2002 4.7L Intermittent Loss of Fuel Pressure

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RobertsRam

Junior Member
Joined
Jul 22, 2022
Posts
4
Reaction score
1
Location
Houston TX
Ram Year
2002
Engine
4.7L
Intermittent problem - loss of fuel pressure. My mechanic assumed a faulty fuel pump and replaced it, but the problem came back. Engine will always fire when cranking (and even did so with the 'faulty' pump). I was able to catch this one time with a fuel pressure gauge connected, and a monitor light on the fuel pump power. The pump power dropped to zero, and the engine continued to run until the fuel pressure dropped to zero.

I hooked a voltmeter to the fuel pump relay control side of the fuel pump relay coil to see if the PCM was de-energizing the relay, but I couldn't get it to fail after that. While running, I see about 1.0 V on the fuel pump relay control. I assume that internally the PCM has some diode and/or transistor drops that account for this level.

I'm trying to use the 2001 Service Manual, because I can't find the 2002. According to the functional description, the CKP signal seems to be the main input that would tell the PCM to keep the fuel pump relay energized while in the running condition. The relay itself seems like an unlikely candidate for this failure. Wouldn't a faulty CKP sensor/signal cause the engine to die immediately? By the way, no codes are thrown by this failure.

I'm stuck, at least until I can observe the relay control during failure. Any similar experiences or ideas?

Thanks in advance
 

Oliver Closehauf

Senior Member
Joined
Nov 21, 2020
Posts
718
Reaction score
457
Location
Cincinnati
Ram Year
2005 Laramie
Engine
Cummins 5.9
This is kind of tangent, do these have an external fuel pressure regulator mounted to the rails or is it built into the pump?
 
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