Misfire diagnosis
Thread Starter
Racer
Joined: Oct 2021
Posts: 436
Likes: 15
From: Maryland
Misfire diagnosis
Hello, all, my 82 is starting to do the bad things again. I think I have a misfire,
symptoms are more shaking at idle, and less power. No wierd exhaust sounds or smoke, so could be something else, but I’m leaning to misfire. I also have new spark plugs in each cylinder.
ecm throws lean exhaust code, but that is prob due to the hole in my driver side exhaust manifold. I am going to diagnose with the following steps
1) attach timing light to all wires, check for inconsistencies
2) pull each spark plug wire with the car running, listen for change in sound
3) if that doesn’t give any clues, compression test each cylinder
4) check fuel pressure
questions:
1) I have a feeling that even if a cylinder isn’t completely misfiring, it might have a weak detonation. How can I diagnose this?
2) any other diagnosis step suggestions?
symptoms are more shaking at idle, and less power. No wierd exhaust sounds or smoke, so could be something else, but I’m leaning to misfire. I also have new spark plugs in each cylinder.
ecm throws lean exhaust code, but that is prob due to the hole in my driver side exhaust manifold. I am going to diagnose with the following steps
1) attach timing light to all wires, check for inconsistencies
2) pull each spark plug wire with the car running, listen for change in sound
3) if that doesn’t give any clues, compression test each cylinder
4) check fuel pressure
questions:
1) I have a feeling that even if a cylinder isn’t completely misfiring, it might have a weak detonation. How can I diagnose this?
2) any other diagnosis step suggestions?
Last edited by nitan2k; Aug 28, 2022 at 10:07 AM.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,150
Likes: 4,260
From: US-PNW
Good to see you back Nitan.
A simple, quick test is to hook up a timing light to each plug wire and watch the strobe pattern - you won't run the risk of getting zapped either! If you find an inconsistent pattern, try switching wires with an adjacent plug that has a consistent pattern to test the wire vs. the plug or distributor tower.
A simple, quick test is to hook up a timing light to each plug wire and watch the strobe pattern - you won't run the risk of getting zapped either! If you find an inconsistent pattern, try switching wires with an adjacent plug that has a consistent pattern to test the wire vs. the plug or distributor tower.
Race Director
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From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
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Cruise-In VIII Veteran
The other thing to check is fuel pressure. These crossfire cars like 12 - 13 PSI. if it drops it'll run like you describe.
Thread Starter
Racer
Joined: Oct 2021
Posts: 436
Likes: 15
From: Maryland
Good to see you back Nitan.
A simple, quick test is to hook up a timing light to each plug wire and watch the strobe pattern - you won't run the risk of getting zapped either! If you find an inconsistent pattern, try switching wires with an adjacent plug that has a consistent pattern to test the wire vs. the plug or distributor tower.
A simple, quick test is to hook up a timing light to each plug wire and watch the strobe pattern - you won't run the risk of getting zapped either! If you find an inconsistent pattern, try switching wires with an adjacent plug that has a consistent pattern to test the wire vs. the plug or distributor tower.
I was actually considering doing that timing light thing too lol. I got wierd looks last time I told a mechanic I did that, so I didn’t write it, but good to know that it’s a valid step. I’ll prob do that as step 1
Thread Starter
Racer
Joined: Oct 2021
Posts: 436
Likes: 15
From: Maryland
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
On some Timing Lights, they have a tendency to falsely show a misfire with the strobe. It's quite common for the instrument to get confused about the HEI signal it is supposed to receive and respond. This scenario often shows up on the harmonic balancer as a mark that may jump around or completely disappear for a fraction of a second. So keep that in mind.
EBay has a Sparkplug Wire Tester for a few bucks that has both female / male ends and a bulb. Simply installing it in-line with a suspected cylinder will show if it's getting juice. It will not show however, if the plug is actually firing. Nor will a timing light. Nor will holding a plugwire to GRD. Those test only show that the cap, rotor, plug-wires are ok. But not the plugs condition itself.
