Fuel Pressure changes as car warms up

[SLO_5-OH]

I've had a fuel pressure gauge on my car for several years and have just started having a problem with the fuel pressure changing by itself as the car warms up. As my car warms up, the fuel pressure drops. If the car cools down after being parked for a while, the fuel pressure will be back up. It is moving around by as much as 8 psi.

Normally the fuel pressure stays at my setting regardless of the engine's temperature. I have changed gauges and regulators and I still have the problem. Has anyone heard of the fuel pump or anything else causing this problem? I try to keep my fuel pressure set at 40 psi with the vacuum line disconnected. I have an 89 GT with a 13 year old 155 lph fuel pump.

 

[Black1987Stang]

I would have to say sounds like the pump's getting tired.

 

[SLO_5-OH]

I'm going to change the fuel filter tomorrow since that is an easy & inexpensive thing to change. If this doesn't work, then I suppose I'll try a new fuel pump. I'll probably move up to a 190 lph since I'm hoping for a 347 next year.

How involved is changing the fuel pump? I'm sure that having the fuel level as low as possible would be a good start. Does anyone have detailed instructions for changing the fuel pump on an 89 GT?

 

[mustangmike6996]

step 1 remove fuel pump

step 3 car runs great

thats all you need to know....

no really its a couple of bolts the hold the filler neck to the fuel door housing and a couple of bolts that hold the tank to the car. make sure you dont break the bolts... when you get the tank down it may be a good idea to flush it out and replace the small pickup filter as well... most pumps come with them (the mesh bag on the sending unit)

oh and when you try to take the o ring off of the sending unit seal to the tank use a hammer and a WOOD block not a screwdriver... safer and will do less damage.

let us know how it works out for you.

 

[Black1987Stang]

It's pretty basic, you might find something in a search if you look.

All you really gotta do is open the gas door, take out the 4 little screws and pull up the gas fill tube out of the tank, take off the 2 fuel tank straps, (14mm I believe), drop the tank, disconnect the fuel lines by taking out the white plastic clips, and disconnect the wire harness....***USE SOMETHING THAT WON'T CAUSE SPARKS WHEN YOU TURN THE RETAINING RING*** You can use a small piece of wood and a hammer or I used a wide flat head screwdriver and wrapped it in electrical tape so it can't make any sparks just in case. Pull the whole assembly out, change the pump which is pretty common sense, just remove and replace, I had to cut and shrinkwrap the new wires in, mine came with a new filter sock and everything else eneded(walbro 255lph). When you put the sock in the hole you might have to squeeze it a little to get it in there. The rest is reverse of disassembly. A jack is nice to jack the tank back up there, I did mine on the flat ground with my friend under there benchpressing it up into place while I bolted it in while in 10 degree weather.

 

[SLO_5-OH]

Thank you for the instructions. I will try the fuel filter first. If that doesn't fix my problem I'll move on to the fuel pump.

 

[SLO_5-OH]

Which fuel pump is recommended?

I changed the fuel filter and still having up-and-down fuel pressure problem. I will now move on to the fuel pump. Does anyone have any recommendation for a brand of fuel pump that is well made and comes with everything necessary to install in an 89 Mustang? I am going to buy a 190 lph since I am hoping to move up to a 347 next year.

Summit sells several 190 lph pumps in the $115-$130 range including BBK, Holley and Trick Flow. The Ford Racing pump is around $215.

Thanks,
Dean

 

[legalize420]

im actually having the same problem now. my pumps fairly new so i dont think thats the problem. are you using a liquid filled gauge? from doing some searching it seems that they are not accurate. even more so when warm. im thinking about ordering a dry gauge soon. for now i would set it at 40 when cold and leave it alone.

 

[mustangmike6996]

have you hooked up an actual pressure test gauge to the rail to verify that the pressure is dropping and raising? look for pressure bleed.

 

[Roland69]

try another gauge, maby another FPR, and a filter is never a bad idea..