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.
Posted via Mobile Device

[LT7A]

I will follow along with interest. My truck is a 93, but basically the same engine. I just replaced the t stat since the temperature gauge was never moving above about 105° and so I figured that t stat was stuck open. I was only using it for short trips. But I just took it on the long road trip and the temperature still didn't rise and I could see that the t stat was closed when cold and open when warm by popping the radiator cap. So I'm assuming that the temperature sensor is the culprit. I hope I don't have any other problems hiding behind it like your situation. There's only the single coolant temperature sensor that is both for the computer and the gauge correct, front of the intake manifold next to the water neck?

[ZacharyDouglas1992]

Originally Posted by LT7A

I will follow along with interest. My truck is a 93, but basically the same engine. I just replaced the t stat since the temperature gauge was never moving above about 105° and so I figured that t stat was stuck open. I was only using it for short trips. But I just took it on the long road trip and the temperature still didn't rise and I could see that the t stat was closed when cold and open when warm by popping the radiator cap. So I'm assuming that the temperature sensor is the culprit. I hope I don't have any other problems hiding behind it like your situation. There's only the single coolant temperature sensor that is both for the computer and the gauge correct, front of the intake manifold next to the water neck?

On mine the coolant temp sensor is in the front intake location you mentioned, and the coolant temp sending unit is in the driver side cylinder head. Not 100% sure if that’s the case for the 88+ trucks but I’m pretty certain you have that as well. I’d check for that, and make sure the wire going to it hasn’t been disconnected or burned on one of the exhaust manifolds
Posted via Mobile Device

[kwmech]

Did you check the timing plug to make sure it is plugged in? Single tan wire with a black stripe near the firewall. Ground wire to the engine block? Was the timing set correctly? Disconnect the wire, set timing to spec, plug wire back together

[ZacharyDouglas1992]

Originally Posted by kwmech

Did you check the timing plug to make sure it is plugged in? Single tan wire with a black stripe near the firewall. Ground wire to the engine block? Was the timing set correctly? Disconnect the wire, set timing to spec, plug wire back together

Thanks for chipping in. Yeah it’s plugged in, I wish it was as simple as that. That one causes an instant code. Mine will go away after I shut off and wont come back on for a long time. Incredibly intermittent. And yes timing is at exactly 0 and was set with the wire unplugged.
Posted via Mobile Device

[Just call me Sean]

I was getting that code after setting the timing on my S10. It had never been set before, the tape was still on the plug. Turns out the plug ends were corroded and unplugging it ruined the contact it was making. I had to clean the corrosion from the connector ends and after that it was fine.

[ZacharyDouglas1992]

Originally Posted by Just call me Sean

I was getting that code after setting the timing on my S10. It had never been set before, the tape was still on the plug. Turns out the plug ends were corroded and unplugging it ruined the contact it was making. I had to clean the corrosion from the connector ends and after that it was fine.

Update-

I went back to the drawing board, disconnected the computer and started through the step by step diagnosis. Went all the way through the algorithm and ended up with “code 42 intermittent”. I replaced the distributor- because a partially failing pick up coil is one of the common culprits I noticed in research when the more common causes have been eliminated. So far I haven’t had the light return. Can’t really say anything about the way it’s running because it’s been running great even with the light. I’m going to be driving it daily to either confirm it’s fixed, or trigger the malfunction again so I can take it to a shop.
Posted via Mobile Device

[LT7A]

Good info, thanks for the update.