Cooling issues revisited

1988 S-10 4.3L "Z" with a new motor 20K or so ago. Also got new waterpump, thermostat and both temp gauges at that time.
A week ago I got the oil changed. On the drive home, I notices my temp guage
registering an uncomfortably high level (265 is the top line, it was at that top line). No boiling over at all or anything, but I threw on the heater on high and got home quick. I have now replace the thermostat (which appeared to be fine) with a 195F as is called for. I also put a new radiator cap on. I have been driving it all week now, and what I am seeing now is after 2 or so miles, the temp gauge will register 265F for 10-15 seconds, and then will quickly drop down 210ish (where it always ran prior to these issues) over the next 30 or seconds of operation. This can occur whether at a stop light or driving down the hiway. Again, never has it boiled over, no change in coolant level, no leaks etc.
So I am curious if my gauge or sensor is FUBAR, or if something worse is going on. I have purchased a mechanical gauge to put in it to see what the temp actually is running. I wanted to see if anyone has any ideas/suggestions, and I also wanted to verify that the dash temp gauge is the one that is in the side of the block and NOT the one on top of the motor near the thermostat. Right? I obviously don't want to do the work twice, and I know if I pull the computer's temp sensor my truck will run like a dog.
Big Chris
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Chris,
You may have some air trapped in the cooling system.
Al
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any suggestion as how to purge the air if this is the problem?
Big Chris

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Generally GM motors that need to have the air bled from the cooling system have one or two ports at the top of the motor some where. I'm not familiar enough with the 4.3 to know if it has them. Some of them are at the top side of the intake I think. Generally you would remove the plug from the bleeding port while it's running and warmed up and wait for coolant to come out with out spitting.
Brian

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I am not aware of any such port on my 89 4.3. I basically have the same truck he does.
I'd check and make sure you got the proper rotation of waterpump-- they make a standard and reverse.
Check for leaking head gaskets. Water in oil/oil in water?
Installing the Tstat backwards, perhaps?
by all means plumb in the mechanical temp gauge and see what it tells you. I would NOT attach it inside the vehicle-- unless you like the idea of trying to control the vehicle as scalding hot water sprays all over you.
How old is the radiator? Might want to have it inspected--- some time with a temp gun might show you you have a lot of blocked-off cores.
Kinda in a hurry, but had a similar problem with my 89 a while back, that I fixed. Don't mind helping out if you give me the above info and do the homework.

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make
Correct rotation of waterpump. Waterpump has worked fine for 20K, and appears to be working fine still (no weaping, circulation visible with radiator cap off).

No water in oil, no oil in water.

Previous Tstat was installed correctly, and worked just fine for 20K. New Tstat put in exactly as old one was taken out. The only thing that is different is that the old Tstat (which was taken out after the beginning of these problems) seemed at visual inspection to flow more freely than the new Tstat. That is just my eyeballing it and seeing how much space is made when it opens though, no real evidence that the new one is flowing insufficiently.

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Radiator is original, though it has been flushed at least twice by me (twice in the past 45K, once on old motor, once on new motor). There are a few dinged fins, but nothing to really reduce airflow. I've run my hand over the radiator (front and back) after bringing motor up to temp, and it appears to be getting evenly hot, though a core may be plugged in spite of that. I'll have to check around and see if I can find someone with a thermo gun. Fun to play with for those of you who haven't used one.

I
Any help is appreciated. This is one of those weird ones, and the idea of melting down my motor is not appealing :-)
Big Chris
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Big Chris wrote:

You can easily check and see if you have an "air in the system" problem on your truck. Check the level of the coolant in the radiator "cold"..ie: after it has sat overnight, and before you start her in the morning. If it's low, top it up, and make sure that you have the level in the overflow bottle to at least the "cold" level. It does not hurt anything to run the overflow bottle a little higher.
Now go drive the vehicle, let it warm up...then let it sit until it cycles again. Where is the level in the rad? If you have a properly working rad cap, a cooling system with no leaks, and no restriction from the radiator to the overflow bottle, it should be full. If not, you will need to investigate those areas a bit more.
4.3's in the s/t trucks don't have bleed screws. If I'm filling one after the block has been completely emptied...I find it much easier to just remove the heater hose that goes into the intake manifold. When you get coolant coming out of that point as you are filling the radiator, you will pretty much have all the air out. You can then run the engine with the rad cap off, allowing the level in the rad to be a bit low (2-3") until the vehicle hits t/stat opening temp. Once you have coolant flow through the radiator, air will automatically be purged, and you can just top up the rad while she is running.
Anyway, hopefully you can eliminate air in the system as your cause and move on to other diagnostic areas.
Ian
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Yup,.that's the way I did it when I put the new motor in my rig. Works a treat.

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SNIP

SNIP
What Doc mentions is available from Stant. They make really good products.
Brian
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Thanks, Brian H
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Bingo, exactly what I was gunna say!
Doc

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How old is the radiator? Put a new one in the 91 S-10 at 130K cause the seams were bad.
Just for kicks ripped off the side plastic tanks------nearly half the tubes were clogged
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