Engine runs rough and when I come to a stop the service soon light comes on. When I acclerate the light goes off. The truck also runs rich.

The temperature gauge runs off of the single or double wire sensor that comes off of the head, the computer uses the sensor by the thermostat. If the computer is reading -40, it's not getting a signal from the coolant sensor by the T Stat. Check your connection, and make sure when you installed it, you didnt' use any thread tape or anything that wouldn't allow it to have metal on metal contact with the block. If you used thread tape, it won't be able to make a good ground connection. This is going to be the primary problem with the engine running rough until you get it fixed...it's loading the engine down with an overly rich fuel trim.

You need a better code reader, ie "Mechanic". The ECM should control "Running rich or lean". Is engine light? Quote, Service SOON or Service NOW. SOON is usually minor, but needs to be found in the codes. "NOW" need to be checked immediately.

Have you checked the computer for trouble codes yet ?

Read the fault codes and replace faulty units.

The knock sensor is famous for causing all sorts of trouble If you have a code for it, it probably needs to be replaced.

It could be a faulty sensor, valve or blockage in the EGR (tube) from a carbon buildup.

in some cases it may be a problem with the ecu

o-2 sensor

TO SAVE YOU TIME AND MONEY.HAVE TRUCK SCANNED TO SEE WHAT CAUSES SERVICE LIGHT TO COME ON.YOU CAN GO TO AUTO ZONE THEY WILL SCAN IT FOR FREE.BUT IT SOUND LIKE YOUR O2 SENSORS COULD BE BAD.

need to have system checked to get trouble code

must have a bad o2 sensor need to check with a scanner auto zone and advance auto parts do it for free.

Like to other guy said, give it a tuneup. Also try SEAFOAM to clean carb up of slug. Also check to make sure the butter fly for the choke has not come loose, causing it the choke it down. Failed choke pull-off.