Quick update on the situation. When I started it up this afternoon the check engine light turned its self off. Which was nice. I kept an eye on the fuel trims, which also were doing better. Mostly keeping to about 10-15% (long term). Also heckled on the 02 sensors which were fine. The trims were also behaving like a more classic vacuum leak, higher on average when my foot was off the gas.

the other thing that was off on the live data was the STFT bank 1 sensor 2, which I think means the downstream O2 sensor was giving readings around 99%, though the voltage from all the o2 sensors was in the normal range. I’ll verify tomorrow. Does that mean it needs a new o2 sensor?

thanks again

It might.

Hard to tell with another known issue skewing things.

Can you tell if it is cycling properly?

Well, last night I spent probably half an hour smoking out the vacuum system for a leak, and found nothing. So I went back at some of the live data and freeze frame data to see if I could find anything. With the engine off MAP was the same as barometric pressure. At a cold idle the MAP was about 12inhg, which I think is plenty of vacuum, and the MAF was at 8 g/*, which I think is high, but makes sense if it’* cold maybe. At the moment the code was thrown the MAF was reading 3.4 g/*, low i think. Could it be a MAF problem maybe?

also thought that fuel injectors could be the problem I guess.

anyway hoping someone could double check my deductions here.

thank you

Need to know engine RPM with the each MAF reading. Would be helpful to know if it is open loop or closed loop also.

Took a couple more complete readings this morning.

At idle:
fuel system 1 closed
fuel system 2closed
calculated load value16.1%
coolant temp 192 degrees
STFT bank 1 -.8%
LTFT BANK 1 11.7%
STFT bank 2 -2.3%
LTFT bank 2 10.9%
Intake Manifold absolute pressure 10inhg
RPM 544
speed 0
ignition timing advance cyl 1 19.5 degrees
Air flow rate from MAF .63 lb/min

at about 45mph
fuel system 1closed
fuel system 2closed
calculated load value 42.4%
coolant temp 192 degrees
STFT bank 1 -1.6%
LTFT bank 1 12.5%
STFT Bank 2 0%
LTFT bank 2 10.2%
intake manifold absolute pressure 17inhg
RPM 1768
speed 43mph
ignition timing advance no. 1 cyl 32 degrees
intake air temp 73
air flow rate from MAF 4.29 lbs/min
absolute throttle position 18%

the long term fuel trims are definitely trending down. They are highest when coasting down hill, today about 16%, a week ago it was around 22% and lowest when accelerating today as low as 7% and a week ago around 14%.

Not sure why they would do that, and I can’t see anything else wrong in these numbers.

thank you again

I wonder what happens at idle if you disconnect the vacuum booster hose and plug it off.

I don’t have a one it’* got a hydro boost system.

Edward

Drat!

So just being thorough:

1. You started to have the issue
2. Among other things, you installed a Duralast MAF from Autozone
3. Still have the same issue, no change

. . . right?

So its a little more complicated.

the MAF was replace like 6months ago, that was recommended to me in trying to fix a misfire problem. I don’t think it was the issue there but it did stop throwing misfire codes with new spark plugs.

it ran fine for the rest of winter, then the P0171 came up like a week and a half ago, and since then the LTFTs have been slowly trending down from 25% to 11% at idle.

and as far as I know all the other live data looks good.

thank you for helping me with this,
Edward

Gotcha.

Yeah, for this the most likely culprits are low fuel pressure from the fuel pump, plugged up fuel filter, intake tract leaks, and bad/dirty MAF sensor.

You've read 60PSI fuel pressure this month, so it'* probably the fuel pump.

It'* an issue at idle, while the car drives normally as far as you can tell, so it'* probably not a plugged up fuel filter.

Your MAF is fairly new, but it'* a Autozone Duralast. This puts it at a 50/50 chance of still being good in my book. Like with your master cylinder issue, the store-brand stuff isn't so great IMO, so maybe this is contributing. Another reason to go with a better brand of parts is to remove a lot more uncertainty when troubleshooting other stuff (like what we're doing now) later.

Your intake manifold gaskets are original as far as we know, so they probably have 21 years and 125,000 miles on them. The LS 4.8'*, 5.3'* and 6.0'* from this time were notorious for intake gaskets going bad after lots of time and heat cycles. They lose elasticity and stop sealing as well and this kind of thing happens. There'* a 50/50 chance thet they leak on the lower half of the gasket, so that means they can't be easily diagnosed with propane and/or smoke because that faces the valley and is inaccessible without disassembly. At this point in the troubleshooting process, I'd place good odds that these need to be changed. I strongly suggest using Felpro gaskets, no matter how great the description of Duralast gaskets is.

On my 2001 YukonXL 5.3 , I used the Felpro MS98016T kit because it comes with all new gaskets necessary to do this job. No piecemeal getting the other gaskets together or reusing anything. This kit is also compatible with your 4.8 .

The 4.8 and 5.3 are the same engine except for the stroke. I put mine on at around 240,000 miles about 12 years and 80,000 miles ago and still no issue. The leaky ones I pulled off were ACDelco, so I don't know if they were original, or more likely got replaced at a dealership earlier in life. The Carfax on it shows lots of dealer visits for maintenance and repairs over the first 160,000 miles. I bought it at 180,000 miles.

It isn't too bad of a job overall, but in a van, there'* a little more contortion involved.