hook a timing light on your #6 plug wire and see if it's fireing consistantly.
Double check that your plug wires are in the correct order in the dist. cap.

[gordoncoll]

thank you for the suggestions:

we have done the test of the consistency of the spark to no 6 and it is consistent

the leads are on ok

head studs, what is the easiest test? same for valve guides, would we not still expect to have some attempt at ignition given the reasonable condition of everything else ?

[smokintr6]

When you replaced the injectors, did you replace all the seals and the plastic sleeves that they slide into? I had a very similar problem with an SC and it was solved by replacing all the O-rings and injector sleeves. Good luck, it sounds like you have checked most of the normal potential issues. I agree that a leak down won't detect worn valve guides, but I don't think the valve guides need to be in great shape to run a cylinder... unless the plug gets oil fouled. What does the #6 plug look like? Also make SURE that the plug wire has snapped onto the plug head. It should make a satisfying snap to let you know you're there. I've been there too!

[gordoncoll]

sorry yes we did replace the seals and o rings, i even spent some time messing about bead blasting all the runners before i put tthem back so they looked nise, we have smoke tested the car as well, we pushed smoke under pressure through the vacuum system and no major issues apeared

it is maddening

[KingSalmon911SC]

Is #6 sparkplug wet? Like smokintr6 asked: "What does the sparkplug look like?".

I'm sure you have; but if not, try another sparkplug & plug wire.

Good Luck!

Richard

[stlrj]

If you have the stock shielded ignition wires, you will never see where the ignition wires are shorting out. It will literally drive you insane.

Only Porsche was insane enough to use shielded ignition cables, no other manufacturer as far as I know.



Cheers,

Joe

[woland]

He

After changing the distributor brushes in my SC I sadly went around with 5-5 1/2 cylinders during whole april. Then I built a key and took off the sparks (Champion platinum) and saw all were ok.



Then took off one by one the distributor caps with the engine running and identifyed the insensitive cylinder (first on the left). Put a new spark and in a 15 miles the engine started running to six.
Now the engine wonderfully purrrs. But it didn't do that istantly: it took around 15 miles.
I suppose that something went wrong in the cylinder chamber or in the valves after so much time going five: it had to SPIT something.

Sometimes I wonder if there's something in this cars, so complicated that looks like a kind of counsciosness

[techman1]

Bad spark plug wire for 6? Fire it out in a dark room/ night and look close. "Visual test for spark" My concern would be the act of hooking the lead up to plug six is pulling/shorting it somewhere. If at all possible, put all new ignition on six and see what happens. Maybe swap with a ignition lead from another cylinder of the same length.

That fails?
New plug, new lead, new cap, new rotor. One at a time.
In reading your problem solving, it seems like all was checked. That is why I feel the act of actually plugging in the ignition wire into six is the issue.

[gordoncoll]

well on monday we have a "glass" visible spark plug to try so we are going to start from basics again !! car will drive me crazy.

thank you all for your replies

[gordoncoll]

just an update

we now have all 6 working, swapped out the plugleadsd no 6 was a really weak lead by the looks of things so we now have 6 :-)

however

there is a miss at between 1-2k revs, only when the car is in 2nd 3rd 4th and 5th gear, it is fine in first and when not under a mechanical load, ie you can rev tthe nuts off it when it is just parked up an no problems !!!! so joy of joy we are now tring to sort that. I will do a new thread.

I just wanted to say thanks for your help here on this thread with this problem