What did you do to your NBS GMT800 Tahoe/Yukon Today?

ive been running this maf from this seller for 6 years now. its a delphi aka oem.

https://www.amazon.com/dp/B000CGFHBY?ref=ppx_yo2ov_dt_b_fed_asin_title

Interesting, they won't ship to my address..
Rockauto has them too, maybe I'll see if they won't ship to me..

Hoping there's something going on with gear sensor..
Since I get knock retard in 2nd, and it shows me in 2nd when I'm not...

mikez71 said:

Interesting, they won't ship to my address..
Rockauto has them too, maybe I'll see if they won't ship to me..

Hoping there's something going on with gear sensor..
Since I get knock retard in 2nd, and it shows me in 2nd when I'm not...

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that kr is likely due to being under or over fueled, or maybe a bad motor or trans mount could cause. if that exhaust manny is leaking it can cause all sorts of fueling issues.

Maybe it is exhaust leak.. I don't hear the typical exhaust tick though..
There is often a clunk coming to a stop..
But I don't notice any an extra noise/vibration in 2nd vs other gears..

False alert on the the gear selection, when I log PRNDL settings, it shows all gears proper.

I haven't seen TCC lockup in my logs, but I did activate it in scanner and stall the motor out in 5 seconds..(too slow?)
My evap purge is usually 100% when my LTFT is high.. (maybe purge happens when on the throttle?)

Reason I don't think it's the MAF, because it should be under-reporting, so O2's would read lean, to cause higher fuel trims.

Hoping it's a weak pump.. And lean spike causes the knock retard.. (unfortunately there's 2 pumps, and OEM cost is high! Can save about $300 if I go with Delphi..)
Will update when I get my hands on a real guage..

For what its worth, I fought a P0101 code on my Denali for a few years. It would randomly pop on when towing or loaded heavy. I would also get knock when hard accelerating hard from a stop, so bad you could audibly hear it in the cab. Experience said it was clogged cats, but it passed a backpressure test with flying colors. All my other tests came back good, even fuel pressure/volume. I did some of the typical things like clean the MAF, swap a known good one from another truck, replace intake gaskets, had my injectors flow flowed and cleaned ect. So, i kind of just learned to live with it.

Last summer when I replaced brake lines, I put a new fuel pump assembly in as my tank was out to do lines and I have not had a p0101 code since.

I'll take fuel pump(s) over cats!
At 150k, I don't mind having a fresh pump.. (one of my worries about my tahoe at 240k)

Did you by chance check fuel pressure while driving?

I did not. I just got a static reading with KOEO.

I would still check cat pressure. I made a tool for checking them. O2 sensors plug, some fittings, poly tubing and a 0-15 psi gauge. Pulled the pre cat o2 sensor, install gauge. Start engine, pressure should be roughly zero at idle. Give a small rev pressure should still be zero or there about. I would have to double check on the spec for these trucks.

The poly tubing holds up fine for short starts. I wouldnt run the engine longer than 30-60 seconds.

Oh I meant when you checked fuel pressure?

@mikez71

https://www.harborfreight.com/basic-fuel-injection-service-kit-64939.html

I like this fuel pressure guage. It's got like a 3' long hose, so you could duct tape it to your windshield and do a load test driving around.


I did replace the main fuel pump on my 04 project suburban last fall. I went with the Delphi. I actually bought the auxiliary pump as well because I suspect the fuel sender on that one failed (getting intermittent low fuel level signal), but I haven't replaced that one. That was my first fuel pump replacement ever, since I started driving when I was 18 in 2001, LOL.

Fuel pressure guage has helped me diagnose a TON of "crank-no-starts" on my 99 Silverado when I had the 4.3, which was the distributor due to moisture. That truck still has the original pump now at 286k...not too shabby

mikez71 said:

Oh I meant when you checked fuel pressure?

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I just got a static reading with KOEO.

Thanks @6speedblazer, good to know!

And @strutaeng Website says they have it in stock near me. If i rent one who knows how long/short the hose is. I'm going to pick it up soon!

I maybe asking you for tips on replacing the pump!

mikez71 said:

Thanks @6speedblazer, good to know!

And @strutaeng Website says they have it in stock near me. If i rent one who knows how long/short the hose is. I'm going to pick it up soon!

I maybe asking you for tips on replacing the pump!

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pump replacement is pretty easy. hardest part is getting that damn filler neck off the tank lol.

one thing i would suggest is to have the nylon fuel line on hand. its the one on top of the tank that connect the sending unit to the metal lines at the cross member. The fitting likes to rust weld itself to the nylon fitting. Both my trucks that fitting pulled apart when i tried to remove it.

a set of the dorman tank strap bolts and clip nuts is also a good idea to have on hand.

mikez71 said:

Thanks @6speedblazer, good to know!

And @strutaeng Website says they have it in stock near me. If i rent one who knows how long/short the hose is. I'm going to pick it up soon!

I maybe asking you for tips on replacing the pump!

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Sure!

I went through 2 of the basic pressure guages from the local parts stores. The hose would eventually get eaten away from the fuel. That HF was bought maybe 4-5 years ago and has held up well, although I'm not using much lately (haven't had many issues, until the 04 FP issue, so it just basically sits in my garage.) The guage needle is also easy to read.

