CHKEE750CF

I have a 1998 GT with 108,000 miles. I have noticed a drop in power above 3500 RPM with the throttle 3/4 open or more. I can free wheel in first to redline while light on the throttle but as soon as I push to 3/4 throttle and get past 3500 the car seems to be retarding the timing or losing a cylinder. At low RPM and full throttle the car runs fine....it just loses it when I am on the throttle hard. I had a stuck injector and replaced all 8 injectors, fuel filter, plug wires and plugs and recently I replaced the coils. Of course any time I have had a misfire the car does not throw codes. I am out of ideas and was thinking that someone would have experienced this problem and could help. Thanks for any help given!

HaV

Is it just over 3500rpms that it happens, or anytime that you are getting on it cant tell what you mean by on it hard vs full throttle? Check the obvious stuff first (you probably already have), but Ive seen Cats cause this when they start clogging up and going bad.. they seem to run fine when your not on it, but the second you get on it, the power just isnt there... If you have changed your plugs/wires and injectors already, then have you looked at your fuelpump? Make sure your airfilter is clean and if its in the stock box make sure their is nothing blocking the passageway into it limiting air.

Dan04COBRA

Replaced your fuel filter yet?

CHKEE750CF

I have replaced the fuel filter and fuel pressure is at 40 psi at idle....I need to check it on the road. The funny thing is the power loss comes with the engine feeling rough like I have a misfire....but under a slow, heavy load it wont do this....just over 3500. To answer the other question it is typically anything above 3/4 throttle. It is like a switch on or off and nothing in between.