90' Intermittent power - Spark problem

[JerryRig]

Hey all I've been trying to track down an intermittent power loss problem.

I've replaced:
Coils
Both 02 Sensors
Spark plug wires
Spark plugs
New FIC Injectors
Assured all Vaccum lines are sealed
New Fuel Pumps
New Fuel pressure regulator

Here is what I've found so far:

Affects Cylinder number #5 ONLY

There is an abnormal continous spark(no change in idle when plug boot is removed)
Sat a Spark plug on manifold while running and there is a strong spark but it seems to be faster than normal.

Audible noise/air leak from cylinder #5 almost like a bad header seal.

I've added Stainless works Headers (there is a nice back-pressure leak where header meets cat i need to get fixed) - symptoms listed were the same with Original manifold.


I'm stumped, some days it runs like a dream and others it's running on #7 cylinders.
One thing I found is when it does run bad from startup, it might eventually run on all 8 cylinders once everything warms up, be it from idling or getting on the road and going through the gears.

I have borescoped the cylinder and the walls look great. It does have more carbon buildup than surrounding cylinders.

So, again, sometimes it runs great with no symptoms listed above. With everything that I've changed the issue still persists at random with cylinder number #5.

[RSLyrick]

Hey, I'd like to try my luck at a diagnosis, if I can help .

Question :

Has this cylinder #5 issue been noticed before replacing all the listed items or after running out of solutions ? (after investigating deeply).

Or have you started replacing parts based on the intermittent power loss symptom only ?

Possible fact :

Significant carbon build up could be related to a poor combustion caused by a weak spark (short in time as you described), not providing enough energy to ensure proper combustion over time.

As the LT5 uses a waste spark ignition system, how does the spark look like on the paired cylinder (cylinder #8). Do you notice a short spark as well that could indicate that one of the four ignition module output is not controlling the dwell time properly ? (assuming you have good spark plugs, coils, wires).

You should also observe carbon build-up in this cylinder as well, I guess. Was it only in #5 ?

So, in other term, I'm suspecting the ignition module going bad...?

[JerryRig]

Yes, it has always been at #5 cylinder.
The paired #8 cylinder seems normal and does affect idle when spark plug boot is pulled while running.

It almost seems like #5 side of the paired coil is "stealing" the spark from #8 as well. The Coils are new Autozone brand. The previous Coils were originals, both old and new ohm'd out normally.

I did talk to Marc Haibek and he said the Ignition Module rarely goes bad. I did explain my situation to him and he thought it still could be a coil at fault, and or the ECM might need to be repaired.

I think if the ECM comes back fine, it's probably the ignition module....and again I guess I would be one of the very few out of Marc's ~800 cars he's worked on.

[RSLyrick]

Understood, let's see if more people can advise on your issue. Otherwise, Marc is your best shot.

However, I'm wondering how a faulty ECM could lead to such erratic ignition behaviour...To my understanding the ECM is using a single wire to trigger coils with specific timing advance / retard. How could only cylinder #5 ignition control fail ? And assuming that this is the case, cylinder #8 would also fail.

Bad coil seems like a more resonable explanation (manufacturing issue, bad contact on side #5 ?).

When a single cylinder is affected I would target parts that drive a single cylinder (spark plug, spark plug wires, injector).

That tickles my mind...I'm very curious about what's wrong with your Z.

Did Marc say something about the air leak sound ?

[JerryRig]

I don't think I mentioned the sound that accompanied the spark issue; doh... I'm sure that would have helped.

Frustrating..I've swapped so many parts that should have resolved this.

Could a failing crankshaft sensor send bad signals causing this problem? It's a fairly cheap part, maybe that's an option as well.

I'm going to take it to the exhaust shop next week and tighten everything up to rule that out.

[RSLyrick]

Originally Posted by JerryRig

Could a failing crankshaft sensor send bad signals causing this problem? It's a fairly cheap part, maybe that's an option as well.

Unless you have a damaged machined slot on the crankshaft reluctor wheel at this specific 8/5 position which would lead to a missing pulse for 8/5, I don't see why. Very rare then. And again, would affect cylinder #8 as well. A bad sensor would cause ignition issue for all cylinders IMO...but who knows...

Yes get this header issue solve. Is this exhaust leak on the same engine side as cylinder #5 ?

Call Marc back about this sound...every single details/observations can have a meaningful impact in a problem solving process, even if trivial for you.

[Great White]

From what I gather, you say that the spark on #5 looks/sounds like it's more frequent than any of the adjacent cylinders at any given RPM, and provides no change to RPM when unhooked?
That coupled with the carbon build up is the only thing that really narrows this down to cylinder #5...
This sounds to me like cross-triggering either between coils, or plug wires, or within the #5/8 coil. If the #5 wire or coil has compromised insulation, it may be intermittently shorting to ground as well. This would all explain why it's so sporadic/intermittent. If this issue was occurring before you replaced all these parts, it may be an issue in the primary wiring before the coils.

If in fact this was occurring only on #5 before the parts were replaced, I wonder if there's a mechanical issue on cyl. 5 like a dead lifter, burned/bent valve, or wiped lobe.

Is the plug on 5 tan, black, or covered in raw fuel?

[Thomas ZR-1]

What about the spark plug electrode distance? Are they all similar? I gap them to 1.25 mm = 0.05 inch as Marc recommends.

[JerryRig]

Originally Posted by RSLyrick

Unless you have a damaged machined slot on the crankshaft reluctor wheel at this specific 8/5 position which would lead to a missing pulse for 8/5, I don't see why. Very rare then. And again, would affect cylinder #8 as well. A bad sensor would cause ignition issue for all cylinders IMO...but who knows...

Yes get this header issue solve. Is this exhaust leak on the same engine side as cylinder #5 ?

Call Marc back about this sound...every single details/observations can have a meaningful impact in a problem solving process, even if trivial for you.

Yes, exhuast leak is on same side as #5 (driver-side)
I can tell from at least the Cat that its been getting hotter by the coloration(like how you take a torch to metal and it turns blue/purple)

I'll try to reach out to him today again, thank you for the help!

[JerryRig]

Originally Posted by Great White

From what I gather, you say that the spark on #5 looks/sounds like it's more frequent than any of the adjacent cylinders at any given RPM, and provides no change to RPM when unhooked?
That coupled with the carbon build up is the only thing that really narrows this down to cylinder #5...
This sounds to me like cross-triggering either between coils, or plug wires, or within the #5/8 coil. If the #5 wire or coil has compromised insulation, it may be intermittently shorting to ground as well. This would all explain why it's so sporadic/intermittent. If this issue was occurring before you replaced all these parts, it may be an issue in the primary wiring before the coils.

If in fact this was occurring only on #5 before the parts were replaced, I wonder if there's a mechanical issue on cyl. 5 like a dead lifter, burned/bent valve, or wiped lobe.

Is the plug on 5 tan, black, or covered in raw fuel?

Yes, no RPM change when unhooked.

I just put on new NGK plug wires and still have the #5 issue.

You think primary wiring? As in a piched wire or loose plug somewhere?

All cylinders look like they're burning great minus #5 which looks like it's burning uneven...and darker.

After talking with Marc he was thinking it couldn't be a valve, valve/spring because the problem comes at random times. I borescoped the #5 cylinder and compared to other cylinders the valves looked closed...enough, nothing abnormal I could spit.
All that to say it could still be one of those problems, I was trying not to take the Cam cover off unless I had to.