I have a '67 Coupe with a 289... Im new here, just got my mustang about 3 months ago. I have a very weird issue. My car gets hot and boils the coolant out, only when i have driven over 65 mph for a period of time. I can drive 60 forever with no issue. I cant figure it out...
I have been searching forums for weeks. I have changed the thermostat, replaced the lower rad hose with one with a spring, upgraded radiator, fan, and shroud. Im lost.... Please help.
By the way, nice to meet everyone.

 

Check to be sure you're not leaned out on the carburetor. Lean condition could cause the cylinders to be to hot, maybe. Just a guess, as the 60 vs 65 thing is... odd.

Other than that, you didn't say you swapped the water pump...?

And... I got nuthin' else. Weird indeed.

 

Yup - water pump maybe, even belts maybe, too lean main jets (plug color?), or timing too advanced would cause my carb sixer to overheat.

 

I had a water pump that the impeller would free spin when the engine got hot. I just happened to check it one day when I was at my wits end.

 

All good ideas.
New water pump this weekend, it is.
I would really like to take her for a drive and not have to worry about getting too hot. I know the 60 vs 65 thing is weird. Tried it out today...
Drove 65 in one direction for about 15 miles, stopped and temp was high and coolant was spewing out of the overflow.
let it sit for 10 minutes, drove the same trip back home @55-60 stayed amazingly cool.

 

Ok, I might sound like I'm reaching. I have a flowmaster dual exhaust, so you cant hear yourself think when you're going down the road. Until you reach that 65 mph mark. Then it gets really quiet.... Hmmm. It's not that its upshifting, the exhaust just gets really really quiet. I don't know if that is a newb thing to say. I'm just confused about this whole situation.

 

What temp thermostat did you install? 190 is preferred. Also do you have a clutch fan or straight?

 

Weird. Something plugging the exhaust? That would sure do it.

Also when you changed radiator did you do one of those prestone flushes? wonder if coolant passages in the block have some gunk in them?

 

Ok the thermo is a 180. Straight fan. Flushed the radiator but not the rest of the system...

 

Also, how in the world do I check for an exhaust blockage??? Thank you all for your help

 

Take it off, and blow air into it. If it came back in your face, plugged.

Other way is to take it off and run a pipe cleaner type thing through it, but I dunno how it'd react in the muffers...

Other way is to replace it all, it's the only way to be sure. But that's a rather Ripley vs Aliens sort of thing to do...

You might can just take off the exhaust and see if the problem goes away, but... I don't think people would be appreciative, nor would your ears.

You might can take off the mufflers by themselves, in which case, the blowing/shoving thing might be doable.

/sounds kinky when I say it like that, sorry.

 

Exhaust is fairly new. I can't imagine that there is a blockage, but I'll check it out. Hopefully someone just pulled the "Beverly hills cop" banana in the tailpipe. That would make my life a lot easier. Haha

 

Do you have a sufficient fan shroud for the motor. This can make a huge difference. Had a similar issue with my old Mopar.

 

Sorry. Just reread your OP. Fan shroud: check.

 

Ok, kind offs new at this. Haven't had carbureted car in a long time. (driven fuel injected jeeps for about 12 years). Im not sure how I would check this. Sorry if it's another newb question. I am planning on rebuilding the carb. Didnt think it would pertain to this issue tho. Is there an easy way to check this out?
Water pump is happening this weekend. Also.

 

Depends on the carb. Some, it's float level, others, it's jetting, others still, there's an adjustment screw... you just simply have to read the manual, if you can find it, for your particular model carb. I'm sure there's plenty in Google.

If it's a dirty carb, then yeah, that WILL cause the problem, as the gunked up carb will not let gas flow right, and leans out the mixture. Leaned out mixture=higher temps, especially at higher RPMS. The gas itself is a coolant of sorts, believe it or not.

So if you were planning on the carb cleaning, I'd say now is the time.

 

Sweet, I'll get on that today. Thanks

 

Ok all,
New plan of attack.
Is there a possibility that the new radiator isn't big enough?? I'm 90% sure that it's a 3 core rad. It was put in right before I bought the car a few months ago.
Either way, I took it to a shop today. The owner has a '66 with a 6 cyl. We looked at our radiators and the core width is pretty close to the same. Does that mean that my rad is too small. I will drop the cash on a 4 core aluminum rad, but I wanna know that it is the problem.
It got hot today and I ran some cool water onto the radiator, cooled down immediately...
To me; that would mean that everything is circulating correctly but the radiator can't keep up...(reasonable assumption?)

 

First off: I am seriously hoping that whoever did the radiator didn't try to use a 2 core in it. Although it might work... Still, no, seriously, why would you bother with that in the first place?

Second question: Automatic or Manual? Might be important, and you'll see why below.

Third question: A/C? Or no A/C?

I'm going to sum up what's been done here for myself, and maybe others.

Problem: 289 overheating at 65, not at 60.

Changed:
Thermostat, 180 degree
Lower Rad Hose
New fan, no clutch
New Shroud

Things that may or may not have been done, but have been suggested:
Carb cleaned
New water pump.
Check timing.

Note: Radiator is newish, and supposedly a 3 core, but not for sure on the cores.
Running water on it hot cools it down... but that's at idle at a stop, so...

