OK. I have a 99 Sierra. In the past I have replaced the heater hoses and water pump on two separtate occasions. Refilled the system and managed to bleed the air with no problem. I just installed an engine out of a 2001 with 40k miles on it. Never been broken into. The only difference I have seen between the 99 and 01 motors is that the 99 has front and rear steam crossover tubes and the 01 motor only has a front steam crossover tube. The rear ports have factory block offs. I transferred the new water pump and thermostat to the 2001 engine. Now on to the problem

I have filled the tank with a mix of dex-cool and water, pumped both upper and lower hoses while filling as well as loosening the steam tube while filling to try and bleed out any air.

Start up the truck turn the heater on max setting and let it get to operating temp opening the thermostat. Vary the engine speed, top off the fluid and put the cap on. It will idle forever without overheating until I go drive it. As long as rpm is 1000 plus everything is fine and steady at 195 degrees. Once it stops to idle at a redlight the temp starts to climb eratically. Guage jumps 10*, then 10 more, then it may drop 10, then back up again. It will continue to do this until it is revved up to about 1000 rpm. Then it goes back to 195 until you stop and let it idle again.

I have replaced the coolant temp sensor, and I checked the fan clutch. It seems to have the proper amout of resistance and is flowing air at idle. Is it possible that I need to add the rear crossover tube? I feel like there is an air pocket causing a lack of water flow somewhere but I never had this much trouble bleeding one of these trucks before....WTF!!!
Please help. thanks

 

Definitely not the crossover tube. It does sound like you might still have an air pocket. One other thing you might want to check is if anything is stuck behind your water pump pulley. Sounds wierd, I know, but it happend to me. Something got lodged between the pulley and pump fins, so at idle it would get stuck and stop spinning til I revved it up. Accidently found that when I started taking the pump off to replace it 'cause I thought the pump was bad. Loosen the belt and see if the pump spins freely.

 

OK I'l try that. Anyone else?

 

What comes to mind from me is that you may need to replace your thermostat, or that you have an air pocket in your cooling system. Another possibility is if you have e-fans and the motor on the fans went out or a relay in the harness went out causing the fans to not operate properly.

 

I thought e-fans too but he mentioned checking his fan clutch. It could be a thermostat not opening all the way, though, being a restriction in your coolant path.

 

The way the gauge jumps erratically it sounds like it is a temperature monitoring problem. You said you already replaced the temp sender. Maybe there is a faulty ground to the gauge or some other problem with it. You could try temporarily using a mechanical gauge to see if that's the problem.

 

Oh yeah, missed that.

Check out your t-stat and try to verify if you gauge is reading accurately.

 

T-stat would be my first suggestion. Trying to remember if the stock t-stats fail open or closed. Sounds like yours could be closing at low rpms.. which could also be low coolant flow pressure (not enough force to push/compress spring). Stock t-stats aren't that expensive. 160s are usually about $55 (I think). I don't know what sensors run, but I haven't heard much of those failing.

 

Most t-stats are designed to fail in the open. Maybe a weak or worn water pump???? I know you said you put on the new water pump, but you have already had issues with them in the past with the old motor. What was wrong with the one on the new engine? Maybe even the clutch in the fan?

 

I happen to have a hypertech 160* t-stat if you decide to go that route.