I'll post my VIN later but pretty sure the engine is stock. A/C system is original, nothing modified with the belt drive.

The temp gauge is hooked up where the stock sensor is for the dash gauge, on the passenger side of the block by the exhaust manifold. It's almost like there is a pocket of air that keeps sitting in that area until I rev it, then is purges it, then comes back. Either that or the coolant isn't flowing well and the cooler stuff is basically just sitting in the radiator until I rev it, then it starts pulling/pushing it through the block. If that was the case though you would think it would just over heat just idling. The fan is rotating the right direction but I never did open up the new pump before I installed it to check the impeller, it's supposed to be new, not rebuilt so I didn't worry about it.

Heat output isn't bad but it never was real good. Before or after the core change. I'll try to measure that at the dash vent on my way to work this morning but awhile back when this first started and the gauge was reading pretty hot it was measuring about 100 at the vent with my IR thermometer.

 

Still sounds like your water pump , its bad or flowing the wrong direction.

 

With the engine at operating temp, the heater core inlet hose should be ~185F to ~190F, and the outlet hose should be slightly lower, ~170F. Air flow at the floor registers should be ~130F. The upper radiator hose, (at the thermostat housing), should be ~200F. Lower hose at the radiator should be ~130F.

 

Those are some useful temps. I'll have to check those. As I remember though at least at the stat housing it was about 200 and I don't remember the other temps, but the heater hoses I don't think were that hot. While driving today the air out of the dash vents in the middle it was between 95 and 105 and that was with the temp gauge reading 200 and 240.

Is there a way I can test if the water pump impeller is correct without pulling it off? Weather is a little less than ideal lately so hoping for an easy way to check.

Oh and the VIN is 1GNDT13Z2N2163650.

 

Your VIN should be:

1 = US Built
G = GM
N = Chevrolet MPV
D = 5001 - 6000 VWR Range and hydraulic brakes
T = Sm Conventional Cab 4x4
1 = 1/2 ton
3 = Four - Door Cab/Utility
Z = CPC (engine procucer) 4.3L V6 EFI which is RPO LB4
2 = Check digit
N = Year (should be 1992)
2 = Morraine, OH (assembly plant)
163650 = Production serial number

Not sure about the layout of the cooling lines to and from the heater core.
An air pocket could explain the funny behaviour of the system. Normally the V6 is absolutely no issue to fill the cooling system.

Not sure if somebody has experience with cooling systems that got a lot of slush or debris in them. I think you have flushed the whole engine and radiator? Sorry no time ot re-read the thread.

Water in the oil?
Oil in the radiator? (grey to brown slush)

 

Good info on the VIN, thx. Coolant is staying clean, oil is clean, still have a very slight leak at the water neck so I'll have to use some sealant on that. Changed the thermostat so many times I think I scratched it up too much getting old gasket material off. Water has to be flowing the correct direction judging how I could see it flow with the thermostat out. Rushes through nice and steady looking with the cap off.

If it's an air pocket which it does sound like then it's like it keeps coming back. Can't imagine the pocket would stay this long without working itself out.

I've flushed both radiator and engine and both were pretty clean, never has been that dirty.

So far the cooler weather is been behaving pretty well but has been a couple times on the highway where the temp got over 140 but then cooled down within a minute to between 130 and 140. When it does that I've been driving a fairly regular speed/rpm on pretty level ground. It's driving me nuts.

 

ok i've read the whole thread and just a few things come to mind ..headgaskets although unlikely, water pump (there not to expensive i would replace again) and a possible upside down thermostat (i have seen this many times) my old jeep always had a overheating problem and when i finally decided to change the t-stat i found that the previous owner installed it upside down! well thats my 2 pennies :P

 

Thermostat is correct. Water Pump I doubt is the issue because the problem stayed the same before and after I replaced. Had gaskets though I might go ahead and do along with intake gasket since it has been leaking oil slightly (outside the engine) ever since I owned it and I'm getting tired of seeing it. That would have to wait until the weather starts warming back up though.