flucuating fuel pressure really bad.

[Foxfan88]

ok i have a mechanical FP gauge mounted in my cowl induction hood. was messing with the car today trying to solve soem cooling problems.

hopped in the car to drive around and i noticed my fuel pressure was jumping around.

the FP would hang around the correct pressure, around 39 psi, and pressure would increase and decrease like normal with engine vacuum. but instead of beind rock solid. the FP would bounce around abour 2-3 psi on what it should.

for ex say it should be 39 psi, the needle will shake from like 37-41 psi. give it throttle and it would increase slightly but still have the 2-3 psi shake to it.

got home and when the engine settled down to and idle the FP really went crazy. jumping from like 30-45 psi.
the needle shaking is VERY fast.
car seems to run fine.

popped the hood and checked my braided line coming from the fuel rail to the gauge and i found no leaks. i couldnt see anything wrong around the engine compartment.

i duno what would cause this.

only thing out of the norm i did recently was burping the cooling system for a while.

i had my car running at idle for about 30 mins with the drivers side jacked up trying to get as much air out of the system as possible. but i dont see how this could cause this problem. i dont know if its been doing it for a while now. i have only driven the car a few times in the last week or so. i havent noticed anything until today.

the fuel pump is a 255 LPH i replaced a month or two ago the pump doesnt have 500 miles on it. i dont see how it could be bad, i got it from summit (summit brand) its a walbro pump.

maybe fuel filter? its been a while since i have changed that....

any more ideas? this is the first time i have ran into a problem like this. at least since i have had the mech FP gauge. since i first got it my FP would be rock solid and move smoothly now its jumping around like its on speed lol.

 

[Foxfan88]

UPDATE!

did a test and i now know FOR SURE its a problem with my car.

i took the mech FP gauge and fittings off of my car and hooked it up to my brothers mustang (bone stock 91lx vert)

ran his car and the FP readings were rock solid. so the gauge is good for sure.

only two things i can maybe think of.... FPR and fuel filter. i would lean towards the FPR, its a bosch or borg warner( dont remember) its an adjustable regulator i got at the parts store. a guy at the store was able to find AFPRs and for alot cheaper than the aftermarket brands, this was 2 years ago when i put it on.

i have a tweecer and tune my car now so i have no need for an AFPR, i can just get a stock one.


and another thing that sucks. after i got done testing with my bros car, i DROPPED the FP gauge on the driveway and it hit hard enough the needle moved, i figured the gauge was screwed up for good. i started hitting it on my palm and the needle FELL OFF .

theres no way to get the black ring off the gauge without tearing it all the crap. so i pretty much ruined the gauge and need to buy a new one. its the little stuff like this that makes you go nuts. at least i got to test my bros car before i broke the thing.

so anyone got any more ideas??

 

[bentley429]

80% sure its the regulator. My car did something very similar, my holley regulator was junk. Just the cheapo $50 one. Aeromotive rail mount now no problems since. If it is your problem look into a kirban think thats the brand they are nice also.

 

[Beasty306GT]

im looking to put a fuel pressure gauge on my stock rail...where do u guys intall yours?? show me some pics if u got any ..i also got to install a regulator

 

[Deleted member 87397]

Could be the regulator starting to go bad. Pull the vacuum line off the regulator and see if it stabilizes. Usually when pressure is erratic like that and it's the regulator, it will level out when you disconnect the vacuum. Take the line off and see how it acts at and while driving it around.

 

[Foxfan88]

im looking to put a fuel pressure gauge on my stock rail...where do u guys intall yours?? show me some pics if u got any ..i also got to install a regulator

the only place to install a gauge on stock rails is via the schaeder valve.( the same valve shops use to check FP)

for my particular car. i have to run a 1/16NPT to -4AN adapter, my braided stainless line running to the gauge is -4AN which seems to be the standard thread size for a FP gauge.

perhaps aftermarket rails have extra holes that can be used to tap fuel pressure, if not it would be easy enough to drill into them.



as for my deal, once i get my new FP gauge in i will start messing with the car, will try the vac line unplugged and replace the regulator and fuel filter (needs replaced anyways)