Also of note: has dual exaust from Flowmaster, and it's loud.
... except until you get up to that 65, then it's quiet?!? <--- Red alert...

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Have I got that right?

The exhaust going quiet at the 65 MPH mark is... disturbing. A lot. In a spooky, creeping-me-out, "Get rid of that thing!" type of way. There is no way the exhaust gets quieter at speed, if it didn't downshift, unless you're breaking the speed of sound or some other unnatural thing such as this.



It sounds a whole like the car *stops working* at that speed... which is impossible, because you can't keep it at 65, of course. Which is the poltergeist situation I envision is going on here.

Beyond that, it seems that by the water trick, there is flow, BUT... you aren't at the speed of engine that causes the issue, so it's really a flawed test/result.

So... run the engine at the RPMs indicated. Which, by the way, is what I'd be trying next: Keep it in a lower gear and see if it's RPM or speed related.

For example, let's say the car currently overheats at 65MPH at 2500RPM. (Whatever it is, substitute that RPM) Now, slow down enough to drop it down a gear, but run at the same RPM. Don't care about the speed.

Does it still overheat or not?
If no, then it's speed related issue.
If yes, then it's RPM related.

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And yes, it will make a big difference. Because if it only does it at 65, it narrows the list to these oddities made possible by the *speed only* (where RPM isn't the cause), as near as I can fathom the possibles...:

Cooling air cannot flow into the radiator because the speed is too great, and there is some sort of restriction of the air flow through the radiator at too high a speed. I highly doubt this is the case, but I must throw it out there, it IS a thing. I expect much derisive posts from this, but there I go, saying it anyway.

Dragging brakes or such. Perhaps the rear gears or an axle bearing, but you'd hear those most likely, and/or feel them stopping. Also unlikely, given what I've read here. And that's because I think it's the next one, below, because the exhaust wouldn't get quiet for this... if anything it'd be louder than normal as the engine tried to overcome the issue.

Transmission troubles. I am going out on a limb here and saying this is an automatic car, and as such, the exhaust thing is making me lean towards a bad torque converter in an automatic or something like that. There is, again, NO WAY the exhaust gets quieter unless the engine is doing something unnatural... like reving weird. A badly operating transmission would not only let the engine do weird things, but would really help explain the overheat in that ITS heat is run into the radiator too. A separate transmission cooler might be helpful in at least determining if it's contributing to the problem or not.

Assuming the auto uses transmission cooling from the radiator in the first place, which I think is a yes.

If it's a manual, then the clutch MIGHT be an issue, but wouldn't contribute heat into the radiator like the auto would. Gears are still in play, but... not as likely, in which case..

If it's an A/C car, the condenser coil is in front of the radiator. It's possible it's restricting the flow of air into the radiator at speed, if it's clogged up or it's fins all twisted to stupid. This WOULD be seen at speed.. Get a radiator fin comb and comb it out maybe. I'm sure this has been done, though...

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Now, that's all Speed. RPM related issues, well, might be the above, but these are more likely:

My favorite, but probably not it: A cam was done, and it's made to race... in which case, high RPM cruising is not it's forte, MOAR POWAR is for drag strip use, and it's overheating because of that? Which would also explain the exhaust issue, come to think of it, maybe... possibly... I dunno... maybe not...

Way wrong gearing in the axle? Making the engine work at a higher RPM than normal? 4:11s maybe? Oh sure, it's break neck fun, but...

Ditto transmission? Both together would be hella fun at a track, but for real life...

Oil break down at higher RPM, causing friction heat? Perhaps change the weight of the oil to a higher viscosity or whatever that is?

Bad bearings somewhere?

Poor coolant flow?

Slipping belts on the water pump, causing said bad flow at higher RPM? (although, I would think you'd hear that...)

Messed up thermostat? *Backwards* thermostat (it is directional, but I'm sure you knew that. )

Carburetor issues? Lean condition caused by wrong jets or clogged fuel circuits?

Fuel issues? Bad fuel pump? Gunked up tank?

Bad timing? Bad points? Bad coil? Bad plugs? Bad wires? Bad condenser? Bad advance mechanism?

Compression? I know, I'm reaching now, but... perhaps a couple of cylinders are having issues... bad head gasket maybe...

Does it have electronic ignition? Bad pickups for the ignition? Bad ignition?

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Can't come up with anything else off the top of my head. Pretty good list right there, and I'm sure you thought of some, if not all, of those... And some of those are not only from left field in relation, but left field in outer space, I'll admit.

That whole exhaust thing... it's driving' me nuts on this. That's a *really* weird result...

Here's the two things I can think of to try:

1) separate transmission cooler. If the heat is being generated from that, this will 'solve' it in that you've isolated which is the problem.

2) remove the thermostat. Yes, pain to do. Results will be kinda important though.

3) New belts maybe.

4) Flex fan. Might pull through more air, but... the shroud might not allow it to work when it flexes...

5) Electric fan, ditch the regular one. If it's a clutched fan, even more so. Matter of fact:

6) If it has a fan clutch, perhaps it's bad...

But first and foremost, the RPM/Speed test. I am really curious what happens now. And whether it's an auto or not, and A/C or not.

/Too much, gang?
//I know, I can really ramble... Sorry.
/// Hey! I SAYS "Sorry!"