my 65 is over heating

i have a 65 289 that suddenly started overheating after a few miles.

so far i have
changed the radiator cap
got a new thermostat
fixed a bleeding bypass hose
flushed the block
flushed my rad even though its new
added an inline catch screen in upper rad hose to see if debris are coming out of block, which they are not
added water wetter
changed coolant ratio
fixed my timing
checked the head gasket

the only other things i can think to do are
bypass my heater core to see if its gummed up and causing the issue
check my air/fuel mix to see if its to lean

WHAT ELSE COULD BE WRONG!!! EVEN 3 MUSTANG MECHANICS SHOPS ARE BAFFLED

Thanks for the support, i miss my car    

bypass the heater core...if thats the problem and i say if cause i'm not sure. change out the heater core and the hoses just to make sure you know everything is flowing the way it should be. and i would pull the thermostat...i remember way back when even a new stat could sometimes be bad.
thermostats dont always work the way they're suppose to so pull it and run the engine with out it. if it stays cool then you found your problem.
these are the only 2 things i can think of for now ..
hope they help  

Temporarily take the thermostat completely out. That will tell you if you have a defective NEW thermostat causing a flow restriction. Something has to really be restrictive to make the engine overheat in just a few miles. Make sure your lower radiator hose isn't weak. Make sure it has an expander spring in it so it won't collapse and shut off flow once you get going too. My guess is that its one of these since either one can cause a serious flow restriction that can make your motor overheat fast after reaching operating temp.  

Make sure water wetter didn't coagulate for some reason and clog some of your radiator tubes. Its a long shot but has been known to happen on rare occassions.
Open your rad cap when engine is cold and look at top of your rad tubes to see if there is blockage.

The heater core, if blocked , will act just like a bypass since there will be no flow in or out of it. That won't cause your car to overheat. Forget about it.

Lean fuel mixture can cause an engine to run hot. Check your plugs. Should be light tan or grey but probably wouldn't be enough to make car overheat in a couple of miles. If it was that lean you'd know it.

What kind of fan do you have? Check to be sure you installed it right so you have air flow through the radiator front to back and all fan belts are tight and in good condition so you don't get slippage and loss of fan/water pump operation.
Make sure if you have an automatic clutch fan that the fan clutch is working.
Make sure your water pump is working. If you have flow, it is.  

thanks JBirch

the 160 hi flow thermostat thats in is fine, i checked it before i put it in and i checked the 185 i swapped out and it was fine as well so its not that.

I dont have an expander spring and was told i didnt need it from my shop and i tend to agree with that since the car was running fine without it before and the hose is new.

I have a four blade fan with no clutch, which was working fine and keeping the car cool. i purchased a six blade and shroud but now have a new issue that the shroud wont fit cuz my motor mounts are old and the engine is sitting lower than it should.

regardless of that all my stock items were working to keep the car cool so i feel like getting the six blade and the shroud is just masking whatever caused my stock items to stop being sufficient

and i do have flow. the tubes are juiced and you can feel the flow when the thermo opens up  

When you say "started overheating", do you mean an actual boil over or the temp guage just registering hot? I don't remember reading that in your post.  

Is the 6 blade fan PULLING air through the radiator. I've seen 'em put on backwards before by mistake. Is the fan about an inch behind the rad core?
I'd still pull the thermostat. Even good ones can suddenly go bad and/or not open  fully until 20 degrees past their advertised opening temp.
It has to be some sort of restriction that causes a rapid heating like you describe.
BTW: How hot was the outside air temp when you noticed the rapid engine temp situation?  

yes boiling over running in the 220s range or hotter  

i havent finished installing the fan yet because the shroud wont fit on cuz my motor mounts are collapsed and there is no clearance on the bottom, but the fan is on right and the 4 blade was on correct as well. i just put the thermo on 2 weeks ago, everything i listed ive done has been over a period of the last 2 weeks and i test the car at sunset when its a cool 70 degrees

i am so baffled beause i did a complete restore 8 months ago. drove about 300 miles beautifully over the first 6 months, and then BOOM all the sudden its overheating  

i understand where you are coming from with pulling the thermostat but the car was working with the standard 185 and the flow that allows so it has to be something else since i put in the 160 hi flow which i tested as well  

The new thermostat may have gone bad or stuck on you even though it is new and been recently tested. Even new ones can fail early sometimes. No thermostat at all is guarnteed NOT to fail since there is no part to go bad. Then you will know for sure.
Its an easy procedure. Partially drain the radiator and remove/replace the two bolts on the thermostat housing. Refill radiator and fire up the motor with the rad cap off to let any trapped air escape. Once it begins to warm up make sure you have flow and then securely replace the rad cap and test drive.  Then you will know for sure. The other side benefit of doing this is that any air pockets trapped in your cooling system that could be helping to oveheat your car will be eliminated.  

