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]

Originally Posted by 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.

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??