Recurring P0101

Recurring P0101

Postby pmou [OP] » July 20th 2020, 3:59pm

I've been having a P0101 that won't seem to go away. I tried searching, and from what i could tell most P0101s that are on this forum are magically solved with MAF cleaner, or seem to be a part of some larger grounding/ignition issue... which I don't think (at this point) is me.

I had a p0101 that felt like a loss of power under load. by that I mean the code was there after this event. I cleaned the MAF with CRC cleaner, re-installed. It then read what seemed quite low (4.5g/s). Before this, it was reading between 8.5 - 11.5 at low/high (cold) idle. MAP reading is within range, ~75kpa @ 8300 ft.

I'm trying cleaning the MAF again just to ensure something didn't get missed when i blasted it.

also in the meantime, i checked the leads. the ground seems good (< .05ohm), the pos lead had full voltage (+/- .05 V relative to battery) however it did have around 100Ohm resistance.. not sure if that could be part of the problem.

At this point, from what i can tell, it seems like the next possible things to check / could be the problem are:

1. Replace the MAF, it's bad
2. Exhaust or intake restriction leading to actual low flow
3. Vacuum leak, less air through MAF but coming from elsewhere
4. Grounds -- seems like MAF is ok, but maybe PCM ground is no good leading to bad reading?

my thoughts are : #1 is last resort hopefully, #2 seems unlikely as it was reading more before the cleaning.. but maybe that was incorrect? #3 - possible, but i think same, wouldn't this mean the MAF reading wouldn't have really changed and problem persists? also, no blower noise under the dash when accelerating. #4 definitely possible, but i'd think i'd be getting other error codes as well

Documenting here for science and to see if anyone has :2: they want to throw in. TIA for any advice

Edit: at one point during a long drive when the P0101 came back, there was a pending P0131 (o2 low voltage B1S1) that popped up pending, but then went away. perhaps could be related
Last edited by pmou on July 20th 2020, 4:26pm, edited 1 time in total.
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Re: Recurring P0101

Postby pmou [OP] » July 20th 2020, 4:07pm

So at this point i'm thinking my next couple of steps are:

1. Reinstall MAF, verify proper installation, check readings.
2. Check for exhaust blockage -- i would think this means pull O2 sensor and see if MAF rate increases?
3. Check for vacuum leaks -- haven't thought through this one, but i have a feeling it's not super simple
4. Check PCM ground

If 2,3,4 show no problems, and the MAF reading is still off.. then it must be the MAF?
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Re: Recurring P0101

Postby tonydobbs » July 20th 2020, 4:29pm

pmou wrote:It then read what seemed quite low (4.5g/s). Before this, it was reading between 8.5 - 11.5 at low/high (cold) idle.


If my memory serves the last time I measured my MAF it was 4.6 at idle, so that doesn't seem too unusual. It's going to measure higher when it's cold and the RPM is higher. If I take a drive today I will check the MAF data and post it here.
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Re: Recurring P0101

Postby pmou [OP] » July 20th 2020, 4:39pm

tonydobbs wrote:
pmou wrote:It then read what seemed quite low (4.5g/s). Before this, it was reading between 8.5 - 11.5 at low/high (cold) idle.


If my memory serves the last time I measured my MAF it was 4.6 at idle, so that doesn't seem too unusual. It's going to measure higher when it's cold and the RPM is higher. If I take a drive today I will check the MAF data and post it here.


that would be great! the reason i thought this was low was because in the factory manual under P0101 it states that the MAF should read " 7-12g/s at ide to 150g/s or more at the time of the 1-2 shift"

lol and now that i'm reading this closer it also states that

a high resistance of 15 ohms or more on the ignition 1 voltage circuit may cause this DTC to set. A high resistance may cause a drivability concern before this DTC sets


I'm assuming the ignition 1 circuit is the red lead? i verified resistance from that pin to + terminal. So perhaps this could be the problem? I will double check that resistance (this was KOEO) edit -- err, key on engine off.. i realized i have no idea how to differentiate the O's for off/on in those abbreviations.
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Re: Recurring P0101

Postby pmou [OP] » July 20th 2020, 6:10pm

OK well -- not sure if i fuzzed the measurements i took or what. went back to the vehicle after MAF was dried. doubled checked all intake hose connections, re-installed. measured wires to MAF -- this time i got about 35ohm on ground lead, 0ohm on + lead. so i think i will look into engine grounds, from the wiring schematic i believe the MAF ends up at G105 or G107. G105 is the one at the back near the knock sensor, which i thought looked OK enough when i was there doing plugs. G107 is labelled "in thermostat housing" so i will try to check that out next. also double check the major engine to chassis / battery near the starter / radiator.
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Re: Recurring P0101

Postby Sailing_Faith » July 20th 2020, 9:36pm

The thermostat housing ground is the one for the MAF... often a problem, both because of corrosion ans because rhe terminal gets twisted when the bolt is tightened....
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Re: Recurring P0101

Postby pmou [OP] » July 20th 2020, 10:25pm

alright, went and poked around a minute to see if I could put eyes on the tstat ground. it's in a bit of an unfortunate spot, but I suppose with the filter box and intake hose out it wouldn't be too bad to get to. still need to spot the engine chassis ground by the radiator, but maybe it makes sense to add a fat ground here as well.

Anyway while in there I spotted this guy:
IMG_20200720_151307.jpg


another angle:
IMG_20200720_151313.jpg


and where does it go?
IMG_20200720_151323.jpg


apparently.. not wherever it used to go. so I'm guessing this would contribute to a vacuum leak. starting to look like going through the vacuum lines could be a good idea...

also if anyone could offer where the other end of this thing is, much appreciated.

