Fuel Pressure Question

B

Brandon

Registered User
My car runs really strong, except when you go WOT and then it'll bog down and then pick up again. I'm thinking it's fuel related. It also blows black smoke, which would lead me to believe it was running rich, but the bogging would make me think it's running lean. What do you guys think? I have 42lb injectors and an AFPR. Should I put the stock FPR back on? I have it at about 41psi static right now.
 
More than likely your problem is not FP related but MAF transfer function related. Need to get an AF reading on the car so it can be corrected.
 
check fuel pressure with engine idling should be 35-40 psi then go to wot pressure should go up to 50psi
 
ok guys well i gotta question now.. i was gonna start a new thread but i figured as long as its on top ill just ask in here.

been trying to get my 94 tuned for the last 3 months chasing one problem after another. finally have it narrowed down to what i believe will be the last one.

static FP with no vac line at idle 39. checks out looks normal
my datalogs kept showing lean under load once you get above 3.75 maf volts. no matter how much tuning couldnt get it to straighten out.
got new seals for my snapon fuel pressure gauge and hooked her up tonight
took her for a drive FP under no load just cruising it seems normal. as soon as you give it a load the comp pulls all kinds of timing out, it goes lean and fuel pressure never really got any higher than 37ish under load.

im running a 255 holley in tank not the FI version tho. stock lines stock rails. stock FPR a full list of my motor build is on my member page.

now my question is do you think the FPR has gone south, pump just not big enough lines not big enough? soo many possibilities.

Thanks guys
Chris
 
XR7 Dave said:
More than likely your problem is not FP related but MAF transfer function related. Need to get an AF reading on the car so it can be corrected.
Click to expand...

Is the only way to do that on a dyno?
 
NR94mtSC said:
ok guys well i gotta question now.. i was gonna start a new thread but i figured as long as its on top ill just ask in here.

been trying to get my 94 tuned for the last 3 months chasing one problem after another. finally have it narrowed down to what i believe will be the last one.

static FP with no vac line at idle 39. checks out looks normal
my datalogs kept showing lean under load once you get above 3.75 maf volts. no matter how much tuning couldnt get it to straighten out.
got new seals for my snapon fuel pressure gauge and hooked her up tonight
took her for a drive FP under no load just cruising it seems normal. as soon as you give it a load the comp pulls all kinds of timing out, it goes lean and fuel pressure never really got any higher than 37ish under load.

im running a 255 holley in tank not the FI version tho. stock lines stock rails. stock FPR a full list of my motor build is on my member page.

now my question is do you think the FPR has gone south, pump just not big enough lines not big enough? soo many possibilities.

Thanks guys
Chris
Click to expand...

Are you sure that the vac line you have hooked to the FPR is referencing boost? It should connect to the manifold return plenum and no where else and it should not have any other lines connected at that point.

Technically all your equipment should be more than adequate. Something is wrong. Process of elimination, make sure the FPR is working correctly and move back from there.
 
XR7 Dave said:
It doesn't have to be on a dyno, but you need a wide band O2 sensor.
Click to expand...

I hear they are pretty pricey....is there a specific one that would be good to get if I decided to?
 
dave i never even thought about that. when we dropped the motor in the port on the plenum was broken off. I JB welded it closed and tapped into the main vac line going thru the firewall near the brake booster.

could this be affecting it? also what can be done to check if the FPR is OK. there is no fuel in the vac line and nothing comes out when the pump runs.

Thanks
Chris
 

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NR94mtSC said:
dave i never even thought about that. when we dropped the motor in the port on the plenum was broken off. I JB welded it closed and tapped into the main vac line going thru the firewall near the brake booster.

could this be affecting it? also what can be done to check if the FPR is OK. there is no fuel in the vac line and nothing comes out when the pump runs.

Thanks
Chris
Click to expand...

You can't do that Chris. You must use a dedicated line tapped into the pressure side of the system. It cannot share line with something else and it MUST see boost.

Brandon, I picked up the Innovative Motosports LM-1 from Modular Depot. Not cheap, but it's either that or a dyno.
 
