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Discussion Starter #1
I've got the lift pump mounted at the tank on the frame. It's been that way for 70k miles. Also, I've got a low pressure light to monitor it.

Lately, it has randomly lost pressure at IDLE after 4-5-10 minutes. It does fine when under power going down the road. This only happens at idle.

What's the Problem here?

Also, I know it's time for FASS but that doesn't explain the current problem!
 

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Im not trying to sound like a know-it-all but i think (or guess!!) what is happening is your lift pump is getting very weak and your injection pump is pulling the fuel just enough to cut the light off!! at idle your injection pump is not pulling enough but your lift pump isnt pumping enogh, while throttle is on the injection pump is pulling!! And that results in injection pump failure!! A fuel gauge my help you figure the amount of fuel your lift pump is pumping!!! I would try checking the pressure at idle and WOT!!
 

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Discussion Starter #3
I've got a pressure guage on the "dirty" side of the fuel filter housing. The pressure light is on the "clean" side of the filter housing. The other day the light came on at idle, I checked the guage and it was on 0 psi. so i shut the truck down. I gave it a second then cranked it up and the guage was at 12-13 psi at idle.

I'll check tommorrow what the pressure is at WOT.
 

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Have you checked all the plumbing? Sounds like you may be pulling air instead of fuel from the tank sometimes. Just a guess.
 

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Either your getting air or your carter pump took a dump!!
 

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This might make me sound stupid, but what about your power source. At idle, you might be losing power, like a bad alternator or something. When you get on it, you regain power...but like i siad...maybe im slow...lol are your lights dimming at all?
 

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Discussion Starter #7
Alright, today I watched the guage and the light and the guage was holding 12-14psi at idle and 11-13psi down the road. At WOT it was no less than 10 psi.

Maybe air but I don't see how. The power source is the original wires extended down to the frame at the tank.
 

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It maybe you TPS too!! call me and I will run a diagnostics test for you!!
 

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This might make me sound stupid, but what about your power source. At idle, you might be losing power, like a bad alternator or something. When you get on it, you regain power...but like i siad...maybe im slow...lol are your lights dimming at all?
This is exactly what i was going to say also. If you have a bad connection on the power or ground source it will cause high resistance and seem that you motor is weak. It would do this at idle because your alternator is not at its full output and this would lower the amperage going to the pump even more. Then when your driving you alternator is putting out full volts and amps and would temporarly fix the problem until your return to idle. Hope this helps. if you understand what i'm trying to imply.
 

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Discussion Starter #10
Tommorrow I'll check the voltage output at idle.

Will, I'll give you a hollar in the next day or so.
 

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I've been checking posts because I had the same problem today. My FP dropped to 8 psi at idle for no apparent reason and after a few minutes it came back up to the normal 14-15 psi. Everything was fine there after but I did throw the P 1693 code. I no wonder were the real problem came from and was wondering if you solved your problem. I never bothered to look at my alternator output so I cant speak for that.
 

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I've got a pressure guage on the "dirty" side of the fuel filter housing. The pressure light is on the "clean" side of the filter housing. The other day the light came on at idle, I checked the guage and it was on 0 psi. so i shut the truck down. I gave it a second then cranked it up and the guage was at 12-13 psi at idle.

I'll check tommorrow what the pressure is at WOT.
A while back when I only had a warning light, I could be driving along and the light would come on for low pressure, I could shut the truck off and start it right back up and the light would be out and would stay out. I ended up replacing the pump and also upgrading to a better lift pump as a main supply, using the carter as the backup
 
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