Check Engine light - Code P0171 and other issues - Updated - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 03-09-2006, 08:25 AM   #1
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Default Check Engine light - Code P0171 and other issues - Updated

It'* long...sorry...hopefully good information.

Going into last weekend my check engine light showed up. Around the same time I also notice a blip in my rpm'* at WOT. Near 4K rpm there'* a superquick hesistation. Found out Advance no longer scans, went to Autozone for a scan. Code P0171 Lean Bank 1 (Front O2 sensor says the exhaust is too lean). Looked up the code in the FSM, it says to look for wires from the O2 sensor contacting the exhaust etc, possible O2 sensor needing to be replaced. Also says it takes two consecutive run cycles failing to turn on the CEL and three good cycles in a row to turn it off. I saw the light a few times, because it didn't fail everytime.

Wires checked good, O2 sensor on a real time scan tool was slow and sluggish on it'* responses. I have no idea if it'* the original sensor or not. Changed it with a new AC Delco, code still coming back quickly now. I'm at my mechanic buddys shop and he looks at it with his Tech2 scanner. He sees nothing out of place. Intermittent problem is becoming a constant. Whatever is causing the issue is going to be easier to diagnose when it'* a constant.
Next check is the vacuum lines. I had one or two that were cracked and one that was not making a good seal. Replaced them with some high temp silicone lines. Code still appears. I'm lazy and didn't check the grounds from engine to pcm as the manual said.

At this point it'* yesterday.and on my way to work I get the P0171 as usual and a P0336. P0336 is a crankshaft error, the system can't read the 18x pulses and defaults to using 3x (basically limp mode). Looking up the new code thinking it may be the root of the P0171, it says, check ignition coils, plug wires, wiring to ICM and crankshaft sensor wiring. I jump the gun and get a crankshaft sensor on my way home from work. When I get home and start following the FSM diagnosis...it has no mention that I need to change the sensor. Starting to pull my coils off to test the primary and secondaries, I notice all the bolts are extremely loose. (Month ago, I pulled them to help a buddy...he put them back on...apparently lightly snugged the bolts). I tighten them down and go for a test drive. No more blip in rpm, code P0336 never returned, P0171 is back again.

Ok, blip seems fixed. Being the only noticable issue..I'm happy. Only saw P0336 once and the FSM suggested checking coils. I found them loose..fixed as far as I know. P0171 still here..why? I spray TB cleaner around all the vacuum areas. No change. Extreme testing time...I hose off the engine while running with water. Serves two purposes, it'll show any vacuum/ignition issues and all the winter road dirt is being washed off. No change in the enging during the hosing. Hmmm doesn't seem vacuum related, or ignition related. Back to the FSM, check fuel pressure....while I know my fuel pump is making noise when I stop...it tests good. FSM suggests unplugging MAF sensor and driving it that way. (OMG you can unplug 3/4 of the sensors and these cars run seemingly fine). I drive for 30 minutes and P0171 is not found. FSM says replace if the code doesn't return. I have a spare TB at home and change the MAF sensor (since it'* clean and does not look bad). No more code after 45 minutes of driving. Drove to work this morning, no codes.

Will continue looking for codes, this problem appears to have been a bad MAF. The MAF never showed a code or gave any indication that it was going bad. Even a trained mechanic that worked at Pontiac during the year of this car was unable to properly diagnose the issue with the information the car was giving. The Factory Service Manual had my answer through methodical diagnosis steps that a dealership mechanic would follow after the quick fix O2 swap didn't work.

I have ordered an Autotap scanner and it'* on the way. If I had it, I would have had files of recorded data and maybe could have seen the MAF sensor acting funny or giving different reading from it'* norm. I kept the sensor and plan on recording data with the good and bad sensors in...I want to know if I could have seen this issue.
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Old 03-09-2006, 10:02 AM   #2
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Wow, quite the flippin' PITA that was. I have heard of MAF sensors going bad without generating a specific code before. I imagine that it may have eventually done so, once it got bad enough to detect. Before then, it might just have been giving the occasional bad reading, which the PCM could interpret as a valid reading. My thoughts anyway.

