Overheating 97, 4.6L Expy
Posting Guru
Joined: Jun 2006
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You replaced the heater core right? Did you fill to proper level? If low, it could cause overheating. You may still have an air bubble in the system too, or worse, you could have a head gasket leak. It wouldn't surprise me with a head gasket if you constantly allowed the truck to overheat and kept driving it.
First off, did you do a pressure test on the cooling system? If not, I would perform one. Replace the overflow cap while you are at it. Do the easy stuff first. Also, did you replace all of the coolant when you replaced the heater core? It is very likely you induced air into the system.
In order to get the air out, take off the overflow resevoir cap and run the truck. Turn the Heat on HIGH. In your case, have someone else monitoring the temp gauge, you don't want this thing overheating while you are under the hood!. Squeeze the top radiator hose to get the air forced through the system. You will need to do this AFTER, the thermostat opens up. Be careful, the top hose should eventually get hot. If you have a bad hose, it could end up bursting. (rare, but it could happen while squeezing) Add coolant if the thing drops below the fill line.
Lastly, if all else fails, get a block test ASAP!
First off, did you do a pressure test on the cooling system? If not, I would perform one. Replace the overflow cap while you are at it. Do the easy stuff first. Also, did you replace all of the coolant when you replaced the heater core? It is very likely you induced air into the system.
In order to get the air out, take off the overflow resevoir cap and run the truck. Turn the Heat on HIGH. In your case, have someone else monitoring the temp gauge, you don't want this thing overheating while you are under the hood!. Squeeze the top radiator hose to get the air forced through the system. You will need to do this AFTER, the thermostat opens up. Be careful, the top hose should eventually get hot. If you have a bad hose, it could end up bursting. (rare, but it could happen while squeezing) Add coolant if the thing drops below the fill line.
Lastly, if all else fails, get a block test ASAP!
Fleet Owner
Joined: Apr 2008
Posts: 24,316
Likes: 5,127
From: 0,0,1
After an 8 mile drive your t-stat would've more than opened and heated up the upper hose. I'm going to say your problem is with your t-stat not opening.
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Mountain Pass
Joined: May 2004
Posts: 226
Likes: 2
Thanks for all of the advise, after I posted my last comment. I went out and started the engine, took off the cap, let it run for about 10 min. I had watched the coolant level go up and down as I gave it gas (from the throttle body) then after I did that the coolant started coming our of the tank. Shortly after the coolant came out the tank was about 1/4 full but the bubbles started to come up and I could feel the upper hose pulsating from all of the air. I did replace all of the coolant, the pressure in the system is constant through out the night, I checked it this morning before I started it and it hissed when I removed the cap. After I got all of the air out I drove it for about 13 miles, I got hot air, the top hose is hot, and good heat coming out of the heater. So I am happy, it looks like the air in the system was the issue after I replaced all of the other stuff. I no longer have the running water sound coming from the dash. WOOOO HOOOOO, and the Broncos are 5 and 0 life is good. Thanks again everyone for all of your help and guidance, this site is the best!!!!!! You all ROCK!!
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Mountain Pass
Joined: May 2004
Posts: 226
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Back to the cooling system! My wife was driving our Expedition yesterday and it overheated on her, same issue as before. The heater goes cold, then the temp gauge red lines. She called me, I told her to pull over and turn off the vehicle and let it sit for a few minutes. She restarted it and temp gauge went back to normal, this happened to her twice on the way home. Could this be the thermostat again, I just replaced it not more than 2 months ago!! I am at a loss it it freaking cold here. I am going to replace the thermostat again today and see if that does the trick. Would it affect anything if I did not have a thermostat?? I don't want to pay to have the system flushed again, should I do that? I know that I am asking a lot of questions but I really need to get this fixed. Thanks for all of your help.
Logistics Pro
Joined: Jan 2001
Posts: 3,740
Likes: 37
From: Central NYS
With the engine all warmed up, shut it off and feel all the fins of the radiator core to see if there are hot and cool spots. (Or better yet, if you have one of those laser thermometer things, check the radiator core for even temperatures.) If the radiator is cool when the engine is hot, the coolant is not getting through the radiator, but the air is, causing the radiator to feel cool. And when you blip the throttle the coolant surges in the overflow tank becasue it can't get through the radiator fast enough.
