sorry, but just to be clear both the dash gauge and the computer both read 160, when the ir gun reads 188?
Gage and computer take temp mid way down the head. The engine is hottest at the thermostat.
Did you do something like put a heater valve in so you could cut the flow to the heater core?
Doing so may cause pressure from water flow to cause high pressure on one side of the thermostat or the other holding it closed or open if you have a high flow pump.
Did you double check that the thermostat is in the correct way (not upside down).
Where is the return line (top hose at fire wall) for the the heater going?
The other side? I have seen people hook the return to the radiator and hook the other line to the water pump. The water pump is also return so the water pressure does not equalize between the intake hot side of the thermostat and return side of the pump. Again a Valve in the heater line may cause the same problem. Toyota had a problem with this (they have a valve on the heater) and they put a bleed hole in the edge of the thermostat to help equalize the pressure at the thermostat to get them to open.
Bottom line on a FI truck is if you don't get the temp up it will not go closed loop. 160 it thinks it is still warming up and it will ignore the O2sensor. Verify by testing for closed loop while it exhibits the trouble.
The only things that could exert that much fuel dump to get the engine to run cold is:
The O2 but that seems to be ruled out by the temp the ECM sees agreeing with other test (gage IR) so it never gets hot enough to be using the O2 for mixture readings.
Thermostat and thats is the most common and just because it seems to work in a pan of boiling water doesn't' mean its good. Water flow could be causing it to hang full open instead of regulating temp drop the $6 for a new one.
Bad ECM running it pig rich.
Vacuum leaks to the MAP sensor making it think you have your foot to the floor (low manifold vacuum) and running it rich for acceleration. At idle and steady cruise you should be seeing about 18-19hg or better. If you see less the motor is shot or you have vacuum leaks.
Bad MAP (see above for how it can act).
The ECM in not smart enough see a misreading MAP. If it sees it (no shorted or not open) it will not trip a code or light.
Bad temp sensor for the ECM (you seem to have ruled out by the test you have done).
Retarded timing. Not likely with they way its all electronic control but easily checked by watching the timing mark with a timing light and raising the RPM to make sure it is advancing with the speed of the motor.
Bad Knock sensor. ECM usualy responds with dumping timing it however would not explain 160 at idle if it is functioning enough no to throw a light.
I'm just a stupid shade tree BTW I am not inhibited by having a fancy computer to tell me what to do like some dealer mechanics I actually took the time to really learn how it works and not be a parts plugger following a computer diagnostic test recomendation.
Dorian knows his ****. If you give him good intell his recomendation are usually spot on...he's one of the last people you second guess on thei site. As otheres have pointed out you have been a bit of a wanker when people have made recomemndations. Some of them gave you very good information. Vaccum leaks and Thermostat is the biggest cause of this issue and cheap enough that throwing one in would be the first place most people would start before chasing their tail.
