tealLT1

iTrader: (1)

So, I was having some other problems with my car after the build, mainly with the clutch hydraulics, but i got all of those worked out. Now after driving the car for a while it was running really strong and reliable until i was on my way home from work yesterday. First it started as a very slight miss almost unoticeable unless you were trying to maintain a specific speed, ex: 45mph. Then all of sudden it was like someone had removed half of the spark plugs! The car had a very heavy miss at idle under light and heavy load at all rpms.

Later when I got the car home I let if cool off for a few hours and checked all the plugs and wires which were also new. The number 5 plug wire had a bad burn spot on it so i replaced the wire with another from a set that a friend had given me.

The miss didn't seem as bad but still nearly makes the car undriveable. I swapped the coil as well and this made no difference at all. I used a temperature gun on all the header primaries and number 5 seemed to be about 100 degrees less than the rest of them. I have not yet run a compression test on the engine, but plan too hopefully tonight or tomorrow.

I guess my question is what is causing this, i have considered the opti, but its a new acdelco unit ~300miles. Any other suggestions? i could really use some help trouble shooting this as i am at a lost. Also during all this no service engine light or codes being thrown.

nitrous2fast

iTrader: (2)

did you change the plug on cylinder 5 along with the wire swap??

Speedy

If your wires aren't burnt you might have a loose (wobbling) rotor in your opti. Did you check to make sure it was properly screwed on when you installed it?

tealLT1

iTrader: (1)

I did not change the plug there appeared to be nothing wrong with it. I did pull it out and attach the wire to it make sure it was getting firing though. If the rotor was loose or wobbling wouldn't more than one header primary have showed to be cooler than just number 5? Would the service engine light come on if there was an actual misfire in the cylinder? I had cracked a plug once in this car when it had the 3.4 in it and the service engine light came on. it had a similar miss to it but this is much worse.

tealLT1

iTrader: (1)

I'll be sure and post the readings from a compression test tomorrow afternoon to see if maybe i can figure out where to go from there. thanks guys

mezzyshreder

Now it has an 8? What engine? If its an LT1, you need to check for a dripping water pump onto the opti spark. It may not be your cause, but its so common, its worth checking, just to be sure.

SS RRR

iTrader: (2)

I'd say concentrate on your ignition before going through the PITA of a compression test. This sounds ignition related and as suggested, coolant on opti, loose rotor and/or plug wires. Easy way to check wires is wait until it's dark, start the engine and see if there is any external spark grounding onto the engine or related components.

tealLT1

iTrader: (1)

I have checked for a leaking water pump but no dice. I also looked at the wires at night and didn't see any arcing after replacing the burnt wire. I replaced the coil and this made no diference either. Hopefully this afternoon i can check the codes and maybe find something even though the ses light isn't on. I was wondering if the icm had gone bad, but is there anyway to check it? or would the car just not start?

SS RRR

iTrader: (2)

I was always under the impression an ICM either works or it doesn't.
You can always try this:
http://shbox.com/1/4th_gen_tech2.html#opti_test

I am still thinking it's opti/rotor related...

tealLT1

iTrader: (1)

that's kind of what i thought about the icm.

results from the compression test are as follows:
1-210
3-210
5-150
7-210
2-210
4-210
6-210
8-205

So cylinder number 5 seems to be a big issue.. im kind of hoping its a valve instead of rings, but we shall see.