Need an oracle! Low speed miss on cylinder #1 only. - Page 2 - ZR-1 Net Registry Forums

bmarcum · 03-13-2021

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 · 03-13-2021

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 · 03-13-2021

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 · 03-15-2021

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 · 03-17-2021

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

bmarcum · 03-29-2021

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 · 03-29-2021

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 · 03-29-2021

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 · 03-30-2021

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

bmarcum · 03-31-2021

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?

03-13-2021	#11
bmarcum Join Date: Oct 2018 Location: Indiana Posts: 24	Re: Need an oracle! Low speed miss on cylinder #1 only. 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.

03-13-2021	#12
bmarcum Join Date: Oct 2018 Location: Indiana Posts: 24	Re: Need an oracle! Low speed miss on cylinder #1 only. 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.

03-13-2021	#13
wfot Join Date: Jan 2009 Location: New jersey Posts: 156	Re: Need an oracle! Low speed miss on cylinder #1 only. 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

03-15-2021	#14
bmarcum Join Date: Oct 2018 Location: Indiana Posts: 24	Re: Need an oracle! Low speed miss on cylinder #1 only. 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

03-17-2021	#15
Young1 Join Date: Sep 2016 Location: Ocala, FL Posts: 880	Re: Need an oracle! Low speed miss on cylinder #1 only. Congrats. It does go that way sometimes. Dunno but it works!! __________________ Steve Young 1991 ZR-1 Quasar Blue Metallic/Black #729 Second Owner 1991 ZR-1 Dark Red Metallic/Black #2040 SOLD 1990 ZR-1 Dark Red Metallic/Saddle #1670 SOLD Double your happiness.....complain half as much.

03-29-2021	#16
bmarcum Join Date: Oct 2018 Location: Indiana Posts: 24	Re: Need an oracle! Low speed miss on cylinder #1 only. 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

03-29-2021	#18
32valvesftw Join Date: Jun 2015 Location: Texas Posts: 530	Re: Need an oracle! Low speed miss on cylinder #1 only. 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.

03-30-2021	#19
Ccmano Join Date: Jun 2006 Location: Sparks, NV Posts: 2,852	Re: Need an oracle! Low speed miss on cylinder #1 only. 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 __________________ 90'ZR-1 #1461 Red/Black Ported Intake, Degreed Cams, OBX Headers & Exhaust, Fidanza FW, Secondary Delete, Custom tune, C6 GS Wheels. Sold 08 C6 Z51 Crystal Red Metallic Sold Vararam Intake, Tune, Sold 05 C6 Z51 Red/Black Sold 90'ZR-1 #1723 Black/Gray Sold but not forgotten 91' Z51 L98 White Vert. My First, you always remember your first. Sold. Last edited by Ccmano; 03-30-2021 at 01:44 PM.

03-31-2021	#20
bmarcum Join Date: Oct 2018 Location: Indiana Posts: 24	Re: Need an oracle! Low speed miss on cylinder #1 only. 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?