I have a chevy 350 in my 88 truck. I have put 3 different thermostats in it and it will always get hot on initial start up.  It is like it delays in opening.  Once it opens it will cycle fine. It red lined tonight after I put in a new 160, The top hose was cool and it was not boiling over.  Yet the gauge seems accurate when it cycles.  any ideas.  Thanks  Tony  

i had the same problem with my 87 gmc. i put about five thermostats in it and the gage would still say it was running hot. i shot a temp gun on the thermostat housing and it was staying around 175 to 180 while my gage was saying around 230. Try doing that, you might have a bad sending unit or something else  

Tony,
I had the same problem with the 350, 330 hp crate motor in my 1968 Camaro.  After trying several thermostats, I finally installed a new high-flow water pump.  The old pump was a remanufactured model that the previous owner had installed.  It was mechanically sound but didn't have the flow required to keep the temperature down at the heads where my temperature sensor is located.  Of course, you will want to make sure your radiator is in good working order, and not restricting flow.
Good Luck,
Dave  

Where is your temp sensor located, in your head or your intake. Is this a problem it has allways had or did it just start. If it just started you may have a crack or a leaking head gasket. If your head gasket is leaking combustion into your coolant and your temp sensor is in your head it will heat the coolant in the head and be cool up top. Had this problem myself. Can you see any bubbles in the rad. Is it using coolant.Try putting a volt meter on it. Put + lead in the coolant and the - lead to - battery post. The combustion will contaminate the coolant and if it shows above 1 volt it may be your problem.  

It sounds like you need to verify the problem first. If it isn't boiling over and the gauge reading is the only indication it is running hot it may just be a bad gauge or sending unit. But before you just go replacing things take some measurements with a temp gun or temp probe. If you have access to a scan tool see what the computer reads engine coolant temp to be.  

Hey Tony,
It sounds simple but try and replace the cap to the radiator. I had a simular problem in my 1984 Chevy P/up and that fixed it after I replaced virtually the total cooling syatem. Good luck. Stan  

It will run all day around town and not overheat. But on the highway it will increase for about 15 miles and then stay pretty warm.  Am thinking maybe radiator or water Pump as already said?   Thanks  

Check your ignition timing.Sounds like it might be over advanced.Cruising in town at lower rpms does fine,but on highway,higher engine speeds for extended periods,acts up.Couldn't hurt to check and won't cost a dime.  

Try drilling a 1/8 hole in the thermostat to let the air bleed out of the cooling system.I have had air get trapped in a cooling system and the water pump isnt designed to pump air there for it cant push any water through the rad.The hole in the thermostat acts like a bypass and lets the air make its way to the rad. where it can escape.  

My neighbor had this problem, we changed the thermostat and that helped a little. Come to find out he had a cooler mounted in front of the radiator, and just having that where it was, you could reach down and grab the lower radiator hose and it wouldn't be hot, but the temp on the gauge would. Just something to look at...but yeah definately check the radiator cap like stan said, more times than not it is something so easy yet frustrating.
Eric