strange misfire/crossfire in V8 with single coil ignition and distributor

Gentlemen,

as quite a fan of the ScannerDanner youtube channel I became aware of this forum and hope to maybe find some help with my troubleshooting problem here:

Engine is a customized V8 with port injection, transistor controlled single coil ignition with Mallory Unilite distributor. Compression ratio is 10.4:1 (->high secondary voltages). The ignition distributor is "dummified", i.e. it just distributes the spark; triggering is done on the crankshaft via an inductive pickup. Engine is new, gaskets are new, coil is new, plug wires are new, spark plugs are new. Firing order is 1-3-7-2-6-5-4-8;

Symptoms are sporadic misfire of cylinder 7 with flame in intake port 7.

Tests done so far:
* Relative compression test shows no problems with cylinder 7.
* Fuel pressure is fine. Fuel injector 7 is fine because I swapped it with the injector from cylinder 6, symptoms stayed with cylinder 7.
* Vacuum in intake port 7 with idling engine is similar to vacuum in intake port 6, with cylinder 6 showing no problems whatsoever.
* Latest test was logging of the intake port pressure of cylinder 7 (with an external MAP sensor) and the signal from an inductive clip on spark plug wire of cylinder 7:



Also I logged intake port pressure 7 and the signal of a capacitive clip on spark plug wire 7:



The inductive and capacitive clips I have rigged together pick up quite some noise, so please just use these signals to relate the pressure signal with the spark in cyl. 7.

The pressure log shows that the flame in intake port 7 starts aproximately two firing events before spark 7 occurs. This is about 180° crank rotation before firing of cylinder 7. At this moment, intake valve 7 is still open. This would suggest, that spark 1 crossfires into cylinder 7.

My Question now is: If this is a crossfire, how can this happen without showing up on either capacitive or inductive signals of spark plug wire 7? How can I proceed with this problem? How can I determine if this is distributor-related or wire related?

I am very grateful for any advice with this problem. I have spent quite some time now trying to figure this out on my own, but I'm momentarily out of ideas.

Thank you for your consideration and time!

Welcome to the forum! We're happy to get the fun problems like this. :lol:

Is this a Ford block? Just thinking about cylinder numbering and the firing order. Does this have cylinders 1-2-3-4 as bank one, and 5-6-7-8 as bank two? That would mean that cylinders one and seven are NOT on the same bank, and not likely to have their spark plug wires routed adjacent to one another. Am I thinking correctly here? There were quite a few years of GM's that had crossfiring issues between wires, which required specific routing to fix.

Thinking about how to confirm your pressure testing results... What about using a test light to short spark out of the distributor cap, one cylinder at a time? If you short a cylinder and the backfiring stops, then you know where the stray spark is coming from. Not sure if this is practical, depending on distributor location and how intermittent the backfire is.

I'm guessing this is a normal distributor cap design, and not like a stupid design like this:



If it's a normal design, then it's tough to get cross firing inside the cap itself without water intrusion or some serious carbon tracking.

Tyler,

thank you very much for your warm welcome.
Yes, you are right and sorry I neglected this information: This is a Ford block, R/H bank front to rear 1-2-3-4, L/H bank front to rear 5-6-7-8.
I agree with you, if my pressure log is right (IF!), then this seems to be a R/H bank (Cyl1) to L/H bank (cyl7) crossfire.
The distributor is of regular design, however it is raher small in diameter:



The spark plug wires are routed not in a radial fashion because of limited space. They run left and right; wire 7 runs across the cap and between the others, not close to wire 1 though.

The flame in intake port 7 occurs at idle in average about every 30 seconds, sometimes twice in a short interval.

I like your suggestion with the test light very much and will try this immediately.

I read about this condition of high ozone-concentration in unventilated distributor caps that benefits crossfires. This phenomenon however seems unlikely to me in the case at hand (?) since my troubleshooting symptoms are already present at idle and right after starting.


Thank you very much for your quick and productive answer.

No problem at all! Eager to hear back about the spark shorting test - I think that'll point us in the right direction? :lol: It makes sense in my head, anyway.

I'd not heard about the ventilation problem, interesting. Still, I agree with you, I doubt the gas could build up quickly enough after startup to cause a problem.

I wonder if the spark is tracking across the rotor arm As an experiment you could try reducing the plug gap on number 1 cylinder to an old school narrow gap say 0.025" to 0.030" (0.625mm to 0.75mm) to see if the crossfire disapears.

Tyler and Andy,

thank you for you helpful advice.
As per Tylers suggestion, I disconnected and grounded spark plug wire 1 (I also for good practice unplugged injector 1).

Unfortunately, I still encountered sporadic flames in intake port 7. This seems to happen more seldom than before, but this may just be my perception.
Along with this observation, what still puzzles me is the fact that I see the "assumed" crossfire into cyl 7 with neither the inductive nor the capacitive clip on spark plug wire 7 (see pictures in my first post).

Tomorrow I have to drive out to another job, but I will continue with this troubleshoot on monday. Next try will be logging the pressure in intake port 7 WITH grounded spark plug wire 1. Any other suggestions are much appreciated.
I'll keep you updated as soon as I have some results.

Thank you for your kind advice.

So, the spark isn't crossing over from the #7 wire. Try the rest of the wires next? Or at least just the ones adjacent to #1 in the firing order, so we can rule out a wire problem.

While you're doing your pressure testing, it might be good to try attaching your MAP sensor to manifold vacuum instead of ported, so we can get an overall better idea of what's going on in the intake during the backfire. I have some overlay software that might be useful on those captures.

Gentlemen,

with the testing I've done yesterday I thing I have grasped the problem with this particular engine:

Further intake port pressure logs on cyl 7 with grounded ignition wires led to the conclusion, that sparks 1 and 8 cross into cyl 7 and cause the occasional flame in intake port 7. With grounded spark plug wire 7 I could also observe some rare flames in other intake ports which I could not identify because these occasions were very rare (and quick).
Considering the fact that spark 1 crosses into cyl 7, this problem cannot be a spark plug wire routing related issue. Spark 8 into cyl 7 maybe, but spark plug wire 1 is very distant (and on the other bank) from spark plug wire 7. Further from the the observation that other cylinders have the similar problem (although much rarer) I concluded that this is a distributor-crossfire situation. I am very sure the spark in the distributor "crossfires" in a radial manner, from the central terminal to other cylinder terminals. The latter are the ones that lead to a cylinder with some vacuum (intake valve open), where the spark gaps are less isoltated by air than in the comression situation.
I reduced the spark plug gaps for 25%, but the flames in the intake ports remained. I guess the ignition voltages are still too high for the installed small-diameter-distributor even in this configuration. I couldn't reduce the gaps even more because I was pressed for time.

I will recommend a distributor with larger diameter to the customer, or alternatively a distributorless ignition.

Lessons learned for me:
* A radial crossfire in this particular distributor cap will require the spark to jump aproximately 1". From the observations and conclusions drawn, this obviously represents less resistance than firing the spark in the correct cylinder after the compression stroke (in this case with high compression of 10.4 and a stochiometric, non-rich mixture). This is hard to believe for me, but probably the only possible explanation.
* A crossfiring spark at the intake stroke can be so weak, that it is not distinguishable from noise (at least with my Bosch capacitive and inductive clips).

Thank you very much for your kind suggestions which surely led me in the right direction for solving this (for myself quite tricky) problem.

I would probably do an end to end resistance measurement on the plug wires. ISTR Paul has a video on this but I can't find it so here is NGK's official test method Link to NGK site

If the rotor arm has a built-in supressor resistor it would also be worth doing a resistance check on that.