gotta 79 f-150 with the 400. just bought it and runs fine except it overheats quickly. i do an initial check and see that whoever recently replaced the thermostat seemed to use an entire tube of sealant when the put the housing back on. so im pretty definitive on that being the source of the problem but i noticed on the housing it had a couple of vacuum nipples with no hoses on them. (at least i think thats what they are) i noticed what seemed to be a vacuum tree behind the carb next to the firewall. so my question is are those vacuum lines and do they have anything to do with the thermostat operations? ive never seen that on a thermo housing before. or do i just need to replace the thermo. im also pretty sure of the thermo being the culprit because after running the hose from it is not warm at all. thanks for any input...

 

The vacuum fitting on the thermostat housing is a ported vacuum switch. It opens\closes a vacuum circuit based on coolant temperature. If nothing goes to it, it's fine to leave it be. It does not have anything to do with your overheating problem. It is used for the EGR system, and evaporative canister purge valve on later models. The important thing is that any vacuum sources that used to connect to this via vacuum hose (typically a ported vacuum fitting on the carburetor) are capped off. Sounds like someone might have disabled your EGR system at one point, which is detremental to the performance of your engine unless the person doing this took the necessary steps to re-tune the engine.

The only way that the sealant used on the thermostat housing could contribute to overheating issues is if the person who installed it, caked so much on there that it actually got into the thermostat itself and blocks water flow. However, it would take a ridiculous amount to do that. The more common concern with using too much sealant here is water leakage.

The vacuum tree behind the carb, next to the firewall, is a manifold vacuum tree. It allows you to hook up several lines to manifold vacuum. It has nothing to do with your overheating problems, or anything to do with the cooling system for that matter. The important thing here is that any unused ports are left capped.

Overheating is commonly caused by gunk in the radiator or coolant passeges in the engine, loose fan belt, thermostat failing, missing fan shroud, late ignition timing, among others. To properly diagnose an overheating issue, you must use a mechanical gauge with the sending unit bulb installed in the block. The stock gauge is a 30 year old rheostat, which is about as reliable as horse droppings, excuse my language. You really have no way of knowing what it's telling you, and even when working properly, it's meant to be a fancy go\no-go indicator. With a mechanical guage, you know the temperature of the engine coolant. With the stock gauge, who is to say how hot "between the O and the R" really is?

 

I'd suspect the thermostat, especially if no heat on the upper rad hose-if you are sure it is overheating.
Never hurts to drain some coolant from the rad, you can get an idea of what the rad cores look like, for buildup-easy check.
If you have a high temp. thermometer, remove the thermostat and boil it on the stove and see if it opens, and when, or if it is slow, etc. It should be +/- 10 degrees of the rating stamped into it.
Thermostats are cheap!
Only other possibility, which is rare, is that the waterpump isn't doing its job of circulating. Usually waterpumps just leak, bad bearings, loose when they malfunction.
My first bet is the thermostat tho.
Good luck!

 

thanks for that. i wasnt being sarcastic when i said a whole tube, theres a ton of it. i just wanted to make sure about the vacuum ports. iv only delt with the 300 I6s so im kinda new to this massive thing under the hood and id never seen vac ports on a thermo housing. appreciate the help... oh and it was to the L and beyond *buzzlightyear impr*