low fuel pressure
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Mountain Pass
Joined: Sep 2010
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low fuel pressure
I am having problems maintaining fuel pressure. I let the truck run for a minuit and i can then get air out of the schrader valve. If I bleed it off, It will run another minuit and then have air in the line. I have replaced the fuel filter, fuel pump, and have applied compressed air from the pump though the line to where I disconnected fro the tank. If I unhook from the tank and put the end in a gallon Jug it still finds air. My return line puts out plenty of fuel.
What else do i check?
What else do i check?
Fleet Owner
Joined: Jul 2006
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From: Mi'kma'ki
the '85 or the '88 truck?
run a hose from the lift pumps suction side into a jug of diesel.
if the truck runs fine for more than a minute/no more air in the lines,then the air intrusion is the sending unit,fuel selector valve,or in the delivery fuel line.
^
the above is likely for your symptom.
below will cause leaks still,but generally just hard starts.since your pump is pushing from here,the engine shouldn't care,you'll just have a mess of fuel,know what i mean?
if the truck still has the issue,then the problem is the lift pump,fuel filter/water separator leak (a few locations here) or the line to it.
if this is the '85,has the stock water separator been replaced with a better unit?
run a hose from the lift pumps suction side into a jug of diesel.
if the truck runs fine for more than a minute/no more air in the lines,then the air intrusion is the sending unit,fuel selector valve,or in the delivery fuel line.
^
the above is likely for your symptom.
below will cause leaks still,but generally just hard starts.since your pump is pushing from here,the engine shouldn't care,you'll just have a mess of fuel,know what i mean?
if the truck still has the issue,then the problem is the lift pump,fuel filter/water separator leak (a few locations here) or the line to it.
if this is the '85,has the stock water separator been replaced with a better unit?
Thread Starter
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Mountain Pass
Joined: Sep 2010
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this is the 85.
The tank is is the bed, been there 6 years problem free. Fuel selector andwater seperator have been removed. I just replaced the fuel filter and lift pump to eliminate those possibilities.
I'll look under the truck for leaks. it is all down hill from the tank to the lift pump so it should drain out if there is a crack in the line.
When I unhooked from the tank and put it in the jug it still found air at the schrader valve.
The tank is is the bed, been there 6 years problem free. Fuel selector andwater seperator have been removed. I just replaced the fuel filter and lift pump to eliminate those possibilities.
I'll look under the truck for leaks. it is all down hill from the tank to the lift pump so it should drain out if there is a crack in the line.
When I unhooked from the tank and put it in the jug it still found air at the schrader valve.
Fleet Owner
Joined: Jul 2006
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From: Mi'kma'ki
how bad is it? i mean will the engine stay running? does it surge and hiccup or is she just hard to start?
what are the symptoms that made you start looking for air intrusion?
must be sucking in air around the filter base someplace probably.
what are the symptoms that made you start looking for air intrusion?
must be sucking in air around the filter base someplace probably.
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Mountain Pass
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the engine won't stay running. It takes 2 cycles of glow plugs instead of the normal 1 and it will start. Then If i bleed the schrader, I can get it to run for 1 min. then hicup for about 10 seconds and then die.
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Mountain Pass
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Fleet Owner
Joined: Jul 2006
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From: Mi'kma'ki
well she's got to be lift pump or higher related because it's running the same way as when running from a jug of diesel off the lift pump.
why wouldn't she still run and just leak fuel after the pump though,since the pump is then pushing fuel from there right? so it's really pointing to a failed lift pump.
maybe your bed tank is putting to much pressure on the diaphragm of the lift pump and this is why even with a new lift pump acts the same way?
is the bed mounted tank a new install? or did it used to run just fine and this symptom is new?
why wouldn't she still run and just leak fuel after the pump though,since the pump is then pushing fuel from there right? so it's really pointing to a failed lift pump.
maybe your bed tank is putting to much pressure on the diaphragm of the lift pump and this is why even with a new lift pump acts the same way?
is the bed mounted tank a new install? or did it used to run just fine and this symptom is new?
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Fleet Owner
Joined: Jul 2006
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From: Mi'kma'ki
man i wish someone else would jump in here lol.what a head scratcher with it running out of fuel from a diesel jug too.
makes ya think there's got to be a pinhole from the suction side of the lift pump.
did you use a whole new line from the lift pump to place into the diesel jug too though?
maybe the return line is plugged up,and that's why she's cutting out......but thing there is,that doesn't explain the air your finding.
it's just like you replaced a failed lift pump with another..........that's all iv got without being there i think lol.
makes ya think there's got to be a pinhole from the suction side of the lift pump.
did you use a whole new line from the lift pump to place into the diesel jug too though?
maybe the return line is plugged up,and that's why she's cutting out......but thing there is,that doesn't explain the air your finding.
it's just like you replaced a failed lift pump with another..........that's all iv got without being there i think lol.
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Mountain Pass
Joined: Sep 2010
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Thanks for the input 250, I know it's got me stymied. I will try tomorrow running a different line to the lift pump. Good thought there. I was running through the same line all the way from the tank. I'll try a short line from a jug right under the engine.
I checked the return line, didn't measure but it was running a full stream.
I can't work on it today, got to take care of a sick kid.
Can I check a lift pump by taking it off and trying to blow through it? It seems like I would be able to tell If I had a hole in the diaphram.
I will look at it tomorrow
I checked the return line, didn't measure but it was running a full stream.
I can't work on it today, got to take care of a sick kid.
Can I check a lift pump by taking it off and trying to blow through it? It seems like I would be able to tell If I had a hole in the diaphram.
I will look at it tomorrow
Fleet Owner
Joined: Jul 2006
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From: Mi'kma'ki
ah ok!
well see,the reason you try a short devoted hose with your diesel jug like this,is to help you find/rule out the line from tank to lift pump (and normally the FSV too in stock setups etc.)
so there it is.in a round about way,you found the problem.your sucking from the lift pump.so in order yo get her to stall out (besides a plugged up return) and see air bubbles,is pre-lift pump.after that,she's pushing,and it was a leak bad enough to stall out the engine from the push side of the lift pump,you'd have fuel spraying all over im sure lol.
so you've ruled out the tank,and left the bad spot that must be giving the issue.the supply fuel line from tank to pump.
that's what makes it hard.........can't see what your doing there.
i think you'll find the problem easy enough now.
i ditched all the old metal fuel lines upon purchase of this truck and went with:
Goodyear SAE 30R7 Diesel Rated Fuel Hose/Delivery & Return
to avoid these issues with mine.15+ yr old metal fuel lines in rusty Maine would have = disaster on the side of the road.
(dual poly tanks too.)
well see,the reason you try a short devoted hose with your diesel jug like this,is to help you find/rule out the line from tank to lift pump (and normally the FSV too in stock setups etc.)
so there it is.in a round about way,you found the problem.your sucking from the lift pump.so in order yo get her to stall out (besides a plugged up return) and see air bubbles,is pre-lift pump.after that,she's pushing,and it was a leak bad enough to stall out the engine from the push side of the lift pump,you'd have fuel spraying all over im sure lol.
so you've ruled out the tank,and left the bad spot that must be giving the issue.the supply fuel line from tank to pump.
that's what makes it hard.........can't see what your doing there.
i think you'll find the problem easy enough now.
i ditched all the old metal fuel lines upon purchase of this truck and went with:
Goodyear SAE 30R7 Diesel Rated Fuel Hose/Delivery & Return
to avoid these issues with mine.15+ yr old metal fuel lines in rusty Maine would have = disaster on the side of the road.
(dual poly tanks too.)
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