lack of fuel at WOT

1st my mods:
3.6" MPS with 660(66.8") s/c belt
DHP PCM w/15* stock timing
AFC 2.2/LS1M
PEMs(Ported Exhaust Manis)
3"DP-Hi-Flo Cat-2.5" U-Bend Delete/Resonator Delete-Stock "Y" and Stock Mufflers
180* T-Stat
[email protected]
4"FWI(Fenderwell Intake)
ZZP Fuel Pump Rewire

With the AFC at idle/cruise they're slightly negative but very close to 0.
heres a scan:
http://scandepot.net/viewscan.php?pk=2388

i tested the Fuel Pressure and its at 40PSI at idle and holds there. when driving and at WOT, it'll hit ~50PSI, decrease as i increase in the RPMs to ~44-45PSI, then it'll spike back to ~50PSI at the start of the next gear then it'll decrease again as it increases RPMs. i also disconnected the FPR vac line and the Gauge went from 40PSI to 48PSI.

I replaced the following Parts to these:
stock Fuel Pump to 40K fuel pump
stock FPR to a used FPR
stock injectors to 50K injectors
New Purolator Fuel Filter

after all of this im still getting Lean O2s and getting as much as 20-21 on my injector cycles and hitting alot of K&R. as much as 5-6* at WOT. i am currently testing the voltage on my fuel pump connector and see if its loosing voltage to the fuel pump. but if anyone has any suggestions for my problem that would be greatly appreciative.
fst100

Edit-also i checked all the fuses and relays and they are fine. also i didnt see/feel any kinks along the fuel lines.

The fuel pressure is too low for the mods. What year is your GTP?

I have nothing less than 45 psi at idle with vacuum connected at the FPR. I see you installed a used FPR but have you considered a higher output like the Casper.

What rating are your fuel injectors in pounds of flow per hour?

98 GTP
i'll be getting another used FPR from a local member.
I was thinking about the casper AFPR, if thats what you're suggesting, but its a bit expensive for now(getting an alky kit in the mail).
my injectors are stock injectors with 50K miles on them. cant remember the stock # they flow, but im guessing 36 or 39 is coming up in my head at the moment.

minimum fuel pressure spec is 48-55psi key on, engine off. It looks like your issue is low fuel pressure.

98 GTP
i'll be getting another used FPR from a local member.
I was thinking about the casper AFPR, if thats what you're suggesting, but its a bit expensive for now(getting an alky kit in the mail).
my injectors are stock injectors with 50K miles on them. cant remember the stock # they flow, but im guessing 36 or 39 is coming up in my head at the moment.

I understand, it was the miles on the injectors. The stock are 33#.

If you can get flow up at the higher rpms or 3 - 5 psi you should be OK but you are low compared to what I've seen. Yes, I was referring to the Casper adjustable.

Try the replacement FPR. They vary in what they flow and put out. The Purolator fuel filter is OK but the AC Delco flows better.

minimum fuel pressure spec is 48-55psi key on, engine off.

Not on a GTP Rich.

The fuel pump has two speeds.

I understand the fuel pump has two speeds, and the PCM primes the fuel rail/injectors using the high speed circuit. This check ensures proper fuel pressure for the vehicle to start. What is the fuel pressure reading key on/engine off? THe fuel pressure reading seems borderline at best while running from what I'm reading. Just my 2 cents.

minimum fuel pressure spec is 48-55psi key on, engine off.

^^^Incorrect.

I understand the fuel pump has two speeds, and the PCM primes the fuel rail/injectors using the high speed circuit. This check ensures proper fuel pressure for the vehicle to start. What is the fuel pressure reading key on/engine off? THe fuel pressure reading seems borderline at best while running from what I'm reading. Just my 2 cents.

Why are you asking me what the proper fuel pressure is for a L67 GTP? If you had L67 experience you would know what the fuel pressure reads in the pump prime position (key on/engine off) and I would not have to explain it. It is not 48-55 psi as you had stated at key on/engine off. Plus there are other reasons why a L67 has a two speed fuel pump in which I am not going to explain here. Also, the L67 fuel pump flows more fuel than one on a L36. Throwing in your two cents in is one thing but erroneous information is another.

As a matter of fact a L36 SE/GT will not read 48-55 psi in the pump prime position (key on/engine off).