I mainly bought it because of the long hose. It's hard to get accurate readings if you have the short hose version working by yourself, especially if you are trying to prime the fuel system more than one and going back and forth between the driver's side and engine compartment.

Bought the gauge and even got a free hat! (Special edition!)

Gauge is accurate compared to my '12 FPCM reading.. Fittings screw on better than the amazon tool, and no varying readings. Long hose is awesome! I love it.
That's the good news..

The bad? news, it seems like my pressure is decent, pretty steady at 55.. might drop to 53 at the most when on the gas..

I get a brief 'squee' noise when I rev it.. I thought it was belt slip noise..
Now I'm worried its a broken cat and the exhaust whistling past..
It happens on the initial rev up each time, and doesn't continually make the noise..

I will do a backpressure test eventually.. I need someone around to rev it while I watch the gauge.
Could still be clogged fuel injectors, exhaust leak near O2 sensor? (Doubt it would leak by both O2 sensors??)
For that matter, both cats are broken/clogged? (guess that's not too odd..)
Perhaps manifold leaks would be my best hope at this point..

I'm pretty defeated for now...

mikez71 said:

Bought the gauge and even got a free hat! (Special edition!)
View attachment 483085
Gauge is accurate compared to my '12 FPCM reading.. Fittings screw on better than the amazon tool, and no varying readings. Long hose is awesome! I love it.
That's the good news..

The bad? news, it seems like my pressure is pretty steady at 55.. might drop to 53 at the most when on the gas..

I get a brief 'squee' noise when I rev it.. I thought it was belt slip noise..
Now I'm worried its a broken cat and the exhaust whistling past..
It happens on the initial rev up each time, and doesn't continually make the noise..

I will do a backpressure test eventually.. I need someone around to rev it while I watch the gauge.
Could still be clogged fuel injectors, exhaust leak near O2 sensor? (Doubt it would leak by both O2 sensors??)
For that matter, both cats are broken/clogged?
Perhaps a manifold leak would be my best hope at this point..

I'm pretty defeated for now...

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55 psi sounds pretty normal to me actually...This is a returnless system, right? 2005? My 2004 is returnless.

When my FP failed, I got 0 pressure readings.

Yep returnless/dead-head.. Regulator in tank I guess..
Will do more poking around with a stethoscope and back-pressure gauge.. maybe after the rainy days pass..

Last night I chatted with Google A.I. on my phone. (typing)

It told me my MAF squiggle is the MAF sensor failing/struggling, despite reading proper airflow at idle and WOT redline in 1st.
(assuming it's not a wiring or reversion issue)
Something about the sensor delaminating and vibrating at higher RPM's.

AND, that the primary suspect for my high LTFT at higher load/RPM's, and knock retard, is DIRTY INJECTORS plateau'ing..
Injectors were on my radar, but since I've never personally had issues, and read few cases about it, it was on the edge of my radar..

Then it told me to check injector duty cycle.. (will test later today probably)
Said around 90%+, injectors would struggle and probably can't keep up with demand, causing a lean spike..
Causing knock retard in 2nd, which has higher load than first, and combustion temps have heated up more.

I know it's based on what the internet and I am telling it, but the answers were very logical and concise..
(And without the repetitive wordiness I'm used to seeing from A.I. responses!)
It honestly makes the most sense to me... If it's right, I will be pleasantly surprised!

So I will focus on logging injector duty cycle..
Which by itself doesn't mean it's the injectors, but if fuel pressure is good, why isn't it getting enough fuel?
(Unless O2 sensors drifted, but they're not lazy)
AI says it should be between 75-85% at redline WOT, but expects mine will be higher, which makes sense when LTFT is 25 and STFT is 15..

I also ordered a new Delphi MAF and a WIX fuel filter.

We shall see...

Well I logged injector pulsewidth, but using a calculator, 18ms at 5200 rpm was about 78% duty cycle only.. Will update when new MAF is installed this weekend...

mikez71 said:

Well I logged injector pulsewidth, but using a calculator, 18ms at 5200 rpm was about 78% duty cycle only.. Will update when new MAF is installed this weekend...

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hpt tuners scanner can scan the injector duty.

top box in white of the bottom left window.

Thanks for all the suggestions Fless, strutaeng, mountie, 6speedblazer, Scottydoggs!
And I'm surely forgetting someone.. sorry if I did!
Good news!

As Fless mentioned my MAF was not reporting right.. Too much at idle, too little at WOT.
Alvin of PCMofNC also informed me that MAF sensors age out..

Installed new MAF. Max LTFT now 9.4%, and STFT also much better. Old left, new right.
Honeycomb pattern looks different. New one has ovalized hex at different angles.. Or it's just deformed from mishandling at the factory..


MAF resistors? look a little different.
Old legs angled 45 degreess, new straight. One new resistor larger.


Idle at stop in drive, the new MAF reads 6.6g/s..(old 8g/s)
Max airflow at 5,184 rpms, new equals 260g/s..(old 213g/s - 226g/s)
Injector duty cycle 17.40ms at 5,184 rpm supposedly 75% duty cycle.

Someone with a LQ9 logged 253g/s at 5200rpm, 20.7ms, 89% duty cycle.
They also logged 265g/s at 5,530 rpm..

Old MAF - airflow signal squiggly, fuel trims jump when driving modes change


New MAF - airflow signal smoother, fuel trims change much less