When at a stop light the engine idles with an uh-uh-uh-uh, sort of a pulsating idle between 900 & 1200 rpm. Not that it pulsates between those 2 numbers, but that's the range it idles in. When it's in park it sounds fine. Pulled codes: 1451. Looked under the bed and found the sensor unplugged, so I plugged it back in. The code didn't clear.

Also I checked vacuum lines, found one and replaced it. Also the IAC tested bad resistance, I replaced it. I noticed too, that the temp gauge measures constantly cold. I realize these problems feed into the computer. Would these idle symptoms be caused by incorrect readings on the temp gauge? I replaced plugs, plug wires. I took it to a mechanic and he found the PVC vacuum line was leaking and he replaced that. Any ideas?

Also, I posted this on another forum. Just trying to get coverage:)

 

The fuel mixture would be rich with a cold sensor. Could be part of your problem. I think that you need to carefully inspect all the vacuum tubing for cracks or splitting. Your truck is at the age where plastic and rubber parts are starting to fail. It doesn't take a big leak to cause idle and drivability problems. While you are piddling around under the hood, Pull the throttle body and clean it.

 

Bill, Thanks. The only thing I haven't done is clean the throttle body which is easy to do. All vacuum lines have been checked out, including the line to/from the PCV valve which was leaking (mechanic did that). The coolant temp sensor is a bear to get to (between firewall and coolant pipe going behind the engine). Sometime I'd like to have a one on one with the engineer who designed it that way, but that's another story for another time. Those frustrations aside, I like the truck.

 

There is a vacuum line that comes from under the AC box under the hood. It loops around the heater hoses and goes into a rubber tube ending in a vacuum pot. I found a small crack in the hard line. It was causing a small surge.

 

Thanks. I replaced that already as it controls the air recirculation control behind the glove box. Bill is right and you are too: Especially the hard vacuum lines are brittle and easily crack over time. Cleaned the throttle body too.