Advice on overheating Chevy K15OO 4.3 engine....?

Question

I have changed thermostat, flushed coolant, replaced radiator cap and still have intermittent overheating issue. It will purge all coolant into reservoir then after shutting off engine for a brief period, coolant will go back into radiator. Water pump good pressure, belts and pulleys fine. Fan has little resistance when cold when trying to turn by hand and same when hot....good or bad? It is a 1991 with 99,544 original miles, otherwise strong engine. No engine coolant in oil, no water coming out of exhaust, no exhaust smell in coolant. Any ideas what to check next? Thanks!

additional info: when i bought the truck it had a bad miss .changed plugs and found right rear plug was loose and had lots of carbon and water scale on the outside of old plug.have found no water in oil.or exhaust smoke,and no sign of outer water leaks from the head or side of the block

Answer 1

Only for troubleshooting purposes I would remove the thermostat completely and re-assemble the housing, then see if it continues to do this crap.
If it doesn't, get a lower temperature setting thermostat, it sounds like that might be the root of your evils, more so on older cars, don't run a high temp thermostat.

I wouldn't drive for too long without a thermostat, but just while you're troubleshooting it, you'll have to take about a 30-45 minute drive someplace, likely the temp gauge will act a bit funny while it stays on the cool side of things, again this would be an indication it is the temp setting of the thermostat.

Answer 2

You wrote: Fan has little resistance when cold when trying to turn by hand and same when hot....good or bad?

This is probably the key. It's probably a fan clutch. You might also want to try to wobble your fan forward and back to see if there is any play there. There shouldn't be.

Some of the other answers are good, too, but I would also check to see that there is not blockage in the cooling fins of the radiator. You can clean this fairly easily at a high pressure car wash.

I hope this helps, as anything else will be a lot more expensive.

Answer 3

I had a 1997 4.3 and it ran particularly warmer than some, perhaps 210 degrees. But if its really trying to dump fluid out your reservoir I suspect it is also spewing it out the cap if you were to take it off even when cold and letting warm up. That sounds like a bad head gasket into an exhaust port that is forcing exhaust gasses into your cooling system. Now have you flushed your block well? Seems your doing everything you can. Now when you fill it and run it to burp the system after radiator was changed did you see the water circulate as it should fast or slow? If slow then flush, if you can and pushing fluid out then yes head gasket. OOPS, not good

Answer 4

Have you tried leaving the radiator cap off and seeing if there is water flowing thru the radiator? You mentioned good pressure on the water pump is that what you meant? Do you have the clutch style fan? Sometimes the clutch will get intermitant and give you faults every now and then. I would probably replace clutch on the fan, I have had that problem before but older model Chevy.

Answer 5

FACT: GM had problems with many 1989 to 1990 head gaskets on the 4.3 v-6 and all the small block v-8's. Don't rule out head gasket failure. By the way, does it actually overheat? The coolant resevoirs function is to catch spilled coolant from the heated engine, and return it to rad when eng cools.

Answer 6

Almost sounds like air flow through the radiatior isn't strong enough, or blockage/restriction in coolant flow. Make sure your radiator is free of debris and blockages. I had a lady that complained her car took too long to heat up, so she put cardboard in front of her radiator, then complained when she overheated.

Check to see that your hoses aren't collapsing also. Worked on a car once that when coolant heated up, the rubber hoses didn't hold their shape, and collapsed on themselves.

That's where I'd start.

Good Luck