silverdo 5.3 lean codes

Howdy all you scanner danners, I have a 2001 chevy silverado with a 5.3. It kicked a p0171 and p0174. Watched scanners video and did a water test and a smoke test and it was the intake manifold gaskets. Light went out, got my smog all was good for about two weeks. Then it kicked on again and I got a p0102 along with the p0171 and p0174. Hmmm....retourqed my intake and then read about this VE test. Watched another scanner video on maf and after testing wires and WOT drive tests and plugging in numbers to a few VE calculators, determined the MAF was bad. All good for 2 more weeks and now I got a P0442 evap small leak and p0171 and p0174. Hmmm.....truck runs great but has high short term fuel trim numbers at idle. As soon as I take off the trim numbers are perfect. Can a evap purge valve cause high trim numbers?

It's definitely possible for a purge valve to stick open and cause fuel trim issues. BUT, GM diagnostics for that condition are pretty sharp - they have a specific code for just such a situation, P0496. A stuck open purge can also cause EVAP leak codes, as you found.

The easiest way to confirm a sticking purge valve would be to get the trims high at idle, and disconnect the purge line from the purge valve, plus disconnect the electrical connector. With the connector unplugged, there should be zero vacuum at the purge valve. If there is, replace the purge valve.

You can also plug the purge valve end and see if your fuel trims improve significantly. If they don't, keep looking for other vacuum leaks.

pinched brake booster line off?

Hooked up the scanner and evap reading was at 0%....disconnected power and tapped on valve and then it started reading. At idle the purge solenoid said 15% and stft's were 36.7% and 32.8%. At 1000 rpm the purge solenoid says 21% and stft's read 3.1% and 6.2% also the Purge solenoid when commanded in the off position at 15% the fuel tank pressur sensor is at -0.84 and when commanded on at 14% the fuel tank pressure sensor is -7.80. I apologize if I'm posting the wrong info but I'm not a mechanic. Just a do it yourselfer tired of being a parts changer.

Sorry. All my pics look like crap. My phone sucks.

I haven't tried this yet. What am I looking for if I pinch off the brake booster line?

A change in the fuel trims. It sure sounds like a vacuum leak from the info you've shared, and one that impacts both banks.

+57.8% Fuel Trim Bank 1(Drivers Side, Cylinders 1,3,5,7)
+61.7% Fuel Trim Bank 2 (Passengers Side, Cylinders 2,4,6,8 )

VERY HIGH Fuel Trims.

What looks like to me is when you activate the purge valve, reason your fuel trims go down is probably due to the fuel vapors you are drawing into the intake manifold which will help rich the mixture. I've seen this go both ways, lean or rich but in your situation, looks like it is adding vapors and lowering the additional fuel command. Your long term fuel trims are still high. long term fuel trims are your computers fuel trim memory. If you have a long term fuel trim (LTFT) of 16% and a Short term fuel trim STFT of 6%, then you have a total of +22% fuel trim. If short term stays at 6% long enough, then the long term will eventually add an additional 6% to its value bringing the short term value to 0 and long term fuel trim to 22%

If you raise the RPMs, do the fuel trims get better or worse? Don't manually activate purge valve. If they get better, than its a vacuum leak, if it gets worse or stays the same (trims might be maxed out), than possibly a fuel delivery problem.

How does the engine run when giving it throttle?

The truck starts, idles and runs great. Has no loss of power. I pull my bass boat up a mountain with no problems. Since I tapped on the purge solenoid and it started working again I pulled a vacuum on it and it holds fine. I guess I'll smoke it again and see if I can find a leak.

paul holt wrote: The truck starts, idles and runs great. Has no loss of power. I pull my bass boat up a mountain with no problems. Since I tapped on the purge solenoid and it started working again I pulled a vacuum on it and it holds fine. I guess I'll smoke it again and see if I can find a leak.

Did you suck on it or use a vacuum pump? I've seen them where sucking wasn't good enough and the pump really showed an issue.

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Lol....I suck but I can't suck that hard. I used a mityvac on it about 6 times and it held every time. Let it sit 3-4 mins each time.