The so called textbook reading of 48-55 is with the engine running at idle and vacuum removed at the fuel pressure regulator. The poster gets 48 psi in that condition.

i also disconnected the FPR vac line and the Gauge went from 40PSI to 48PSI.

The 40 psi reading is enough to start the engine. I've seen readings in GTPs ranging from 40 to 45 psi with key on/engine off. Mine reads a solid 43 psi in this condition with an Actron fuel pressure tester. However, readings will vary from GTP to GTP. The poster's problem stems from insufficient fuel pressure or flow at higher rpms. If you review his first post a peak reading of 50 psi at WOT or full SC boost is low. I get 52 - 53 psi. The median for a GTP is 52 psi.

A Casper adjustable fuel pressure regulator makes up for that deficient area especially if the engine is modded.

Also, note that the poster has a ZZP fuel pump rewire kit which is supposed to illiminate the fuel pump resistor and allows the fuel pump to get a higher input voltage and current wise due to the poor stock wiring. Casper makes one similar called the Hotwire. The purpose is to prevent a lean condition.

From what I can see he is 2 - 4 psi low for what is typically normal. However, in an unmodded GTP that might work out fine. Where I agree is his readings are low as I previously stated.

couldnt get the FPR from my friend to work, his was an 02 while mine is a 98. seems like the 97-98 were different than the 99+ >.<

will i tested the fuel pump adapter. there are 4 wires on the fuel pump.
purple-fuel level sensor
grey-fuel pump sensor
black-ground
black w/white stripes-cant remember but its irrevelant in this situation

took a multimeter and probed the pin on the grey wire and grounded the other probe needle to the chassis. i had someone prime the fuel pump. supposely, the voltage from the fuel pump sensor should read ~10-12volts, but mine didnt even move, not even when we started the car. now im thinking its an electrical problem. but i cant seem to find where the wiring harness goes under the trunk.

couldnt get the FPR from my friend to work, his was an 02 while mine is a 98. seems like the 97-98 were different than the 99+ >.<

will i tested the fuel pump adapter. there are 4 wires on the fuel pump.
purple-fuel level sensor
grey-fuel pump sensor
black-ground
black w/white stripes-cant remember but its irrevelant in this situation

took a multimeter and probed the pin on the grey wire and grounded the other probe needle to the chassis. i had someone prime the fuel pump. supposely, the voltage from the fuel pump sensor should read ~10-12volts, but mine didnt even move, not even when we started the car. now im thinking its an electrical problem. but i cant seem to find where the wiring harness goes under the trunk.

The black w/white wire is also for the level sensor but as you said it is irrelevant for checking the fuel pump.

Maybe you are referencing to a bad ground. The gray wire is the pin B input to the fuel pump (hot). The black wire is pin C to the fuel pump (ground). Is the multimeter set for DC Volts?

Try putting the red lead on gray/pin B and the black lead on black pin C at the connector plug.

How is that ZZP rewire kit wired into the circuit?

The other thing you can do is go point to point and check for continuity between the connector at the fuel pump and back to the fuel pump hi-speed relay in the engine compartment fuse box. However, you will need a one long meter lead to do this.

BTW - It would be impossible to get the car running or have any fuel pressure reading if the wiring was really that bad like an open or short.

Edit added:

The fuel pressure regulators are supposed to be the same for '97 - '03 GTP models. My book shows the same part number.

"Maybe you are referencing to a bad ground. The gray wire is the pin B input to the fuel pump (hot). The black wire is pin C to the fuel pump (ground). Is the multimeter set for DC Volts?

Try putting the red lead on gray/pin B and the black lead on black pin C at the connector plug.

How is that ZZP rewire kit wired into the circuit?"


i did put the red lead on the grey/pin B and black lead on the black/pin C, and still no reading. The ZP Fuel Pump Rewire is wired in the inside of the car, i opted and did not do the 100% method of extending it all the way to the engine bay. Still, the Fuel Pump Rewire is wired correctly, otherwise my fuel pump wouldnt' even power on with the key turned.

