P0171, P0174, P0420.. Help!!

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by nmartin4149, Feb 3, 2013.

  1. nmartin4149

    nmartin4149 New Member

    Hey everybody,
    I have a 2000 Silverado 5.3 with the codes P0171, P0174, P0420. I am completely stumped right now. My mechanic replaced the intake manifold gasket, which was blown, as well as the MAF. I know P0420 is catalytic converter but here's the thing. The truck, after having all that replaced still wants to stall in the garage and whenever I try to take it over 2000rpm is acts likes its dying. I also ended up replacing my EGR valve and cleaning the IAC and nope, still runs like crap. But the real kicker to me is that the stalling and everything is intermittent. I also believe it does it more when the tank is less than half. It the original fuel pump with 193,000 mi on it. So I'm not sure. I read that the catalytic converter could cause it to stall, but would that be intermittent? Someone help! I'm not sure where to go now. I don't have a tool to test the fuel pressure and I'm not sure that I want to take it back to the mechanic since he didn't fix it the first time.
    My next question is what is the average reading from a MAF? I bought a scan tool with live data and it generally pulls 3.6-7.8gr/s. I also changed the PCV and inspected all hoses.

  2. a.graham52

    a.graham52 Rockstar 100 Posts

    check for a vacuum leak around the intake. with the engine running do you hear any hissing or anything that could be a leak? theres a few hoses that run to the intake that if not hooked up properly (and will need to be removed to do intake gaskets) will cause a poor running condition. place to start. your 420 code means the cats not working properly (nothing to do with exhaust flow). with yoru scan tool you ahve, monitor sensor one and sensor two (forget which bank the 420 code is for, i almost think its bank 1), and you should see sensor one voltage going high and low very quick. sensor 2 should NOT be doing this and should have a smooth consistent voltage. if your tool does graphs that would be slick for looking at that.
  3. j cat

    j cat Epic Member 5+ Years 1000 Posts

    your codes indicate a lean condition since the intake was repaired it could be he screwed up. also the fuel pump pressures could be low . engine on 57 psi is what is good . spec is 53-60psi . key on engine off should be 60-62psi steady and not droping.

    did the coolant get into the combustion chamber/crankcase ? if yes then the O2 sensors get damaged. the upstream sensors are the controlling sensors. the downstream sensor just are for feed back and cat converter operation checks.
  4. shibby2oo8

    shibby2oo8 Member 2 Years 100 Posts

    Do a fuel pressure and flow test. 190k on a dephi pump is unheard of. My guess is a fuel prob after the intake work. Also the intake can warp and the ne gaskets would not cure the air leak. Post the engine off, running and driving fuel pressures and whether or not it dropped during the flow test.
  5. nmartin4149

    nmartin4149 New Member

    Thanks for the responses. My dad is gonna drive it around tomorrow to a couple of places to get it re looked at. I'll be driving a new Hyundai Tucson lol. A lot different than ours! I looked at the scanner today and all the O2 sensors are reading like they should, we replaced all 4 amonth ago. The STFTM1 and 2 were fluctuating between -3&3 while the LTFTM1 and 2 were around 23. Is it possible the fuel pump could cause a lean code? And yeah 193k is lucky I guess! Just changed the brakes 30k miles ago, original too! This is the first problem we've had with the truck and would love to keep driving it. I drove home from work tonight and it was fine if I kept it under 2000rpm. If I go above, it feels like its boggin down. I can't do the pressure test because I don't have the gauge. :grrrrrr:
  6. shibby2oo8

    shibby2oo8 Member 2 Years 100 Posts

    Rent a gauge from az or advance. A lean code means there is too much air(oxygen) then normal
    in the system. This can be caused by adding air ( leaks, bad sensors, etc) or taking away fuel. Since it seems to act up under a load i think its not getting enough fuel. A vacuum leak would show most at idle not at 2k rpm.
  7. TRPLXL2

    TRPLXL2 Epic Member 5+ Years 1000 Posts

    Check your exhaust manifold gaskets, we had a blown gasket on one of our headers, and it took the Bank 1 STFT and pegged them out at 37. If you are driving it like that, you will for sure foul some plugs, AC Delco's do not do well once they get fouled.
  8. nmartin4149

    nmartin4149 New Member

    Well, had a smoke test done today, intake is still leaking. The shop is gonna look at it tomorrowfor free since they said the gasket would fix everything. :) also fuel pump too:/ my pump was running 39psi at idle..
  9. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner 1000 Posts

    Well I would say that you were getting a two sided problem. too much air and too little fuel. hopefully it will get fixed this time.
  10. j cat

    j cat Epic Member 5+ Years 1000 Posts

    sorry to hear that all my guesses were correct. multiple failures . hopefully they will fix it properly .

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