my '96 nissan pickup started running hot and I was not losing fliud. The fan comes on, the oil is not milky so I changed the thermostat. The first day I drove it and ran it for a total of 25 minutes. Temp was fine. Second day drove for 10 minutes and ran hot. Drained and flushed radiator ran hot again after 10 minutes. After it cooled, I did have some water loss. Could this be the water pump or could I have screwed the thermostat installation up?

Just to make sure it's not the thermostat remove t - stat put hose back on vehicle and let run to see if it still run hot. If not you have a bad thermostat but if still runs hot try spraying your radiator fins with a garden hose and nozzle better yet a pressure washer. Fins on radiator and if you have A.C condenser coil can be clogged with dirt and bugs causing low or no air flow across radiator and A.C condenser coil.

You also said you have a little water loss can you tell were it is coming from ?

I figured 'boiling out'..idk. It did not start running hot until after I ran the a/c for a few minutes

Is your A.C condenser coil in front of or behind your radiator ?

behind

Still do the test I mentioned.

I would also check your fans to make sure their working properly as well. You might have a little water loss due to the fact when you drained it and did your flush you there was some air in the lines so that would be normal to see it drop a little after the flush. It could be highly likely that you have a bad pump as well. I would start with the fans as I have had the same problem and ended up being the fans.

Thanks. I will clean the rad fins and check a/c lines and fan. Then pull out the t-stat. What would be the first clue that the water pump is bad

Also I see you mention if you screwed up putting in thermostat . Trust me it is easy to put the thermostat in the wrong way sometimes. Take out and check.

On the fans when the vehicle is off of course just give them a little spin they should have a very mild resistance on them when you go to spin them. If the water pump is bad it should be leaking around the housing is usually a sign..

the AC coil was in the front not the back

All water pump has what they call a weep holes on bottom of pump so if pump starts to leak it will leak from bottom of pump from that hole.

Yes that could be your problem since the A.C condenser is in front of the radiator maybe that's why it over heats when you turn on A.C condenser being partially clogged with dirt and bugs A.C condenser getting more air flow then radiator.

My 95 Nissan pick up is running hot pushing steam or liquid out the over flow there no water in oil or oil in water