About the FPR, i mistyped that, the FPR i got from the local came from the L36, which is very different from the L67. he nor I knew of the difference(didnt have a manual unlike some of us :-p) im gonna rule out the FPR since I'm not getting any voltage from the fuel pump sensor when i should >.<
a friend i know recommended taking an 18 gauge wire and use it to 'directly' connect the battery to the grey wire fuel pump sensor since its alread hot. im gonna have to cut the grey wire to then connect the 18 gauge at the fuel pump adapter. of course i'll put a 15amp fuse in between the battery and the grey wire. im also gonna hook up a switch so i can turn the voltage on whenever and see if there is actual voltage going into the fuel pump sensor. it'll look something like this:
battery (+) ->15amp fuse holder -> On/Off Switch -> Fuel Pump Adapter

once that's in i'll take the car out on a drive and also hook up my Fuel Pressure Gauge Tester and check on whats my FP at WOT. To see if it'll go up as high as 52psi as the target with my mods.

What do you think? any other suggestions?

"The other thing you can do is go point to point and check for continuity between the connector at the fuel pump and back to the fuel pump hi-speed relay in the engine compartment fuse box. However, you will need a one long meter lead to do this."
Yea i dont have a long enough cable to use unfortunately >.< i forgot to mention, i did replace the Fuel Pump Relay as well as the Fuel Pump Hi-Speed Control Relay. Still a No Go

It sounds like you are on the right track and your troubleshooting technique looks good but I do have a reservation. The problem (checking voltages at connector) doesn't make sense. With the connector at the fuel pump connected do you hear and feel the fuel pump prime at key on/engine off? The last time I checked when ignition is switched to on fuel pump prime only lasts about two seconds. Therefore, there will only be voltage at the fuel pump connector for a few seconds. You can repeat by taking ignition to off then back to on while checking the voltages.

when i turn the key to the "On" position, the pump will prime, i can hear it even if the middle fold-up/down elbow rest is closed off. it'll last for ~2-3 seconds, so the pump is priming the line with fuel. trust me i went and checked the red lead to grey pin and black lead with either chassis or black pin for the 2-3 seconds the pump was priming. i know this doesnt make any sense to me either. its just mind boggling.

Edit- yes i turned the key on and off testing different ways to see if theres voltage going. i even, again, started the car to see if there was voltage, although i think im killing my starter-turned key to "Run" and didnt click a couple of times in different instances >.<

when i turn the key to the "On" position, the pump will prime, i can hear it even if the middle fold-up/down elbow rest is closed off. it'll last for ~2-3 seconds, so the pump is priming the line with fuel. trust me i went and checked the red lead to grey pin and black lead with either chassis or black pin for the 2-3 seconds the pump was priming. i know this doesnt make any sense to me either. its just mind boggling.

Edit- yes i turned the key on and off testing different ways to see if theres voltage going. i even, again, started the car to see if there was voltage, although i think im killing my starter-turned key to "Run" and didnt click a couple of times in different instances >.<

Oh yes, I agree definitely mind boggling. You might have to check the voltage to the pump with everything connected. There is some idiosyncracy in your system. Places like Radio Shack or any electronics supplier should have special test lead prods where you can insert the sharp tip into the respective wire without causing too much damage. Also, I've used dressmaker pins inserted into the wire and alligator clips to hold the meter leads.

From what I can see the wiring from the engine compartment fuse box to the pump appears to be good since the fuel pump does operate. But it would be nice to know if you are getting full voltage to the pump with no voltage drop due to degraded wiring or high resistance. At least for piece of mind and ruling it out as a problem. Good luck!

2 things are running through my head. 1)good connection from the underhood fuse box to the fuel pump adapter because fuel pupmp runs and primes, but gets no voltage at the grey pin/copper wire itself w/a ground source. 2)you mentioned the fuel pump has 2 speeds, please elaborate on what each does and when they turn on/off?

Edit-i did test everything connected with the meter, trust i did, thus the use of cutting through the rubber insulation and testing with the bare copper wires on both the grey and black wires. like i mentioned, i think, a few posts up, i also rammed the red lead into the opposite side of the pin on the grey as well as the same with the balck wire. still no voltage reading

2)you mentioned the fuel pump has 2 speeds, please elaborate on what each does and when they turn on/off?

