What to look at, for fixing lopey idle and running rich?

Okay guys, y'all have helped me out on the brake issues on this truck, now I need some assistance/recommendations on the idle and running rich issues.
1997 C3500 L31 5.7/4L80E/4.10 FF.
Began running rich and having lopey idle, worse in drive than in park. All I did was put some gas from a different Kroger than I usually go to, because I was closer to that one, and only put in about 8 gallons. Filled it twice since then, once with Shell and once with Kroger gas from the usual store. Mileage is down a bit, was 11 on my Austin trip last spring, down to 9.6 this year. Truck has been tuned up a year ago with new plugs,cap and rotor, and new fuel filter. When you give it some throttle (most running other than at idle) it runs great.
Is it possible or likely that the CTS sensor in the intake manifold decided to be "done" at the same time as I fueled up? Scratching my head, need some ideas.....thanks!

Fouled plug or 2 would be my first guess.

618 Syndicate said:

Fouled plug or 2 would be my first guess.

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Okay cool thanks! I'll look at that in the morning.
The thing that is stumping me on this, is I hadn't been working under the hood. Just fixing on the front door. So it's very unlikely that I touched something under the hood.....

Shìtty gas can foul plugs.

HotWheelsBurban said:

CTS sensor in the intake manifold decided to be "done" at the same time as I fueled up?

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if it was bad/failing I think it would through a code for it

618 Syndicate said:

Shìtty gas can foul plugs.

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Especially if it had a good dose of diesel in it.

For driveability issues, always check for DTCs first (including stored DTCs).

I'd look to see what the fuel trims and O2 sensors are doing.

It hasn't thrown any codes, and I don't have a scanner. I'll go to my local O'Reilly's and have them check for stored codes.

You can check that CTS with an ohmmeter. Easiest to do when the engine is stone cold at ambient temp, you would expect a reading that would match near that. Resistance value chart should be in the service manual but it's also been posted around the web a bit. Then of course once engine is fully warmed up you'd expect it to match near coolant temp.

Fuel pressure test would be wise as well, though I wouldn't necessarily suspect it in your case. Always good to know and since it's easy on a Vortec.. why not.

And oh yeah, just because it IS a Vortec.. got your old distributor cap you could throw on as a test?

Richard