That's my guess. And I'm sticking to it.
Senior User
Joined: Jun 2005
Posts: 125
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Other than the heat going cold and the needle pegging does the vehicle actully exhibit overheating symptoms? The needle pegging and then quickly returning to normal suggests air (steam) in the system which would read as a much higher temperature vs. the water/coolant mix. Occasionally the air is gap is next to the temp sensor and causing the pegging of the needle, once the air gap migrates away and the sensor is immersed in fluid the temp wil return to normal. The only other thing I can think of and this would explain the cold heat is the water pump is cavitating. Cavitation is where the pump (impeller) is turning in an air gap and cannot pump fluid, if this is the case the cooling system stops circulating and the vehicle would overheat (actual overheat, not just a pegged guage) and the top hose would not be hot and the heat would not work.
Just my thoughts.
Just my thoughts.
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Mountain Pass
Joined: May 2004
Posts: 226
Likes: 2
Today I was driving it and the same thing happened to me, the air went from hot to cold then the temp gauge red lined, and the coolant light turned on. I pulled over let it sit for a minute turned the key to on (didn't start it) and waited til the temp gauge started to go down (about 1-2 minutes total) started it back up and it was fine and the heat worked. I drove for about 1 mile, and I felt the air getting cold again so I slowed down put it in neutral and coasted just to see if it would get hot, and nothing so I put it back into drive and the air warmed up and I drove home (from that point about 20 miles.) With no issues. I just don't understand why when the temp gauge red lines antifreeze spills all over the place, I went and bought a new motorcraft thermostat today and will install it in the morning, I have also though about getting a new radiator just to see if my issues will go away. I hate to throw money at this issue but I have 255k miles on my baby and it is probably time to start replacing things like the radiator. I'll check the heat across the radiator in the morning and then make my decision about replacing the radiator after I do that.
Senior User
Joined: Jun 2005
Posts: 125
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These kinds of intermittant problems are a real challenge, almost like a puzzle. I hate to start replacing parts when I don't really know what the problem is. These are just some more thoughts to consider:
1. Thermostats usually work or not, true or false? I've not heard of stats that randomly open and close.
2. Coolant doesn't randomly freeze and unfreeze in a couple of mintutes.
3. Radiators don't work fine for 20 minutes and then randomly stop working for 1 minute.
4. Heat blows hot when hot coolant is circulating through the heater core.
It is clear that you are are having short periods when your coolant is not circulating. The problem occurs randomly. What type of problem would cause a random circulation stoppage? I can't think of anything other than air trapped in the system causing a cavitation of the water pump.
Alloro is an expert on these issues, maybe he'll chime in with some thoughts. I don't think the thermostat will change anything unless the air is bled properly.
1. Thermostats usually work or not, true or false? I've not heard of stats that randomly open and close.
2. Coolant doesn't randomly freeze and unfreeze in a couple of mintutes.
3. Radiators don't work fine for 20 minutes and then randomly stop working for 1 minute.
4. Heat blows hot when hot coolant is circulating through the heater core.
It is clear that you are are having short periods when your coolant is not circulating. The problem occurs randomly. What type of problem would cause a random circulation stoppage? I can't think of anything other than air trapped in the system causing a cavitation of the water pump.
Alloro is an expert on these issues, maybe he'll chime in with some thoughts. I don't think the thermostat will change anything unless the air is bled properly.
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Thread Starter
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Mountain Pass
Joined: May 2004
Posts: 226
Likes: 2
My buddy just called me this morning and told me that he was having the same issue that I am having. He turned off the heater and it doesn't seem to be overheating on him, but when he had the heater on it was redlining on the temperature? Is this a coincidence? I am half tempted to just take it in to the mechanic and have them fix the issue because I am at my wits end with this. I have tried everything that I can think of, running the engine with the tank cap off for 30 minutes and squeezing the hoses to work out all of the air. I am not sure where to go from here or even what to tell my friend what to look for or even a remedy as to what he needs to do to fix his problem. 