Glad you found it and gave such a detailed write-up. Might help us to diagnose someone else'*.

This also shows the value of following the FSM diagnosis sequence, IMO. Now you can return that crank sensor.

Off topic: Getting an Autotap, huh? Cool beans!
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Old 03-09-2006, 10:37 AM   #3
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I wanted the Autotap to record data during that rpm blip. I was bothering because of how quick it was happening, I couldn't see anything on a scan tool.
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Old 03-09-2006, 10:40 AM   #4
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glad that you finally got it worked out.....i kept wondering how many more codes your car was gonna spit at ya
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Old 03-09-2006, 07:58 PM   #5
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P0171 has returned tonight. Fuel tank was very low and the pump is whining louder than the supercharger on full boost. Pump has been noisy...but was reading good pressure. Going to look over the fuel pressure tests again and see if I missed something.
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Old 03-10-2006, 07:52 AM   #6
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Dang it! Did you find out anything last night?
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Old 03-10-2006, 08:43 AM   #7
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I quadruple checked the vacuum lines. And took it ouf for a cough "test DRIVE". Getting rough with it didn't help. Still got the code.

Drove 23 miles to work this morning and no code.

Autotap is supposed to be delievered today and I will be able to see and record data to understand what is happening. Tracing wires and wiggling harnesses is next. Also looking the PCM and connections in order at this point.
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Old 03-10-2006, 08:48 AM   #8
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Quote:
Originally Posted by BillBoost37
I quadruple checked the vacuum lines. And took it ouf for a cough "test DRIVE". Getting rough with it didn't help. Still got the code.

Drove 23 miles to work this morning and no code.

Autotap is supposed to be delievered today and I will be able to see and record data to understand what is happening. Tracing wires and wiggling harnesses is next. Also looking the PCM and connections in order at this point.
Sheesh! This is the kind of stuff that makes the classic old carbed non-computer car guys laugh and point. At which point a wrench up side the head is in order.

Good luck!
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Old 03-10-2006, 09:49 PM   #9
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You may think I am nuts. However hopefully the pieces of my current puzzle are coming together.

To and from work today..no codes..
On my way to meet up with Toddster tonight my car bucks once, and the DIC dings the 3 times, "check oil level" is what it says and it goes away instantly. I had great oil pressure and no signs of an issue.

Meet Toddster, his wife and friends..great dinner with BC people. On my way home..I'm staring at a lovely check engine light (I'd bet money it'* P0171) and my laptop with Autotap scanner is in the trunk due to eating dinner and not wanting it stolen.

I'm home and thought, that oil level sensor dinged once leaving work about a month ago. It was a warm day. Hmmmmm car acts up when it'* been up to temp for a while....hmmmm. What one thing did I say I was lazy about? Checking the grounds!!

I open the FSM.. Section 8A-14 is grounds. Interesting tidbit of information here. The MAF (only thing that seemed to keep the code from coming up) is on the same ground as the oil level sensor and the PCM. UREKA!!!!!!! Following the figure reference in the 8A-201 section are pictures that show you where everything is located. This ground is near the oil pressure sending unit on the backside of the motor.

I am getting changed, putting the car on ramps and heading to look over/clean that ground. I will check and clean any others in the engine bay that I am aware of while I am under there.

More info and test results to come.
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Old 03-11-2006, 01:02 PM   #10
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Ok... I took the ground off. It didn't look fantastic, and it didn't look bad. Cleaned it super well and put it back together with a dab of dielectric grease for good measure.

Instantly had a code and got another low oil level. Hmmm WTF? I checked the connects at the PCM and then the car idles poorly. Reseated them. Car was running poorly..I reseated the connections again.

Called Randy..and I can swap another pcm in to try it.
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