Edit: looks like it could be this? https://www.rockauto.com/en/catalog/gmc ... +hose,6908

"HVAC control panel vacuum hose" #15967970
Last edited by pmou on July 20th 2020, 10:33pm, edited 1 time in total.
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Re: Recurring P0101

Postby Leeann_93 » July 20th 2020, 10:29pm

It's hard plastic, yes?

https://www.astrosafari.com/viewtopic.php?f=34&t=4468


It runs across the engine to the driver's side head, to the canister thingy the PCV connects to. Common issue; heat makes it crunchy and then it breaks.
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Re: Recurring P0101

Postby pmou [OP] » July 20th 2020, 10:35pm

Yes hard plastic, looks like it's the part no I referenced. beat me to it!
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Re: Recurring P0101

Postby pmou [OP] » July 20th 2020, 10:48pm

Found this image here that is helpful: https://photos.google.com/share/AF1QipP ... NINEhfT3l3

Are all the vacuum lines aside from that one just some generic hose? I see a part for the check valve as well, but otherwise no parts listed on rockauto
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Re: Recurring P0101

Postby Leeann_93 » July 20th 2020, 10:57pm

That one is the only one with a part number and it's the only one that's plastic.

For all else, you buy a chunk of 'windshield washer line' at your LAPS (usually in a box or plastic snap thing with a few feet), then snip what you need. 7/16" rings a bell, but double-check.
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Re: Recurring P0101

Postby sixsix » July 20th 2020, 11:00pm

Local Auto Parts Stores carry both kinds of tubing/hose and helps if you carry some pieces in w/ you.
Couldn't hurt to change that little check valve either - LAPS here also.

That diagram has been uploaded to the site so often w/o regard to Year, Gen or Intake style.
There are some vast differences - but that one is the most common - Good Luck.
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Re: Recurring P0101

Postby pmou [OP] » July 20th 2020, 11:20pm

So, perhaps slightly off topic -- but I'm looking at the grounds.. is there not just a fat AWG 2/4/6 wire somewhere that grounds the block? There must be, right? how else would enough juice go through the starter? I'm having a heck of a time finding any mention of anything existing from the factory. Anything i find here about updating grounds seems to go back to something about the big 3 and people's own locations for engine-chassis / engine-body grounds.

Am i missing something?

Also re: vacuum lines.. they are all pretty crumbly so i think i will be replacing all of them here shortly.. Looks like mine doesn't have the "tee" that is referenced in that picture from what i can tell. i can see the remnants of the HVAC line resting across the top of the block.
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Re: Recurring P0101

Postby Leeann_93 » July 20th 2020, 11:27pm

Battery cable. Grounds to core support.
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Re: Recurring P0101

Postby pmou [OP] » July 20th 2020, 11:55pm

Oh duh. ok well in case anyone ends up here in a search, it's probably staring you in the face.

IMG_20200720_164749~2.jpg


I also located the main block ground which is just above the support bracket for the PS CV axle, forward of starter. that looked pretty good as well.

Also TIL that the resistance measured on your multimeter might change based on the direction of the leads .. with negative lead on the ground at MAF and pos lead on the neg battery terminal, I got 0ohms. Similarly, with POS lead on hot wire at MAF and neg lead at post terminal, I got 0ohms. reverse both of those and I get 35o and 100+ohms respectively. so I think I'm going to rule out electrical/grounds. :shifty:

Will report after all vac lines are done.
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Re: Recurring P0101

Postby pmou [OP] » July 21st 2020, 3:34pm

I just want to check also, would this contribute to the incorrect MAF readings? as in, would this hose being broken allow additional air into the intake system past the MAF? i can't quite tell at this point what it connects to and what the vacuum source is
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Re: Recurring P0101

Postby Birdshake » July 21st 2020, 5:17pm

That broken vacuum line you have is the red line in the diagram in this thread. Its easiest to get to with the doghouse removed.
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Re: Recurring P0101

Postby pmou [OP] » July 21st 2020, 5:21pm

Birdshake wrote:That broken vacuum line you have is the red line in the diagram in this thread. Its easiest to get to with the doghouse removed.


yep that's it. i've been on the fence about taking the doghouse off to get to it, you think it's best though? It seems like (at least for me) that some of the vacuum lines in that diagram are not there gen 3 2003 GMC. I can't seem to find anything that lines up directly with the gen 3s, so i may do a separate post about what exactly i find for gen 3 folks in the future.

If i do have to take the doghouse off once again, i'm wondering if i should just do the PCV and knock sensor while i'm back there, still haven't touched those.
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Re: Recurring P0101

Postby Birdshake » July 21st 2020, 6:57pm

My old '03 and current '05 are the same as that diagram. The PCV in the '02+ doesn't really need to be replaced There's nothing in it to break down. I did drill a bigger hole in mine though. If the knock sensor isn't throwing a code I wouldn't touch it. I had a bad MAF throwing a P0100 in my old '03 it would cause the engine to stall. I replaced it with one from a picknpull and it ran fine without that issue till I retired it.
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Re: Recurring P0101

Postby pmou [OP] » July 21st 2020, 7:14pm

Interesting.. actually now that I'm looking at that, I think the difference may be because I don't have rear heat.

AFAIK I don't have the little T that I circled in blue here, but I will double check.

heater vacuum routing~2.jpg
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