oxfordgt

LOL Yup thats fuel pressure. Remember I said above that that was just for the gauges. Do you see a fuel pressure gauge in your dash?

cliffyk

From the factory shop manual, Instrument Cluster Diagnostic Mode:[/align]

fausty

bro im on the same boat bro. myproblem is when i rev more than 4k my car stalls and feels like im runing out of hp. someone says its prob the fuelpump and ive notcied the last two days my gas has been runnning out relatively fast i put 20 buks today that gave me a little bit more than half a tank and i drove to school 8 miles away and my gas is at a quarter tank.i need to fix this asap

rogerwilco357

ok i have a 97 with vortech and chip etc. now i must not have enough anti-freeze because my car over heated and spilled out a gallon or so and my gauges all aftermarket read like this 40 on thefuel pressure, oil 60 on start to like 33 or so then the heat up to 220 shut her down and let it cool dropped the fluid and filled it up with fresh anti freeze .michigan so the temps jump alot . then it never passed the 150 on heat great then i read this string ..ok i looked at my pressure the next day oil fine temp fine gas pressure from fluxuations 40 39 37 20 stayed there all else fine then engine light came on ..Now the whole time the car is stable at 900 on the rpms so the 20lbs on the gauge didn't make sense. I turned the car off and back on she stayed nice and idled but the gauge now went from 20 to 0...but still running lol..i unplugged the battery and reset the cpu and back to 40 all warning lites off now etc the abs light due to some brake work done ,,another story but sorry for the long story but i read your string and wonder if the cold weather plays a factor in the gauges? anyone ..? hope yours works out i checked the setting on my chip all is fine for the time ..michigan and mustangs what a mix wish i could buy another car for the winter but the economy has me bent over..happy holidays and merry christmas to all especially those serving in the military and there families.godbless and take care .
rogerwilco out...
p.s. any other michiganders here ?

U S Marine

I had to go in the way back machine for this one! I don't interact too often, because of searches, but I'm hoping some one can help me out.

I have an 03 GT, stock, with SCT tune, that has a hot start issue. (Did it before tune also) Doesn't like to start, even after priming it 3 times, until you depress the accelerator, then it starts. Figured I'd start troubleshooting the fuel side. I installed a mechanical fuel pressure gauge and started a datalogger. The fuel pressure gauge was anywhere from 23-30 psi at idle and 40 with no vacuum. The datalogger showed 39-40 psi at idle and did not rise on revs, but would fluctuate on decel. None of this information is under load, just parked.

Could this be a faulty FRPS? I wouldn't think that there would be 25% difference between the two. Any suggestions are appreciated.

Tony

cliffyk

Does it do this after running it fairly hard and then letting it sit for a few minutes (like running into a jiffy store to get a 6-pack)?

If so it is normal--the fuel heat soaks and vapourizes in the rails causing a lean condition on the hot start. There is a function in the tune (Cranking AF Ratio) that can be changed to make the mix richer at high coolant temps--also running premium fuel in the summer can minimise the problem.

U S Marine

I don't run the car hard; but it does sit for a few minutes.

I do understand the heat soak/vapor lock that can happen. That's the reason I started depressing the accelerator, to purge the air. I'm sure the Cranking AF Ratio would have to be established by the tuner, meaning; it's not an end user option(?). Also, the tune is a 93 octane tune, so I run 93 100% of the time.

Got any ideas on the difference in the readings between mechanical gauge and the electrical reading?

Thanks,
Tony

cliffyk

The mechanical gauge is reading the actual rail pressure that can vary from 28 to 40 psi depending on manifold vacuum--or even more if the PCM senses the fuel may be hot.

The psi provided by the PID (datalogger) will always be around 40 psi, or 53 psi or so if the fuel is hot. The PCM adjusts the actual rail pressure to maintain a 40 (or 53) psi drop across the injectors. So if the manifold vacuum is high (say 20 inHg = -10 psi) then the PCM will set the rail pressure to 30 psi to maintain a 40 psi differential across the injectors.

Another issue is that depending on the quality and full scale range of the mechanical gauge it could easily be off by ±3-5 psi at any reading; that would be an overall accuracy of ±3 to 5% of full scale for a 100 psi gauge, about par for a non-lab grade mechanical unit.

U S Marine

Makes sense. I knew the mechanical gauge had some room for error; but I figured it might give me a quick, on the spot, indicator if I was having a fuel related issue.

Would you say that the fuel system is operating as it should then? Also, while doing this, I shut the car off and during crank up, it did not start. Mechanical fuel gauge read 30 psi.

cliffyk

I have never measured fuel pressure while cranking, however that sounds as though it is in the ball park of where I would expect it to be--was that during a "hot engine no start" condition?

How does it start when cold?

Also, you are correct that the Cranking AF cannot be set via a typical handheld or PC based tuning system--however there are "end user" tuning systems that do allow access to any parameter of the tune--like SCT Advantage and Delta Force's Commando...