Engine Bad radiator cap?

91GTstroked

5 Year Member
Jun 14, 2007
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Hey guy,

First hiccup with my new 93 mustang. Previous owner said coolant was low. And he said that he believes he overfilled the reservoir. Which it is. It is past the hot mark when the engine is cold.

So I let the car sit overnight and take the radiator cap off. It still has pressure and leaks coolant all over.

I believe the cap is the culprit. That valve at the end just dangles. It should be spring loaded correct?

I also can't figure out why they used red coolant? Unless it's that dirty, but it looks more red than brown.

Below is a picture.

Thanks!
 

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KRUISR

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Apr 16, 2015
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That lowest disc I believe is to allow coolant from overflow back into rad. I would replace the cap if it is in any doubt.

Then to get the coolant level right, I start the engine and let it idle and warm it up (with the cap off) until the coolant starts flowing out of the cooling tubes. The coolant level in rad should drop half way or more in the rad tank. Start topping it up until it fills and keeps rad full. After a while, with the engine still running, the coolant will want to burb out of rad. This is when I put the cap back on. At this time I usually fill the overflow to the cold full level and call it a day.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
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If there is pressure when opening the cap I'd say it works :shrug: can't tell what is loose.
Red coolant??
GM used a red coolant, I wouldn't use it, the guys at the shop btched about it alot.
Of course we need a pic, by we I mean me :pop:
 

90sickfox

Wasn't a pretty sight...and I've got big hands
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Mar 2, 2015
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A cold system shouldn't have any pressure. When the system heats up the valve in the cap allows coolant to overflow into the bottle. As it cools down that valve is supposed to allow the coolant to pull from the bottle and into the radiator.

Radiator caps are cheap. I prefer the lever vent ones by Stant. It's got a red lever so if it hot you can flip that lever and bleed the pressure out before opening the radiator.
 
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KRUISR

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Could the pressure you are seeing when the car is cold be vacuum as opposed to pressure? That would mean your system is not allowing the coolant from overflow bottle back into rad. Replace the cap.
 
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91GTstroked

5 Year Member
Jun 14, 2007
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Could the pressure you are seeing when the car is cold be vacuum as opposed to pressure? That would mean your system is not allowing the coolant from overflow bottle back into rad. Replace the cap.
Possibly. Hard to tell, both the radiator and overflow tank were full. But previous owner added fluid.

I'll buy another cap, but I forgot I can use the cap off my 90 coupe and test it. I know that cap works.
 
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91GTstroked

5 Year Member
Jun 14, 2007
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Update:

I put a new radiator cap on, seems to be working. Unfortunately, now I might have a leak at the water pump gasket. I suspect to much pressure in the radiator and it caused a leak at the weakest place.

The whole radiator needs a flush. I'll basically be getting all the fluids changed.
 
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