Fuel pump won't stop priming

[Explorer_Mooch]

Hey, all. First time poster, here. So here's my problem: When I turn the key to my 91 Explorer, the fuel pump starts priming, but never shuts off. The engine will run, but just barely. I'm guessing that it is either the PCM or the cam posish sensor. Is this a common problem with Explorers of this year? How can I tell if it's the PCM or the cam sensor?

 

[MrShorty]

IMO, the first step in checking the CMP or the PCM would be to pull codes from the computer to see if the computer can see the problem.

In this case, though, a sensor problem isn't the first thing I would suspect. The first things I would suspect/check:
a) short to ground in the wire from the coil side of the relay to the PCM.
b) relay mechanically stuck in the closed position.
c) Someone has jerry rigged a wire to bypass the relay.

 

[lqy125]

I would check the fuel pressure first, if it is low, then there are two possibilities, first is the pressure regulator failed, the fuel return to tank without any resistence (no pressure buid-up to trigger the pump to shut down and low flowrate for fuel injector), second is that the fuel pump failed to generate pressure, caused by internal leak or worn gear, need a new pump.
Hope this will help.

 

[unclemeat]

I would replace the fuel pump relay. Use a motorcraft replacement. They are about $15 and take about 15 minutes to replace. It is located under the fuse box, under the hood. It is the green one in the middle. The relays go bad and the fuel pump does not shut off. It is a common problem.

 

[Explorer_Mooch]

Igy125, are you sure there is a pressure transducer in the pump to tell it to shut off? For cars of this era, I think the pump is always on and the excess fuel just gets bypassed to the tank. If it were the fuel pump relay stuck on, then the car should still run fine, I would think, just the pump would always be on as long as the car was on.

 

[BrooklynBay]

The pump is only supposed to turn on for a few seconds when you turn on the electrical system, then turn off. It only runs continuously when the vehicle is running. The computer controls the pump's operation, so it knows when the vehicle is not running (no pulse signal from the crankshaft position sensor).

 

[lqy125]

Explorer_Mooch: It seems Brooklybay answered your question already. There is a presssure transducer in the fuel deliver system to ensure proper pressure buid-up when the key is in 'on' position. You used the word 'pump priming', it is just to build the pressure in fuel line.
Since you said that your pump is continuous priming, but engine run barely , I do not think there is any problem with your electric system, if the relay stuck at close position, then your truck should run fine, but you may shorten your fuel pump life.

So check your fuel line pressure first, it should be around 30 psi (not sure, you can check your repair manual).
Also, you can take the vacuum line out at the top of pressure regulator and put some vaccum (that will increae regulator pressure) to see if the engine run better.

 

[BrooklynBay]

I don't think there is a pressure sensor. When the engine is turning, then there is a pulsed signal going to the computer (CPS). This tells the computer to keep the fuel pump turned on. The 3 second timed pulse is preset in the computer to prime the fuel system. Your system keeps running, and doesn't have that 3 second pulse. It might be a stuck relay, or an internal short in the computer. Most likely it is just the relay. Refer to Mr. Shorty's post.

 

[marragtop]

Start with the relay. It's <$10 and 5 minutes to install.

 

[unclemeat]

Start with the relay. It's <$10 and 5 minutes to install.

Explorermooch,

If you have a 91 Explorer and you are having fuel pump problems replace the relay, if it is not bad this time next time it will be. The fuel pump relay is a common in older Explorers.