You’ve got questions, we’ve got answers. We work with the Summit Racing tech department to help you tackle your auto-related conundrums. In this week’s Mailbag, we’re troubleshooting a Pontiac Trans Am Tuned Port Injection engine that’s experiencing both idle and off-idle stumbling.

Q: I have a 1988 Pontiac Trans Am GTA with the 350 engine. I installed an SLP three-inch cat-back exhaust system, 180-degree thermostat, and a K&N air filter. The car has 50,000 miles on it and has never been in the rain or snow.

Recently, I have noticed that when I start the engine cold, it stumbles and dies out. On the second try, it starts and runs fine. I have also noticed the acceleration is not what it used to be, especially with the air conditioning on. The car will also stumble and try to die from a standing start with the transmission in Drive. It seems like the engine isn’t getting enough gas at idle.

I did the last tune-up on the car about four years ago. Could the problem be dirty fuel injectors, a bad fuel pump, idle speed control, or something not fuel system-related?

chevy 305 tpi v8 small block engine on rolling engine stand in a shop
(Image/Richard Holdener)

A: The Tuned Port Injection engines in these cars have a reputation for developing both vacuum leaks and dirty throttle bodies. Either of these can cause the idle and off-idle stumble problems you’re having. We recommend looking at both to find a solution to your problem.

Also, be sure to check your fuel pressure. It should be 36 psi at idle.