04 GTO engine w spark to only every other cylinder?????

[truckfan2]

This is the oddest thing I have encountered working on LS engines. The engine is in a road course 67 firebird that was running a stock 04 gto LS1 getting the crap beat out of it for the last 2 seasons without issue. My buddy decided to stroke the motor and added a BTR intake, fast fuel injectors, and a new holley 92mm dbc throttle body. The motor was dynoed at 550 hp at Speedway Motors engine shop before he took delivery of it to ensure no issues with it upon installation. When he tried to fire the motor he mentioned that is ran terrible. I mentioned that we will have to do quite a bit of tuning for all the changes so I headed over there with my laptop. Very long story short is the engine is running on 4 cylinders? Spark is missing on 2 cyl on each side? Cylinders 1, 7, 4, and 6 have no spark which corrolates to every other cylinder in the 18726543 firing order???

I tried his ecm on my test stand with another stock LS1 and it was fine.
Swapped coil packs from side to side with no change.
Ran a whole new standalone wiring harness to the motor and no change.
Swapped several spark plug wires around with no change.
Put in the tune that was running last summer with no change.
Installed another new GM crank sensor with no change.
Pulled out the crank sensor and verified the reluctor wheel was not loose or damaged.


We are out of ideas so I figured I would see if anyone else has ever seen this?

[blindsquirrel]

Reluctor is flipped around front-to-back on the crankshaft.

[truckfan2]

Do you think the MSD box would have been able to ignore this? The motor was dynoed with a 750 carb and a msd box to run the coils. I agree it really seems like a reluctor issue but the dyno numbers confuse me....

[truckfan2]

Reluctor is flipped around front-to-back on the crankshaft.

I just found another dudes post from last year with the exact same issue so yes, it sure sounds like the reluctor is on backwards. Motor coming out today, thanks for the reply man!

[blindsquirrel]

The 24x reluctor is much more sophisticated than a plain toothed wheel, it's more like a resolver. The MSD box likely only uses the one main reference notch and then assumes (or ignores) the rest, while the stock PCM is gathering a lot more information about actual position during a single revolution.