85 Firebird intermittent lack of power



I have an 85 Firebird V6, 2.8L, tuned port injection, we bought it new and I have changed the oil every 2000 miles, it still runs great, except… The fuel pump gave out, I replaced it with a new Bosch, in the process I tried to replace the fuel filter, the line was rusted, it broke. I had to replace about 2 ft. of metal line with nylon tube, I also had to install a fuel filter with quick disconnect fittings, the filter is not the original but some generic because the original metal threaded fittings are gone. It runs great up to about ½ throttle, if I try to accelerate more than that it loses power and almost dies. If I turn off the ignition and restart the engine it runs fine for a while, full power all the way to full throttle. I’ve noticed that if I keep the fuel tank full the problem happens less often.
My theory is that the fuel pump is aerating because of not enough restriction in the fuel filter because it isn’t designed for that car, if I keep the tank full it is harder to start the aeration and stopping the pump and restarting it stops the aeration for a while. If I could get a fuel pressure gauge and be able to see it while I’m driving that would be great, but how? Also, what does the fuel pressure regulator really do? Does the fuel pump run all the time the engine is running? It also sounded like an electrical problem but it idles well and it is very smooth up to about ½ throttle. Could I just have a bad pressure regulator or does my theory sound like it makes sense?


I think you are on the right track…it sounds like a fuel starvation problem. The first thing to do is to have your fuel pressure checked. Finding the proper fuel pressure for your vehicle shouldn’t be a problem. You will probably find it low. The problem could be an improperly adjusted fuel pressure regulator, bad fuel pump, fuel line problem or improper/clogged fuel filter.

You probably need to go back to a metal fuel line and have the proper fuel filter installed. I would advise against plumbing a fuel pressure gauge/fuel line inside your vehicle because of the danger. A broken or leaking fuel line inside your vehicle can hurt/kill you in a variety of ways. Your fuel pump runs most of the time that the engine is running except for idling. A fuel injector system requires a lot of pressure and that requires that the fuel pump be operating.


The fuel pressure regulator does what it name implies. It regulates the fuel pressure at the fuel injectors.


When idling, the regulator allows more fuel to be returned back to the gas tank because there’s no demand for power from the engine. As the vehicle accelerates and more power is demanded from the engine, the regulator restricts the flow of fuel back to the gas tank. This causes more fuel to forced thru the injectors as the engine produces more power.

I’ve seen fuel pressure regulators stick where when more power is demanded from the engine, the regulator fails to restrict the flow of fuel back to the gas tank. This then causes the fuel pressure to drop at the injectors until it reaches a point where the engine won’t go any faster.

The fuel pump runs all the time at a certain pressure. It’s the regulators job to control the pressure at the injectors.



You need to consider both fuel pressure and flow. I think you have a restriction somewhere and likely in an area you recently repaired. It would only take one fitting that reduces the diameter of the fuel line to reduce the fuel flow significantly. When the motor needs more fuel than the line can deliver the pressure will drop and that will shut down the motor.


How much rust was sloshing around in the tank when the fuel pump was replaced?


Not much rust, there’s a Newton’s law of physics that says rust on the bottom of a tank tends to stay at the bottom of the tank.
Thanks for the input, I connected a fuel guage, duct taped it to the windshield and took it for a toot. I stepped on it and when the problem happened the pressure went from 36 to less than 10 ! I’m replacing the fuel pressure regulator and will see what happens.


I did the same thing as you did Bird-on-Perch, taped a fuel pressure gauge where I could see it, then drove the car, years ago, w/a VW Rabbit. The fuel pressure dropped like a rock at the exact moment the car lost power. A very telling test indeed. On the Rabbit, it had that CIS fuel injection system, and when I took it apart I found the fuel metering valve was sticking. After I cleaned it, the fuel pressure held rock steady and he loss of power problem vanished.

I think you are on the right track to a solution. You will probably just have to do a few experiments of changing things around to see what the problem is that is causing the fuel pressure to drop. It could be the fuel pump. Or the regulator, or some kind of constriction or a fuel hose collapsing.

Is the fuel pressure regulator a separate gadget? If so, there should be a vacuum line going to it. Have you removed that vacuum line and inspected to see if there is gasoline in that line? If so, that would mean the rubber diaphragm inside the regulator is leaking and the regulator needs replacing.