What's causing my misfire?
Thread Starter
Heel & Toe
Joined: Jun 2014
Posts: 15
Likes: 0
What's causing my misfire?
HISTORY
96 LT4 6spd Corvette. Just finished changing the plugs and wires to MSD 8.5's and NGK BR6ES's gapped to .035 (planning on running a 75 shot). When I screwed in cyl #2 I realized that I cross threaded about 2 threads. Had to rethread the hole. During the process I used plenty of petroleum jelly to catch any metal shavings. The plug is seated well and does not visually leak.
SYMPTOMS
The car starts and runs great. Turns the tires over with a full tank of gas in first gear now from a roll. Sounds like it should. Once it gets above 180F it starts sputtering and misfiring. At about 210F it starts hesitating and misfiring pretty frequently. When you give it past 10% throttle, though, there is absolutely no hesitation or loss in power. At the higher temps timing sounds way off.
I'm thinking I fouled plug number 2? Any other thoughts or ideas? Car has 35k miles and the original optispark, but the lack of misfires during all driving conditions would tell me otherwise. Please help!!!!! I'm desperate......
96 LT4 6spd Corvette. Just finished changing the plugs and wires to MSD 8.5's and NGK BR6ES's gapped to .035 (planning on running a 75 shot). When I screwed in cyl #2 I realized that I cross threaded about 2 threads. Had to rethread the hole. During the process I used plenty of petroleum jelly to catch any metal shavings. The plug is seated well and does not visually leak.
SYMPTOMS
The car starts and runs great. Turns the tires over with a full tank of gas in first gear now from a roll. Sounds like it should. Once it gets above 180F it starts sputtering and misfiring. At about 210F it starts hesitating and misfiring pretty frequently. When you give it past 10% throttle, though, there is absolutely no hesitation or loss in power. At the higher temps timing sounds way off.
I'm thinking I fouled plug number 2? Any other thoughts or ideas? Car has 35k miles and the original optispark, but the lack of misfires during all driving conditions would tell me otherwise. Please help!!!!! I'm desperate......
Thread Starter
Heel & Toe
Joined: Jun 2014
Posts: 15
Likes: 0
Thank you so very much for your reply! I was wondering if you might elaborate on how that could be causing my trouble? Also, wouldn't the scanner use the voltage output of the sensor to measure temperature (similar to what the gauge cluster is doing)?
Thanks again!!!!!
Thanks again!!!!!
Melting Slicks
Joined: Jun 2013
Posts: 3,065
Likes: 39
Thank you so very much for your reply! I was wondering if you might elaborate on how that could be causing my trouble? Also, wouldn't the scanner use the voltage output of the sensor to measure temperature (similar to what the gauge cluster is doing)?
Thanks again!!!!!
Thanks again!!!!!
Thread Starter
Heel & Toe
Joined: Jun 2014
Posts: 15
Likes: 0
Ahhh. That makes perfect sense.
I pulled plug 2 and plug 4. Plug 4 looked right. Plug two not so much. Looked extremely lean. Reinstalled the plugs and started the car at 140F and it had all of the hes
itations it did at 2xx. I'm confused.
I pulled plug 2 and plug 4. Plug 4 looked right. Plug two not so much. Looked extremely lean. Reinstalled the plugs and started the car at 140F and it had all of the hes
itations it did at 2xx. I'm confused.
Melting Slicks
Joined: Jun 2013
Posts: 3,065
Likes: 39
Okay it looks like it's probably going to be a temperature sensitive injector, or coolant entering the cylinder. Test the ohms on that injector, and tell us what it is. Do this after the engine is warmed up.
Thread Starter
Heel & Toe
Joined: Jun 2014
Posts: 15
Likes: 0
Checked resistance on the CTS. 405 ohms at 165F. Dead on. I'll check the injector real quick. I'll test at the temp its at now, as its misfiring right now.
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Melting Slicks
Joined: Jun 2013
Posts: 3,065
Likes: 39
That seems to be okay. That's with it hot right? Next step is to do a hot compression test, and leakdown test if available to you. And also, a coolant system pressure test would help too.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Yikes. WATCH OUT, OP!
You're problem isn't likely injectors or "Coolant entering the cylinder" Where did THAT one come from??
FAR more likely, the problem is in the ignition system. I see that you changed the wires and plugs. I don't feel that the stripped plug threads would contribute to or influence the symptoms. I'd be more inclined to check/test the distributor, ICM and coil. How many miles on the car? When was the last distributor maintenance/replacement?
When the symptoms occur...when it's missing and hesitating, what does the tach needle do? Stay steady at what seems to be the correct RPM? Or does it flutter/sag or fall out of whack w/the RPM of the engine?
You're problem isn't likely injectors or "Coolant entering the cylinder" Where did THAT one come from??
