Help diagnosing a P0172 + P0175

impala454 · September 15, 2016

I've got a 2006 Silverado Flex Fuel 5.3L and have had a P0172 + P0175 on and off for the past couple of weeks. It's got a K&N cold air kit and I cleaned the filter (which needed it pretty bad). When that didn't help I cleaned the MAF sensor. That also didn't help (and it seemed like it was pretty clean). I pulled this data with my scan tool and it's obvious the fuel trims are way off, but I wonder if it's more likely an O2 sensor or thermostat or injectors or what? I found it odd that one out of the four O2 sensors is showing a lower voltage. Is that normal? Here's the data:

certified22 · September 15, 2016

Probably faulty intake gasket do the codes come and go especially with increase and decrease in weather tempature

impala454 · September 15, 2016

Yeah they do come and go. Ugh I hope it's not. Is there any other low hanging fruit I can try first? Or more definitive check to see if it's intake gasket?

anumber1 · September 15, 2016

Intake gaskets really aren't too bad.

impala454 · November 29, 2016

I have new data to add to this. I hadn't been seeing any issues except the trouble codes themselves (engine was still running fine). Now I'm beginning to notice it's a little rougher to start up. I put a fuel pressure gauge on the tester port and noticed the fuel pressure with the key on but engine off comes up to about 40 psi or so and slowly drops off to about 10 psi. Once the engine is running it stays at 55 psi almost exactly. One thing I noted (which may just be my ignorance of how the fuel system works) is to initially connect the gauge I ran the engine and then flipped my kill switch (a fuel pump cutoff) but the engine kept running! It ran a little rough but it kept running and wouldn't shut off! Just curious why this would be. Anyways, the main question I have with this new data is are those fuel pressure readings ok?

Coby7 · November 30, 2016

You seem to be getting plenty of fuel but it looks like all the air going in is not being metered, check your intake between your air filter and throttle body inlet for kinks or leaks. You're not getting enough air.

impala454 · November 30, 2016

Yeah there's just so many different pieces that could be the problem. I will definitely check my connections just to be sure that's not the issue. I just don't want to go on a wild goose chase replacing every piece in there. MAF, MAP, O2, intake gasket, thermostat, etc. And I've seen some say it's a weird computer issue where it needs to be reprogrammed. I guess I'd like to see if there's some sure-fire data from this fancy OBD reader that can give me a clear answer of which part it is. Of all those things, which ones may cause a slightly more labored startup?

impala454 · November 30, 2016

https://docs.google.com/spreadsheets/d/1RNeO3zbJUyd5AxTMXU5Xn37AD_D87m7Au_6PpmsFNTo/edit?usp=sharing

Here's a data log from this morning, you can clearly see in the short term fuel trims it goes from good readings (low single digits) to rich (-19 or so) back and forth. Any thoughts?

Coby7 · November 30, 2016

Well it's not your intake gasket because revs would be much higher than 570. It could be a gummed up throttle body though where it would set the codes at idle because of the black coke obstructing airflow when butterfly is almost closed, this would explain your low RPM, hard start-up an rich mixture at idle.

impala454 · November 30, 2016

Ok! I'll have to give the TB a closer look. Thanks.

Coby7 · November 30, 2016

Have a look in around the butterfly to see if you have a lot of soot.

impala454 · December 17, 2016

I finally got to doing this. The throttle body was pretty dirty. I took some data from one of the hard starts before I cleaned it. See below. After this I replaced the MAF sensor and also cleaned the throttle body. My first start after doing that was still just as hard. Seems if it sits for a while it's a hard start. Otherwise no problem at all, and no problem while running. Still analyzing the data from the start after the part swap and cleaning.

impala454 · December 17, 2016

And here's the same data after I swapped the MAF and cleaned the throttle body. (Though just sat in the driveway and popped it a couple times)

impala454 · January 3, 2017

Still playing around with this. Now read somewhere else that a stuck injector could cause the startup issue? Anyone have thoughts on that?

Ogantone · January 11, 2017

Wow, sounds almost the same problem I'm having with my suburban. I changed the o2's. Now running lean code, with hard to start after sitting awhile. Are you Running rich?