overheating at idle

[rjkuljis]

My truck has the 390 engine. Started up and was letting it warm up. Truck was parked and idling while I was working on something else. I heard the radiator pressure cap go off and steam everywhere. At first I thought it was a hose. Turned off and the pressure cap reset. The engine was hot but the lower radiator hose not even warm. Is this an obvious issue? Would it be a thermostat or pump issue?

[sparky72]

I’m going to guess your radiator is clogged.

[MAK]

First thing I would do is remove the thermostat and run it again to check for water flow. If the lower hose was not hot then it sounds like no coolant is flowing. I’ve never had a clogged radiator but I’ve had a lot of faulty or stuck thermostats over the decades.

[rjkuljis]

Yes. Going to try that. It heats up really fast and just started happening so probably thermostat..

Wil, update post when I figure it out for anyone with same issue. Seems like this would be obvious just thought I would ask because there is a lot I don't know..

Thanks for replies

[ddcrim]

Set the thermostat in a pan of water and bring it to a boil. You can then watch it open, hopefully all the way.

[rjkuljis]

This one is pretty obvious.. I went to drain the coolant and found that there was not much there! Coolant level was fine before the last recent trip and no need to think it was off. I did pull and test thermostat and put it back together as everything was working. Added plain water and let truck idle. Back to a normal temp when idling and no overheat.. Noticed that one of the freeze plugs has a very small leak so that solves mystery of the missing coolant.. So need to fix that and put the proper coolant back in and good to go!

[MAK]

Sometime we are lucky and it turns out to be the simple thing that should have been the first to check but had already dismissed.

[cep62]

When you change the freeze plug flush the block out . It's amazing how much crud can be in there .
If it was mine I'd pull at least one one each side.