Whats up guys,

Haven't had a properly pressurized cooling system in forever now. I have 87 s4 n/a. Typically has no pressure you can squeeze the hose off of tstat housing easily, sometimes it will be pressurized though and idk why. Pictured below, my cooling system has the flat tstat cap on the tstat housing and the pressurzied(the bulky one) cap with the line to the overflow tank on top of the radiator. One thing I assume is a major problem is that whenever I remove the tstat housing cap the coolant will drain into the overflow tank, which is bad obviously. So I suppose my question is why would the coolant be draining into the overflow with the cap is off. I replaced both caps and that didn't work. Both bought from Atkins. I am confident that I have NO other leaks or places that coolant gets in or out besides into the overflow tank. Damn near about to spend all the money I have the get a koyorad just so I dont have to deal with this stock pressure system.

Thanks, Stephen

 

You might have a bad thermostat. If the thermostat is stuck in the open position, than your coolant will never reach normal operating temperature. If your coolant never reaches normal operating temperature, than your cooling system may not reach the normal operating pressure.

 

Feel like its probably not the tstat. Over the last years that has been replaced like 7 times. Also I did get an aux coolant temp sensor so I could verify the temperature of the coolant, it hits 200f or higher depending on ambient temp. It expands, it just expands into the overflow tank before there is time to build pressure. idk unless there is backpressure from the expansion tank cap, i think I just need to get https://www.atkinsrotary.com/store/8...6-15-17YA.html.

 

Although, since I will need to drain coolant to do that, I will probably test tstat at the same time

 

Update:

Went for a drive and when I got back I used a vice grip to pinch the hose to overflow tank so that I would be sure that no coolant was leaking out of the system. Just opened the tstat housing cap and waited to see what would happen. Well when I first opened it was maybe an inch below the top of the cap. However as I revved the motor, the coolant level would suck down and then never rise back to the same level. I just kept refilling the coolant and revving the engine for damn near 20-30 minutes. Havent driven the car yet, but for anyone having the same issue, give this a try.

 

Literally just updating this in case anyone has a similar problem. I know I never found a threat with problems like mine...

The above post was the solution, just keep bleeding it with the upper tstat cap off, it will take a long *** time because your replacing small *** air bubbles, but I took it out for a drive last night and under all situations where it used to heat soak it now holds temp.

Steps taken
1. replace tstat housing cap, make sure its a flat cap.
2. replace the radiator cap, the pressurized cap (for the coolant overflow)
3. Run car up to temp, go for a drive so you know the coolant is where it should be. Use vice grip on overflow hose so that no water drains out of the system.
4. Set the heat to max, so that coolant fills the heater core and heater system.
5. Open tstat housing cap and fill to top. Using the throttle cable behind the intake, rev the engine for 5 second intervals at a rate where all the coolant gets sucked into the motor and then slowly reduce the revs so that the coolant doesn't explode out and into your face. Keep doing this until the car is full of coolant, it took me over an hour to reach this point.
6. Go for a ride and enjoy those sweet rotary noises.