So I replaced the pulled the upper intake and fuel rail this weekend to address the problem from post #1. I replaced the upper intake gaskets and the gasket below the fuel rail.
I cleaned the MAF while I was under the hood.
Unhooked the battery to clear the codes.
I restarted the engine, and noticed a smoother idle after the initial relearning of the computer.
I drove around in the pasture a lot to check fences and cattle and even do some playing around. I drive over 150 miles and no check engine light.
But today it comes back on!
I sprayed a fair amount of carb cleaner under the hood to check for vacuum leaks, but I can't find anything.
I had the code re-read by an idiot at AutoZone, and it says there's 2 codes there but he could only figure out how to see one: P1131
, one of the original codes.
I began looking for anything that might jack with a sensor's reading and discovered a very small exhaust leak.
When I had my transmission rebuilt over 30K miles ago, the shop never replaced the donut gasket after the cat and the rest of exhaust system. The entire mating surface is not leaking, just one section about an inch long has a gap the width of a penny which I can feel exhaust escaping when the engine is running.
I'm leary to point the finger at this, because this has been leaking many thounsands of miles before the Check Engine light ever came on. But it upstream of an O2 sensor...
If you can't tell, I could use a little direction.
The plugs were replaced about 15K ago with MotorCraft plugs. Wires are still original. Fuel filter replaced about 30K ago. The IAC was replaced about 10K ago. Again, the truck's got 120K on it.