2004 G3500 Box Truck CEL

[rpaoness]

Set the random cylinder misfire again idling it to roll off the ramps. I wonder if it’s something about very low speed high low that somehow makes it skip. Not something I felt, but I’m not that observant. Reset and we’ll see how soon it returns. At 274k miles and 3k miles a year it’s probably not worth chasing until it gets more frequent. 6k on plugs. Could try coils or injectors, or ignore.

[rpaoness]

Finally got a couple minutes to look up parts. Interesting that looking back on the freeze frame history I posted in the first couple posts on this thread they were all approaching 25% long term fuel trim.

I had installed an Amazon-ordered Delphi MAF since I think I needed the truck kinda right away. Interesting that this new one has not had a "real" MAF code, but I do agree it's a likely culprit.

Straight up GMPartsDirect listings showed nothing available. Most avenues showed no GM/ACDelco instock anywhere. I finally went to the AC Delco website with just my year/model info and came up with these two:

19420212 for sensor only - appears to be discontinued

Full senser with housing:
GOLD/PROFESSIONAL
Mass Air Flow Sensor
Reman Mass Air Flow Sensor
Part# 213-3553 - ACDelco
Part# 19137066 - GM
Per Veh: 1
Years: 2003-2004

With that part number, it appears to be available on GMPartsDirect for about $83 w/$29 core charge, $16 shipping:
https://www.gmpartsdirect.com/oem-parts ... wtdjgtZ2Fz

And GMPartsGiant, About the same $, $20 shipping:
https://www.gmpartsgiant.com/parts/gm-s ... &filter=()

These guys have it for $57 w/ $29 core, $8 shipping:
https://www.gmpartsstore.com/oem-parts/ ... CqEALw_wcB

I haven't bought from gmpartsstore.com before, but I guess I'm willing to give it a shot.

[kevm14]

The other possibility is unmetered air.

[rpaoness]

Ordered. Hopefully it works out.

[rpaoness]

The other possibility is unmetered air.

It would kinda suck if I find a between-MAF-and-throttle body leak when I go to install after spending $75...

[kevm14]

But if you have two different MAFs doing the same thing, that may motivate you to track down an unmetered air leak. I didn't really evaluate the data with the question of could it be a vacuum (after throttle body) or unmetered air leak (between MAF and TB). Vacuum I'll say no because I manually idled it down to 450 rpm or something. That likely wouldn't have been possible with an active vacuum leak.

With vacuum, the higher the vacuum the more you'd see high fuel trims.

That leaves the other kind of unmetered air leak and during very light loads (like idle) there isn't a lot of delta P to want to suck air from the wrong place. With this type of leak it would present as the opposite of a vacuum leak - it would be worse the higher the mass air flow.

[kevm14]

Did you ever unplug the MAF and observe fuel trims? We could have tried that as well. However, I submit that the grams/sec was too low as reported by the MAF. So that's a real thing. Whether it is a leak or not was addressed by my previous post.

[rpaoness]

Looks like I ordered some phantom listing. They canceled it and supposedly it's now discontinued.

[rpaoness]

I was on RockAuto ordering mainly thermostats for the pickup (seems very slow to warm up when it's cold out), wipers and stuff for most of the fleet, and somewhat on a whim decided to try a "Walker" brand MAF that was listed for the box truck. With the air tube it was like $58 and I had good luck with the Walker cat that is on the truck so I figured I'd try it. I also now have the Ediag elite, so I should be able to track MAF readings a little more easily (I haven't installed the app yet - hopefully there's some plotting or tracking available).

Do you have a good source for expected MAF readings on a high mileage 6.0L? I know you thought they were low, but I thought I'd try both MAF's and see how comparable at least the two I have are.

[kevm14]

No but I found this:

https://www.hotrodders.com/threads/6-0- ... st-4694247

What you're really looking for is fuel trims in the single digits when you add short term + long term across a variety of load conditions.

Make sure the MAF is not installed backwards in the intake.