Read a few articles about plug gap last week. For decades the std gap for HEI was 0.045 as opposed to earlier 0.035 with points.
The article went on to say that 0.045 does not give the coil enough time to recoup before the next firing?
Because I believe 20% of what I read on the net, I re-gapped to .040 (happy medium)
EBay has a Sparkplug Wire Tester for a few bucks that has both female / male ends and a bulb. Simply installing it in-line with a suspected cylinder will show if it's getting juice. It will not show however, if the plug is actually firing. Nor will a timing light. Nor will holding a plugwire to GRD. Those test only show that the cap, rotor, plug-wires are ok. But not the plugs condition itself.
Read a few articles about plug gap last week. For decades the std gap for HEI was 0.045 as opposed to earlier 0.035 with points.
The article went on to say that 0.045 does not give the coil enough time to recoup before the next firing?
Because I believe 20% of what I read on the net, I re-gapped to .040 (happy medium)
Burning Brakes
Joined: Dec 2005
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1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,150
Likes: 4,260
From: US-PNW
On some Timing Lights, they have a tendency to falsely show a misfire with the strobe. It's quite common for the instrument to get confused about the HEI signal it is supposed to receive and respond.
...
Read a few articles about plug gap last week. For decades the std gap for HEI was 0.045 as opposed to earlier 0.035 with points.
The article went on to say that 0.045 does not give the coil enough time to recoup before the next firing?
Because I believe 20% of what I read on the net, I re-gapped to .040 (happy medium)
...
Read a few articles about plug gap last week. For decades the std gap for HEI was 0.045 as opposed to earlier 0.035 with points.
The article went on to say that 0.045 does not give the coil enough time to recoup before the next firing?
Because I believe 20% of what I read on the net, I re-gapped to .040 (happy medium)
What does the article mean about the gap affecting the coil's recovery time? I'm not seeing the connection to how gap affects that - an HEI thing again?
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Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
Evidently, the longer or I should say wider the gap / longer spark takes more juice from the coil, so to speak. The more juice going out, needs to recover for the next firing. This is known as dwell. And I know it's important part of the old "points / condenser" style IGN.
I never thought of a more modern HEI having dwell limitations. Kind of makes sense. I guess you can't have a constant 50,000 volts? IDK
So the article says to close up the plug gap a smidge and not worry about complete burn of the flame front.
Who Knows?
It was another one of those hot rod, how to get more power stories on the net. Also known as "click-bait".
Take it with a grain of salt, I guess. Interesting story, none the less.
I never thought of a more modern HEI having dwell limitations. Kind of makes sense. I guess you can't have a constant 50,000 volts? IDK
So the article says to close up the plug gap a smidge and not worry about complete burn of the flame front.
Who Knows?
It was another one of those hot rod, how to get more power stories on the net. Also known as "click-bait".
Take it with a grain of salt, I guess. Interesting story, none the less.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,150
Likes: 4,260
From: US-PNW
Evidently, the longer or I should say wider the gap / longer spark takes more juice from the coil, so to speak. The more juice going out, needs to recover for the next firing. This is known as dwell. And I know it's important part of the old "points / condenser" style IGN.
I never thought of a more modern HEI having dwell limitations. Kind of makes sense. I guess you can't have a constant 50,000 volts? IDK
So the article says to close up the plug gap a smidge and not worry about complete burn of the flame front.
I never thought of a more modern HEI having dwell limitations. Kind of makes sense. I guess you can't have a constant 50,000 volts? IDK
So the article says to close up the plug gap a smidge and not worry about complete burn of the flame front.
Safety Car
Joined: Jun 2017
Posts: 4,786
Likes: 425
From: North Georgia, USA
Another useful tool to find which cylinder(s) are misfiring is the infrared "laser" thermometer. Point it at the exhaust manifold or header pipe, about a half inch from the head. All the manifold sections or header pipes should be about the same temperature at that location. If one (or more) are more than 50°F cooler than the rest, that cylinder is misfiring.