alrite will do when i have a chance, need to go get a new thermo gasket first  

alrite guys i lost the thermo and installed the lower hose spring and installed my six blade fan and shroud but im still running hot. also checked if belt is tight and if water pump is good and they are. also made sure im not running to lean an dim not.

so what gives, im out of ideas  

Flow check your radiator again. Sometimes engine slag/rust can break loose from the cooling passages and clog some of your radiator tubes after a few engine heating and cooling cycles. I had a brand new radiator go bad in just 800 miles from that stuff. It can also clog the catch screen you installed in your upper rad hose and block flow. Sometimes conditioners can too.
Do you have to keep adding water or does the coolant level remain constant?
Wondering if you are sucking water into the motor via a bad gasket or head.
Check your spark plugs for any signs of abnormal operation.

Now that you lost the thermo be sure you have flow by opening your 14-16lb rated radiator cap when cold and watching to see if water is moving across the radiator with the engine running. If it is moving pretty well then you are most likely OK with water pump and coolant passage flow. Let cool and drain some coolant out of the radiator until it is below the tubes in the top tank of the radiator.  With the rad cap off shine a flashlight down the rad cap opening and really check out the tubes. If there is crud in any of the ones you can see you can figure there are are three or 4 more you can't see. If you have clogged tubes have a rad shop rod out the radiator to regain cooling efficiency. If tubes are OK and flow is good with no air bubbles then test the 160 thermostat in a pan of water on the stove to 160 degrees using a cooking thermometer. You can buy one cheap at any grocery or dollar store. Make sure your 160 thermo really starts opening at 160 and is FULLY open not too much beyond that and reinstall it.

Make sure you are using no more than 50/50 mix of coolant/antifreeze. Water is better at dissipating heat than antifreeze. I think you did this already but just want to be sure in case.

You did a compression check. If you had two cylinders next to each other that are low you may have a bad head gasket. Check for air bubbles in your coolant when you look down rad cap hole for coolant flow. That can be an indicator of a head or head gasket leak. Run the engine up to normal operating temp with the rad cap off to let any air pockets trapped in the system escape that might have created a coolant passage block.

Check your timing again to be sure it is not too advanced and causing the motor to run hotter. Idle timing should be around 6 degrees BTDC stock and about 10-12 degrees BTDC with a mild street cam. No more than 36 degrees total with vac advance hose disconnected and plugged. 48 to 50 degrees with vac advance connected. Check your spark plugs for any abnormal indications or water. Check you exhaust for excess water vapor when warmed to normnal operating temp. You might be sucking water into the combustion chamber and out the exhaust.

Make sure your brakes (including the parking brakes) aren't dragging or causing rolling resistance that overworks the motor.

How is your torque converter/tranny working if you have an auto trans? Sometimes a too tight converter (stock is 1700-1900 rpm) or tranny will pre-load the motor down and cause it to run warmer. You should only have about 100-150 rpm drop from nuetral to drive @ normal warm engine idle speed. More than a 200 rpm drop could mean something is off. Should be around 625 rpm in gear and around 775 with a mild cam in nuetral.  Less if totally stock.  

What kind of radiator do you have?
Do you have A/C with a condenser in front of the radiator?
How many miles on the motor since rebuild.
Any non-stock equipment on or in the motor? Like aftermarket cam, etc.
What is your normal idle speed?
What is your idle manifold vacuum?
Does the motor get hot on the highway or just in town in stop and go traffic?
Does it overheat if you drive it when the outside air temp is cool too?
Was the engine bored more than 0.040 inch during rebuild.
What kind of oil are you using?  

AV3NG3D:
i have a 65 289 that suddenly started overheating after a few miles.

so far i have
changed the radiator cap
got a new thermostat
fixed a bleeding bypass hose
flushed the block
flushed my rad even though its new
added an inline catch screen in upper rad hose to see if debris are coming out of block, which they are not
added water wetter
changed coolant ratio
fixed my timing
checked the head gasket

the only other things i can think to do are
bypass my heater core to see if its gummed up and causing the issue
check my air/fuel mix to see if its to lean

WHAT ELSE COULD BE WRONG!!! EVEN 3 MUSTANG MECHANICS SHOPS ARE BAFFLED

Thanks for the support, i miss my car  

How exactly did you check for a head gasket? If you didn't do the chemical reaction test to the coolant you haven't checked thoroughly enough.
I'm suspecting a blockage in the passages or a cracked head/block. (I hope not though)
jlg