99 Gt Intermittent Driveability Problems


New Member
Sep 8, 2016
Hey guys, New member, i have used this site's knowledge in the past on many issues ( thanks for that), but I've come to a problem I need to reach out to you guys. I am on many different forums, bikes, motorcycles, chevy truck etc.. so let me start by saying thanks for looking at a new post by a new member. I have looked through the stickies and searches and have gathered lots of information but no solution as of yet. Paying my respect to the OGs here first.

1999 Gt 4.6 stock engine, auto, modified exhaust (no mufflers), bbk cold air intake.

Problem - Service engine soon light on, extreme Intermittent drivability issues (stalling at lights, hard time starting meaning multiple key cranks to get engine to turn over,sometimes unable to start at all, max speed of 25mph at some points, rough idle, surging idle, loss of power.)

Auto zone is unable to read my codes. scanner shows multiple codes (not specific codes, just a number ranging so far from 1 code to 6 codes) when plugged in, then unable to read at computer inside. Multiple locations, multiple times. No one knows why. Possible clue to problem i don't know?

Firestone pulled my codes. o2 sensor bank 1. I back probed all sensors and got readings fluctuating between .9v-1v, but i bought a new one (today) and have yet to replace it (i need the F***** tool).

Note* i can clear the SES by disconnecting battery, it goes away then comes back within 25 miles or so, on the way to work.

It stalled exiting off the freeway on me, would not start back up, i checked fuel pressure at the rail with my knife, pushed down on the shraeder valve, got nothing. cycled the key to prime ran back to the front nothing. Called my dad had him come and hold the valve down while i cranked it and no fuel came out for multiple cycles. finally air started coming out, then fuel, then started. took it home.

Put a fuel gauge on the rail at the valve. key to on position, 0 Pressure, cranking 0 Pressure. multiple times with no success. tried same "bleeding technique" as earlier started up again, put gauge on, 15 psi. Psi immediately drops to zero when engine is turned off.

I know about the Check valve or PPRV in the line, and i know people deliberately delete them so i don't see this causing a problem, an annoyance more like it. I have replaced the Fuel pump 2x in this vehicle. first time i had gotten a bad pump form rock auto, they sent me a new one. It is totally possible it is the fuel pump, i am just skeptical because it was replaced less than 3 years ago. i used some fresh gas with injector cleaner in it and I also replaced the fuel filter today with a motor craft one. moving on.

FRPS im not sure how to test exactly, i do know however, when i remove the vacuum line from the sensor and plug it there is NO increase in pressure, which i think there should be. However when everything is connected and idling, if i remove the electrical connector from the fprs it shudders and almost stalls, then corrects itself. this tells me it might be working???? maybe not

Other things i have done.

to rule out plugs/coils/injectors i did a cylinder balance test by removing one injector connector at a time and noting vacuum/rpm changes in cylinders to pin point dead cylinders. ALL 8 cylinders responded dramatically to removal, leaving me to believe i have no bad plugs, coils or injectors.

A leaking injector in terms of o rings is not likely, i don't smell gas, or notice any leaks. i can use a dye test to be sure. A stuck open injector would likely cause a fouled plug or dead cylinder, which i tested previously. correct?

Like i said o2 sensors checked ok, but i have a p1131 code, and i WILL change my bank1 sensor tomorrow.
IS there a relationship between fuel pressure at start up with a fuel system operating on an open loop though??

I have cleaned the MAF sensor, the erg valve, the IAC valve, checked vacuum lines for leaks (which is difficult but I'm almost positive there are none), sprayed brake cleaner around intake for leaks as well.

One thing i noticed is if i remove either the FRPS or IAC, i get immediate engine response changes. The MAF has zero change in rpm, throttle response, etc. IS that normal, would that be normal with a good MAF and bad o2 sensor?

There is a chrome diaphragm looking piece in line the fuel line into the throttle body, i was thinking of removing that line and inspecting but have not yet.

with all these things being said, after my car eventually started up (on 15psi) i drove around the block and it was fine. came back home turned it off. couldn't get it started again. INTERMITTENT is a huge part of this problem. i can see no pattern here. I m pretty sure i have working components yet very low fuel pressure. p1131 reads

- Faulty Upstream Oxygen Sensor Bank 1 - Fuel pressure high or low - MAF sensor dirty/defective - Vacuum leak on engine - Leaking fuel injector or fuel pressure regulator - Faulty Powertrain Control Module (PCM)

does low fuel pressure mean bad fuel pump, i figured a bad fuel pump would affect both banks.

IF you have made it this far, thank you for taking time out of your day to help me. I am perplexed to say the least. or just naive to think a rock auto fuel pump is good quality. I am open to any and all comments suggestions. thanks again.
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10 Year Member
Sep 15, 2006
St. Louis, MO
Change fuel filter and then re evaluate the fuel pump. I've heard of problems with the pprv delete. Sometimes people don't use the correct high pressure fuel line, the line they use kinks, or the line collapses under pressure or suction from the pump doing its job. Couple of maybes to ponder.