Here's the deal. My fuel pressure seems to wander (TBI). When the engine is cold, the pressure is around 23psi. Once the engine warms up pressure reduces to about 20psi. If I shut the car off and restart it, I might get lucky and see 21psi if not it will be 19psi and gradually creep up to 20psi after a couple of minutes.

Anybody have an idea what's goin' on?

My fuel pump is fairly new (255lph Granatelli), fuel filter is a few years old and regulator has a new spring.

 

It may be the gauge itself. The liquid filled gauges are only accurate at room temperature. As they warm up the indicated pressure drops.

Even warming the gauge with your hand or a hairdryer will show the affect.

RBob.

 

I've heard about the guage temp before, thanks for confirming that

The reason I was thinking it may actually be changing pressure is that I seem to be chasing my tail with my idle AFR. I'm running open loop with all OL vs. CTS equaling 14.7 (OL vs CTS + OL vs VAC = 14.7). Idle AFR is never consistant.

Any thoughts?

Oh Yeah, 90# injectors @ 25psi, Stock O2 in stock location

 

Running 90's at 25 psi is on the edge. Another issue is the stock TBI code. It really isn't setup to handle variations in manifold temperature (among additional deficiencies). Could also be switching in and out of sync/sync fueling modes. With the stock ALDL for data logging you are going to be hard pressed to get any useful data.

RBob.

 

I've been noticing the same problem with my fuel pressure. When I start the car cold, FP is at about 14 psi. After warming up from driving a while, it drops down to 11 psi. I have a small 0-30 psi gage in the supply line where it jumps from steel to rubber in the left-front corner of the engine bay. I'm running a new 190 lpm walbro pump.

At first I thought it was the gage was lying to me due to a temperature induced shift, but I'm starting believe the gage is correct because when I datalog, my blm's start out at one level, but seem to creep up (about 5 digits) during a 1/2 hour drive. Maybe the IAT is getting hotter and that's affecting things. I sure hope there's not problem with the pump, god forbid I have to drop the tank again.

I'm going to run a FP test tonight. Without starting the engine, I plan to jumper the fuel pump relay so the pump runs for about 1 hour while keeping an eye on the pressure. Thay way engine temp will not be a factor.

By the way, my Haynes manual says that one of the ALDL connector terminals ('G' I think) is supposed to be the fuel pump test terminal. That terminal is not populated on my 1991. So to run the FP test I have to pull the fuel pump relay and jumper terminal A to terminal E. Is there an easier way to do this without pulling the realy?

 

Look for a red wire with a connector on the end of it. Under the hood, it may be near the fuel pump relay, could also be on the opposite side. Jumper that to the battery+ terminal to turn on the pump.

Can test the gauge by warming it with a hair drier.

RBob.

 

I took Rbobs advice and looked for a single red wire under the hood. Nothing near the fuel pump relays, but guess what . . . over on the passenger side right next to the set timing connector was a short red wire sticking out out of the wire harness. So with a leap of faith and pucker factor at about 9.5 I applied battery voltage. Sure enough, on went the fuel pump. Thanks RBob, I've been looking for the fuel pump test terminal for quite some time. That will come in handy down the road.

I ran the pump for about one hour. Kept a charger on the battery to keep it from dying. Pressure started at about 13.25 psi, dropped to 12.75 after about 5 min, down to 12.0 after 10 min, then stayed at ~12 psi for the rest of the time. So it seems my pressure drops 1 to 1.5 psi after startup. Is that normal?

I could hear a hissing noise as fuel was flowing through the FP regulator, but I figured that was normal. Just for kicks, I gradually crimped down on the return hose until the pressure started to go up. Pump seemed to have no problem holding 20 to 30 psi. I did notice the hissing noise in the regulator went away after the pressure went to above 15 psi. Not sure it means anything, just an observation.

Maybe this is all normal and I just never noticed the small pressure drop until recently. Or possibly something with my pump or pressure regulator???