I can't get a 171 and 174 engine code to clear, help!


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Old 03-03-19, 06:38 AM
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I can't get a 171 and 174 engine code to clear, help!

I have a 01 Chevy Silverado 4.3 and trying to clear these codes and nothing seems to work. So far I changed the MAF sensor, gas cap, fuel pump, fuel filter, smoke test for vacuum leaks, and even put in a new fuel injector unit. I'll clear the engine light and on a cold start it'll come back on within a few miles, I'll clear it again and can drive the truck without the light ever coming back on until I shut it off and cold start it again in the morning. Not sure what else to look for, and ideas?
 
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Old 03-03-19, 06:55 AM
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Common Causes of P0171 and P0174 Vacuum Leaks

This is the most likely cause of too much air being taken into the system. The source of a vacuum leak could be one of many things, including (but not limited to) cracking, wear, or a hole in any of the following:

Vacuum hoses
Air intake boots (see image example below)
PCV hoses
EGR valves
DISA valves
A loose dipstick (or one with a broken seal)


Seems like a tough one to find!
 
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Old 03-03-19, 06:59 AM
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Watch this video. See if it explains your problem:

https://www.youtube.com/watch?v=sk3orMCcpWI
 
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Old 03-03-19, 07:17 AM
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Try disconnecting the battery for a while and make the computer relearn the system. I had a random misfire issue on my 2000 Silverado and that worked for me.
 
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Old 03-03-19, 07:19 AM
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Do you also happen to have any oxygen sensor codes?
 
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Old 03-03-19, 07:46 AM
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Reason I ask... one clue can be your oxygen sensor data. If you have a scanner with live data, start the car and observe the O2 sensor data bank 1, sides 1 and 2 when it is at idle. If the data does not oscillate between .1V and .9V like a sine wave, that is an indication of a vacuum leak. Mine was fine on the highway but it would flatline and throw the P0171 and P0174 codes at idle.

A lot of people think that a separate seemingly unrelated oxygen sensor code means you need to replace the oxygen sensor, when in reality it may be working just fine and is trying to tell you something about the engine.

In my case, my problem was a vacuum leak... replacing all the gaskets on the intake manifold spacer and the isolator bolts that hold it down fixed ALL the codes and restored the O2 sensor function. (Course, that's a known problem with my Ford engine and plastic intake spacer).
 
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Old 03-03-19, 07:58 AM
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I sprayed cleaner all around my hoses, intake, and valves and got no change in the idle, plus the smoke test showed no leaks, you think I should pull and clean the EGR valve next? I did something someone told me to try when I changed my MAF, they said to unplug it while running and see what it does. My engine runs fine and the idle stays the same plugged or unplugged, is this something it should do?
 
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Old 03-03-19, 08:02 AM
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Wirepuller38.. I have great fuel pressure
 
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Old 03-03-19, 08:06 AM
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XSleeper.. I don't have a scan tool but bringing it to my friends shop Monday and hopefully I can find out more. Seeing that it's reading lean bank 1 and 2 would that mean both 02 sensors are bad?
 
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Old 03-03-19, 08:14 AM
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I tried the starting fluid trick and got no change... but in my case it was still a vacuum leak. They can be hard to find, even with a smoke test.

I just watched one video on youtube, his 2001 4.3L had a vacuum leak in some hard to access area. Other posters seemed to have this same problem too. I'm guessing it's something like that... just very hard to find.

Like I said... there is usually NOTHING WRONG with the O2 sensor if it acts that way. When it flatlines at idle it is working perfectly, telling you there is more air getting into the engine than what your MAF is detecting. Those 2 clues together are a vacuum leak, plain and simple.
 
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Old 03-03-19, 08:41 AM
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suspect you may have an intake gasket leaking or plenum gasket would check them again on a cold engine as they are more likely to leak when the engine is cold.
 
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Old 03-03-19, 10:38 AM
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I'm leaning towards a vacuum leak myself, I'll try checking for leaks around the intake again. I just put a new spider injection unit in and all new gaskets on the plenum, checked when we started it and had no leaks but I'll give it a second look. I tried getting the engine light off by disconnecting the battery for 15 minutes but it would clear, any other way to get the light off? I just put on a new MAF and wanted to see how long before the light came back on after a cold start. I'm going to have another smoke test done, maybe the mechanic missed something. By the way for my own understanding, will I get a change in my engine idle when I unplug my MAF, it seems to run the same if it's plugged in or not. Sorry one more thing, after I connected my battery and started the truck my fuel gauge is all the way past full and doesn't move, is this something the computer has to reset by driving it? Thanks guys..
 
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Old 03-03-19, 10:53 AM
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Keep in mind vacuum leaks can be anywhere you have vacuum lines. Emissions controls near the gas tank... brakes, dash board, its hard telling where the leak could be... at any connection or tee. At 18 yrs old, those vacuum hoses are likely getting brittle and dry. If you haven't gone to Youtube and typed in "4.3L vacuum leak" I would certainly recommend that you watch some of the problems that other people have had and try to rule them out. Don't assume it is something on the engine. It could be something as simple as a loose air cleaner hose that is AFTER the MAF sensor.
 
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Old 03-04-19, 03:59 AM
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Thanks for all the info and I will check out Youtube. Yesterday I took it into town about 6 miles with the engine light on after installing the MAF and reconnecting the battery. After I came home I parked it for about 4 hours and had to go to store, started the truck and the engine light was off. Drove it 10 miles to the store, came out started it and the light was still off so I drove around for awhile and put another 40 miles on it. It's been sitting all night so I'll go out and give it another cold start this morning, if the light stays off I'll put another 30 miles or so on it then run into the shop to see if it's ready for inspection. If that doesn't work and the light comes on again it's getting a good smoke test this time, I think the last mechanic gave it a quick once over and probably missed something. I'll keep you posted, thanks again for all your help..
 
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Old 03-04-19, 10:09 AM
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I got lucky, problem solved! I don't have to do a smoke test, put on another MAF and everything cleared, the first MAF must have been faulty from the factory, just my luck. Thanks everyone for all your help, I learned a lot from you guys in the past few days....
 
 

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