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Still getting HORRIBLE fuel mileage please help me

99GPGTP

99GPGTP

New member
heres the story again. 3 years ago trans went out. finally last summer i got money to fix it. went to load car on trailer and fuel pump was bad so i had to replace pump. i put a AEM 320lph pump in my assembly. car fired right up. i removed engine and trans. replaced the trans with a shift kit and 2560 stall converter. put new oil pan gasket and timing cover gasket on engine while it was out. put back in car and ever sense my mileage has been horrible. on hwy with cruise set the best i have gotten was 19.8mpg average around town has been 13-15 depending how i drive. i also put a ZZP 1.0 pcm in while i was doing the transmission. all other mods were done before trans went out and car use to get 28-30hwy and 22-24 city.

list of new parts i have tried and didnt help any
new mass air
new o2 sensor
new map sensor
tried my original stock computer again
tried an after market adjustable fuel pressure regulator setup
Different injectors
even tried a 255lph walbro pump


is there anything else i need to try. i know at idle in park my Fuel pressure is 55psi when i rev throttle it jumps to 70 psi.

i feel that i need to put a stock fuel pump back in the car. i work in a shop and nobody can figure it out, my long term fuel trims are about 12-17 at times at idle. i can get any data that is needed. i just need help to figure out whats going on.
 


You say the fuel pressure goes up when you rev it. Is the car in park when you do this? Is the boost bypass valve working? If your running boost all the time it would consume fuel.

Did you try putting the stock PCM back in?
 
the ltft? + or - ?

that stall maybe to high too. stock is 1800 for a gtp.

met up with a local and his zzp 1.0 tune had him at +12 pretty much all the time, and thats lean as hell.
 
I was thinking the fpr you are using is set too high which is dumping too much fuel, but...your trims are lean so that doesn't make sense. Unless you have an exhaust leak. Personally I would put the stock fuel rail and regulator back on. Not sure why you need an aftermarket fpr?

And your pump shouldn't matter as long as the fpr is doing its job.
 
i forgot to mention i have sense gone back to stock FPR and no exhaust leaks. bypass valve is working correctly. and yes the fuel pressure increases when i rev the engine in park no load. i believe the ltft was showing +12-17 but i may be wrong. i will go by work and get the scanner and post numbers. i can even text screen shots if needed. unable to post pic for some reason. as far as the stall. that shouldnt kill fuel mileage that much. once in lockup the stall is irrelevant
 
but during stop and go local driving your pissing mpg away.

my local mpg suck too, 14 to 16 tops. im all stop and go here tho. on the open highway im 24 and up easy.


those trims are super lean. same thing that local i met up with had going on with a zzp 1.0 tune. all i found changed was the pe file. and the trans. so i installed the pe file in my car. my trims took a crap (went lean) and it started to get kr. so in the trash that file went lol
 


your city fuel mileage isn't any worse than what a lot of us get. driving around like a grandma i'm lucky to get 15. but i can still get 28 - 30 @ 70 mph. i'd love to know how you ever got 22-24 mpg in the city in a GTP. they were only rated for 16 -18 from the factory.
 
EHB said:
your city fuel mileage isn't any worse than what a lot of us get. driving around like a grandma i'm lucky to get 15. but i can still get 28 - 30 @ 70 mph. i'd love to know how you ever got 22-24 mpg in the city in a GTP. they were only rated for 16 -18 from the factory.
Click to expand...

^^^^


Sent from my iPhone using Tapatalk
 
22-24 was actually combined city /hwy. Straight city was 19-22 depending on how i drove. Currently at idle my
ltft is -8 to -13 fluctuating little
Stft is -1 to 4 fluctuating little
Ect 204*
Map 6.4psi
Tp.45volts
Maf frequency 2691hz.
 
those f/t's at idle? the short term should be close to 0.0 at idle. whats the long fuel trim at wot? short term should be 0.0 at wot.

driving around the short term is all over the place and its normal.
 
Stft 0 wot
Ltft -10 wot


Cruise at 50mph
Stft -1 to +3
Ltft -17

At idle right now they are
Stft 0 & drops to -6 then back to 0
Ltft holding -22
 
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well. ZZP sucks ass once again.

thats all out of whack for sure. but thats a canned tune for ya.

you 100% sure you have no vac leaks?
 
Have double checked and triple checked. Use propane to check while watching scanner and nothing shows up
 
Do you Want me to put stock computer back in car? And post results. It only got 1-2mpg better with it. I inly run fuel from BP grade 91-93 octane
 
id put it back in, let the trims learn all over again. then see where the trims settle back in at. if they are closer to normal you know the tuned pcm is doing it.
 


you want neg numbers not that high wot tho. wot should lock in at 0.0 around 40 to 50 % throttle. but it will hit 0.0 any time you get into the gas for a while till it learns. and that takes like 30 miles of driving or so. do a ten mile trip turn it off for a few minutes, then re start it and go 10 more miles, and repeat.

neg means its rich, pos means its lean.
 
Your trims are about maxed at idle. Pcm can only compensate up to 24% I think. If its not the tune I would pull the fuel rail and make sure you don't have a leaky injector.
 
its still learning.

like i said they all do that as they learn. a little into the gas and the ltft goes to 0.0 when it stops locking in to 0.0 at less than 1/2 throttle they are close to finishing learning.
 
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