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Discussion Starter · #1 ·
Holy **** this has been driving me crazy. The 428 in my '68 fastback would always get hot only at idle and it was driving me insane. I tried literally everything and had just accepted that I need to keep the RPM's up at idle to prevent the temperatures from creeping up. It was only until I started researching pulleys for a power steering swap that I realized that there was an issue.

Turns out I had the wrong water pump pulley. The correct one is over an inch smaller in diameter. Now that the fan is spinning at the correct RPM at idle, my problem is solved!

IMG_1378.HEIC by armon7, on Flickr
 

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I'm glad you fixed your issue. Perhaps it is because the water pump is spinning faster instead of the fan. Maybe it wasn't able to pump enough water with the larger (slower) pulley.
 

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Interesting...I just posed a thread with a similar problem. You’re giving me something to think about. Mine’s a 289, but I wonder if pulley size could be a factor...
 

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That great that you reported back with the solution, most peeps with issues start a thread and you never hear how it turned out

I'm not sure if I have ever seen pulley size discussed on these overheating threads, nice job!
 

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Good work figuring that one out.
Good learnings there for everyone.

It’s crazy when you have been chasing down this gremlin for a while and you feel like you have tried all the easy stuff so it MUST be something complicated!
Turns out it was a simple fix all along.
That crap will drive a person insane and frankly sometimes take some fun out of the hobby.



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You got to love this hobby and forum. Like IndyFive-O I appreciate the report on the solution to the issue. Well done and thanks for the info
 

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1" smaller in diameter is a lot faster in water pump rpm.....
Good catch. Wonder what the application was for the pulley you removed?
Can't quite make out that engineering number.
 
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I’m feeling good about the cooling in my 390...finally. I, too had issues overtime. First I had a pin hole in a freeze plug. Then found the temporary rubber freeze plug was leaking, so I got lucky and tapped a metal freeze plug in the motor. Still was getting hot. Then I switched from brass to aluminum radiator. Still not quite right, but improved. Then I went with 7 blade BB fan and heavy duty clutch.(had a 6 blade n clutch) It ran cool, but would lose a little water overtime and get hot after a week of driving. I finally noticed that my stupid lever radiator cap was not fitting correctly on the Chinese aluminum radiator. It had a funky neck on it. Replaced the radiator cap and now it’s humming along at 185-90* and constant, and I’m not losing any water. Next I need to install AC and see how cool it stays. This summer has been a thorough test as it’s been hot as he** here in S.Texas. But if I can blast down highway, then get into traffic and the temps don’t exceed 205 or so, I think I’m doing fine. I’m glad to hear you have yours resolved as well. I, too have pulleys that seem correct, but not 100% sure they are correct and matching. Can you post up the part numbers and sizes of you pulley arrangement? It would super helpful.
 

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Don't forget that when you install the AC condenser, your best cooling will be with the gap between the condenser and the radiator around the edges sealed. This will keep the air flow through the radiator smooth. Leaving an open gap between them reduces the air flow through the radiator.
 

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I hate cooling problems. Fortunately nothing Mustang-related for many years.
Dealing with these issues on my truck right now when an engine fan exploded on Monday....
 

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Discussion Starter · #11 ·
I'm glad you fixed your issue. Perhaps it is because the water pump is spinning faster instead of the fan. Maybe it wasn't able to pump enough water with the larger (slower) pulley.
I think it's actually a case of the fan not spinning fast enough to move sufficient air at idle, although I'm sure the water pump turning slower didn't help either. I had an electric fan on the car for about 2 weeks that managed to prevent the car from overheating at idle until the fan(s) decided to seize up and leave me stranded.

1" smaller in diameter is a lot faster in water pump rpm.....
Good catch. Wonder what the application was for the pulley you removed?
Can't quite make out that engineering number.
Per Mustang Tek:

C8AE-8A528-B

Applications:

68 390 428 Ford Std., PS, XC

69 390 exc/PC Ford Std.

69 390GT Fairlane Std.& XC from_1/6/69

69 390GT Mustang std.&XC from_17-feb-69

71 390 Ford w/42amp

7 1/4" diameter

Overall depth : 2.40"

Ford Service Part Number ID C8AZ-8509-A

The replacement C6AE-8509-A pulley is 5.86" in diameter!

I hate cooling problems. Fortunately nothing Mustang-related for many years.
Dealing with these issues on my truck right now when an engine fan exploded on Monday....

Holy crap!
 

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That was a great catch on the wrong diameter pulley. I wouldn't have ever thought of that. I seem to recall under drive pulleys being all the thing for the 80's fox body cars to get moh powah out of them. Seems like they would be under turning the water pump and alternator at the least doing that.
 
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When I got my car the belt setup was all screwed up. The crank pulley was only 2 groves and the belt for the Vintage Air was running off a third grove on the water pump. Very Rube Goldberg.
We put on a three grove crank pully, but never thought about the water pump pulley. Just went out in the garage and checked!! Mine is ~6 inches.
PS I've never had an overheating issue.
 

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I was wondering the same thing. I know a couple of my friends that had them and they would have to turn their headlights off at idle so the alternator would keep up.

Glad you got it fixed up HappyStang!

That was a great catch on the wrong diameter pulley. I wouldn't have ever thought of that. I seem to recall under drive pulleys being all the thing for the 80's fox body cars to get moh powah out of them. Seems like they would be under turning the water pump and alternator at the least doing that.
 

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Don't forget that when you install the AC condenser, your best cooling will be with the gap between the condenser and the radiator around the edges sealed. This will keep the air flow through the radiator smooth. Leaving an open gap between them reduces the air flow through the radiator.
Thanks to @happystang for posting the solution. On behalf of future readers, I want to express my appreciation.
@Rufus68, can you give more details about how to have zero gap between the condenser and the radiator? I have about an inch gap and believe my installation is stock. Thanks.
 

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Thanks to @happystang for posting the solution. On behalf of future readers, I want to express my appreciation.

@Rufus68, can you give more details about how to have zero gap between the condenser and the radiator? I have about an inch gap and believe my installation is stock. Thanks.

Read this thread that was posted on a different forum. If you read his adventures until post #25, you will see how he solved his idle cooling issue by sealing the gap between his condenser and his radiator.
https://fyi.boardhost.com/viewtopic.php?id=9835
 

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That's awesome OP!!



I think it's actually a case of the fan not spinning fast enough to move sufficient air at idle, although I'm sure the water pump turning slower didn't help either. I had an electric fan on the car for about 2 weeks that managed to prevent the car from overheating at idle until the fan(s) decided to seize up and leave me stranded.

That is interesting. The donor 5.0 I am using had under-drive pulleys on it. I replaced them with stock Ford pieces to avoid any potential issues with slowing down the water pump. But I also installed an electric fan. maybe I'll put the under-drive pulleys back on.
 

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With +50 years of who knows what happening on these cars and motors.. it's not surprising you had an odd pulley. I had the wrong one on my alternator! Go figure.

A good reminder with overheating issues to check everything. Including pulleys ;)
 
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