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Mountain Pass
Joined: May 2004
Posts: 226
Likes: 2
I changed the Tstat, bought it from Ford, I ran the engine with the cap off for about 15 min. Things seemed to be fine for about a two weeks then the same issue temp gauge red lines and the heat goes cold. Now the top radiator hose is so stiff that I can't squeeze it (not even a little bit) and it's cold. I was checking the oil and I was about a quart low so when I went to add some oil I noticed there was a thick milky film on the bottom of the oil cap. I think that I have a bad head gasket, could this be the problem that I am having (getting air into the system, and slowly losing coolant) I am at the end of the road with this issue, but it needs to get fixed. How do I tell for sure that it is the head gasket? I can't see anything leaking from anywhere, there are no puddles under the vehicle after I run it and park it. Could it also be a craked block?? I know that air is getting into the system because when I accellerate I can her it in running through the heater core, even after I purge the air out of the system.
Thanks again for any guidnace and advise that I get. HAPPY NEW YEAR!!!
Thanks again for any guidnace and advise that I get. HAPPY NEW YEAR!!!
New User
Joined: Jan 2007
Posts: 9
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Overheating issue
I have been having the same issue with the heat going cold and then the engine overheats for a matter of seconds and then drops back down to normal and the heat comes back. I changed my oil the other day and noticed the same milky white substance on the bottom of my oil cap. I have changed the cap on the coolant reservoir/degas bottle, installed a new water pump, new thermostat, new fan clutch and flushed the system. I then purged the system on a hill so the coolant reservoir was on the high side. I did this over the course of 2 days, about 45 minutes each time. This lasted for about 3 days and then the symptoms came back. However, sometimes the heat goes cold and the vehicle doesn't overheat. This could be due to the fact that its about 10 degrees here in Iowa. I hope someone comes up with a solution to this problem because I have tried everything. I guess the next thing is to see if my head gasket is blown. 
I hope not!
I hope someone comes up with a solution to this problem because I have tried everything. I guess the next thing is to see if my head gasket is blown. I hope not!
Postmaster
Joined: Jan 2009
Posts: 4,584
Likes: 3
From: Chesapeake, Va.
I changed the Tstat, bought it from Ford, I ran the engine with the cap off for about 15 min. Things seemed to be fine for about a two weeks then the same issue temp gauge red lines and the heat goes cold. Now the top radiator hose is so stiff that I can't squeeze it (not even a little bit) and it's cold. I was checking the oil and I was about a quart low so when I went to add some oil I noticed there was a thick milky film on the bottom of the oil cap. I think that I have a bad head gasket, could this be the problem that I am having (getting air into the system, and slowly losing coolant) I am at the end of the road with this issue, but it needs to get fixed. How do I tell for sure that it is the head gasket? I can't see anything leaking from anywhere, there are no puddles under the vehicle after I run it and park it. Could it also be a craked block?? I know that air is getting into the system because when I accellerate I can her it in running through the heater core, even after I purge the air out of the system.
Thanks again for any guidnace and advise that I get. HAPPY NEW YEAR!!!
Thanks again for any guidnace and advise that I get. HAPPY NEW YEAR!!!
Pablo, I just looked at some of your old posts and this issue has been going on since march of '08. I think it's about time for you to bite the bullet and go to a radiator shop as it's clear to me that as much as we all would like to be able to help you find the problem and fix it we have not been much help and I for one would like to know the cause and cure for this problem that seems to plague a number of us owners. If you do, let us know the results.
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Mountain Pass
Joined: May 2004
Posts: 226
Likes: 2
Brintonr, I had the head gaskets changed about a month ago.....no problems since then. I had several people telling me that it was a clog in the system, the radiator was bad, the thermostat was bad, the heater core was bad and on and on. In my case the head gasket was the culprit. At first I flushed the system, then I changed the heater core and T-stat (that seemed to work for a few days), then I figured that the T-stat was bad so I changed it again, same issue. I then said #$%@ it and changed the head gaskets. That seems to have done the trick, like I said it has been about a month and it has not overheated on me since. The heat works fine it doesn't go hot then cold or vise versa, the temp gauge stays where it should. If I were you I would seriously look at the head gaskets!! Let me know how it turns out, best of luck. HEAD GASKETS were the problem for me. Thanks guys for all of your tips and guidance, this site is the best!!