I recently purchased a 2003 Blazer and it is throwing a P0174 (lean on passenger's side) code.

Fuel pressure appears to be at least one of the causes of the lean condition. After changing the fuel filter, fuel pressure at the schrader valve reads about 51psi when the engine isn't running but the key is in the on position. When the engine is running, the pressure gage bounces quickly between 45-55psi at idle but holds steady at about 51 psi at higher RPM's. I've been told fuel pressure should be >60psi at the schrader valve. Fuel pressure at the fuel filter is at ~85psi when the pump first starts running. It drops down to 67psi 10 minutes after shutting off the pump. I've noticed the fuel level sensor has an intermittent (and rare) failure causing the low fuel light to illuminate.

Here are fuel pressure theories with my comments:
1) faulty fuel pump - makes 85psi and holds pressure...although this is under a no-flow condition. Maybe under a high-flow condition it is not able to make 85psi.
2) intermittent open electrical circuit to fuel pump - could be an intermittent loss of ground or power which is also causing the fuel level sensor to fail intermittently. it doesn't make sense that the schrader valve pressure would be consistently low though...if the pump was running correctly sometimes, at some point the schrader valve pressure should be in spec.
3) faulty fuel pressure regulator - most posts I've seen about fuel pressure regulators failing involve leaking fuel into the intake which causes a rich condition (not a lean). It is possible the regulator is stuck or the spring has lost it's stiffness causing an excessive volume of fuel to recirculate back to the tank.
4) restricted fuel line - there is only a pressure difference in a restricted (pinched) fuel line when there is active flow. When there is no flow, there should be no difference in pressure before and after the restriction. Since the schrader valve pressure is low when the engine isn't running but the pump is active (no flow), it doesn't seem like a restricted fuel line would be the issue.

Is there another reason pressure at the schrader valve would be low I didn't list?
Based on the information I listed, would you recommend I:
1) replace a part (like the fuel pressure regulator)
2) perform additional testing before replacing anything else (if so, what tests?)...I don't have a scan tool that gives live trim data but have a PLX Devices KIWI 2 on order that I plan to use with a torque app.


I also looked for vacuum leaks and did fine an open vacuum line that went to no where. I didn't see where this line is supposed to go, so I just plugged it for now. I found a diagram showing there is supposed to be a vacuum tank in the driver's fender but I don't see any evidence of a tank located in the fender. Is there supposed to be a tank this line connects to? If so, is it hard to locate this tank? If this tank isn't being used, could that cause or compound the lean condition?

 

valve pressure way to low

https://blazerforum.com/forum/tech-a...agnosis-23038/

 

I agree that Schrader valve pressure is too low.


I looked at the diagnosis thread and all the attached snapshots are unreadable.


If I know that line pressure at the filter isn't too low but it is at the Schrader valve, can I jump to the conclusion that the fuel regulator has failed or could it be something else?

 

I'll also agree the regulated pressure is too low.

Annoyingly, I can't find a volume test specification for the fuel pumps on our trucks (or from GM more generally).

I did find more complete specifications from two aftermarket sources for fuel pumps for these vehicles, and those specs included flow rates. AirTex claims a minimum free flow rate of 50gph, and Spectra claims 48.6gph for their pumps for this application.

If you want to test for restrictions, hook up the gauge set to the Schrader valve, attach a hose to the bleed port on the gauge, get a 1 quart mason jar, and put the open end of the hose in that. Then jumper the fuel pump, and use a stopwatch (or the second hand on your watch or clock) and time how quickly the thing fills the mason jar when you open the bleed valve. 1 quart in 20 seconds (or 1 pint in 10 seconds) is 45gph, and I'd call that a "pass" for fuel flow.

After verifying fuel flow is unrestricted, it's down to the fuel pressure regulator.

 

Lets say there was a restriction in the fuel line or the fuel pump can't maintain pressure at a high flow rate but the pressure regulator is in spec. In that scenario, as load increases, fuel pressure would drop. However, under a no flow condition (engine not running) there shouldn't be a pressure drop from the filter to the schrader valve lower than standard regulated pressure. That's not the condition i've observed...schrader valve pressure is low even under a no flow condition. Does this sound right or am I misunderstanding the system?

 

You're actually correct there. I was either "overthinking it" or maybe "underthinking it."

I was starting off thinking, "what if the restriction is such that 'normal' volume down the return pipe isn't available." I didn't think that all the way through, though. In that case, the regulator would close up some, to maintain pressure, and you'd have less flow down the return pipe, but correct pressure.

So if your pressure is low at the Schrader valve, but good at the pump, then it pretty much has to be the fuel pressure regulator.

 

Bump on the mystery vacuum line

 

The answer is yes to the question highlighted in green. The stem it connects to is difficult to see but easy to feel. It's next to the abs motor on the firewall side.