I had my L98 replaced in my GTA by GM performance engines and have had terrible gas mileage ever since. Like around 8 mpg. I disconnected the brown with black striped wire and put my timing light on it and the timing mark on my harmonic balancer is off by 2 inches from the saw tooth towards the drivers side. I tried bringing the mark up to the saw tooth, but the engine bogs down terribly. What could be the problem here ? Could it be GM performance assembled the cam to the crank a tooth off on the gears with the timing chain ? Funny thing is the car idles @ 600 rpms with no miss. There is hesitation though when I open the throttle body fast.

 

Dude, what cam is in that engine? If it is a flat tappet cam in there and you are using the stock distributor then the gears on the distributor/cam are different. From what i remember about this is when i was into the Ford 5.0 motors. The dist. gear on the H.O. was steel , designed to work with roller cams and the flat tappet cam motors used an iron gear. If you mismatch them i don't think you will ever be able to time the car correctly. It will just tear up the gears eventually. So, basically you do not want to run a flat tappet cam with a dist. designed to work with "roller" cams. Check into it.

 

Also, are you using the stock flexplate and harmonic balancer? They should both match.

 

This is a stock L98 complete from GM Performance

 

Sounds like you have a damper with a 1:30 timing mark, but a timing cover with a 12:00 timing tab.

Line up the engine to #1 or #6 TDC (same thing, just use whichever is easiest) and mark your damper to match your timing tab.