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fuel pressure issue

SgtMarshal

SgtMarshal

Active member
when I try to start my car, most of the time it wont start unless I turn the key and let the fuel pump prime first. I know it could be a fuel pump, but it could also be the fuel pressure regulator. how do I isolate between each potential fault? the other thing I have noticed is that under increased engine load my O2 readings from the narrowband sensor lean out in PE mode, and sometimes there is kr other times there isn't. My fuel pump might not be able to keep up with the demand. just want to make sure before I start spending money on parts I dont need.

Xp cam, ported L32 heads, full size intercooler, 42 lb injectors, north star maf and tb, fuel pump rewire, ported gen 3 and a machined lim.
 


Check fuel pressure. Since you also have a regulator you can, while the engine is running unplug the regulator vacuum line and see that the pressure rises. Check the specs for that. Also when you shut the car off monitor how long it takes for the fuel pressure to drop. It should take a few hours.

Jeff
 
if the fuel pressure drops is it more likely to be the regulator? we checked the pressure, and it would check out at the pressure it was supposed to be, but then drop off a bit
 
It's hard to say, you could have a bad check valve in the fuel pump. When you remove the vacuum line from the regulator how far did it go up? I seem to recall a spec of 39.5 at idle and 45 at atmospheric (line removed), but done quote me on that. The other thing is that your fuel pressure should rise 1# per psi to maintain the differential pressure. So for instance at 10psi boost with my example spec the fuel pressure should be 55psi. My car is returnless and the pressure is 65 psi all day long no matter what. So for mine the net pressure at 10 psi would be 55psi.

also recheck your re wire of the pump and make sure you don't have some of the wiring burnt up. Check at every connection because that's where you will have resistance build up. Resistance causes heat and voltage drop.
 
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What is a good fuel pump rewire to get? I have a 00 GTP that does the same thing as OP but have changed the regulator twice because I thought the first one I changed was faulty to begin with. I have a drop in fuel pump assembly coming in the mail now and figured I would try a rewire as well at the same time.
Thanks in advance,
Tate
 


the rewire increases the voltage the fuel pump receives. the oem fuel pump loses voltage because of the design/materials of the harness. any aftermarket fuel pump is going to need a rewire so it gets the voltage it needs.
 
A good rewire to get? Man I have no idea, if I was going to rewire the fuel pump I would just buy some 12 gauge wire a relay and relay holder (if you like) a fuse and a fuse holder. Start with the basics first check your base fuel pressure as mentioned above.

if you do rewire the pump make sure to use a quality relay such as a Bosch or Potter&Brumfield. None of that parts store crap, and if you do then buy an extra.

basically the wiring would be as follows:

87 from battery and fuse
87A not used
30 to fuel pump
85 ground
86 old fuel pump wire to turn on the relay.

I also recommend buying a diode suppressed relay or socket or adding a diode across any relay coil you have a 1N4001 is fine.
Jeff
 
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^^^ that is good cheap advice. I would also measure amp draw before and after, that will tell you how clogged the filter was.

Jeff
 


I did check fuel pressure, but I've never checked pressure, and the guy I was with didn't really know what he was doing either. we looked up the specs and the pressure would check out fine, but then bleed off. the problem is getting worse, so sooner than later the car probably wont start at all. to get it started now I have hold the accelerator down almost all the way.

this afternoon I plan on checking the pressure with someone that know's what's going on.
 
Well what where the pressures? It's supposed to be like I said about 39 and 45 with the hose disconnected. Was it close?
 


J57ltr said:
Well what where the pressures? It's supposed to be like I said about 39 and 45 with the hose disconnected. Was it close?
Click to expand...

38 to 51 with the engine running and 48 to 54 with ignition on. With the engine running it measured at 52 and was at like 50 with ignition on. I don't think the gauge was connected good the first time I measured. Current theory is a leakyour faulty injector is causing the problem
 
Possible, but you should have a plume of gaseous smelling cloud when you start. Did you just leave the gauge connected to see how long it took to drop?
 
I lengthened my fuel gauge hose to be able to drive with it. Two ways you can tell if you are running out of fuel is to check WB O2 or driving fuel pressure.
 
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