I recently bought a 1978 Ford Bronco. After purchasing it and driving it 120 mi. back to where I live it starting running real hot with the needle almost pegged on the end of the hot line on the gauge. I proceeded to pull on over about 30 mi down the road in an attempt to assess the situation. After pulling over and lifting the hood I noticed a small drip from the back of the radiator cap. I allowed it to cool some, then removed the cap. I noticed some stress cracking of the gasket but the radiator was still full of water so I decided to go to the nearest parts store and replace it with a pressure release cap to raise the boiling temp a few degrees.Upon starting the vehicle and proceeding down the road the vehicle was running at about half gauge temp and slowly creeping hot. The gauge reached almost to the danger zone again and proceeded to kick back down to 3/4 temp. I'm thinking sweet, it's probably just a sticky thermostat, I just need to closely monitor the temp until I get home. It maintained the 3/4 gauge reading on the temp for another 60 miles and then creeped into the danger zone again at which point I proceeded to pull over. I allowed it to cool down and checked the water level to see if I was losing coolant. I drove the truck 10 miles with it in the danger zone to get it to the nearest services. I was concerned i could've blown a head gasket in the process of doing this. The radiator was still full. I two stepped it back to my house with it cold the last 30 mi so I could more properly diagnose it the morning. I woke up and went and got a thermostat and gasket. After removing the old thermostat I decided to boil it to see if it was opening. It opened. I'm pretty sure it is good but said forget about it and replaced it anyway. After reassembling the thermostat housing I started the truck the check for overheating, and again, It overheated. I purchased a reusable thermostat housing gasket and decided to boil the new thermostat to see if it was a manufacturers defect and again, it opened. I let this thing cool down and opened the cap. The radiator was still full. So I ask myself if maybe I installed the thermostat backwards, I have changed many in the past and never installed one backwards. I could be wrong but am pretty sure the thermostat goes in spring side into the block. This was how the old was also. I reinstalled the thermostat and opened the radiator. I refilled the tank with water to full and ran the truck with the radiator cap off to see if coolant was circulating and noticed it was not. Maybe I have a bad water pump. I removed the ps assembly and alternator to check for shaft play, there was none. There was also no drip down from the weep hole either. Maybe in some alternate universe with pixies and fairies the water pump is bad with no normal indications of failure and for 16 dollars decide to replace it anyway. Low and behold, the truck overheats again. At this point I have dealt with the obvious even though all the equipment I have checked and replaced showed no signs of failure, even the light cracking in the radiator cap gasket was not significant nor cracked through the gasket, It should and most likely could still hold pressure, I decided to think about the non-obvious problems such as electrolysis, radiator clogging and temp gauge malfunction, radiator hose collapse, etc. I even looked into proper draw from the cooling fan, which is mounted to the factory location on the water pump, There is no aftermarket electric fan. Sometimes on these older vehicles not having a fan shroud to direct air can cause overheating at an idle. It has a properly installed fan shroud and hence is most likely not the problem. The fan is also clutchless which will eliminate a common problem of overheating due to a bad fan clutch. Voting for the radiator to most likely not be clogged because usually radiators crack and fail before they get clogged I narrowed it down to electrolysis or bad temp sensor. Seeing as electrolysis is merely speculation and very difficult to identify without taking the motor apart I checked the gauge against my buddy's laser temp and determined it is functioning correctly. Here comes the "strange" part. I got a wild hair up my butt and decided to remove the thermostat and check for overheating again. After removal of the thermostat and running the truck at idle for over 45 min the temp gauge never even reached normal operating temp and the truck truck stopped overheating. I would normally say forget about it and run the truck without the thermostat, but the problem is the truck does not get warm enough to even idle without the choke engaged. I do not understand what is going on as I have never encountered this problem. Everything in the cooling system checks out including the parts I replaced on a whim. They were still good. I need the thermostat in and am now asking for some suggestions to remedy the problem. I am not a novice and have been repairing vehicles for years and have never encountered a problem like this. Any suggestions would be greatly appreciated. Thank you.

 

Since your overheating seems to be related to cruising speeds, I'd suspect the radiator is restricted. And that you've changed the thermostat (spring does go down) with no improvment. At idle your not going to generate the heat as you would at speed, even given the air movement through the radiator, and a minimal or blocked circulation probably would keep the temps within norm during idle/low RPM times.

Did you lower the radiator level and look at the bars for deposits blocking the passages? When it's runnning at temp but not overheating, pull the cap and run the RPM's up fairly high, does it want to push coolant out of the radiator neck? or try and collapse the lower hose?

When it starts to run hot, check the upper hose, then the lower hose,, any real noticeable difference in the temps? If so on these checks, I'd look again at the radiator for a restriction. Also, look for any gas bubbles pushing up/out through the coolant during idle, and again at a fixed cruise or off idle speed..

