I have had a really bad miss/stumble/bogging lately after the car has been running for like 15-20 mins. A bunch of people suggested checking fuel pressure. I have the proper test kit but need to know how to use it to tell if its the problem im having. Any one do this before and know what to check for?

 

Ok, so I tested it and was about 39.5 with key turned on, supposed to be 41-47. Then at running temp idle it was about 37.9, not the desired 3-10 psi drop. Where should I go from here? DOes this mean its the cause of the problem, the margins its off by are not much at all. People that had fuel pressure issues before, what was your fp?

 

Have you replaced your fuel filter?

 

Try measuring the pressure this way:

Start the engine. Pull the vacuum compensation line off. Then read the pressure. That's when you want 41-47psi. Then reconnect the vacuum line and see how much it drops.

Tape the gauge to the windshield. Take it out on the road, up to max RPM/engine load. It should hold within 1 to 2psi of what you got without the vacuum line connected.

 

When I measured the fuel pressure with the engine running and fpr vaccum line off, it jumped to about 45 psi (from the 40 psi it was at idle with the hose on). But then when I got out and started driving it around, it would hold the 40-42 psi under low rpm driving, but when I got on the car, the fuel pressure would drop to about 36-38 psi, what could this mean?

 

When you open the throttle, the fuel pressure should increase all the way to the value you got with "no vacuum". If the fuel pressure drops when you open the throttle, either the fuel pressure regulator or the fuel pump is screwed up.

 

Mine would stay pretty steady in the low-40 psi range but then close to WOT it would decerease slightly below 40 psi. I didn't think such a slight variation could mean I had problems. How do I know the fpr is shot rather than the pump?

 

Could be time to change O2s? Seems like with an OBD1 that FP at 38 +- would give you more power with the lean burn anyway.

I gained about 17 RWHP when I dialed in 38 on my OBD11 (unfortunately the PCM adjusted the injectors to compensate for my FP adjustments)

I am running stock now, but PCMFORLESS adjusted my A/F to provide me that missing power. Probably wrong as usual, but that's the way I sees it.

 

id like to know how you know if its the regulator or pump that is bad after these tests?

 

The typical problem with the FPR would be a broken diaphragm. In that case, you would have fuel in the vacuum line. Did you notice any when you were testing the pressure?

 

No there was no sign of fuel in the vacuum line when I disconnected it from the fpr.

 

One thing I don't understand though, if my stock fuel pump is going out, wouldn't that cause my motor to be starved for fuel (especially at WOT in the high RPM), causing a lean condition? When I logged a few runs with a friend of mine we had the wideband O2 sensor hooked up to my exhaust and it showed my AFR to be fine (even slightly rich) at WOT alll the way to redline.

 

You're basically telling us that the 45psi (no vaccum) fuel pressure drops to 36psi at WOT. That's a 9psi drop, and not within the acceptable range. Whether that is causing your problems - miss/stumble/bogging - is unclear. You asked how to measure the fuel pressure, and I told you how.

Does the miss/stumble/bogging occur only at WOT, or at various combinations of throttle position and RPM?

Stock programming (you haven't told us if its stock or its been "tuned") will generally produce an A/F ratio in the range of 11.7:1. If your tune is "stock", is that what you saw on the wideband?

There are any number of things that will cause the problems you have, and fuel pressure isn't necessarily the cause. But your fuel system isn't performing up to spec. You need to find out why. When was the filter changed last?

 

I think you're confusing me with the original poster I am not having any problems (knock on wood) it's just that I instaled a cam kit in my car last summer (made a new sig for mods) and it may be down on power so I'm looking at some things. Since the fuel pump is stock, 10 years old and prone to failure from what I read on the boards I decided to test it out (found this thread while searching) and got the results I posted above. However I also got a reading in the mid-12's for my AFR at WOT on the wideband. I do have a PCMforless tune.

I just wasn't sure if my slight rich condition at WOT could be compatible with my diagnosis of failing fuel pump that I got from testing the pump.

 

My mistake.... guess that's what happens when someone hijacks the thread. Sorry I wasted your time...... and mine