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Old 05-07-2013, 08:19 AM   #1
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Missfire Issues

I bought my '97 Z28 this past November. It currently has 161,202 miles on it. I had the plugs & wires replaced this weekend because I was getting a misfire code from my #8 cylinder. It's now moved to the #5 cylinder. Any ideas as to why this is happening? I'm thinking the Opti-Spark is bad considering I have no idea if it's ever been replaced. Any suggestions would be greatly appreciated.
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Old 05-07-2013, 09:07 AM   #2
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Could be type of plug or Could be a bad coil.
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Old 05-07-2013, 09:38 AM   #3
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Quote:
Originally Posted by lmb247 View Post
I bought my '97 Z28 this past November. It currently has 161,202 miles on it. I had the plugs & wires replaced this weekend because I was getting a misfire code from my #8 cylinder. It's now moved to the #5 cylinder. Any ideas as to why this is happening? I'm thinking the Opti-Spark is bad considering I have no idea if it's ever been replaced. Any suggestions would be greatly appreciated.
I would pull #5 plug and check the gap.
Also check / test the wire for #5
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Old 05-07-2013, 01:29 PM   #4
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The plugs are AC Delco. I rechecked the gap yesterday and they all check out. What's weird is that it doesn't sound or drive poorly. In fact, it's pretty quiet. I might just have to bite the bullet & replace the ICM, coil, cap, & rotor.
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Old 05-07-2013, 01:34 PM   #5
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You can have the ICM tested at most auto part stores, have them run the test several times to build up heat in the ICM, heat is what causes them to fail and when it gets heat soaked is when it would show signs of failure. A cap and rotor very rarely fix an issue with an opti. Most of the time when an opti fails its due to the optical pick up.
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Old 05-08-2013, 10:42 AM   #6
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So I took the Z28 back to the shop & he's going to pull the injector. The reason being is that the intake manifold gasket was replaced. I'm hoping it's just a matter of the injector not being seated properly.
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Old 05-12-2013, 06:24 PM   #7
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Check all your plug wires. Make sure none are burnt. I had a bad misfire because my wires were touching the exhust manifold on the passenger side near the egr tube.
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Old 05-12-2013, 10:26 PM   #8
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Ignition coils man.......
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Old 05-13-2013, 09:33 AM   #9
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Ignition coils man.......
LT1 only has one coil.
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96 T/A, Black/Graphite Leather, T-Tops, 6 speed, 10 speaker Delco/Bose "Monsoon"
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AIR delete. Bored .030 over.
Comp cams Xtreme Energy LT1 266HR-14 street cam and springs..
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Old 05-14-2013, 05:22 AM   #10
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So last Tuesday I took it back to the shop. The mechanic said one of the wires on the #7 injector was causing the misfire. He fixed that & I got the car back. Then yesterday as I'm driving home from work the check engine light flashes & it's running rough again. I go to O'riley to scan it & it gives me a p0300 random misfire detected code. I obviously need to view the live data to rule out a specific cylinder but this is getting aggravating. Why would it drive fine for a few days then run like shit again?
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Old 05-14-2013, 07:11 AM   #11
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Update: Just got my scanner back & it says I have a #7 misfire again. So now I need a meter to check the voltage going to the injector. Or can I swap injectors & see if the misfire jumps to that one?
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Old 05-14-2013, 07:38 AM   #12
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Update: Just got my scanner back & it says I have a #7 misfire again. So now I need a meter to check the voltage going to the injector. Or can I swap injectors & see if the misfire jumps to that one?
The injectors have power to them at all times, the PCM grounds the injector to pulse it.

You need a fuel injector noid light to test the circuit.
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Comp cams Xtreme Energy LT1 266HR-14 street cam and springs..
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Old 05-14-2013, 01:03 PM   #13
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Quote:
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The injectors have power to them at all times, the PCM grounds the injector to pulse it.

You need a fuel injector noid light to test the circuit.
So you unplug the the injuctor & then plug the light into the pigtail?
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Old 05-14-2013, 04:50 PM   #14
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Quote:
Originally Posted by lmb247 View Post

So you unplug the the injuctor & then plug the light into the pigtail?
Yes...
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Old 05-14-2013, 05:23 PM   #15
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Yes...
Can the same thing be accomplished with a meter to measure the voltage? I read on another site there should be 12-15 volts going to the injector.
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Old 05-14-2013, 05:46 PM   #16
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Can the same thing be accomplished with a meter to measure the voltage? I read on another site there should be 12-15 volts going to the injector.
System voltage goes to the injector, and you can read the voltage to see if in fact it is getting voltage, but.what needs to be checked is that the PCM is grounding it to pulse it, the pulse can be so short that a meter might not pick it up,
Hence the need for the noid light.

You can read the ground ( PCM side) of the harness with a meter in order to see if it is shorted to ground and keeping the injector open.

You can read the resistance in the injector with a meter to see if it is open and not working.
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Total engine rebuild at 187,020 miles.
AIR delete. Bored .030 over.
Comp cams Xtreme Energy LT1 266HR-14 street cam and springs..
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Old 05-15-2013, 02:29 PM   #17
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So the injector ohmed out @14 ohms & the pigtail has 12 volts going to it. I cleared the fault & let idle in my work parking lot. Once it got to running temperature it started throwing the p0307 code again. Maybe the wires are getting too hot and it's losing voltage?????
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Old 05-16-2013, 05:09 PM   #18
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Here's the culprit:

Click image for larger version

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The dude who installed the plugs & wires effed the #7 wire up. It was melted & he tried to rectify that by wrapping the wire in plastic conduit & eletrical tape. Moron! I rerouted the new wire so that it didn't rest up against the exhaust manifold.
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Old 05-16-2013, 06:27 PM   #19
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Thank you to everyone who responded to my post. I appreciate everyone's insight and knowledge.
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