Hello & Welcome,
Can you tell me what the "normal range" is on the gauge?
Have you checked to see if the radiator cooling fans are working?
Middle of the gauge sounds about right for normal operation. I don't suspect that you have a thermostat problem because that is where the gauge should be.
The cooling fans are mounted behind the radiator. There is a possibility that they are not coming on at the right time because of either a fan problem or a relay problem.
Here is a site that talks about the cooling system fans on a similar GM van (identical really). It gives details including schematics of the system along with relays and fuses involved. I would check that system to make sure everything is working properly.
One other thing you can try is to turn on the AC system and see if the temperature comes down after a couple of minutes. This is assuming that the AC system is working properly because switching on the AC will also cause the fans to run.
I would also check that the cooling system is absolutely full, when the engine is cool. This should be checked at the radiator cap and the coolant should be right up to the very top. Also, the overflow bottle should be about half full at all times, or at least between Full Cold and Full Hot marks on the bottle.
I hope this helps. Please let me know what you find or if you need more information.
Take Care, Greg A.
For the AC not working, it could be because the cooling fans are not working, but then the AC would work at highway speeds. There could also be a freon leak. The condensers on these particular years (mounted in front of the radiator) were prone to a leak developing at the lower passenger side of the mount. There was a bulletin out to use a different style mount so that wouldn't happen.
But I would definitely recommend checking that cooling fan system. The fans have the job of keeping air moving through the radiator at lower speeds, and if they are not working, it could cause this problem.
If you have had the freon leak out, it is a good possibility that the condenser had/has that problem. Here is another picture of a leaking condenser in the spot I mentioned.
Please let me know what you find related to the fans.
Take Care, Greg A.