P0171 code won't go away. Terrible gas mileage

vizions

Original poster
Member
Oct 13, 2012
6
I have an 08 Trailblazer LT 4.2 with 61k on it. Recently it threw a light and the code was p0171 which is lean on bank 1.

I have replaced the upstream o2 sensor, cleaned the MAF and throttle body with no luck, Pulled the plugs (looked fine)

A couple symptoms I've having, A rough idle while in D only and at a complete stop RPM's drop to about 600 and starts to idle rough but acceleration is normal. Now an odd one is while I'm driving and doing about 40-45 with no foot on the gas my RPM's will drop from about 1100 to 6-700 instantly and instantly jump back to 1100 with an clearly felt jump. My biggest problem is the gas mileage is absolutely terrible.

In addition now, I've had a P2270 (02 bank 1 sensor stuck lean) code pop up and a misfire on cylinder 6 code pop up. I've just dumped a bottle of Techron in last night on a empty tank to see if any relief, it hasn't. Any help would be much appreciated
 

CaptainXL

Member
Dec 4, 2011
2,445
Welcome to GMT Nation ! Glad you found us.

Which brand 02 sensor did you use?

I would also check for spark at #6 coil. Know how to test for secondary spark? Take the whole coil out with spark plug and then ground the base of the plug to the block. Unplug the fuel relay and crank engine. See what you get.
 

McGMT

Member
Jun 17, 2012
621
I am wondering if there is an issue with the secondary air injection valve. Like it is letting air in and leaning the engine out.

That or you have a vac leak, the intakes like to come unbolted sometimes so checking the intake seal and all vaccuum lines would be a good idea also
 

vizions

Original poster
Member
Oct 13, 2012
6
Used a Bosch o2 sensor. I'll have to check for leaks but I don't believe there is any. I also swapped coils with 3 and 6 with the plugs and haven't had an issue with a misfire other then the one time for about 10 seconds and it returned to normal.
 

gmcman

Member
Dec 12, 2011
4,656
Do you have access to a vacuum gauge? You may not hear the hiss but could be presesnt.

Does it only stumble at idle with the brake depressed? Reason I ask is the brake diaphragm could be ruptured..not likely but something to check.
 

vizions

Original poster
Member
Oct 13, 2012
6
Dont have a vacuum gauge and it does it if the brake is depressed or not. Also it's not a real rough idle but is noticeable.

Could it be my downstream o2 sensor?
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
These trucks don't like the Bosch O2 sensors. It has something to do with the heater not being in spec with the GM requirements. IIRC, Roadie did some checking on it.
 

gmcman

Member
Dec 12, 2011
4,656
If you had a dead cylinder you would have a misfire code. The fact you have a shudder and a lean condition I would suspect a vacuum leak or maybe a sensor. I would definitely check for a vacuum leak first.
 

SAR85

Member
Jan 31, 2012
74
vizions said:
Should I take that sensor out and switch with somethin else?

You should replace with an OEM sensor (AC Delco) if the Bosch is not working correctly.
 

vizions

Original poster
Member
Oct 13, 2012
6
After researching for days, could it be the map sensor? I've read that the map sensor has been known to cause this code and bad gas mileage
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Could be the MAP. A few minutes with an experienced tech using a higher end scan tool can examine a lot of sensors.
 

CaptainXL

Member
Dec 4, 2011
2,445
I see you swapped 3 and 6 coils on the OS. Did the misfire follow? Do you now have a P0303? You will probably have a P0300 as well if the codes weren't cleared before swapping coils.
 

b.paine

Member
Dec 4, 2011
15
I took my 07 I6 Envoy to the local dealer and he said the code 0171 meant a bad intake gasket, and now it was running lean. I am 75 years old and the car belongs to my daughter. The tech said the very cold morning( 30 degrees F) caused the gasket to leak....the fix would be $ 500.....I dont believe it. I asked if the manifold bolts cound be tightened but he said no....he said she could burn up her engine if it ran lean. Any suggestions, please:frown:
 

vizions

Original poster
Member
Oct 13, 2012
6
PROBLEM HAS BEEN SOLVED, after reading above post I went and checked my intake manifold bolts. They were so loose I was hand tightening them back up till a socket was needed(very loose) with the worst ones towards the back (cylinder 6) no wonder i was misfiring on that one. Cleared the codes and haven't come back on since. On a side note some of the bolts were an absolute pain to get a socket on to tighten. First place I'd look if that code comes up now.
 

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