How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Arench Your Own Question
Arench
Arench, Master Technician
Category: RV
Satisfied Customers: 1490
Experience:  30-years experience auto and truck repair.
13630601
Type Your RV Question Here...
Arench is online now
A new question is answered every 9 seconds

1991, P30 RV 454 engine. Fuel pump used to stay on seconds

Customer Question

1991, P30 RV 454 engine. Fuel pump used to stay on for 17 seconds after key turned off. Now it stays on several minutes. I have replace all three relays and fuel pump modual.
Submitted: 1 year ago.
Category: RV
Expert:  Larry Welch replied 1 year ago.
Hi Larry here. How hard is the engine to start after the pump shuts down and you try to start again?
Customer: replied 1 year ago.
No problem. I'm presently un plugging the fuel pump. When I need to leave I plug it back in. The pump will be off then. The only item in the circuit that controls time is the computer. It's design is to keep on for 17 to 20 seconds to over come vapor lock related to that engine. What circuit am I missing and why does the fuel pump have three leads in the plug?
Expert:  Larry Welch replied 1 year ago.
I don't think you are missing anything. The timer is malfunctioning. Don't forget to rate and thanks for the question.
Expert:  Arench replied 1 year ago.
I saw this question open and thought I could help.My name is ***** ***** I will try to help you with this. Lets start with some background and some facts which you have wrong. The 7.4 EFI in 1991 was troubled by vapor lock during hot driving conditions due in part to low design fuel pressure of 12psi.Some modifications may have been made that are not visible on your trucks wiring diagram. Please keep in mind I can not anticipate changes that other people may have made to the truck. Since I can not see the truck myself I have to hope and pray the truck's wiring is stock. Sometimes time reveals that problems stem from modifications made by others which I have no way of knowing. The factory had two or three power sources for the fuel pump. One is the fuel pump relay which is controlled by the PCM. Time is instant on with the key and instant off. Two is the oil pressure switch. The switch closes with engine oil pressureto reduce the circuit load on the fuel pump relay. This can stick closed causing the problem. Three is the hot fuel module (sometimes called the fuel cycling module) which you may or may not have. This is designed to run the fuel pump after engine crank/stall during hot soak. Time delay 20 seconds. Two devices, the fuel pump relay and the oil pressure switch are easily unplugged for diagnosis. The hot fuel module is powered by fuse 5. [guages] Use the process of elimination to see which action stops the fuel pump. If it turns out pulling the gauge fuse stops the fuel pump then unplug the hot fuel module, usually found near steering column or taped to the harness near the pcm. Here is an example of what it looks like, a black box with 3 wires. http://noes.net/hfm.jpg