Need an oracle! Low speed miss on cylinder #1 only.

[bmarcum]

Okay sorry for the lengthy post but I need some help. I have a LT5 shop manual but I can?t find it at the moment. I have a 1990 ZR-1 #103. The Z has about 109K miles on it at the moment. This is my second Z and I do love these cars.

My Z has developed a low rpm ignition miss. It started on the interstate in 6sp gear lug. I hadn?t replaced the plugs in a while so I replaced the plugs that evening (not expecting that to be the problem). I pulled the intake the next weekend and found the wire resistance off (9K) so I replaced all the wires. The wires had 30K on them and needed replaced anyway but didn?t fix the problem. I recently changed the coils to MSD 8224 coils.

Now comes the scary part. The miss is intermittent. Sometimes it does it misses cold, sometime it misses hot, and sometimes it doesn?t miss for a while. But it never dies or has starting problems, just a low rpm miss. You can hear the miss at idle, so I started pulling plug wires and found that the spark on cylinder one was weak/intermittent. All connectors on the ignition module are original but seem to be clean and holding tight. So I replaced the coil for cylinder 1/6. Also didn't fix the problem.

How does the ignition system fire cylinder 1 vs 6? It seems to only miss on cylinder one. Also when the rpm?s are above 3/3.5K it pulls through the miss and run well. When I pull the wire on cylinder 6 (same wasted spark coil) it has a strong spark. This really confuses me. It only misses on cylinder number one? Does the computer/ignition module give a separate command for cylinder 1 vs cylinder 6? I thought in the wasted spark ignitions that it just fired the coil on the same command for cylinder 1 and 6.

All other cylinder have good spark at idle. What could I be missing? If the timing pick up was bad, it wouldn?t always be cylinder one missing. Has anyone seen an Ignition module weak on only one cylinder? Can the computer cause this? Anyone have an ignition module for borrow/purchase? I know I am asking for a very rare part. Any thoughts? I am stuck at the moment. The way the problem started, I don?t think it is a control wiring shorting. I also don?t know how to test the ignition module or wires. Any help is appreciated.

Thanks
Brandon

[LGAFF]

Original injectors?

[bmarcum]

No I replaced the injectors 4ish year ago. They had failed due to the ethanol issue. Also the computer has been replaced at that same time.

[LGAFF]

MAP sensor hose checked, etc?

[bmarcum]

Yes I believe the map hose is fine. How would the map cause an ignition miss? I am only seeing a weak spark on cylinder #1 intermittently.

[LGAFF]

Reading the manual

-One interesting point is a faulty vehicle speed sensor can cause a high speed fuel cutoff(as in the vehicle thinks its going faster than it is)

Other points were discussed, bad injector, injector harness, ignition ...check compression, etc

[LGAFF]

Yes I believe the map hose is fine. How would the map cause an ignition miss? I am only seeing a weak spark on cylinder #1 intermittently.

You know its weak spark or think? Bad map sensor hose will cause stumbling and missing ......

Surprised there are no codes, you need a tech one or scanning software to read what the engine is doing when it happens

[bmarcum]

I noticed the miss driving down the interstate in 6th gear at low rpm. Then it became common. I was able to identify that it is on cylinder #1 by pulling all plug wires one at a time. It was easy to see that the miss is cylinder #1, but that is where I am stuck. I have replaced ignition coils, plug wires, and plugs. If this were a distributor engine (90's fords were notorious for similar ignition module problems but not a single cylinder) I would put a module in without thinking about it. Has anyone seen a ignition module have this symptom?

I will check/ replace the map hose tomorrow.

Thanks
Brandon

[Corvette95]

You say you replaced the coils recently. What is recently and did this happen before or after the coil change? Also,have you checked it in a dark environment for arcing? How did you determine weak spark / no spark on #1?

[Paul Workman]

FUEL - AIR - SPARK Take NOTHING for granted. Just because something was replaced, it is not proof that "something" didn't go bad again!

If, as you say, the miss is narrowed down to a particular cylinder I'd want to get it up to operating temp and measure injector resistances again. (BTW, you replaced the injectors...with what, exactly?).

And, too, I had a miss on one cylinder that a compression check lead to a burnt valve (stemming from a bad injector, I believer!) Have you done a compression check yet??

:happy1:

[bmarcum]

I am checking the weak spark by pulling the plug wires and letting it arch to the valve cover bolts. That is how I know the spark is weak on cylinder #1. All other cylinders have great spark! Also it is intermittent so I know it is not compression. It maybe a wire, but I don't know where. I am going to check the grounds and the four wire connector to the coil plate. These is a weird one.

[bmarcum]

Does anyone know if the control module sets dwell for the coils individually? I feel from all the parameters that I have I am dealing with an ignition module that has weak component related to how GM handled control trig for the coil/dwell. I can't get comfortable with the risk as this is a very expensive piece to just change. If I know that dwell was set separate for each of the four wires to the coil plate then I would be confident and just replace it.