FAR more likely, the problem is in the ignition system. I see that you changed the wires and plugs. I don't feel that the stripped plug threads would contribute to or influence the symptoms. I'd be more inclined to check/test the distributor, ICM and coil. How many miles on the car? When was the last distributor maintenance/replacement?
When the symptoms occur...when it's missing and hesitating, what does the tach needle do? Stay steady at what seems to be the correct RPM? Or does it flutter/sag or fall out of whack w/the RPM of the engine?
Melting Slicks
Joined: Jun 2013
Posts: 3,065
Likes: 39
Yikes. WATCH OUT, OP!
You're problem isn't likely injectors or "Coolant entering the cylinder" Where did THAT one come from??
FAR more likely, the problem is in the ignition system. I see that you changed the wires and plugs. I don't feel that the stripped plug threads would contribute to or influence the symptoms. I'd be more inclined to check/test the distributor, ICM and coil. How many miles on the car? When was the last distributor maintenance/replacement?
When the symptoms occur...when it's missing and hesitating, what does the tach needle do? Stay steady at what seems to be the correct RPM? Or does it flutter/sag or fall out of whack w/the RPM of the engine?
You're problem isn't likely injectors or "Coolant entering the cylinder" Where did THAT one come from??
FAR more likely, the problem is in the ignition system. I see that you changed the wires and plugs. I don't feel that the stripped plug threads would contribute to or influence the symptoms. I'd be more inclined to check/test the distributor, ICM and coil. How many miles on the car? When was the last distributor maintenance/replacement?
When the symptoms occur...when it's missing and hesitating, what does the tach needle do? Stay steady at what seems to be the correct RPM? Or does it flutter/sag or fall out of whack w/the RPM of the engine?
Thread Starter
Heel & Toe
Joined: Jun 2014
Posts: 15
Likes: 0
Car has 35.5k miles and has never had issues or maintenance in regards to the ignition. The tach is calm during sputtering. I reconnected all the boots yesterday. Otispark is dry and the water pump isnt weeping. Car has only seen rain twice and it was last year. I'll try to check the injector pulse tomorrow. Again, I'd like to express my sincere gratitude for everyone's help!!!!!
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
As a mechanic with all of your..."experience", what color are the porcelain on most plugs, removed from most EFI vehicles? (Pretty damn white).
Melting Slicks
Joined: Oct 2000
Posts: 2,158
Likes: 9
From: N.E. WA
I would consider working backwards.. ie..put in the old plugs. Did it help? put on the old wires. You didn't change much. I just like to keep it simple. If the old parts don't help then you can at least eliminate the plugs and wires.
I don't know the NGK plug line up. Is the 6 one step colder for nitrous?
I don't know the NGK plug line up. Is the 6 one step colder for nitrous?
Melting Slicks
Joined: Jun 2013
Posts: 3,065
Likes: 39
Well "gee" not that this is relevant here Daniel, but if there is no spark, what creates the combustion that would color the plug? IOW, how is the plug going to get "colored" any more than another plug, in the absence of combustion? THINK.
As a mechanic with all of your..."experience", what color are the porcelain on most plugs, removed from most EFI vehicles? (Pretty damn white).
As a mechanic with all of your..."experience", what color are the porcelain on most plugs, removed from most EFI vehicles? (Pretty damn white).
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Negative on both counts. Go back to auto skool.
That is one possibility. I would focus my attention on the ignition. I had the same symptoms that you are describing. It was my OPTI and I WAS ABLE TO PROVE IT before spending money to replace it. I'm not saying that it is for sure your opti....I'm saying that it's ignition related, and that a diagnostic process should ensue to determine which component it is.
With my 86 there is specific routing for the plug wires to prevent misfiring. Since you replaced yours and now have a problem maybe it could be this.
That is one possibility. I would focus my attention on the ignition. I had the same symptoms that you are describing. It was my OPTI and I WAS ABLE TO PROVE IT before spending money to replace it. I'm not saying that it is for sure your opti....I'm saying that it's ignition related, and that a diagnostic process should ensue to determine which component it is.
Last edited by Tom400CFI; Aug 4, 2014 at 01:31 PM.
Thread Starter
Heel & Toe
Joined: Jun 2014
Posts: 15
Likes: 0
Originally Posted by
:iagree: That is one possibility. I would focus my attention on the ignition. I had the same symptoms that you are describing. It was my OPTI and [URL="http://forums.corvetteforum.com/c4-tech-performance/3020087-92-lt1-gets-warm-then-dies.html"
:iagree: That is one possibility. I would focus my attention on the ignition. I had the same symptoms that you are describing. It was my OPTI and [URL="http://forums.corvetteforum.com/c4-tech-performance/3020087-92-lt1-gets-warm-then-dies.html"
I WAS ABLE TO PROVE IT[/URL] before spending money to replace it. I'm not saying that it is for sure your opti....I'm saying that it's ignition related, and that a diagnostic process should ensue to determine which component it is.
Plug 2
Plug 4