hey all first time poster here. I am having the same issue with my 98 GT. First project car ever and looking to help! I want to keep this thread ongoing i have scoured the internet for a solution. I have been going through the whole checklist.
Ny specifics is power loss. I bought the car used with an automatic transmission issue. I replaced the trans. And car drives and has power under light load. As soon as i go uphill or stomp to get on highway it bucks and loses power. If I shut the vehicle off and turn back on I will have normal power back until under load again.
I changed spark plugs, wires, coil pack, no change.
Cleaned the throttle body, and maf sensor no change.
I checked fuel pressure and turn key on without start, pressure will go to 36 psi. If I start the igniton pressure drops to 28 psi at idle. If I disconnect the vacuum line it shows 38 psi. Haynes manual shows I should be between 30 and 45 psi at idle, and between 40 and 50 psi with vac hose off. Also if given any throttle it drops slightly to about 36 and will only go as high as 33 psi with some revs.
I suspect it is fuel system since when I turn the car off and back on, the pump will repressure the lines and give me power until it struggles to keep up again. Bought a fuel filter and fuel pump and attacking those next. Will post results.
Other specifics about the car from previous owner is
magnaflow cat back exhaust, and what appears to be an aftermarket upsized throttle body. Anyone know these upgrades would cause a lack in fuel? Maybe I should upgrade to high flow fuel pump?
Hopefully I can find what solves my issue and can help other forum users down the road!