Ok guys, I'm new to this Forum but I'm sure I'll be here a lot... I have had a ton of DTC's involving my truck running lean, mass air sensor failure, O2 sensor failure... I cleaning my MAF did nothing so I replaced it with a Granatelli. No change. But I think I learned that blindly throwing parts at it won't always fix it.

So here it is... I have read a bunch of threads about codes P0171 and P0174; both left and right banks running lean (which I have) and I looked for vacuum leaks and found none. Then I thought that maybe I need to get my EPROM's re-flashed because of the mods I have made. (K&N cold air, Flowmaster, Hypertuner, and the Granatelli for cold air intakes) I checked fuel pressure and found I am running at about 40 psi (at idle) insted of in the 60's AND... Whe I hit the throttle while the pressure tester was attached the fuel pressure dropped to about 36 psi. I have read that a drop in pressure when you hit the gas is the result in a blown diaphram in the regulator but the low pressure is a result of a fuel pump dying.

Need help quick guys as my tags have expired and I need to fix this in order to pass DEQ and renew my tags.

Thanks so much

 

What year is the truck? What size is the engine?

 

off the top of my head I would suspect O2 sensor or as you looked for vacuum leak...but more info on the vehicle will help..

 

^^^^^^^^^^ I am going to go with in2pro and say it sounds like alot of your problems are from the bad 02 sensor.

 

More info about my truck...

'04 GMC Sierra Ext cab 4X4
Engine is a 5.3 V8. The truck has almost 75000 miles.
I also had Codes for both O2 sensors at the same time which I thought I could rule out because I figured "no way did both O2 sensors quit at the same time" Do you think I lost both of them? I also thought that the fuel pressure should be somwhee in the low 60's psi

I know that the fuel filter is inside the tank with the pump but is there any other secondary filters that I could check before I drop the tank?

 

you have a 04 so you might have a fuel filter outside of the tank. follow the fuel line and see if there is a round metal canister some where along the frame. you truck should have 4 o2 sensors.

 

Ok, so are the O2 sensors interchangable? Can I swap them around to try to figure out which if any are bad? and what about the fuel pressure. I thought that was a "smoking gun" for all my troubles.

 

O2 sensors are not your problem. They are what is seeing the lean condition. The fuel pressure is way to low. The spec is 55-62. If the pump is not capable of producing more pressure the the regulator has no effect. You most likely have a bad pump. To prove it you need to pinch the rubber section of the return line closed (completely) the pressure should sky rocket (close to 100 psi or more). If it does not you need a pump. Go with your initial thought two O2 sensors at the same time is highly unlikely. 90% of 02 sensors sold/ replaced are not bad.

 

Those are some great tips MDTAHOE. Thanks so much, one last thing though... My dad has a spare fuel pump from his 01 Sierra (same engine, 5.3) Will that fit? I know that the filtering system will be different as they began to put the non-seriviceable fuel filters in the tank with the pump starting in 2004. I have read about two or three different types of pumps depending on the number of connections. How do I know which one I have? TCF or TCU?

 

Replaced the fuel pump...

All is well, no CE light. Starts great, Runs great