A healthy system with the cap off and heated, usually will drop the level 'slightly' for a few seconds on a throttle snap or bringing the the RPM's up until the level stabilizes again..

 

I completely agree with our moderaters views, however, do check for proper airflow through the radiator also. If this has fan clucth, there may be problem. However, many of these did have the flex blades; and it still doesn't solve the problem of overheating at speed, as there should be suffient airflow at that speed.

 

Thank you for your responses but if you had actually read the entire post the problem was occurring at all speeds including freeway. Coolant level, radiator cap, water pump, thermostat, hoses, radiator, and coolant overflow tank are all 100% functioning and short of the coolant hoses, replaced. The problem is still occurring and has not been solved. It runs just below the hot line at the top of the normal section on the gauge. The air drawing through the radiator from the fan should be more than enough considering I can feel the air hitting me in the face behind the drivers door with the hood open. There is a ton of air moving. I going to cap off the heater lines and see if a circulation problem is occurring through heater's radiator. i wouldn't imagine if would make that big of a difference but i guess it'll never hurt to check. Any other suggestion would be appreciated. Thank you.

 

Well,, I 'actually' read the whole post : /

You asked questions, sometimes answering your own. Not flaming you, just an observation, and to help with a diagnosis we need to ''prove out what 'is not' a cause and then try and figure out the possibilities. Like you stated,, 'quote' ..to see if coolant was circulating and noticed it was not.. Then you stated the radiator is 100% functional, how did you check it? Or, was it just not heated enough, and the thermostat still closed when checked? And, radiators 'will' certainly plug/restrict without cracking ect. Seen them plug topside (can even be to the sides of the cap hole where not visable) and on the bottom side after pulling the tank..

I asked if you drained down the coolant level to spot view the bars for obstruction/capped with deposits, or, if you see any bubbles coming up from the heated coolant. No reply.

Also, do you ever see any steam escape at the tail pipe after the truck sits after being hot/warmed, cools, and then on a restart? If a head, head gasket, block problem sometimes you'll get a puff or more of steam at the pipe until it burns off. Generally if compression can leak into the cooling system, after an extended heated sit time, the pressure in the sealed cooling system will push coolant back into the cylinder/s where the bubbles came from

If removing the thermostat keeps it from overheating (you only mentioned checking and a more normal temp at idle) will it overheat with it still out and at extended freeway speeds? The heater core actually 'helps' cool the heated coolant, in fact, on a system thats border line hot, sometimes turning the heater ON will drop the temp level slighly.

Since the air flow question seems to be off the table, can you gauge the upper hose, lower hose, and top/bottom of the radiator as far as temps?

I've replaced and rodded out more than a few of these radiators because of restriction. Like the question I asked before,, with the cap off and heated, and then you raise the RPM 'fairly high and quickly', you should not only see the coolant flow on by the neck, but the level should drop slightly until it stabilizes again..

It just sounds like its not flowing enough coolant. Heads been off at any point?

 

You know, Mr Hayapower is absolutely, completely right on this. I wanted to go a little deeper in this, because I sat at work and thought about it. It's nice to know I agree with someone sane(r) then me.
I think this is an internal engine problem, quite simply. I've worked on these block my whole like, and sometimes, you hit a really weird one, like this. Let me explain why.
1) You've mentioned that you have a ton of airflow (which ok, I admit was a weak answer, but I didn't believe it myself), and it happens at highway speeds.
2) this is something that shows up more often under a load; and,
3) It not happening in any predictable way. It sure doesn't seem t-stat or waterpump related. What else remains?
The block and heads. I'll bet this is one of these real bitches that just doesn't want to cooperate. Fords don't normally do this kind of crap, but when they do, it's usually something that is intangilbe to get to. Like this.
I had an old '87 Ranger (not the same enging, but will become clearer) that I put a brand new Ford waterpump onto. The impeller came off on I-25 going down the road, and it overheated. I knew (even as a tech) that it was happening, but as I couldn't pull over without at time, I limped it home. Took it to work, and ran it a little.
Never once, not once did this truck ever misfire, and after I put yet another new (warranted, no less) waterpump on my truck that under a load, it would do exactly what you explained. Almost to a complaint, my truck did the same things.
Usually on a 2.9L engine, when they overheat enough to crack something, it's the heads. Not in my case. The number 1 clyinder was cracked all the way down the block, and it ran fine and Never Lost a DROP of water.
You've done all the usual and some of the unusual stufff. That is what I believe is wrong for my 2 cents.
One other thing that crossed by mind, I remember from years ago. Every once in a while, and only on 302's, I came across a bunch of these from the late 70's that would blow head gaskets because some of them left the factory with inaccuarte torques. A new set would cure it. May not be the case.
And this is such an odd one that I would bet even if you got it torn down, you may not even see something with the naked eye. They sometimes can hide like that.