Fuel Rail pressure + small idle issue

[Justin G]

Hello All,

Quick background on the vehicle. 2004 LS 115k miles on it. I have been getting a CEL, I forgot the actual code it is throwing, ill post it next time it comes up, but it is "fuel pressure circuit high voltage" or something along those lines. I mentioned it to a mechanic last time i took it in and he kind of blew it off. Anywho while driving around town my fuel rail pressure (relative to manifold) is between 54 and 56 psi. But when I get on the highway it immediately jumps to 74.2 psi and stays solid while on cruise control. if i am going highway speeds and push on the gas the FRP drops back down to 54ish as it accelerates. If i hold the pedal steady it will bounce from the 54's to 74.2 with no real pattern or rhyme or reason. Some times when i first start the car and A/C is on I get a whiff of gas for a few seconds and then goes away. Any suggestions on where to start with this? Could this be a leaking fuel rail? or im kind of hoping just a sensor that went bad.

I am not sure if it is related or not but this morning while i was sitting in my car i noticed that while idling every 10-15 seconds or so it sounds like the exhaust kind of "flutters" for a breif moment, sometimes associated with a momentarily drop in engine RPM. The A/C and heat were off while i was sitting there so it's not anything associated with the A/C compressor flipping on that usually drops rpm briefly. This is much quicker than that. It almost sounds like a misfire but when I checked it, it didn't show any misfires in any cylinder over the last 10 driving cycles. I bring this up in case these 2 issues are related in any way.

I travel a lot for work so while I can do most things myself, I don't really have much time available to tinker around with things.

Thanks for your help.

 

[joegr]

Are you measuring fuel pressure with a physical gauge, or via an OBDII tool?
If you turn the key to run (don't actually start the engine), does the fuel pump stop after a second or two?

 

[Justin G]

I was just measuring it with an OBDII tool. Ill have to check that fuel pump thing, but at this point I assume so, the car starts and runs ok. No noticeable loss in power while driving.

 

[cammerfe]

Some years ago I installed several Auto Meter gauges in my '02 LS. (See photos in 'Land Speed LS' in the performance section.) The fuel pressure gauge is the best analog Auto Meter money can buy and complete with isolator. It's tapped into the cross-over line between the fuel rails, and was installed when I first went to nitrous oxide and was using gasoline as the fuel to go with the 'spray'.

The observed fuel pressure has never been 'steady'. It changes constantly and seems to run from a low in the high 20s to upwards of 60 or so. The engine hums along with nary a hickup of any sort. I've come to believe that it's the ECU 'playing' with itself.

KS

 

[joegr]

Yes, the PCM decides on what fuel pressure range is needed from moment to moment. It is not regulated to any constant value. Also note that the PCM looks at it as fuel pressure relative to the manifold pressure. The PCM controls pressure by commanding the REM to run the fuel pump at higher and lower power levels to get the pressure into the range needed. Fuel is then fine tuned by adjusting the injector on times based on the pressure reading (relative to manifold pressure/vacuum) and calculated fuel need.

 

[Justin G]

The ODBII tool that I am using displays FRP relative to manifold pressure. So the numbers on the op are all relative to manifold pressure. I don't expect the pressure to be constant, but it's odd that when its between 52-56psi it bounces around in that range and that is to be expected. But then it will suddenly jump to exactly 74.2psi and stick there, usually until there is a new throttle input. I guess I expect it to be in that 52-56psi range and not "completely steady" but to just bounce around in that range. I am not sure what is causing it to read 74.2 psi, no bouncing around in this range either. It hits 74.2 and stays on exactly 74.2 until it drops to the mid 50's.

 

[joegr]

I would expect that 74.2 is the reading when the pressure sensor (or wiring to it) is failing. Test/verify the wiring to the sensor and if that is all good, replace the fuel pressure sensor.

I doubt that it is responsible for your idle complaint, but you never know. (If I had to place a bet on that one, I'd go with marginal COPs. 2nd guess would be vacuum leak.)