mjricha

Ive serached and searched and cant find anything that addresses the weird problem I am having. Other threads seemed have similar problems and tried those fixes before making a new thread. I just bought a 91 f150 4.9l manual. The guy who had it before me had bypassed the electrical fuel system and just hard wired the fuel pumps (through the fusebox) to turn on when the key is on. So I go to fix it, and this is what it does. No power to fuel pumps when truck is turned on. I replaced the pcm and the fuel pump relay and the inertia switch and it worked fine for three days. Then the truck was dead when I started it and the fuel pumps no longer work. Bypassed the fuel relay and the fuel pump kicks on. So I got out the haynes and traced all the wires. They all seemed fine. I have dual tanks btw and they both seem to pump fine, and the fuel tank switch works fine. So my question is what could this possibly be? I ran into this problem because my exhaust smells like gas, and was trying to find the reason im running so rich. Other than that the truck runs fine. Ive been driving it for a week, and other than the exhaust smelling bad I havent had any problems. Any help at all would be appreciated!

Rusty but trusty

Fuel pumps are not gonna cause you to run rich. Failed o2 sensor, failed or removed catalytic converter or engine not reaching proper operating temp will have the exhaust smelling like fuel.

billiards101

Amen.I second that..

Chris_1

iTrader: (1)

Hi ! Newbie here (but I've been around these trucks as long as they've been around). I don't know if this will relate to your current problem, but you want to fix that wiring properly anyway, 'cause it's set up the way it is for a reason.
The PCM and it's controlled components run at lower voltage (about 6.5). My understanding of the fuel pump system (and anyone can feel free to correct me ) is this : The F/P relay is a 2 stage setup. When you first turn the key to ON one side of the relay is activated by the PCM and delivers 12 volts to the pump for approximately 2 seconds and stops. Then, if the truck is running (and only if it's running ) the other side of the relay is activated and delivers 6.5 volts to the pump continuously.
Not having it wired that way can possibly cause things to fry, or at least work harder than they were supposed to. Also your fuel pump should shut off when the motor isn't running, even if the key is still on.
Something I found once was that one of the wire connections under the relay was poor and it was causing intermittent fuel pump failure. It's hard to get at, you basically have to take apart the relay box under the hood to get at the underside of the relays but it's a definite possibility (especially if things work when you bypass it by hard - wiring things ) Something to check if all else fails anyway

pon55

So check the relay.

mjricha

Thanks for the Input guys. Im going to test the 02 sensors today and see if that will clear up my running rich problem. I switched out the relay, and it blew a few days later. The problem is, the engine bay was covered with grease and stuff (from previous owner), so I cleaned it all out, and right after that was when the new relay blew. I dont know if its because of that (water in the relay) or if its the wiring. I have taken the relay box out and can see the undersides of all the harnessess. They all look ok. If I jumper the relay the fuel pump will turn on when the key is on. Plus, testing it with the volt meter shows twelve volts when the key is on as well. I am not getting any readings off the relay harness that show 6.5 volts. Im just going to get another relay. I also ordered wire to re wire the relay, I just have to wait to get it in the mail.

Nitehawkjr

The PCM doesn't send voltage to the relays it furnishes the ground. Also I believe the timer is in the PCM not in the relay. So I would be checking for a ground problem if you have 12 volts going to the PCM relay and the pump relay.

5Rangers

Thank NH for saving me from making the correction post. I read and thought , here we go again.

mjricha

thanks NH! I just ended up rewiring the entire fuel system. Started up and runs just fine now. Havent checked the 02 sensors yet though. Ill let you guys know what I find. Thanks again!