tech ? non mopar

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moparshaker

moparshaker

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hope this is right thread for this. i bought a 96 s10 with a 4.3 cheap since it needed a auto tranny. well i'm getting a code p0171 , p0174 which states its running lean on the upstream exhaust on both sides. i've checked this out which covers alot , but short i replaced both upstream and down stream o2 sensors when tranny was done and new cat. the mass air flow is new and i've checked for vacumn leaks and can't find one by spraying carb cleaner around intake and everywhere else on top. book states that low fuel pressure could cause this too. i checked fuel pressure at shrader valve and have 40 lbs, haynes manual says i should have 60-65 lbs could it be this simple as pressure. the guy i bought it from had papers for alot of work from a shop and the fuel pump was replaced at one time , but some of the other work wasn't up to standards i would want but thats another thread. the truck runs fine with no hard starts or misses , just this code that keeps popping up. does anyone think that the 20-25 lb differance could cause this code? sorry to ramble but any input would be great thanks tim
 
Did you or they replace the fuel filter? Probably on the inside frame rail drivers side.
 
After replacing the fuel filter, test the pressure again.

I had the same codes on my 03 Denali and found a forum that discussed this problem as a defective intake gasket. They said that GM re-engineered the gasket and offered for sale thru the parts Dept. the revised gasket is a different color than the original, so they can't be confused with each other. The leak is supposedly on the inside of the engine in the lifter valley, so traditional leak tests don't work.

My codes only appeared on freezing cold days, so if reset during summer, they were fine all summer long. I tried a simple fix, used Lucas rear main seal fixer additive in my oil and have never had the problem again.

Good luck!
 
the pressure i stated is the pressure at shrader valve after new filter. its 20-25 lbs lower than the manual states it should have. i'm going to replace fuel pump this weekend and if it doesn't clear codes i will pull intake this winter to replace all gaskets. i just wasn't sure if the 20 lb differance could cause this code. tim
 
I think it can Tim, but I really don't have enough experience with fuel injection to be 100% sure. The pressure you stated is during engine idling - right?

Sorry and good luck!
Bryan
 
It probably will not clear the codes by itself. You will have to do that.
 
the pressure is at idle and if i crack the throtle the pressure doesn't fall which means the pump is good, but like i said the pressure is nowhere near what the manual states. i'm no fuel injection expert either lol. but the code does state that low fuel pressure can cause this code. so maybe the previous owners shop put a wrong pump in it.
 
i bought a good pump from rock auto so i'll know saturday if it will fix code. wish this truck just had a throttle body instead of the plenum style intake, lol
 
the unit is an airtek oe from rock auto. installed this unit before in a dodge truck and van and had no problems
 
you know there is a fuel pressure regulator don't you? unless your fuel system runs deadhead to the injection rail you have a pressure regulator which comes back to tank. if working properly when you snap the throttle open the fuel pressure should increase marginally. the pressure regulator is actually controlled by manifold vacuum the higher the vacuum the lower.the fuel pressure? a simple test would be to cap your fuel return line at the rail. have your gauge hooked up and just turn your key on see what your fuel pressure rises to. if memory serves you should see between 75 and 80 pounds deadhead. I would hate to hear you had to change a tank unit pump and not need it.
 
the 1 thing about the fuel pressure regulator it is inside the plenum on this system so i would have to pull top of intake off on this and if i'm doing that i should replace it anyways. i thought of that too. pressure does go up slightly when i crack throttle. the pressure just seems to be big differance then what should be at idle. hoping its just pump
 
sorry just made the association of 4.3 and spider ok, now I know you got a vortec. you should still have an exposed fitting for return to tank. I've also gotten away with gently restricting the FLEXIBLE return plumbing with needle nose vise-grips. should see similar results. good luck.
 
well fellas i just wanted to follow up, been busy but first chance to post since i replaced fuel pump. well the replacement of the pump cleared the codes , even though the old pump was working fine i quess the pressure was 2 low so i was getting the lean code on both upstream banks. so been driving since sunday and no codes. i did notice that the sock on bottom of old pump was dirty and non replaceable , which was weird. i think the previous owner got taken at his car shop and they never replaced pump. the new pump i put in has replaceable filter sock on it.
 
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