This one's got me scratchin' my head. I have an '84 Trans Am, ZZ4 with aluminum heads that have been ported, jet hot coated shorty headers, a different cam than originally came with it, and I'm running an oil cooler. It's got a brand new radiator (plastic tanks with aluminum 3 cores), a 16" electric pull fan on the inside and a 12" electric push fan on the outside. I have a 195 degree thermostat (newly replaced).
Here's the scenario: When I'm sitting in traffic or driving around town I usually dont have any problems at all and it stays at 180/195 with no problem, but as soon as I get on it or drive on the interstate/highway for periods of time it will start to rise gradually to 220, 230 even up to 250 (I've only let it get to 250 once!). When it gets hot though, it still runs great, no bubbling or funny smell, no symptoms of overheating at all, just those high numbers and as soon as I start slowing down to come to a stop the temp will come down steadily back to 180/195. I originally thought it was the factory gauge, so I hooked up my auto meter gauge and while the factory gauge *was* about 10 degrees off, these numbers still seem too high. -Oh, also, when I installed the auto meter gauge to the sending unit, I hooked it up in the head where the factory one was located instead of the aluminum intake.-
If anyone has any insight into why it's doing this or how to fix it, I'd greatly appreciate it.

 

Since temp is fine at stop lights id say thermostat is fine. im guessing your not getting enougj air through your radiator. i had this happen in a foxbody i built. the fans wer putting out enough at idle, but couldnt keep up at wot or even cruising speed. Make sure you have a shroud on there. i removed the electric fans on the stang and put on the original with regular fan and problem solved. i think it was just the shroud though.

 

Do you have the plastic "chin spoiler" bolted on under the radiator support (and is it in good condition). They are CRITICAL to getting good airflow (and therefore cooling) through the radiator at highway speeds. Some cars you can take 'em or leave 'em, but not on a 3rd Gen F-body. Must have.

 

Check for leaf clogging the radiator and check that the factory air-dam is in place and mounted properly.

 

As long as I have worked on cars Im almost embarassed to say that is something that I missed!! Ive only had the car on the road for a few months. And for a while I really never drove it anywhere. Well one day in a Publix parking lot about 2 months ago I ran over something when I looked back it was my front air dam. I went and picked it up, checked it out(still in very good condition) hung it up in my shop and thought nothing of it. Drove the car around for weeks no problem, then I went to go to my Parents (which is about 40 miles almost all HWY) about a qaurter of the way there the temp started going up. Im going to make sure the electric pull fan has a good shroud and reinstalle the front air dam.. Its raining right now in the sunny PanHandle of Florida (Ft walton Beach, Between Pensacola&Panama City Beach) But Its still mid 60's! Thanks Bud

 

Yeah I ended up pulling the condenser out also because it looked like it was all nasty,(no leaves though).. As for as the air-dam good call it fell off a couple months ago (before I really ever really took it on the highway) I hung it on my shop wall and didint think nothing of it!! But when the rain stops here in the good ole Sunny Pan Handle of Florida.. Im gonna reinstalle it!

 

Yeah Im gonna check and make sure that there is good pull and flow off air if not I will make and installe an shroud. I also need to put my air dam back on which fell off a couple month ago... This car has been in storage for 16 years.. so just been back on the road for about 4 months now. The body only has 17389 original miles on it... A friend of mine bought it new in 84 drove it for 2 years (he owened a very big custom shop) they pulled it apart and started to build it to run on lake beds or the salt flats, there was alot of money spent on it "suspension,brakes,transmision,motor... then he died... And it sat in the shop.. and someone stole the motor,anyway 10 years later his wife finally sold it to me, and I built a motor and slowly put it back on the road! Its kinda neat because the interior is like brand new! And the whole time it was stored it was on jack stands and climate comtrolled.. I gonna put somemore pics up soon.(I put a couple up all ready)but anyway thanks for the suggestion!

 

Edit... posted in the wrong thead.

 

Still got that 3rd gen?
Im in Pensacola!
Always looking for fellow 3rd Gen'ers