Starting problem, decreased gas mileage on 97 suburban

[mlhempton]

Hi All. Here's what I need help on.
In 2008, I took my 97 suburban 4wd on a cross country road trip. During that trip it passed 200,000 miles. It got 17/18 mpg highway consistently throughout the whole 5,000 mile trek. In 2009, I took it on a 700 mile trip and averaged 14/15 mpg highway, with 209,000.
In August, I had a friend swap out the engine of a 97 suburban with a rebuilt 74 block bored .40 over, with the top end of the 97. When I turned the truck into him, it ran and drove fine, no starting issues. Just my oil pressure was tanking fast due to piston clearance and worn cams and bearings, and rings. So basically a whole rebuild. So I turned it into him at 219,000 miles. I got it back and it has a nasty misfire on startup, like it's getting too much gas and stalls out unless you keep the accelerator down for a second, then it catches and runs fine. It's a consistent problem. Cold, it'll start fine, but after that, it needs to be tapped to keep running at first. I took it on a 100 mile highway trip a couple months back and it returned 12 mpg the whole way.

So my problem is this. The general consensus is that my catalytic converters are clogged and that is affecting my gas mileage. I won't deny that, but I'm not convinced that's the only issue.
I believe the starting problem is related to the coolant temperature sensor, because the temperature gauge on the dash is off. It reads 230-240 hot, but the computer says 195. However upon my attempt to swap out the sensors, I found that the sensor for my truck is different than the one in the truck now. Maybe a mix-up, maybe not. But the connector that connects the sensor to the computer is made for the sensor on there and not for the 97, which doesn't make any sense.

Oh, and I forgot to mention. I have 2 failed oxygen sensors (bank 2 so probably not affecting gas mileage), a bad knock sensor, and a bad control valve body. The valve body problem's always been there, doesn't hurt my gas much. And the computer's telling me the system's running lean, which also could affect my gas mileage.

In a nutshell: where the hell did all my gas mileage go, and should I use the 97 sensor or the 74? I'm thinking 97 but I may have to switch out the wires.

She's at 226,400 and still going.