XR7 Dave is right!

Brandon........I just did the intake gaskets on my 94. If I remember correctly there is another tap right around the inlet air temp sensor that should do the trick. You should be able to tee into it with your FPR vacuum line and be good to go. I believe the line you tap in to is for the EGR.

Hope this helps!

Ed Springer

90 Black XR7 5-speed: 70mm TB, Fresh Air Intake, '94 Supercharger, 5% SC Pulley, Raised SC Top, I/C Fan, Under-drive Pulleys, 190 l/h Fuel Pump, 36# Injectors, Headers, Magnaflow Muffler (no cats) with stock resonators, 3:27 Gears, Ripper Shifter, 73mm CL MAF (Blue Tube), Magnacore Wires, NGK Iridium Plugs and Rear Spring Air Bags.

94 Red SC Auto: 70mm TB, Fresh Air Intake, Stock 90 SC Pulley, Raised SC Top, I/C Fan, Under-drive Pulleys, 190 l/h Fuel Pump, 38# Injectors, Headers, Raven Muffler (no cats) with stock resonators, 3:73 Gears, 76mm CL MAF (Gold Tube), Magnacore Wires, NGK Iridium Plugs, TRANSGO Shift Kit, Pro-Torque 2500 Stall TC, Haydon Auxiliary Transmission Cooler, Tokico Struts & Shocks, American Racing 17” Sniper Wheels, Michelin Pilot Sport A/S Tires and 235 RWHP with 315 LB-FT Torque.
 
thanks again dave.... never even clicked duh. anyway im gonna swap the vac line and see what happens.

as far as the lm1. thats what im using and i like it alot. it also has the capabilties to datalog up to 5 low voltage inputs and rpm plus your AFR for up to 44 minutes.

definitly worth the 350 plus whatever i paid for the rpm converter

Chris
 
im not so sure your problem is vacum . if you had no vacum on regulator you should be getting full fuel presure at 55-60 psi ,did you say the fuel pump you had was not for efi?? if so thats a problem fuel pumps made from carbs dont run pressure high enough to run efi car . an efi car needs pressure and volume when car is idling or under light load the high vacum lets unused/unneeded fuel return back to fuel tank using fuel pressure regulator
when car is under heavy load /wot vacum drops closing regulator allowing fuel pressure to increase with little to no fuel returning to tank . so if you are having problem at wot take a closer look at fuel pump if you are not running a in tank pump make sure to remove pump in tank and replace with new pickup tube ,there are alot of good companys making in pump tanks i would recomend airtex #e2044 it sells for 59.00 from autozone and flows 170lph and has a max psi of 100psi .
 
jwhit said:
im not so sure your problem is vacum . if you had no vacum on regulator you should be getting full fuel presure at 55-60 psi ,did you say the fuel pump you had was not for efi?? if so thats a problem fuel pumps made from carbs dont run pressure high enough to run efi car . an efi car needs pressure and volume when car is idling or under light load the high vacum lets unused/unneeded fuel return back to fuel tank using fuel pressure regulator
when car is under heavy load /wot vacum drops closing regulator allowing fuel pressure to increase with little to no fuel returning to tank . so if you are having problem at wot take a closer look at fuel pump if you are not running a in tank pump make sure to remove pump in tank and replace with new pickup tube ,there are alot of good companys making in pump tanks i would recomend airtex #e2044 it sells for 59.00 from autozone and flows 170lph and has a max psi of 100psi .
Click to expand...


reread my post i was using the 255 lph holley pump for fuel injection. its not a forced induction model pump.

the problem was the vacume line. i switched it this morning with my boost gauge vac port and now the car goes pig rich... the way it should be with that calibration. furthermore having my boost gauge hooked to that port showed that dave was correct that that port doesnt see boost only vac. im assuming that is what controls the vacume operated accesories in the car. there for it needs vac all the time not pressure.

anywho now i get to tune her for real and hopefully find sumtin in the low 13s ....... oh well.

Chris
 
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