4.9l fuel pump runs constantly; engine won't start - Pirate4x4.Com : 4x4 and Off-Road Forum
 
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Old 05-12-2013, 12:47 PM   #1 (permalink)
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4.9l fuel pump runs constantly; engine won't start

Got a 93 F150 with a 4.9l


Started the truck up and drove it out of the garage. Once out, it died in the driveway like it was being starved for fuel. It started a few more times and ran for a few seconds, then died Now the engine turns over, but it will not start.


With the key on and engine off, the fuel pump runs constantly. With the key off, the fuel pump shuts off.

With the fuel pump relay pulled out, with the key on, the fuel pump does not turn on. (I've read through searching that if a bad PCM, the fuel pump will kick on with the relay pulled, indicating a bad PCM)


Because the pump does not run without the relay, this led me to believe it was the fuel pump relay. I replaced the relay with a new duralast unit from vatozone. With the new relay in place, key on engine off, the fuel pump starts, and then shuts off like it should.


Tried to start the truck, it burped a second like it was trying to start, then nothing. Now with key on, the engine turns over, but does not start.

Also, with the new relay installed, with the key on, the pump runs constantly, just as it did before I replaced the relay.


What could be the cause of this?
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Old 05-12-2013, 03:22 PM   #2 (permalink)
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Just replaced the fuel filter... It needed to be replaced regardless...

Put the new relay in and turned the key on. Fuel pump started, then shut off like it should. Few seconds later it kicked back on and only shuts off when the key is turned off...

Any recommendations?
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Old 05-12-2013, 03:29 PM   #3 (permalink)
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Well sumbitch... Went back out and tried it again, and it fired up after a few cranks... Guess I didn't give it enough time to fully pressurize the fuel lines..

I'm going to say that replacing the filter had more to do with it than the relay...
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Old 05-12-2013, 05:59 PM   #4 (permalink)
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Wow, outside OSR?

It has been awhile for this vintage truck for me;

Does yours have the low pressure pump in the tank and the high pressure pump mounted on the frame rail on the passenger side? If so, one of those could be going wonky on you as well, next time that happens check for pressure at the fuel rail? I think 93 was the cutoff when they went to high pressure pumps in the tank only...

Glad you got er going.
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Old 05-12-2013, 06:32 PM   #5 (permalink)
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If the PCM is bad then it will ground the relay causing the pump to run constantly. Every now and then this happens if I get a bad write while doing a custom tune.


The pump can't run at all if the relay is pulled.



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Old 05-12-2013, 06:42 PM   #6 (permalink)
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Quote:
Originally Posted by Whitetrash303 View Post
If the PCM is bad then it will ground the relay causing the pump to run constantly. Every now and then this happens if I get a bad write while doing a custom tune.


The pump can't run at all if the relay is pulled.



.


This was my understanding as well, hence why I pulled the relay to see if the pump would run with the key on. It didn't, so I assumed the relay was stuck.


Is there a sensor upstream of the filter that tells the PCM that the system is pressurized, thus kicking off the pump? If not, what tells the pump to shut off before the key is turned?


If I were taking a wild-ass-guess, I'd say the filter wasn't allowing enough fuel to pass through to reach the correct pressure at the rail, which would keep the pump running. But I know very little about this fuel system, so I am just thinking out loud on this...
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Old 05-12-2013, 06:59 PM   #7 (permalink)
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The PCM has no idea if there is fuel pressure. It just energizes the pump 2 seconds to make sure there is pressure to start the engine. I worked on a late 90s exploder that had the same problem, turned out that a bad ignition wire was arcing to a nick in the fuel pump relay trigger wire from the PCM and killing the PCM.
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Old 05-12-2013, 07:22 PM   #8 (permalink)
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Correct, the PCM gives a 2 second burst at key on to pressurize the system. After that it only runs if it gets pip signal from the dizzy.
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