Coolant building pressure under cap and causing it to leak from cap

Lawdawg86

Member
Sep 30, 2020
84
12
18
35
Arkansas
So on my 2000 3.8 5spd, I replaced the timing cover gasket, water pump gasket, EGR gasket, and valve cover gasket, the valve cover gaskets, and the main seal was bad and leaking so I changed it too. I flushed the coolant system, while running the air full blast on heat, and even filled it up beforehand with the vent open until the bubbles stopped coming out, closed the vent and then went and started it with the cap off, and revved it to around 2500 and topped it off. Now when I get it warmed up it starts to leak at the top of the cap so I got a new pressure relief one and it’s doing the same. Should I have closed the cap before starting and not let any air get sucked in as I was starting it or what? Sounds like air in the system to me, but who knows. It’s not hot, I’ve checked the oil for water and there is none. I even took an oil sample to verify, and there was no coolant or water. So what’s the deal? The backstory on this car is, I bought it from a mechanic friend of mine that gave it to his daughter who drove it for a while, he’s a well known and great mechanic, but anyways the clutch and quadrant went out. He stopped working on transmissions before this, and her being her went and bought a new Stang. So this one sat for 3-4 years. When I got it I put a new billet quadrant in it, new cable, clutch, fuel pump, and filter. I then got tired of the oil leak on the driveway so I fixed that problem. Bought a new thermostat and gasket today(can’t get it on, been raining all day and it’s 43 degrees outside), so my question is what am I doing wrong here? Was the steps I took wrong on the flush, and fill to get all the air out and it’s causing my radiator to become pressurized? I noticed when it’s running it won’t go over into the overflow tank which is strange, but when I shut it off that loss of pressure sends it to the tank. Seems kind of backwards to me, but this is the first Ford I’ve ever owned so who knows. What’s y’all guess? What should I do or check next?
 
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Lawdawg86

Member
Sep 30, 2020
84
12
18
35
Arkansas
So on my 2000 3.8 5spd, I replaced the timing cover gasket, water pump gasket, EGR gasket, and valve cover gasket, the valve cover gaskets, and the main seal was bad and leaking so I changed it too. I flushed the coolant system, while running the air full blast on heat, and even filled it up beforehand with the vent open until the bubbles stopped coming out, closed the vent and then went and started it with the cap off, and revved it to around 2500 and topped it off. Now when I get it warmed up it starts to leak at the top of the cap so I got a new pressure relief one and it’s doing the same. Should I have closed the cap before starting and not let any air get sucked in as I was starting it or what? Sounds like air in the system to me, but who knows. It’s not hot, I’ve checked the oil for water and there is none. I even took an oil sample to verify, and there was no coolant or water. So what’s the deal? The backstory on this car is, I bought it from a mechanic friend of mine that gave it to his daughter who drove it for a while, he’s a well known and great mechanic, but anyways the clutch and quadrant went out. He stopped working on transmissions before this, and her being her went and bought a new Stang. So this one sat for 3-4 years. When I got it I put a new billet quadrant in it, new cable, clutch, fuel pump, and filter. I then got tired of the oil leak on the driveway so I fixed that problem. Bought a new thermostat and gasket today(can’t get it on, been raining all day and it’s 43 degrees outside), so my question is what am I doing wrong here? Also, I don’t think I’ve ever heard the fan on, except when the AC is on. I’ve thought about wiring it to a hot on key on and letting it stay running to keep it cool all the time. Was the steps I took wrong on the flush, and fill to get all the air out and it’s causing my radiator to become pressurized? I noticed when it’s running it won’t go over into the overflow tank which is strange, but when I shut it off that loss of pressure sends it to the tank. Seems kind of backwards to me, but this is the first Ford I’ve ever owned so who knows. What’s y’all guess? What should I do or check next?
 

Lawdawg86

Member
Sep 30, 2020
84
12
18
35
Arkansas
So I took it to the shop that worked on it previously and they were supposed to look into why it was doing what it was doing, and after some drama between them and a associate unfolded I decided to take it home and diagnose. I haven’t filled the degas bottle to where it’s supposed to be, wasn’t sure that was a big issue, I’ve got a new thermostat I’m fixing to put in it and check. Anyone know if this could be causing the issues? It’s not overheating, the fan hasn’t kicked on, it’s just leaking past through the old and new radiator cap. I’m not familiar with fords or mustangs whatsoever so not sure how v6 mustangs work. I’m a Chevy v8 builder and Diesel engine builder I’ve read the way to refill it after a flush, but do they mean fill it from the vent also, and then once the bubbles or whatever stop coming out of there close the vent, leave cap off, and then turn the car on with heater on high and let it run and top it off that way?
 

OldSchoolNoe

Member
Nov 13, 2019
11
2
13
32
California
If you have about $120 you can get the UView 550000 which makes refilling the cooling system/purging out air ridiculously fast. You also need an air compressor.
I did a vid on this tool, (Skip to 2:56)
View: https://www.youtube.com/watch?v=BdBKOaZN998&t=67s


Or if you want to go economical (but lenghty) you can buy the EPAuto Funnel from Amazon for about $20
Here's a vid I did on it (Skip to 6:20)
View: https://www.youtube.com/watch?v=0IUl7mtZE28&t=260s



These solutions are just to purge air out of the system (Which could be the culprit). There's more causes like incorrect cap, bad radiator, thermostat not opening, thermostat put in backwards, loose radiator hose, and a few others that can be narrowed down.
 

Lawdawg86

Member
Sep 30, 2020
84
12
18
35
Arkansas
Well by me flushing the entire system twice, and then replacing thermostat, opening the air bleeder nut and filling the radiator up till all the bubbles stopped coming out of the bleeder and a steady stream coming out of both radiator and bleeder, I shut the bleeder, went in started the car, it had sucked about a half a gallon back in so I revved it up to about 1500-2000 rpm and poured some more in before filling the cap, after that I never once had a problem again. I sold it, bought a 94 GT Convertible that I’m either gonna fix it up or sell it, I haven’t decided. I paid $500 to an old lady that it was her late husbands car and she had it sitting under a tarp. The front two leather seats are a little cracked in some spots, and paints a little scuffed, but the motor runs strong sounds good and has 134k miles on it