Fuel pressure question

281pony · Aug 31, 2004

the search isint working so i couldnt do that.

i have an 88 gt, SD car still. its been running like **** lately. i had a "bump" in the 1500 rpm range and some weird backfiring above 3000 rpm so i figured it was in with that. well finally my spark plug wire fell completely off, and upon reinstalling it the bump and backfires went away.

i have a 70mm tb, cobra intake, 1.7 rockers, bosch injectors, bbk headers, full exhaust, ect.

i can bury it in first gear from a stop and it doesnt spin to give you an idea, then winds pretty slow, especially with 3:55's. we've gone through everything it seems like, and cant seem to put a finger on it.

it seems to bog below 3000 rpm and then it picks up after that a little better, but still not like it should. does this sound like it could be fuel pressure related? i have a gauge under the hood from the previous owner, but im not sure how accurate it is. it was at 35, and i put it at 31 and it seemed to help. i thought it was supposed to be at roughly 35 psi idle, so i didnt adjust anymore. my cousin was behind me on the highway and i passed someone and he saw black smoke.

based on that, any suggestions what it might be causing me to lose so much power? or should i take it in for a tune maybe? it really feels like there should be another 80 at the wheels.

HISSIN50 · Aug 31, 2004

it sounds like a management issue, if you are running better at higher revs. if you were fuel starved, losing spark, etc, those normally manifest at higher rpms.

your fp sounds a little low. isnt stock about 38 psi (vac off)? if you have doubts about the gauge, i would hook up a diagnostic gauge, to compare readings.

any codes?

good luck.

281pony · Aug 31, 2004

38 with vac off? damn. what should i be at during idle then?

havent had a way to run codes. we have checked my timing, plugs, wires, airfilter, vac leaks, ect.

would the black smoke indicate anything? my cousin said carburation issues would cause black smoke. im at a loss, ever since i had the car its seemed low on power but everything seems to indicate fine.

281pony · Aug 31, 2004

35 psi with vacuum off. 31 at idle.

not understanding what is so easy to neglect, but is causing such a power loss

HISSIN50 · Aug 31, 2004

im not sure i follow you - it is really late - sorry.

the FP will drop about 3-4 psi when the vac is reattached. i still think 31 is a little low with the line on.

remember the EEC adapts. so if you run with more pressure, it shortens the injector pulsewidth.

black smoke is an indication of running rich. but if you were on the gas at all, that is not too unusual.

how is your vacuum? stockish motor should have 15-20 inches at idle. since you are still SD, you should be in that area (or else you will be messing the with puter's 'head'). im sure you know this - im just too tired.

good luck.

all you need to pull codes is a test light, as i recall (as you likely dont have a functional CEL). it can only help with diagnosis.

281pony · Aug 31, 2004

i turned over the key, but didnt start it, and checked the fp. 35 psi. is that incorrect? with the car running, at idle i have 31 psi.

ill have to see if i can get some codes run, i dont really have access to any of it here though. about the only small thing i havent replaced is the coil. if it isint related to that or fp im lost.

not real mechanically experienced though, so hopefully i can get some codes run and go from that.

Daggar · Aug 31, 2004

My idle fuel pressure is around 40 psi. Have you made any recent mods? If not then I'd suggest going through your vacuum system with a fine toothed comb. I recently had a similar condition at around 4500 RMP after installing a VAC/Boost guage and failed to install the proper orifice in the line going to it.

Michael Yount · Aug 31, 2004

The fuel pressure with the car idling, and the vacuum line OFF the reg (plug the line) should be 38-40psi. When you plug the vacuum line back up to the reg, it should drop the pressure down into the low 30's (32-34) depending on how much vacuum your engine makes at idle.

Lylebrewer · Sep 1, 2004

Michael Yount said:
The fuel pressure with the car idling, and the vacuum line OFF the reg (plug the line) should be 38-40psi. When you plug the vacuum line back up to the reg, it should drop the pressure down into the low 30's (32-34) depending on how much vacuum your engine makes at idle.

Concur with the numbers. I see about 31-32 at idle on both of my mustangs (89 & 94) rising to about 41 as the throttle opens (or remove the vaccum hose). My gauge calibration might be a pound or two off. Had a problem with the 94 a while back. The car would idle fine. Ran like crap above idle, would not pass a Amish Buggy. Computer was throwing all kinds of codes. Went through a lot of stuff. Finally threw a pressure gauge on the schraeder valve on the fuel rail. Only had 20 psi. Thought my brand new fuel pump had a pressure reg valve suck open. Dropped the fuel tank and found that the S hose had come partially off the discharge port of the pump. Had to double up the hose clamps. Lyle

Fuel pressure question

281pony

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