TBI 5.7 dreaded code 42 saga

[ZacharyDouglas1992]

Ok so I am going to do a comprehensive post on my struggle with this issue since I’ve seen 100+ threads about this issue with very few solutions, and rarely the same solution at that. It’s mostly for informative purposes but if anyone can chime in feel free as this has been a migraine for many.

Ok so, this is on my 87 R10 half ton. The issue started with occasional low idle and shut off if I sat at a stop too long in gear. It ran ok but not great otherwise. Did a visual inspection under the hood, noticed that my coolant temp sensor was broken and spinning in place (the plastic inner portion). Once I replaced that, the chaos began.

1) replace CTS- Cold start and idle immediately improved, but I was soon greeted with code 42 and quite a bit of idle surging.
2) checked IAC. Spring broken, mechanical failure. Replaced and idle surging largely improved. Still code 42.
3) did some routine voltmeter and vac gauge testing. Replaced TPS as it was way out of spec.
4) replaced distributor (very sloppy) and used acdelco module and dielectric goop. Ran even better, still code 42
5)replaced damaged knock sensor connector
6)replaced prom with performance unit (had planned to do already when I had time, figured may as well rule out prom). Surprisingly better response and acceleration. Still code 42
7)performed step by step diagnostic for code 42. Checked for grounding of esc circuit. Checked all connection points. Everything voltage wise is within spec and I don’t have any grounding issues. Ran the spark plug wires as far as humanely possible from the esc wiring to avoid any rogue voltage reading.

Truck is running fantastically now, all around. But still code 42. I’m at a loss. All I haven’t changed is the coil itself (it’s the small cap with separate coil) & the knock sensor (but I did the tap on the exhaust manifold test and it was responding accordingly)

I only get the code after the truck is up to temperature. Never cold or at start up, and its not every time I drive it. Sometimes it’s 100+ miles before I see it again. It still runs fine, a bit more sluggish maybe.

Took it to a mechanic, which I’m always reluctant to do. He had it for a week, couldn’t figure out the issue either. He swapped in 2 known good computers, and the issue would still reoccur.

If anyone has anything to add or anything I missed, please please chime in. Feel free to make me feel as stupid as necessary. I will take the help in whatever form it comes. For all of the above steps I diagnosed before throwing parts at it. It seems like the bad CTS was masking a bunch of other issues and Once it was functional, they revealed themselves. Thanks in advance. If nothing else, I will post the solution if I ever find it. Even if the solution is that I LS swapped the MF.
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