Fuel Pump Modifications on a 1985 Pontiac Fiero 3800
Once the Fiero was fit for road use and I was able to get a few miles on it, I found that it would have a tendency to cut out after 30 minutes driving in hot weather. It would start again but then need nursing home without letting the revs drop below 2000rpm and accelerating slow enough for the fuel pump to cope with. It seemed odd to me that the fuel pump would work erratically, I expected it to either work or not and would have thought that being surrounded by a large mass of cooling gas would make it less sensitive to temperature. To rule out the relay, and other potential electrical issues, I installed a bypass switch by running some 12 gauge wire from each side of the power terminals (numbers??) on the fuel pump relay on the firewall behind the driver in the engine bay to a switch in the cabin. The plan was then to flick the switch when the car cut out, bypassing the relay and essentially wiring the pump directly across the battery. This should cause the pump to run continuously, if it doesn’t run with the switch activated, the pump is the problem.
On a day when it was about 23 degrees I took the Fiero on a test drive. No problem getting to the airport 35 minutes away but on the way back, it cut out again on the highway! I tried the fuel pump relay bypass switch but could not get the fuel pump to run, had to get a lift home with the CAA, pretty embarrassing but at least I had enough justification to replace the fuel pump.
Did some research on fuel pumps and found that an AC Delco EP381 pump (that fits a 1993 Astro van among other things) should be a direct replacement for the Fiero pump and can handle taking the 3800 motor up to about 300HP. I went over to CarQuest to see whether they had anything, and picked up an equivalent pump (a Delphi FE0114) that they had on the shelf for about $130.
Then I dropped the fuel tank and unfortunately had filled up with gas on the test drive! The pump just slots in where the old pump sat but when I got to the wiring, I found that the connection was different and there was no connector with the pump in the box. So back to CarQuest where they found me a replacement inlet screen but had to order the connector. With the connector installed, I tidied up the wiring and installed three spade connectors on the fuel tank wiring so that I can easily disconnect them if I need to get the tank out again.
After a bit of research, I decided that I would follow the recommendations to hot-wire the fuel pump and eliminate the dodgy connector that was old and cracked. I picked up some 12 and 14 gauge wire from Canadian Tire along with an in-line fuse. I found the stock Fiero wiring a bit confusing because it uses a signal from the oil pressure sender, apparently it shuts off the fuel if the engine loses oil pressure. I have since found out that the wire from the oil pressure sensor doesn’t actually shut the fuel off but rather it provides a second live feed from the battery that goes through the oil pressure sensor and into the relay – for some reason? I do not yet have an oil pressure sensor so I decided to delete this and wired it up with a direct power feed straight from the alternator to the relay, then returned the output from the relay straight back to the fuel pump through a 20A fuse. Only the output from the PCM controls the fuel pump relay. I also spliced into the ground wire that goes to the fuel tank and added another ground to the chassis because this also serves the fuel pump.
With these mods and the new fuel pump, the Fiero ran with no more issues regardless of how hot the weather got.