A couple of days ago, my 1991 F150 4.9L engine started sputtering and died while driving. On investigation, the fuel pump relay was chattering intermittently. The fuel pump is audible when I turn the ignition to on. For a reason I don't understand, the truck started again and I managed to get back home. Yesterday it started fine and I ran an errand driving about five miles. It ran poorly, but ran. Today it won't start at all.

I've replaced the fuel pump relay, but it still chatters intermittently when I turn on the ignition switch.

I also disconnected the fuel line downstream of the filter and there seems to be plenty of pressure there.

Also, noticed that the fuel pump seems noisier than I remember.

Got any ideas?

Thanks,


Tom Herrick

 

If the relay is chattering, you have a bad connection either to the control voltage or the power supply of the relay.

 

Additionally, the fuel pumps do not stop after turning the ignition to on like they usually do. Typically, when I turn on the ignition switch I can hear the fuel pump for a couple of seconds then it cuts off. Since this problem began, the pump stays on as long as I keep the ignition turned to on.

Further, when the relay chattered, it sounded as though something else nearer the injectors was also chattering; but, it could just be my aging ears...

Thanks,

Tom Herrick

 

That usually means you have a bad EEC.
Bad caps.
I bet you can not pull codes either.

 

When you first turn the ignition on but do not have the engine running that is called key the on engine off (KOEO) mode. You hear the fuel pump run for a few seconds until it stalls against the head pressure in the fuel system. If you hear it continuing to run that means that it is not able to build enough pressure to stall the pump. You probably have a leak somewhere in the fuel system, possibly either a leaky injector or a bad regulator.

Pull the vacuum line on the regulator, if there's gas present in the line, replace the regulator. Clamp the flexible portion of the fuel return line closed, if the fuel pump then stalls like it should in KOEO, replace the regulator.

If neither of these works, clamp the flexible portion of the fuel supply line closed, if that causes the pump to stall at KOEO, you have a leaking injector.

The noise you're hearing in the fuel rail is gasoline either bypassing the regulator or leaking through an injector. I find it improbable that an injector would leak that badly so I tend to suspect the regulator; they are a known service area. If it is an injector, pull the plugs and look for the one that is very, very fuel fouled and replace the injector in that cylinder. do not drive the truck in the meantime, you will ruin the walls of that cylinder.

If neither of these works, the problem is with the pump or its controls, but that seems unlikely given the new relay and the other symptoms.

Good luck

 

The Fuel pumps run for two seconds, they do not turn off against the head pressure in the fuel system. They are timed by the PCM for a two second run time. The PCM (EEC) does not know how much fuel pressure you have, it only checks the output wire (Brown) of the fuel pump relay. The Fuel pressure for an eight is regulated at 35 to 45 psi depending on the vacuum the engine has and 45 to 55 for a six, so there is no way for it to build pressure to a head high enough to stall the pump.

The only way a pump will run after the two seconds is for the PCM (EEC) to receive a pulse train form the Ignition system, this is a safety feature.

For testing the fuel system on 95 and older trucks you need to ground pin #6 of the DLC (test connector).

If you are not careful when clamping the flexible portion of the fuel supply line closed the pressure is going to go very high and more than likely destroy the selected pump if not rupture a line.

 

Bingo.

A no start condition with a fuel pump that continues to run is almost always a bad ECM. If, as suggested, you are unable to pull codes as well, that's the final nail in the coffin.

A clattering fuel pump relay is the traditional way GM vehicles alert the tech that the ECM is bad, but it happens with Fords as well.

Check for codes. If you can't get any, replace the ECM.