Rough idle, occasionally dies while driving, running out of ideas....

[tomb]

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Wow! Thanks for the detailed response! Way back in 1995 when I took a couple auto classes, we did a lab where we used an old oscilloscope. It could cancel one cylinder at a time. We recorded each rpm drop and compared the results. If one or two cylinders have no drop then it is a good indication where the problem is.
I hope the shop finds your truck's problem. Intermittent symptoms give few opportunities to isolate the issue. If you had waited before changing parts until you absolutely understood the problem. It would be a lot cheaper. I’m not knocking you, I just got burned on a Camry by swapping parts before I understood the problem. I eventually found a couple intermittent ignition coils that only failed when cold.
I don’t want to jinx you, however if this problem comes back I'd keep a scanner close by for a while. I would love to see the short and long term fuel trim readings when it is acting up.
Low RPM is a symptom of misfire. Your rpm drops too much when the A/C cycles (for a 5.7l)
If the trims isolate a bank, then it would be logical to try to isolate it down to cylinder.

 

[tomb]

As for the long crank start, try on first start turn key to run without cranking it. Wait 5 seconds. Shut it off, then run again for 5 seconds, then engage the starter.
If it starts right away, it means your static fuel pressure is bleeding off. I believe they should hold at least 45 psi all the time with power off.

 

[MouthForWar]

Update: Fuel pressure at the TB tested within spec after priming (12.1-12.6, depending on whose test you go by..mine or the shop's lol) and is not bleeding off at all.

Replaced ignition module. Also double checked plugs; turns out one of the ACDelcos I put in there on recommendation actually cracked, so back to NGKs it is for me. Shoulda known better. No fix with either though.

However, I just tested the MAP again...here's some interesting info:

Test 1: 5V reference is good. However, test 2: supply is at 3.7V with key forward (engine off); should be 4-5V. Test 3 (this is the damning one): Supply bounces from 3-3.7V while idling at OT; should have dropped to 0.5 to 2V. It's nowhere even close.

So the MAP failed both supply tests, one by a decent margin. Will be replacing it this weekend. Will update then.

Also: all plugs indicate a fairly lean condition, fwiw.

 

[tomb]

Awesome! I love it when I find something out of spec!

Sent from my SM-G930V using Tapatalk

 

[MouthForWar]

Right? First definitive lead I've had in a while.

I don't wanna jinx it, but it can't exactly be helping things.

Won't have the cash for it for a couple of days, but we'll see then.

 

[Supercharged111]

You should have a 5V signal from the ECM/PCM, a return signal to the PCM that ranges from 0-5V, and a ground. Don't forget that at WOT you WILL NOT see that full 5V because of your elevation. In fact, 3.7V sounds bang on for 7000'. If there is no 5V signal coming from the computer a new MAP sensor will not fix anything. A scanner that shows live MAP sensor data would be the smoking gun here.

 

[tomb]

Dang, that makes sense.

 

[MouthForWar]

Again, the 5v reference signal from the ecm is good, so definitely no issues there. Read at like 5.03.

The voltage at idle on the return bounced all over like crazy; would not sit still at all. But there was very little change overall from engine off to engine on.

One quick note, I unplugged the MAP and the stumble disappeared, for what that's worth.

Edit: Guess I should have clarified the variable voltage is on Pin 2; I guess thats the "return". Forgive the incorrect terminology. The book refers to the signal from the ecm as the "reference" and the return as the "supply", for whatever reason.

 

[tomb]

Assuming when disconmectied the pcm substituted a default signal. Since the issue disapeared, its hard to imagine that you are not in the right area.... does the Map have a part number on it? Is it the right part?

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[MouthForWar]

Alldata says OEM is 12569240.

Part on truck is 039-1325. It's stamped as GM, but the number doesn't fit the typical OE/GM 8-digit #s.

Trying to find out more right now..