Chevy truck cooling issue

It's been one of those days...had to come home early because someone pulled out in front of daugter on her way home from school..now I have some cosmetic repair on her car , but she is O.K . so that was good ...figured I would repace the heater core in my 1/2 ton chevy with a 4.3 v-6 that the wife drives..repalced the thermostat already and it was still fluctuating on the temp gauge from startup it will go to 250 + before the thermostat will open , and even then it will fluctuate between 195-230 bobbing and weavin' the whole time....buddy tells me the heater cores plugged , replace it , ok spend 35 bucks put it in tonight , get a flat in the process start the truck , let it warm up while I am changing the flat , finding the spare is flat also ...the gauge goes up to 230....I shut it off and finished the tire feasco..(spare held air after I pumped it up) took it for a drive and the temp gauge went to 250 and stayed their longer than I would like , the thermostat opened came down to 195 then back up to 210 and sorta lingered...Any ideas????Does the system need to be "burped"?? The only thing left is the water pump, and the previous owner replaced that within the last year, and the truck is not driven that much. Any help would be appreciated ...it been a long day, off to look for Reatta parts, thanks Shawn

Drill a small 1/8" hole in the thermostat to allow the hot water to circulate to open the stat.

Still sounds like a thermostat issue. I'm trying to think how a plugged heater core could cause the temp to fluctuate and I'm not coming up with an answer.

I had a '92 Chevy fullsize with a 4.3 and the plastic side tanks for the radiator developed cracks over time. After I replaced each one (a few months apart because I was too cheap and not smart enough to replace them both at teh same time thinking "the other one won't crack like that - yeah right!) I went thru 2 thermostats and went thru the "burping" process over several days if not a week. Until it was fully burped, when I would shut the truck off the heater core sounded like a coffee percualtor for a few minutes. Eventually that all went away and maintained a steady decent temperature. I wish I knew what the proper technique really was but there is something about that system that is different.

Good luck,
Bill

On my 93 with 6.5 turbo diesel, the sending unit failed, so I replaced with one from a parts house. Then it showed overheat. Checked it against a non-contact thermometer, and was not overheating. Tried another parts house sending unit, same problem. Finally bought a OEM ACDelco sending unit and now it works fine!

Try a good radiator cap.They can drive you. crazy.

My 89 chevy 350 did that every time I changed the thermostat. It would just about red line before the thermo would open up the first time. Everytime I second guessed myself thinking maybe I put it in upside down, then just like that she'd open and cool right off. My uncles 88 chevy 305 ran at 210 for 260,000 miles. Then someone rear-ended him while returning a fertilizer buggy and bent the frame rails, so that was the end of that truck. No fault of the motor. Drive it for awhile, just keep it local. If it doesn't get any hotter than 210 or so, I'd say you're ok.

you need to fruther diagnose the problem, first i would install a mechanical gauge on the engine and make sure the engine is in fact running that hot and its not a faulty temp sending unit, then check the fan clutch on the engine and make sure it is working corectly and locking the fan on when it needs to, if there is fluid on the outer housing its shot, then while the engine is cool remove the radiator cap and start the truck and watch the coolant cycle look for any air bubbles indicating combustion gases entering the coolant from somewhere, also see that the coolant is indeed circulating through the radiator, also look at the front of the radiator are the fins plugged with bugs or grass? if the ac condensor is mounted in front of it check it also, the heater core wont affect how hot the engine runs

Air in the system will do what you are seeing if the coolant level is really low. I would assume that you filled everything and rechecked it after running. A water pump can do the same if there is gasket material blockage. Leave the cap off and let it run, you should see water/antifreeze flow once it warms up a bit. The fluid level will drop about an inch when its running, There should be a flow arrow on the thermostat, make sure its in correct. There is no need for drilling, if GM needed it there, they would have drilled it for you.

On my 72 C10 stepside hotrod I found that the
bottom rad hose was collapsing causing an overheat when standing in traffic. Put on new hose and it runs normal now.

Any ideas????

Click to expand...

Shawn

As 'ericlb' has said, you need to verify that the engine is actually overheating it's possible that you have a hyper-active temperature sender or dash gauge.

Since this vehicle is computer controlled, and if you can get your hands on a data scanner, it's really quite simple to diagnosis a possible dash gauge malfunction by comparing the 'ECM' coolant temp sensor value with the dash gauge value...

If a Scanner isn't available, another method would be to use a laser pyrometer aimed on the thermostat housing during the early run time temp fluctuation.

Also, look for a steady stream of air bubbles in the radiator (head gasket) & check to see if the coolant flows with force & doesn't dribble, I've seen some slipping & rotted water pump impellers that have caused similar problems.

Good luck!



[size=9:10145a91d7]Scott[/size:10145a91d7]

Have you looked at that clutch hub?

Allan

Had a 95 Jimmy at two years age that did that for the next six years.
I put every kind of replacement stat in there. The engine would over heat once then run cold in winter and over heat in summer if trailering.
Every stat came out stuck 1/2 open???
Changed the water pump, making certain it was the correct version. Rad hoses, viscous drive, rad you name it.
Still did it after the 1st, 2nd and 3rd engine rebuilds. Nothing stuck in the water jackets, passages etc.
Never did find out why, would like to know however.

I had a pickup that did that and my cenex station had put the thermostat in upside down.

Get a new radiator we don't go there in this house.

Thanks guys...will check a few Items mentioned and report back later...as for the plugged heater core,it will not allow sufficent flow of water out of the engine upon warm-up causing the high temps in the manifold ( where the temp sender is at) until the thermostat opens.

You right Goose, thats a head scratcher.

You right Goose, thats a head scratcher.

A customer's mid-90's S-10 with a 4-banger acted the same way 'til he fially traded it off.

Check for split in water pumps plastic impeller.Test the thermostat on the stove in a tuna can.A meat thermometer can be used to check opening temp plus it can be used to check temp of top tank of radiator.Air can be trapped when the system is filled,water level will drop when the thermostat opens.Ive found thermostats in backward often.Many posts on which way does the thermostat go on the forum.A plugged heater core would provide little heat.

Not needed, most systems have a by pass hose.