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I built a 67 corvette with a crate LS3 480 HP engine. I did some external mods relocating the coils off the valve covers, custom cold air intake, and a mail order tune from PCM of NC. It runs on a speed density tune. Car ran great for the first 900-1000 miles or so then began to misfire(?) and run rough. The problem has gotten progressively worse. I have Taylor Thundevolt 8.2mm wires. I checked them all and found one to be bad, so I replaced it but no change in the roughness of the engine. I pulled the plugs and they all look like this. Does this look normal or is it giving me any clue what the problem may be. My next step is to run the engine with HPtuner connected and get a data log and send it to PCM and see what they say. What else would you check?
Check ALL connections in case any might have worked loose. There's a reason the coils were put on the valve covers. Nice short wires and easy hookups...
Start with basics. Do compression / leak down of all 8 holes. Check fuel pressure during a brief rev up. Scan for codes, did any sensors drop out? What type of coils? If aftermarket, the bad wire you found may have caused a layer short in an aftermarket coil. If the spark has nowhere to go, it can jump through the insulation within the coil itself and short. Check all 8 plug wires for firing with an old timing light. Try a power-off reset within the ECM.
Thanks for the comments. I pulled the plugs and see one not looking like the others. Spark plug wire is ok. I'm using the stock coils, so I'll swap out a new coil and check the injector next. Would the black plug indicate no spark, or no fuel?
Thanks for the comments. I pulled the plugs and see one not looking like the others. Spark plug wire is ok. I'm using the stock coils, so I'll swap out a new coil and check the injector next. Would the black plug indicate no spark, or no fuel?
A black plug would normally indicate TOO MUCH fuel. Check for an injector hanging open.
You can tell it's firing because the electrode and tip show signs of heat, but further down the ground strap, the black is unburned fuel. The porcelain is also black-er than sin.
You have an abdunance of fuel in that cylinder or mechanically there is something wrong.
I'd do a leak down test on ALL cylinders and see if there is anything that matches up with your plug colors (low = dark plug, etc.).
Then swap injectors and see if the dark plug follows the change (or replace it if you feel like spending money).
A data log with HP Tuners is a good idea. Post it up on the HP board and see if anyone will take a look at it for you and give you some advice.
Ok, so I moved the injector to a different cylinder and the spark plug turned black, so problem with the injector. One spark plug wire was making contact with the exhaust manifold and arcing. So replaced injector and installed all new spark plug wires with heat sleeving and ceramic spark plug boots. Used a few wire clamps to make sure the wires stay off the manifolds. Engine runs smooth again, finally. Replacement injector from the dealer was $107, found new one on ebay for $27. Thanks to everyone for your help.
Cool beans. Thanks for the follow up. Make sure to destroy/discard the suspect injector so it does not find its way to another engine and cause another problem. Same with the bad wires.
I too often see things like these taken off then just laying around the garage. At work, we have a guy that will put a bad test lead back in the drawer, I am about to fire him over it.
02-02-2019, 09:36 PM
LS3 hot cam crate engine misfiring or else? 