Have you replaced the dizzy cap lately? I've gotten a rash of bad HEI dizzy caps for both straight sixes and V8s in the past couple years. Almost every "coil in cap" dizzy cap I've gotten has had misfiring issues. They don't make them like they used to. On the 6 cylinder I had this issue with we went with a 1975 6 cylinder HEI dizzy with the separate coil (not in the cap) and found a good cap for that design (and we used a MSD Blaster SS coil with that). On the V8, we ended up going with a performance aftermarket distributor, again with a separate MSD coil.
Have you replaced the dizzy cap lately? I've gotten a rash of bad HEI dizzy caps for both straight sixes and V8s in the past couple years. Almost every "coil in cap" dizzy cap I've gotten has had misfiring issues. They don't make them like they used to. On the 6 cylinder I had this issue with we went with a 1975 6 cylinder HEI dizzy with the separate coil (not in the cap) and found a good cap for that design (and we used a MSD Blaster SS coil with that). On the V8, we ended up going with a performance aftermarket distributor, again with a separate MSD coil.
Thread Starter
Racer
Joined: Oct 2021
Posts: 436
Likes: 15
From: Maryland
So I did step 1, checked firing with the timing light. No weird patterns on any of the wires. Took it to a mechanic earlier today for wheel balancing (separate issue), he said he noticed it was smelling very rich. So I'm thinking that maybe the ECM is making the engine run rich to counter the lean readout its getting?? If this is the case, then I think I should finally get to fixing the exhaust leak I've always had. Made a separate thread here.
Thread Starter
Racer
Joined: Oct 2021
Posts: 436
Likes: 15
From: Maryland
Another useful tool to find which cylinder(s) are misfiring is the infrared "laser" thermometer. Point it at the exhaust manifold or header pipe, about a half inch from the head. All the manifold sections or header pipes should be about the same temperature at that location. If one (or more) are more than 50°F cooler than the rest, that cylinder is misfiring.
Have you replaced the dizzy cap lately? I've gotten a rash of bad HEI dizzy caps for both straight sixes and V8s in the past couple years. Almost every "coil in cap" dizzy cap I've gotten has had misfiring issues. They don't make them like they used to. On the 6 cylinder I had this issue with we went with a 1975 6 cylinder HEI dizzy with the separate coil (not in the cap) and found a good cap for that design (and we used a MSD Blaster SS coil with that). On the V8, we ended up going with a performance aftermarket distributor, again with a separate MSD coil.
Have you replaced the dizzy cap lately? I've gotten a rash of bad HEI dizzy caps for both straight sixes and V8s in the past couple years. Almost every "coil in cap" dizzy cap I've gotten has had misfiring issues. They don't make them like they used to. On the 6 cylinder I had this issue with we went with a 1975 6 cylinder HEI dizzy with the separate coil (not in the cap) and found a good cap for that design (and we used a MSD Blaster SS coil with that). On the V8, we ended up going with a performance aftermarket distributor, again with a separate MSD coil.
Melting Slicks
Joined: Feb 2007
Posts: 2,709
Likes: 269
From: S.E. MI
St. Jude Donor '15
I had my motor out of the car to freshen it up and noticed the lobe worn. I always had a miss I couldn't get rid of. I ended up getting a crate motor.
You could check the lobes with a dial indicator on the rocker arms to measure the total lift on them while rotating the engine by hand
You could check the lobes with a dial indicator on the rocker arms to measure the total lift on them while rotating the engine by hand
Thread Starter
Racer
Joined: Oct 2021
Posts: 436
Likes: 15
From: Maryland
Thoughts on this? Potentially our cause? Im thinking I should still compression check too while im here. I'll make a separate thread here:
https://www.corvetteforum.com/forums...post1605624399
https://www.corvetteforum.com/forums...post1605624399
Last edited by nitan2k; Sep 2, 2022 at 12:56 AM.