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Q: overheating issue , not all the time. on 1991 Chevrolet G Series Van (G30)

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Carpet cleaning van. Cleaning unit is van powered by a driveshaft engaged with an electric clutch. Van drives fine with no temperature issue.
When I engage the system it will slowly go to overheat. When the system is engaged the RPMS are ramped up slightly, to about 1800.
Flushed system. Replaced fan clutch, thermostat, water pump. Water is circulating well.
There are no leaks I can find but I am loosing coolant. After it cool down and I open the radiator cap I've lost a lot of coolant.
No car image 94a1663db56199c5353592009e34aaa51078a2469bed068bb8d6f0ba43accf97
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Sounds like at this point, you are seeing the result of an air pocket in your cooling system introduced probably when you replaced the thermostat and water pump.

Purge the air out, refill the coolant and try again...
I've done that numerous times. Since this is a carpet cleaning van, , there is a heat exchanger in the cooling loop. There is a bleeder valve on that.
I am topping off the radiator, with the van running, watching the flow. Later, after running the engine there is always fluid loss, with no noticeable leak. The hotter the outside temp, the worse the situation. Quicker to have the engine overheat, with more fluid loss.
Yesterday, while cleaning, I failed to notice the temp going to critical. Loss lot of fluid, but not on ground through leaks, resevoir overflow. This ruined Temp gauge. It had been ok until this incident.
Let truck cool down for long enough to open radiator , refilled . Truck started and seemed to run ok across town. Traffic, very hot. Not much fluid loss.
This morning it was down a little, after sitting all night.
OK...well, another unfortunate possibility is that the coolant may be getting ingested into the engine. If you have no sign of leak, and you are constantly adding - it must be getting into the engine. the 350 isn't necessarily known for head issues I don't believe, but anything could happen on any car. You've pretty much ruled out everything else to this point... Have someone check your coolant for the existence of hydrocarbons (using an exhaust gas analyzer) perhaps. Good luck.
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