Technically it is a two speed fuel pump based on the input from the two relays, the fuel pump relay and fuel pump speed control relay. The PCM is what actually controls the hi-speed relay and the switching between low and high speed. Basically, it has a low speed and high speed but it only has one input at the fuel pump connector. In low speed the fuel pump resistor is operating in the circuits. Therefore, the 12 volt input to the fuel pump will vary depending on the conditions. One reason for this dual mode operation is to meet the demands of the supercharger to insure an adequate fuel supply to the fuel injection system. Although the injectors are actually rated at 33 lbs of flow per hour they are classified as 36 lb injectors. The GTP fuel pump is a high flow and flows a lot more than one on a L36 Grand Prix with NA Series II 3800.

Therefore, the real reason why GM uses this fuel pump configuration on L67s is this:

Due to the volume of the fuel pump the two speed setup of the L67 fuel pump was designed mainly for in tank emissions reasons and to help keep the fuel cooler in the fuel tank. As the fuel was not continuously returning to the fuel tank at high pressure/flow it caused stirring up of the fuel in the tank and warming it as well. This created more in tank emissions so the GM engineers gave it a slow speed setting for when the extra fuel is not needed and to reduce tank emissions.

with the zzp fuel pump rewire, it prevents the low speed and bypasses the resistor, running only in high speed correct? also like the 'mod' of bending pin85 on the fuel pump high speed relay to let it continuously run in high speed mode. i was just thinking if the 2 speeds, mainly the high speed was causing the lack of fuel pressure at WOT.

with the zzp fuel pump rewire, it prevents the low speed and bypasses the resistor, running only in high speed correct? also like the 'mod' of bending pin85 on the fuel pump high speed relay to let it continuously run in high speed mode. i was just thinking if the 2 speeds, mainly the high speed was causing the lack of fuel pressure at WOT.

I know of the ZZP rewire kit and I believe it is similar to the Casper Hotwire kit which I am familiar with. They are supposed to byass the resistor and you will run in high speed only. And the wiring is improved. In your configuration you should have only high speed.

You did say you installed a new fuel pump. How long has the rewire kit been installed with the new fuel pump? Was the fuel pump GM or aftermarket?

i recently istalled the rewire about 2 months back. the fuel pump is a stock gm fuel pump i got from a reputeable junk yard who specializes in grand prixes-ed morad.

i recently istalled the rewire about 2 months back. the fuel pump is a stock gm fuel pump i got from a reputeable junk yard who specializes in grand prixes-ed morad.

I've done business with Ed. He is above board but hopefully the fuel pump was good and not marginal. Thats one of the problems with salvaged parts. :dunno:

Quote from bnaylor,

Therefore, the real reason why GM uses this fuel pump configuration on L67s is this:

Due to the volume of the fuel pump the two speed setup of the L67 fuel pump was designed mainly for in tank emissions reasons and to help keep the fuel cooler in the fuel tank. As the fuel was not continuously returning to the fuel tank at high pressure/flow it caused stirring up of the fuel in the tank and warming it as well. This created more in tank emissions so the GM engineers gave it a slow speed setting for when the extra fuel is not needed and to reduce tank emissions.[/QUOTE]

I always wondered why they had that relay in the first place, and no one has been able to explain it. Thanks for the clarification Bob.

will an update, the reason as to why i got no voltage reading from the fuel pump sensor was because i was checking the original sensor on the car, remember that i installed the zzp fuel pump rewire? will aparently when i hooked that up, i had to cut the grey wire under the passenger kick panel and connect the fuel pump reiwre in its place, thus making the grey wire i was checking totally useless lol. so i checked the sensor off the new wire from the zzp fuel pump adapter and i do have ~12volts when the pump is priming for the 2-3seconds. However, the problem still reamins that i am lacking fuel at WOT. I hooked up my FP and went out for a WOT run. first of all, FP at idle is ~41psi. at WOT. once the engine downshifts to the lower gears my FP is at ~51-52psi and it'll slowly drop as i increase in the rpms(assume im in 2nd gear), then it'll shoot back up to ~51-52psi when i shift into 3rd gear, and it'll again slowly drop in pressure as the rpms increase in 3rd gear. thoughts?

will an update, the reason as to why i got no voltage reading from the fuel pump sensor was because i was checking the original sensor on the car, remember that i installed the zzp fuel pump rewire? will aparently when i hooked that up, i had to cut the grey wire under the passenger kick panel and connect the fuel pump reiwre in its place, thus making the grey wire i was checking totally useless lol. so i checked the sensor off the new wire from the zzp fuel pump adapter and i do have ~12volts when the pump is priming for the 2-3seconds. However, the problem still reamins that i am lacking fuel at WOT. I hooked up my FP and went out for a WOT run. first of all, FP at idle is ~41psi. at WOT. once the engine downshifts to the lower gears my FP is at ~51-52psi and it'll slowly drop as i increase in the rpms(assume im in 2nd gear), then it'll shoot back up to ~51-52psi when i shift into 3rd gear, and it'll again slowly drop in pressure as the rpms increase in 3rd gear. thoughts?

