fuel pressure dropping

racer5252

Member
just noticed my fuel pressure is dropping to 30 psi while cruising at 55 mph.if i accelerate it will climb a few lbs.It will increase with boost. I only noticed this because i got lots of knock at partial throttle .A few days earlier i was running 22psi of boost with NO knock. . If i go out now it will be 40 psi but will drop after i drive it awhile. This is a fuel pump from Full Throttle with less than 1000 miles on it.Also has hotwire on it. Any ideas?
 

earlbrown

runs with scissors
at cruise you're a vacuum situation so it will be lower than the base setting. I doubt you're pulling 30inches at cruise though unless you're doing a throttle chop.

Do the easy stuff first. Change the fuel filter and see if you get lucky.
 

TurboTom72

Boost Addict
How long ago was that pump purchased? There was issues withn the Walboro pumps a while back. I changed mine as a precaution and got one of the bad ones. Not driving the car more than a couple hundred miles a season, it took a while before it failed. Changed out with a pump from Red Armstrong, and no trouble for several years now.
 

krom

Alky graveyard parking.
Found a similar problem on a pump a while ago, it was a loose fuel pump line in the tank.
 

racer5252

Member
Do I have to check voltage at the tank ? Just put a new filter on and still the same.
Pump was purchased around 2009. Suppose to be updated version . Last Walboro went bad.
 

TurboTom72

Boost Addict
Updated, as in after the recall? I'd still throw a guage on the car and verify the 1 to 1 rise in fuel pressure. Checking voltage at the pump isn't a bad idea either. How old is the fuel filter? Was the car running good and this just started all of a sudden?
 

racer5252

Member
yes, it was after the recall.I do have a hood mounted fuel pressure guage and it does raise 1 to 1. I have never noticed before that it goes down to 30 psi at cruising speed. Today the lowest it would go is 34psi. Still lots of knock. Yes, this started all of the sudden
 

TurboTom72

Boost Addict
Are you just running the scanmaster, or do you have a powerlogger? Maybe start taking some log files or checking your ecm data readings for issues.
 

87GNSteve

Just another pretty face....
If i go out now it will be 40 psi but will drop after i drive it awhile. This is a fuel pump from Full Throttle with less than 1000 miles on it.Also has hotwire on it. Any ideas?
This sounds like a pump going bad after it gets some heat.
 

TurboTom72

Boost Addict
X2 on the fuel pump being flakey. That's what the 20 questions were about. Just trying to get all the facts first. That's basically what my recalled pump was doing before it finally started shitting the bed completely.
 

Mr.Spool

Well-Known Member
change the fuel filter,tests volts at the pump,drop tank and inspect wiring,replace the pump if needed.dont drive the car and let it knock.the alky probably covered your 22 psi blast,thats what is nice about having 2 separate fuel systems.
 

Mike Licht

I was here first
First of all check that pump pressure goes up one to one with boost. Base FP setting + whatever boost you are running. If it does, most likely the pump is fine. Do you have a hot wire kit? It is not impossible if the base pressure is set to say 42 for the cruise vacuum to pull it down. What is your block learn at cruise?
 

racer5252

Member
fuel pressure seems to be raising 1 to 1. but it starts knocking at half throttle. I do have a hotwire kit. Key on getting 11.6 volts to the pump. i did notice when i turn key on i hear the pump run then stop but fp gauge is still at 0 psi until i start it. bl at cruise is 122
 

racer5252

Member
Well just installed a complete stainless steel fuel pump assembly from Racetronics and it is still doing the same thing. I did notice the fuel pressure drops to 0 after sitting overnight. I put a new accufab adjustable FP gage in last year. Do these go bad? Any other ideas?


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PSID6

Active Member
Have you checked for kinks in your fuel line? The previous owner of my car had a mechanic change out the intake manifold gasket and put a pretty severe kink in the inlet fuel supply line. I would get 3 to 4 degrees of Knock Retard until I changed out the fuel line then all the knock went away.
 

we4Mateo

Double you eee what?
First of all check that pump pressure goes up one to one with boost. Base FP setting + whatever boost you are running. If it does, most likely the pump is fine. Do you have a hot wire kit? It is not impossible if the base pressure is set to say 42 for the cruise vacuum to pull it down. What is your block learn at cruise?

This sums it up to me. If the fp rise is right and depending on your base setting, 30psi isn't that low under decelerating vacuum. What is your base psi? I think you have another issue causing knock. Have you check alky pressure under boost yet? My alky pumps only last 2-3 years and I use it a lot. If they sit, the alky pumps go bad even sooner.
 

we4Mateo

Double you eee what?
fuel pressure seems to be raising 1 to 1. but it starts knocking at half throttle. I do have a hotwire kit. Key on getting 11.6 volts to the pump. i did notice when i turn key on i hear the pump run then stop but fp gauge is still at 0 psi until i start it. bl at cruise is 122

The pump should stop after a key on before start, but you should see fp at that time. That could just be a bad checkvalve though, not a big deal to me. Your BL is good too so look elsewhere for your knock issue.
 

racer5252

Member
I installed all new stainless steel fuel lines a few years ago but will check them tonight


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