Mustang Misfire

[jeremyc]

1966 Ford Mustang with a 302ci V8 which has HEI fitted (High Energy Ignition - coil in distributor).

It has developed an intermittent misfire that seems to get worse when the engine gets hot.

From what I can see using my spark tester it has a weak or no spark on cylinder number 1 (all others appear to be fine) and will lose spark on this cylinder for a few seconds before stalling at idle (when running I can pull the ignition lead to this cylinder and it makes no difference to the rough running). Spark becomes stronger at higher revs but with occasional misfire/backfire.

So far I have tried and tested:

• swapping the HT lead to cylinder 1 with one of the others: no change.
• swapping spark plug in cylinder 1 with one of the others: no change.
• swapped out the ignition control module within the distributor: no change, and dwell meter shows dwell angle increasing with revs as expected.
• static test on the primary and secondary coils: within spec for impedance.
• static test on distributor pick-up coil: within spec for impedance.

Any ideas on potential causes and/or other stuff to check to try and narrow down causes are welcomed.

Why would I see the problem on one cylinder alone? It seems like an ignition issue rather than fueling or compression, otherwise I'd expect the problem to be across all or more cylinders.

[E-bmw]

Not being familiar with HEI I may be talking rubbish, so please beware of that ignorance on my behalf.

You say the spark to No 1 is weak, so what is exclusive to no 1?

Is there any potential induction issues that could be isolated to no 1?

Is the dizzy good as I have seen them with corrosion/erosion on one more than others?

I would be replacing dizzy & rotor & any condenser for cheapness & making sure that side is all good.

I would also likely be changing coil as once again, they are cheap enough & that is pretty much all of the HT side known good.

[Belle427]

i would check out the pick up coil, assuming its magnetic check for signs of corrosion.

[jeremyc]

E-bmw said:

You say the spark to No 1 is weak, so what is exclusive to no 1?

Is there any potential induction issues that could be isolated to no 1?

Is the dizzy good as I have seen them with corrosion/erosion on one more than others?

I would be replacing dizzy & rotor & any condenser for cheapness & making sure that side is all good.

I would also likely be changing coil as once again, they are cheap enough & that is pretty much all of the HT side known good.

Thanks for the ideas.

• I can't see any induction issues that could be isolated to cylinder no. 1 (other than a valve problem I guess). It has a relatively new Edelbrock carb on an inlet manifold common to all cylinders.

• Distributor also looks relatively new, and inspection inside the cap doesn't show any corrosion or cracking (nor signs of sparks tracking). I cleaned the contacts for good measure.

• The distributor is points-less (a pickup coil detects shaft position and triggers the integral ignition control module) and also has the condenser and coil internally. Replacing the distributor unfortunately isn't a particularly cheap option, so I was trying to get a more definitive diagnosis before throwing (expensive) parts at the problem.

I believe the engine has had a rebuild relatively recently (before my ownership) and many of the parts would have been new at this time (distributor, carb, inlet manifold and heads).

[GreenV8S]

Do you have enough travel on the dizzy to turn it by a whole cylinder's worth? If so, you could prove definitively that the problem is within the dizzy.

Have you proved that the spark strength is only changing on the affected cylinder? I suppose it's possible you're getting a false reading for some reason and the cylinder misfire is the result of some other problem.

[jeremyc]

I thought I'd round out this thread with what is hopefully the final resolution.

Further diagnosis and trials included:

• swapping out all the spark plugs
• checked the switched +12V to the distributor and fixed (by-passing the ballast resistance wire which is not required by HEI).
• checking ignition with a timing light. When failing, the "revs" displayed on the timing fluctuated wildly, on all cylinders (not just cylinder 1); the light itself also fluctuated at the same time.

At this point I decided to hand it over to the professionals as I didn't have the facilities/time/confidence to take the distributor out and to pieces in order to fit a new pickup coil (which I had already sourced as a likely cause).

I can take some comfort from the fact that my diagnosis was correct - the pickup coil appears to have been failing when hot - and that it wasn't a straightforward job to swap. IGS American in Surbiton did a great job, and also cleaned and adjusted the carb so it appears to be running very sweetly at the moment.

Now, on to the other little jobs ...

[Belle427]

looking good.

[stevieturbo]

Cool cars.