Radiator Overflows when engine is turned off in 98 GT - Mustang Evolution

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Old 06-05-2008, 10:17 PM   #1
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Radiator Overflows when engine is turned off in 98 GT

Hi, guys, I was driving home after changing the transmission fluid in the manual trans in my 98 GT. I doubt this has anything to do with my problem but I figured I would mention it in case I accidentially screwed up something without realizing it. But on the way home after about 10 miles I noticed some steam and realized the temp guage was higher than usual sitting right at the L in Normal.

So I pulled over turned off the car, the coolant was boiling out of the overfill and it dumped at least 1/2 of it before it stopped. I let it cool down until the following day, filled it with water and drove it to a mechanic right down the road and he let it finish reaching normal temp and figured out there was a short in the fan plug and it wasn't coming on right. So he replaced the entire fan shroud and the plug and it appeared to be working just fine. The temp guage stayed right at the R on normal in the exact spot it was before the trouble with the fan.

I drove it for about 2 or 3 days and the coolant light came on, so I checked and it was about 1/4 full in the overflow. I figured it was just still low from losing some coolant while the mechanic was looking around for the problem. So I filled it with water and didn't think anything else about it for a few days until the light came on again.

This time the coolant was totally empty in the overflow and I filled it back up with water and started checking it everytime I turned off the car. I realized it would start pouring out of the overflow right after I turned the car off. So I took it back to the mechanic and he did a pressure check on the cooling system and said it showed up fine with no leaks. And he said I probably just needed a new cap.

I went to autozone and got a new cap and some antifreeze. The clerk suggested it had air trapped from when it overheated and needed to be burped. So he went out to the car with me and started pouring the antifreeze in with the engine running it bubbled a few times and he said it was just working air out of the system and he continued to fill it for a few minutes until the bubbles stopped. Which scared me because I have heard bubbles usually mean headgasket leaks. He said he doubts thats the problem. He was driving a Cobra and said hes got lots of experience with 4.6s and after talking with him for a few minutes he really seemed to know his stuff.

So I have driven it for a few days now and even with the new cap it seems to be doing the same thing it might overflow some while running but it really seems to start right after I turn off the engine. I'm not sure if all of the airs out of the cooling system. I stopped adding coolant a few days ago and have kept an eye on the level. Although its still overflowing it hasn't even dropped to the top of the level float.....

I'm scared to drive it even though thats the only problem, aside from the overflow it runs great. It will occasionally blow small puffs of smoke but its done that since I bought it, its not constant and theres no sweet smell. I was told that since it doesn't have cats the smokes probably just due to it running rich.

I have been keeping an eye on the oil and watching for bubbles in the radiator in case it could be a headgasket problem. But theres no signs and the mechanic tells me it would be running hot. I was also told to get a new thermostat and flush the radiator which i'm going to do tomorrow. But I just wanted to check and see if you guys have any other suggestions.


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Old 06-06-2008, 08:35 AM   #2
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Re: Radiator Overflows when engine is turned off in 98 GT

could possibly be a blown head gasket, where the piston compresses, the compressed air blows by blown gasket into the cooling system, therefore pressurizing the cooling system... just first thought
hopefully it's the thermostat not operating/stuck or a split radiator end tank


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Old 06-06-2008, 09:40 PM   #3
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Re: Radiator Overflows when engine is turned off in 98 GT

Hey, just wanted to update everyone. I stopped adding coolant a few days ago and it appears to have stopped overflowing. I check the level anytime I see streaks where the coolant came out and although it did overflow this morning when I really got on it after I let it cool down the level was still normal (its not even dropped to the top of the float). I have driven it 100+ miles today and it appears to have totally stopped. I must of just been adding to much coolant for the cap to hold pressure once the engine gets hot after its turned off.

I'll keep an eye on it for the next few days and keep everyone updated. But it seems to only do it at times when I really got on the accelerator and then turn off the engine without driving very far. But if anyone has had something similiar happen from filling the overflow to the very top, plz let me know. I hope thats the only problem, hehe.

Thx Adam
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