: Why high fuel pressure?

07-23-09, 10:46 PM
Hi, all. After running great for most of a long trip my 66K 02 STS flipped me the lamp on the way home. I found P0172 / P0175 rich codes and the long term fuel trims were down to almost -20% with the short-terms hitting -10% or so. Hooked up a gauge to the rail; sure enough, fuel pressure is about 60PSI all the time, with or without vacuum (no fuel in the vac fitting). I replaced the regulator - for nothing. Still 60psi! My search showed mostly threads about low fuel pressure, not high. Ideas? Thanks very much for taking time to respond.

Best regards,

07-23-09, 10:56 PM
Check for a blocked or crimped return line. That would be the line off of the FPR back to the tank. Might try blowing through it or maybe some compressed air.

07-25-09, 12:21 AM
Well, that wasn't very satisfying. The return line showed no visible damage or collapse; some of it is metal and other sections are 'plastic.' I was able to disconnect all the quick-connect joints and blow through each section all the way back to the tank under the access plate in the trunk. I also put a rubber hose on the tank fitting and was able to blow bubbles in the gasoline. Nothing seemed remotely blocked. Yet, the fuel pressure is still 60psi. When you start the engine it goes to about 37psi, but within a minute it jumps all at once up to sixty five or so and then settles around 60. It's as though something is collapsing. Yet, the plastic sections of the fuel line seem very rigid - unless maybe they delaminate inside? Any and all guesses welcome... Thanks.

07-25-09, 10:04 AM
Never heard of this, but maybe the FPR is stuck or not getting any vacuum. Put the gauge back on it and watch it at idle. Pull the vacuum hose off of the FPR and see it there is vacuum at the hose (there should be). If not, put another hose on the FPR nipple and pull a vacuum with a hand held vacuum pump or even such on it to apply vacuum. Does the pressure fall? If so the FPR is good and if there is no vacuum at the hose, then that is where your problem lies.

07-25-09, 12:56 PM
Thanks, Ranger. All good ideas. When you first start the engine the regulator is working properly; pressure is around 37 or so. If I pull the vac hose it jumps to almost 50. Once the - whatever is occurring - occurs, then the vac on the regulator ceases to have any effect; pressure is always 60. It's as though something is just strangling the return path and when the pump deadheads it makes 60psi. It's certainly not the metal sections of the return line, so that leaves the plastic pieces - one in the engine compartment and one from the tank to the metal piece in the area of the fuel filter. I'm going to try bypassing those one at a time - just haven't yet figured out how.

07-25-09, 01:55 PM
Is there a piece of old O-ring jammed in the return side of the FPR cavity on the fuel rail ??

You may have to pull the quick connects and FPR and blow back through the rail connector with 100# shop air.

07-25-09, 04:09 PM
If you have 60 psi in the fuel system you would have a LEAN condition!!!!!!!!!!!!!!!!!!!!!!!!!!!! :eek: YES LEAN :thumbsup:

•Turn ON the ignition, with the engine OFF. Fuel Pressure reading = 41-47 psi

•Start the engine. Does the fuel pressure drop 3-10 psi when the engine is started?

If it does not drop
•Disconnect the vacuum hose from the fuel pressure regulator.
•Observe the fuel pressure gage.
•Idle the engine.
•Apply 12-14 in of vacuum to the fuel pressure regulator with a hand held vacuum pump.

Does the fuel pressure drop 3-10 psi when the vacuum is applied?

If it does not drop
Replace the fuel pressure regulator

07-25-09, 04:33 PM
When I tested the injectors for hydro-locking I found the following

Adding 10% to the max fuel pressure (5 psi) was just ok

Adding 5% (2 to 3 psi) worked out

Adding 15% (7 psi) we started to see a lean condition

Adding 20% (10 psi) was bad bad. Ran like crap. That would be about the 60 psi mark

Now the 95 to 99 injectors are a different story

07-25-09, 10:17 PM
AJ...........Sounds like the high fuel pressures actually hold the injector SHUT ????

07-25-09, 11:21 PM
Fuel supply is a simple hydraulic system.

Fuel tank, pump, main fuel line and the return line, regulator, injector.

A high fuel pressure after the regulator can be because of:
-defective regulator (even new parts are not always 100% ok)
-wrong regulator psi, please compare the numbers on the regulator.
-vacuum line (check also the the other side of the vacuum line at the manifold, check if your manifold has a crack etc)

What is your battery voltage? Do you read around 14.5 V? Fuel supply pumps have a battery voltage depended flow, means the higher the voltage, the higher will be the flow, with the higher flow there will be higher pressure delta at the regulator. between 14 V and 15 V you could have up to 10% higher flow depending on the pump and system.

All theory but please check again the regulator, if it sits right and the part number is correct etc. And the vacuum hose. When you pull the vacuum hose from the idling engine, the engine rpm should make a change for about 4 sec, the o2 control will manage the correction after that.

With the vacuum hose, this fuel system maintains always the constant delta fuel pressure between the manifold air pressure and the fuel pressure, so the fuel injection mass calculation in the ecm will be only depending on injection valve opening time.