i posted this in the FTE equivalent chevy forum, but have not gotten any answers, so figured lets try the brain trust here.

91 S10 PICKUP 4.3 ENGINE, NO AC, 73,000 original miles.
this is my neighbors car, i am trying to fix it
the water pump was leaking last week. the daughters boyfriend put the new pump in.
took it out for a ride and she said it was fine.
sunday, the temp gauge spiked at 260 and CEL came on.
flushed engine, heater core, and put new thermostat in because the engine was full of rust. tried to flush rad, but the core was plugged with rust that was disturbed when the system was drained.
infrared temp gun on radiator told me rad was plugged.
ordered new rad and put it in with new thermostat, took for a ride and same thing. temp gauge spikes at 240-260 degrees.
shut it off for 1 minute, and gauge drops down to 180, then slowly climbs back up to 240-260
pulled t-stat, and temp gauge sits at 110-120.
yesterday i shot the block with infrared and got 180 at t-stat neck, 200-210 on head at temp sender. gauge was reading 240-260
today i put new temp sender in, still reading 240-260. but the engine temp sensor at t-stat reads 179 as per the scanner.
sitting in traffic, or driving down the road, it reads the same, 177-184.
so i know the truck is not overheating.
what i can not figure out is why the temp gauge spikes between 240-260

so to recap, everything was fine.
water pump started leaking, was changed and temp gauge started reading high.
the truck now has a new water pump, radiator, temp sending unit, and 180 degree t-stat with an air purge hole drilled in it.

anyone have any idea what is going on with this thing?

 

Does the temp gauge read high no matter what the temp is? Does the Chevy gauge read high or low if the sensor wire is disconnected? If it reads high when disconnected, maybe there is a high resistance in the connection between the terminal and the sensor, or perhaps the terminal itself to the wire.

 

in dash gauge reads high when hooked up and engine running. disconnect sensor and it drops to nothing. hook it backup and it pegs out.
engine itself runs rite at t-stat setting, 177-183 with a 180 t-stat in it.
BUT take t-stat out and dash gauge stays down around 120 degrees.

 

check all engine and wiring grounds in engine compartment, i didnt hook one up once on a 97 chevy and all sorts of crazy stuff was happening.

theres arre allot of grounds.

 

My guess would be an incorrect or flaky sender or a bad ground. Try checking resistance through the sender at cold, and with engine running at correct temp as measure by infrared. Check those readings against a spec for the sender if you can find specs. The fact that the gauge is reading different than the sender is seeing and presumably wasn't doing so before the initial repair kind of points that way. You might try running a length of wire clipped directly to the sender body (not the terminal) and connecting the other end to the ground at the gauge itself as a way of testing for grounding issues.

 

also sensor thread no sealant or teflon tape, it grounds thru block if its a single wire sensor.

 

yup, i saw that s soon as i took the new sender out of the box.
these chevys have 2 temp sensors. one for the computer, and one for the gauge.
the computer unit is reading perfect. the gauge unit is the problem. i was thinking bad ground this morning. i have to wait until she gets home at 4 pm to see if the block to body ground got broke. but i think it is highly doubtful because the computer sensor works fine.

 

i doulbt it too since the starter spins.

 

Is there a special dance that has to be done to reliably purge air out of the cooling system on a Chevy 4.3L V6?

Had a Chrysler V6 that to purge, you first unscrewed the coolant temp sensor from the front of the block, it was a two-finger reach. Then fill with coolant mix until it flowed out that hole, then screwed in the sensor quick. Any other refill method would leave an air pocket. No amount of running it with the radiator cap off, etc. would purge it of air. Special air-purge routines are common.

Often the effect of an air pocket is a gauge temp sensor reading high, often intermittently, as the temp sensor starts reading the metal temperature, with no coolant on its tip, just air.

 

i really cant see it being an air pocket because the t-stat is drilled, and when i put the new sensor in i got a steady stream of coolant out of the head/block while the hole was open.
i am not sure if it is in the head or block, it is hard to get to or see with the exhaust manifold in the way

 

wonder if the head passages are blocked some and not letting coolant circulate well enough. did you hit the head the sensors in with your lazer temp guage?

 

yup. gauge reads 240-260, infrared reads 200-210 at sensor.