i posted this in the chevy forum equivelent of FTE, but have not got any answers.
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?

 

Blocked heads most likely, I had an AMC do this once. A reverse flush may help, basically you need to run a hose through the thermostat housing and out the water pump inlet with a lot of volume. GMs of that vintage are terrible about coolant crud, people would add green coolant to the dex-cool and it would be instant black sludge.

 

very first thing i did when i saw the rust in the radiator Jared.

 

ya its probably dex cool problems.
Search for dex cool issues in google.

I've had a lot of problems with gm vehicles using dex cool.
It eats away head gaskets, intake gaskets, etc, and it also reacts with any other coolants, and forms a sludgey grit that plugs the rad, heater core, god knows what else.

If you are not 100% certain that theres no dex cool in it, I'd flush it really good a bunch of times and replace it with something else.

 

it is a 91, and had green coolant in it. probably what was put in at the factory by looking at the condition of the coolant system.
it took close to 1/2 hour to flush the block and heater core before i got clear water out.
i have narrowed it down to a bunch of bad grounds. i have it working semi decent, but still have a few ground wires to replace/clean.