[wfot]

I have an experience that might be worth looking into.
I was going thru a phase of trying a bunch of different types of spark plugs.
Found a set I liked, after a week or so, the power seemed "off". the idle was not smooth full power was down.
Checked the usual suspects and found everything looked good. Decidedchange the plugs and ad found one wires was not fully seated on the plug... I thought "my bad". pushed the wire on fully..idle good.. power good.... "got it" ......
Nope. A day later same situation and the same plug wire was pooped off again..WTH???
answer, a washer stayed down in the head.. so 2 plug washers in the same hole... idle speed was not enough to push the wire off the plug but 7k rpm was.. I noticed when looking at the other plugs and found one plug was missing a washer...it was not really noticeable looking down the plug hole.. long screwdriver and a magnet got everything back in order... Now I ALWAYS always check the plugs for their washers when they come out.

If you find your plug wire is unseating occasionally from the plug this might be worth looking at as it is $0.00 investment..

John

[bmarcum]

I hate this kind of update, but I wanted to close the loop with everyone that made suggestions. I feel that I should give a little of my fathers and my back ground. My father was a master tech with ever certification known to mankind for GM and Ford in the 70-85. He is very specific to find the problem and if possible turn the problem on and off before the final repair. I wasn't able to do that. As we all know this can be hard under the intake. I have grown up working on high horsepower engines for the past 30 years and I am only 43. I am a mechanical engineer and absolute Corvette, boat, and performance nut. This cylinder #1 miss was driving me crazy, but in a good way.

Short story is the problem is fixed. Although the repair procedure is not 100% known. Below is the list of repairs.

I believe the main culprit to have been either contacts on the coil primary triggers on the base plate or too wide a spark plug gap ~0.45 on all plugs.

Other repairs
1. New plugs (not the fix) but I did reduce the to the correct 0.35 gap
2. New spark plug wires (not the fix)
3. Clean coil base plate connections and sanded connections
4. Ohm all coil connection with wires ~0.2 k ohms
5. Replaced map hose
6. Check all other vacuum hose and clean under the intake

Thanks again for everyone thoughts

[Young1]

Congrats. It does go that way sometimes. Dunno but it works!!

[bmarcum]

The Gremlin is back. Grrrr! I am thinking about the ignition module, but not sure that it could be the problem. I know Jerry's Gasket has NOS, but they are $1300. I understand and appreciate his pricing, but it would be great to find a used one since I am not sure it is the problem. Anyone have a used earlier (or late if you have it) ignition module they want to sell? I have replaced the plugs, 1/6 coil and wires. I have also checked, ohmed and cleaned all coil primary connectors and base plate. I checked all connector plugs for the ignition module. I hate this because I can't turn the problem on and off. It is 100% an ignition miss that is intermittent and all ways goes away above ~ 3,500 rpm. The miss doesn't seem to have a relationship with temperature. Anyone want to part with an ignition module. I will resell it to you if it isn't my issue.

Thanks
Brandon

[wfot]

I reread the post, I saw that you stated your grounded out the #1 and #6 plug wires and found that the #1 had a weak spark, correct?

I think that means it is that coil is bad. there is no reason for a coil to fire half good/bad other than the coil being bad. new parts do not mean good parts. I would start with a swapping coils and retesting for spark. IE. move the 1/6 coil to another location and retest for spark on 1/6 and then test for spark wherever you moved that coil from for spark and try to reproduce the weak spark there... won't cost anything to try and if it is a bad coil, just replace that one coil.

John

[32valvesftw]

John has a great suggestion If that does not yeild any clues maybe the issue is not electrical?
Per your original post "It seems to only miss on cylinder one. Also when the rpm?s are above 3/3.5K it pulls through the miss and run well."
What happens above 3/3.5K? Your secondaries open up for one thing, and if not the additional fuel may helping the atomization in that cylinder and make the problem go away. If you are in a position to do when it acts up, pull number 1 spark plug and see if smells like gas and then compare it to number 6.
I would also tape a fuel pressure gauge to the windshield and drive around until it happens, maybe have your Dad with you so someone is always watching that gauge.

[Ccmano]

Sent you a PM.

Find someone with an oscilloscope and confirm that you do indeed have weak and intermittent spark on one cylinder. Simply looking at a sparkplug firing is not definitive. Like you have said, a single cylinder with bad spark would be very unusual for a wasted spark system. They usually come in twos.

H
:cheers:

[bmarcum]

Great point on the oscilloscope. That sounds like a great Idea. What is best for the LT5? I have a friend of a friend that has a old Sun engine analyzer for traditional distributor systems. I don't think it would work for the LT5 but I understand your point. What set up is best for the LT5 to see ignition pulse on each cylinder?

[Ccmano]

Great point on the oscilloscope. That sounds like a great Idea. What is best for the LT5? I have a friend of a friend that has a old Sun engine analyzer for traditional distributor systems. I don't think it would work for the LT5 but I understand your point. What set up is best for the LT5 to see ignition pulse on each cylinder?

Can't say I've used an oscilloscope on an LT5. A modern oscilloscope with an induction pickup (like on a timing light) that can be moved from wire to wire would be best. This YouTube video might help. Interesting to note that the guy talks about half a coil pack not firing.
https://youtu.be/Kr8TIc820SA
H
:cheers:

[32valvesftw]

Switch two coils and see if the "weak spark" follows the coil. Switch plug wires and check again for weak spark, then switch plugs same thing. Going back to basics, do all the plugs look the same in so far as color and build up on the plugs.