That makes sense now. So far from what I can see you look OK and I do not see any real electrical or mechanical malfunction.

With the mods I have on my GTP I could not operate properly without the Casper AFPR. I'm reading at least 3 psi higher than you and have significantly more mods. Other than that your only other option is looking into a Walbro fuel pump. Neither solution will be cheap.

yea i know neither would be cheap. however based, on the AFPR, ZZP acknowledged that on the grand prix's its basiacally usefless. im just curious on how you have yours working if it is? here's an exerpt from their site:
"Adjustable Fuel Pressure Regulator

On many boosted cars fuel pressure needs to be raised as boost is added to compensate for extra air. The Grand Prix uses 2 methods to control fueling which render an AFPR worthless. The MAF sensor on the car reads the airflow and tells the PCM how much air is coming in and therefore how much fuel to deliver. As you mod and increase flow the MAF reads more and increases fuel. The stock FPR changes fuel pressure with boost. As your boost increases so does your fuel pressure. By changing this function by replacing the factory regulator you make your car impossible to tune. Adding fuel pressure doesn't change the maximum pressure because your stock pump is limited to low 60's in psi. So even though you might add 5 pounds at idle and cruising you're not adding that much at WOT. This makes tuning the car a nightmare. All the ZZP cars run stock regulators. If you have an aftermarket unit on your car, sell it and go back to stock. Thank us later..."

here's the link http://www.zzperformance.com/grand_prix/articles1.php?id=23

now i can upgrade the fuel pump to a walbro that'l flow better, but since im not making or planning on making 300+whp, again ZZP suggests keeping the stock pump. bare in mind i did replace my stock fuel pump with a 40K mile fuel pump from a reliable parts place(ed morad). here's their(ZZP) reason:
"Walbro fuel pump

If you have an M90 on your car then your stock pump is capable of flowing enough fuel. Our 97 GTP put down 410WHP using a stock pump, our 2000 Monte Carlo (L36 200HP stock engine) put down 387WHP with the stock pump. Changing your fuel pump increases the chance of pump failure or problems due to hoses coming off. Until you plan on making over 400WHP, keep the stock pump."

so basically maybe i should get a whole new FPR as a replacement, maybe not, i wont know unless i do it. other than that, what would cause the pressure to drop as i increase in rpms?
baylor, im not bashing your suggestions, so dont take it to heart. i just dont want to spend money where i may not even need to spend, i just want the car to run smooth and 'normal'

You have to take what ZZP says with a grain of salt. What works for them may not work for you and vice versa or "the idiosyncracies in YOUR car". They said the same thing about using the stock AC Delco ignition wires over aftermarket like Taylor or MSD. Yet they now use aftermarket wires like Taylor. The AFPR does makes a difference and you can see the results with a fuel pressure gauge hooked up. I get a lot higher readings than using stock at high rpms therefore I can use the stock fuel pump. Or I can adjust it to stock settings. At least I have manual control.

On their explanation of the MAF sensor although it sounds good it is not applicable to me. I have an aftermarket calibrated LS1 style MAF sensor. I do not have a MAF sensor mounted on my throttle body. I can use the extra fuel pressure and flow.

Either way I do not believe that you will find a cheap remedy for whatever your problem is. To start I suspect a marginal fuel pump.


baylor, im not bashing your suggestions, so dont take it to heart. i just dont want to spend money where i may not even need to spend, i just want the car to run smooth and 'normal'

lol. :lol: You can knock me all you want or bash my suggestions all you want within reason. But keep in mind my GTP works and has a proven track record.

Good luck!

will thans for the clarification. seems though since u have an aftermarket maf, the AFPR would work fine, but how would it function say in my case, with the stock maf?