check engine, temperature gauge fluctuations

Recently the check engine lights up on 1997 150 4.2 comes on, with this I see temp gauge rise, then it drops when I turned heater on, but begins to rise then drop repeatedly. When I get off highway and let engine cool I see coolant level very low, so I top up then run truck, gauge did not run hot but check engine stays on, also don't see any coolant leaks anywhere. Any help much appreciated Thanx.

maybe a small leak over time your coolant seeps away,
now the cumputer sees temp rising and sets a code for
low input to ect.engine coolant temp.

Thanx, must have to bring in to shop to get "check engine" light to shut off

I've got a similar problem. My Service Engine Light is on. Error Code says Not enough flow thru EGR. My temp gauge goes into the red zone after approx 15 min. then varies when I turn on the heater. I'm wondering if the EGR system is to blame. The resevoir (degas) is churning and bubling violently with antifreeze trying to escape.
Does anyone have any thoughts on this?

If you have bubbling in the resovoir, sound like a cracked head or blown head gasket.

Thank you for your prompt reply. Over the weekend with Hayes service manual in hand I went through the checks for the error code, ie. Vacuum Regulater, DPEE, etc.. Found out the bottom vacuum hose to the regulator was disconnected (why the bottom nipple on the vacuum regulator is so short is a mystery to me). I haven't had time to check it out during the afternoon sun but will do today. Also, as a big point of interest that I left out in my original messege, I do not have a fan shroud. Somebody had removed it. I have one ordered and should be in today. Hopefully it will have a lot to do with the overheating problem.

Any thoughts?

Bob Thornton
Pushn_Tin