Fuel pressure

Robs96 · Feb 13, 2007

I recently replaced a known bad fuel pump that was causing a miss on my '96 GT. It put out 30lbs of pressure prior to replacement and with the vacuum hose disconnected at the pressure regulator, it flucuated between 36 and 38lbs with a very unsteady needle. I now have steady pressure with 30lbs and 38 with the vacuum disconnected but the car still misses at idle when at a stop and the car isn't completely warmed up. The car runs great when first started, then idles bad when warmed(but only at a stop), and idles fine after running for awhile. Even after the car is warmed and idling fine. it bogs down on acceleration and when the rpms get into the higher ranges. the speed picks back up and it runs. If I accelerate with light pedal, the car takes off okay and speeds up fine. Anyone have similar problems and if so what fixed it? Could this be a bad pressure regulator and does anyone know what the pressure should be? I would look in the Haynes manual but my daughter put it to better use as a coloring book.
Parts that have been replaced:
IAC
fuel pump
maf
driver side O2's
spark plugs
fuel filter
coolant sensor
both ign. coils

ironmedic · Feb 13, 2007

think simple: air/fuel/spark

you changed out the fuel components and spark. what about your MAF sensor? what codes are you throwing? also, what mods do you have done?

Robs96 · Feb 13, 2007

No mods, no codes, fairly new maf sensor. I think the problem still lies somewhere in the fuel system, but I'm not positive. The car missed through all speed ranges before I replaced the fuel pump. Now it does what I posted earlier. I think my fuel pressure is still low but I don't have a long enough attachment for the fuel pressure gauge to check it while I'm driving.

Robs96 · Feb 14, 2007

So if no one has any ideas then can someone tell me what the fuel pressure should be at idle and then with the vacuum hose removed from the regulator?

Smokeurhonda · Feb 14, 2007

How much vacuum are you pullin? Check that first, sounds like you might have a leak. If you put the vacuum gauge on and the needle fluctuates rapidly then there is you answer. Also what you can do to check your fuel regulater is pump a hand vacuum pump to the regulater, at different amounts of vacuum you apply to it you should see a change in fuel pressure.

ForPointSex · Feb 14, 2007

My 2002 mustangs tries to have 40-50PSI. It never falls below 40psi. Did you replace the spark plug wires?

Robs96 · Feb 14, 2007

50k on the current plug wires. I'll have to pick up a vacuum pump and give it a try.

Red2000GT · Feb 14, 2007

Yeah 30-38 pounds sounds low. According to my Predator,mine never dips below 40,hot or cold engine.

LaserRed01GT · Feb 15, 2007

Well the book says:
Key On Engine Off (KOEO) Fuel Pressure 25-40 psi (172-275 kPa)
Key On Engine Running (KOER) Fuel Pressure 35-50 psi (172-275 kPa)

BUT, most healty cars will have at least 39-42 PSI at idle with the vacuum hose on, and with the vacuum hose off (WOT) you should not be below 43-44 PSI.

Robs96 · Feb 15, 2007

Yeah with a new fuel pump and still only 30lbs at idle, I'm not sure whats going on. I haven't vacuum tested it yet so that may tell me something I didn't know.

LaserRed01GT · Feb 15, 2007

Robs96 said:
Yeah with a new fuel pump and still only 30lbs at idle, I'm not sure whats going on. I haven't vacuum tested it yet so that may tell me something I didn't know.

If you just replaced the pump, it's probably the FP regulator. The diaphragms inside just give up after a while.

Fuel pressure

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