Lizard928

Ok 93 GTS auto.

Stock engine management system.

Back story is that the car had bent valves they have been fixed car was put back together then found the front crank seal needed to be replaced.
Did that car back together.

Car runs now but not on all cyl. I can remove the plug lead or inj lead from all cylinders except #6 with a change in running condition.
So in simpler terms. If I pull the lead off cyl 1,2,3,4,5,7,8 the idle becomes rougher.
Cyl 6 this has no effect.
I have removed the injector and it has been tested as in excellent condition by doc inj.
I have verified voltage at inj plug is same as others.
Can hear inj click as I plug it in with engine running.
Have peeled boot back and inspected wires which are in perfect shape.
I have swapped the spark plugs between cyl 5/6 and 6/7 no change
I can hear the arcing if I lift up on the plug while running.
Verified I have good spark on plug with plug out of engine.
I can see visible arcing in the daylight around the plug leads.
I tried another plug lead on this cylinder with ZERO effect
did a compression check cyl 5 170 psi 6 170 psi 7 170 psi
I only checked these 3 to give a comparison/ baseline for the compression tester. Cyl 5 and 7 have enough to fire and are the same as 6. So based on the guage there should be more than enough to fire.
Forgot to mention, confirmed plug wire routing and that 6 is on the right post of the distributor.

So can anyone see what I must be missing?????

Engine was brought up to full running temp and ran for 5 min with no fuel and I cleaned the plug off which was damp but did not reek of gas. So fouling of the plug and cyl should have been eliminated too.

Any suggestions will be helpful!

John Speake

Damage to cam/lifters of that cyl when valves bent ?

dprantl

If you have spark, fuel and compression in the cylinder, there must be some kind of damage in that cylinder that does not affect compression. This is especially true on a batch-fired fuel car where the brain cannot individually control each injector. Or thinking out loud... it could be a faulty EZK retarding just that cylinder like crazy, but that's very unlikely.

Dan
'91 928GT S/C 475hp/460lb.ft

the flyin' scotsman

Is #6 plug wet when you pull it? If not perhaps the injector is plugged.

Lizard928

Considered that, but if that was the case then compression numbers would be off.

Lizard928

I can't see there being damage that would allow for compression but prevent combustion.
My though was similar to your in that the 1 cyl might be being fully retarded by the EZK, but that wouldn't explain how rough the car was.

Lizard928

As stated in original post, I removed the injector and took it to an injector place who checked it right there and they said it was pretty much as new.

However I may have found the cause.
I just tried yet another plug lead yet this time seperated it from the harness. Cyl 6 still has no really noticable affect when unplugging inj or pulling plug lead (don't try that at home ).
Yet with this other lead on the engine the car revs smoothly and a great deal of the vibrations are gone.
So I think that this car must just be very sensitive to bad ign leads!

I think that before much else is done new fuel hoses and leads are in order.

Thanks to everyone for racking their brains on this and if you have any other comments please feel free to share them

danglerb

Can you get a probe or ir sensor down to measure the EGT?

Does the resistance on the wires and boot things measure OK?

the flyin' scotsman

Re-read the first post....................now I see it.

I'd swap the injector perhaps over to #1, replace the spark wires if possible and inpsect/ replace the rotor, disti cap.

I'm sure you checked but to be absolute #6 is on the PS disti very close to #7.
From left to right the wires are 4,6,7 and 1.

Mrmerlin

Colin run the car at night and look for sparking wires my guess is that you have a few bad plug wires, as previously stated inspect the caps and rotors as well, since the plug #6 is wet then you know that its getting fuel. so the injector is working.
Testing the plug wires with an additional plug can be done by removing the wire from the plug in the engine and inserting a new/good plug and grounding its tip, leave the old plug in the engine so you wont have a fuel spray shooting out of the cylinder.
When you replace the wires also replace the plugs, if one or more was wet there is a good chance it could be fouled inside the insulator, I have seen this happen where new plugs were installed with poor ignition wires and then after a few mins the cylinder would go dead, Replacing the plugs cured the problem as well as replacing the wires

76FJ55

Double check firing order/spark plug wires?

Tails

I would check the resistance of the connectors to see that they are within resistance specification tolerances. The HT wires are copper with minimum resistance values, so resistance only via connector caps. This design is so that the resistance of each HT circuit is the same regardless of the varying lengths of the HT wires.

Check the connector caps on each HT lead for corrosion resistance and/or internal arcing.

Check the distributer caps for cracks, tracking and spark errosion on the rotor and pick up heads.

When you reassemble HT leads with caps connectors check the total resistance of each lead and compare values.

Clean up rotor end and pick ups to minimise resistance.

As Mrmerlin said, check under the bonnet at night in a dark garage and see whether there is arking to earth from the HT circuits.

If motor runs smooth then all is OK, if not then you may need to replace HT leads, or faulty connectors or faulty distributor caps or coils.

Also check out condition of coils.

All above resistance values and procedures are contained in WSM.

Tails 1990 928S4 Auto

Lizard928

The leads and plugs will be being replaced as well as inspecting/replacing the caps and rotors.

And I already stated earlier that I could see the arcing in broad daylight! So no need to check at night with a water mister.

And firing order has been confirmed to be 100% correct
I know 6 and 7 are close but I have confirmed they are all where they should be.

zoltan944

What he means by looking at night is looking for arcing while everything is plugged in correctly. If you see any 'fire flys' you know you have a bad wire or multiple bad wires.

Lizard928

Which can be clearly seen during the day with everything plugged in near the edges of the cam covers......