over heating problem

[wintercamo]

1992 Yukon 350 eng. swapped out b4 with older model (think) still center mount valve covers. someone done a shoty eng. swap but runs good....
But my problem it will over heat, check gauge light will flicker a few times and temp will be just hitting red for a few seconds and then it gos back down then its back up and down repeat but gos to 210 and back down. how temp guage looks
now you would think thermostat have put 3 in new 195 180 165 degrees all the same thing have drilled holes in it now pretty much made it free flowing not over heating but have no heat.
Long story short thermostat not opening up soon enough and not maintaining a constant temp. any help or suggestions

 

[OR VietVet]

Grab both hoses at the heater core and see if same hot temp. If not then have a plugged heater core and no, before you even ask, a flush is not going to fix it. Replace it. The temp fluctuation could be caused by a radiator that is clogged also and not removing heat evenly or air in the system or not having a good quality thermostat or is working correctly and the heater core is clogged and the temp sensor at the drivers side head needs replaced.

 

[wintercamo]

thanks for replying .... the heater hose,rad,and core are all clean heater worked great before thermostat swaps or drilled holes in it. I changed out temp unit on side of block cause was busted from eng. swap just guessing on that, from a used 95 model [our donor truck] problem might be there??? but had a small leak found it on lower hose someone didn't tighten hose clamp maybe sucking air?? and they used a starter bolt with out a nut even for the motor mount bolt......this is what I am finding as going along on it. should get to wrenching on it again next few days
.....which temp. unit controls idle the one on intake the 2 wire? and one on block does it just control temp. gauge on in side of truck??

 

[OR VietVet]

The one at the intake/thermo area is the sensor for computer and the one on the head is for the gauge and called a sender.

 

[livingez_123]

try using a mechanical temperature gauge to make sure you're getting an accurate reading sometimes the computer can be way off from what actual reading is