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Po171 code keeps coming up, I changed the fuel filter. Pressure at fuel filter is 45, pressure at fuel rail is 45. Pressure is 50 with pressure relief valve disabled. Diaphram is good at pressure relief valve. Code keeps coming up during in warm weather when things are good and hot. Do I need to replace the fuel pumps? Are they weak? Is it trying to vapor lock on me? Mass air flow has been cleaned. Intake throttle body is cleaned. Can't find any vacuum leaks. 460 gas motor. Help?

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Did you check the o2 sensor bank 1 ? cause de p0171 code say sistem too lean bank 1 do you need replace this sensor and get a better fuel saving . or maybe is disconected or the o2 sensor wires could by cut o burn, i think about you problem is not the fuel sistem . clean again carefully the maf sensor without broke the tiny couple wires thks

Posted on Aug 07, 2010


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Have you checked the fuel pressure at the rail ?
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98 f-150 check light po-170&po171 help

!70 and 171 are lean mixture codes, I'd start by changing the fuel filter. It's located under the truck, inside the frame rail, just about right under the drivers seat. The fuel lines are under pressure so you have to relieve the pressure before you disconnect the lines. You can let the vehicle sit for at least a 1/2 hour with the key off, or find the pressure test port on the fuel rail (on top of the engine) and bleed the pressure off there. You are going to need a special tool(s) to disconnect the lines. Helpfull hint push the line on to the filter while you insert the release tool(s), a little WD-40 or penetrating oil helps. Keep in mind your dealing with gasoline, keep your hot trouble light far away so there is no chance of gas splashing onto the hot bulb.

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1997 GMC 1 ton 4x4 hard to start after it has warmed up. changed fuel filter no help No code to be read We have notice that the fuel in the rail is vapor not a liquid. Is this normal and if not what would...

The fuel pressure should hold at or near the 45 lbs after you shut the truck off.
You have one of three problems causing this.
One, the fuel pressure regulator is leaking internally and allowing fuel to return to the tank via the return line. To verify, use a pair of pliers and gently pinch the return line off. Turn the key on to the run position, the pump will run for 2 to 5 seconds and pressurize the line. Watch the pressure gauge, it should hold pressure. If it does, change the regulator.

Two, the check valve at the pump is weak. To verify, turn the key to the run position, when peak pressure is reached, gently pinch the pressure line and watch the pressure gauge. If the pressure remains near the peak pressure the check valve is bad. Replace the fuel pump.

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Any one of these failures will cause the "vapor" issue you describe.

Good luck, let me know what you find.

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