Engine What Radiator Cap?

Discussion in '1994 - 1995 Specific Tech' started by 95Vert383AOD, Oct 25, 2012.

  1. I'm almost certain i burped every bit of coolant out of the motor after changing the intake gaskets.

    So here's the issue. If parked the car will stay dead on 180. (where its supposed to be) If i start driving it will creep up to around 210. Even on the highway. I have a Stant vented radiator cap. 16psi. I lift the vent arm with the car reading 210 and nothing. I unscrew the cap and no pressure. I'm not losing coolant. So i figure my Stant took a dive.


    I've seen Motorad and Stant vented caps. I'm not excited about the stant cap using plastic inside the cap. I'm not sure if the valving on the Motorad is plastic or metal. I'm open to other brands but i like the venting feature.


    I did some googling on this but wanted to know what you guys are doing. They say 16psi caps were introduced when higher temp thermostats were introduced for emissions purposes. I use a 180 degree thermostat and my SCT chip is programmed to kick the fan on at 180. The argument for a lower PSI cap is supposed to be that the higher pressure puts higher pressure on everything from hoses to gaskets and if you don't run above 180 then a 13psi is better.

    I just wanted to see what you guys say. Recommendations welcome.

  2. The cap pressure is just the relief pressure value. If the coolant isn't being heated to the point where the pressure is 16 psi nothing happens. The coolant just circulates through the system somewhere between 1 atmosphere and 16 psi. Now back to your observation. If the coolant temp is 210*F there would be pressure and it would be noticeable when the valve is released or the cap is being removed. You can't have one and not the other. What were you using to measure the 210*F?
  3. I have an electric autometer gauge with a sensor at the drivers side front of the intake lower. I know it works fine because the fan kicks on at 180 as its been doing always. I also confirmed the temps with a laser thermometer. I know it sounds crazy but the motor will get just under the 210 mark and will have no pressure. revving it with the cap off does show the coolant is moving but at idle with the cap off it never overflows.

    BTW the Boiling point of water is 212....and have Mark viii fan on the radiator itself. That's probably why its not boiling.
  4. For the Stant to do its job, the entire coolant system needs to be able to hold at least 16psi. If you have a screw clamp that's not tight enough and 0nly holds say, 12psi then the coolant can't reach the proper pressure that the system was designed for. I had this problem with a GT I've since sold. I bought a pressure tester and played "whack-a-leak" for a while until the system held a steady 16psi.

    Basically, coolant held under pressure (isn't allowed to expand) will hold more heat per degree of temperature. If you release the pressure, the temperature goes up even though the actual energy absorbed by the coolant hasn't changed. Same principle behind boiling water with a lid on the pot - the water won't visibly boil until you remove the lid.

    tl;dr if your coolant system can't hold 16psi, the coolant will heat up faster then it should. Pressure test it and get back to us.

    So my Stant Lev-R-Vent looked suspect. The metal disk that sits in the center was loose and hung down about 3/4"....I got a new cap where the metal disk sits alot closer to the rubber. I drove about 5-7 miles and the temp stayed a hair past 180 but didn't climb to 210 like before. Hopefully this fixed it.
  6. That's only at 1 atmosphere at sea level. Lower at higher elevations and under vacuum and higher under pressure. It's the under pressure that makes it almost explosive like when the cap is removed and the coolant temperature is above 212.
  7. Yes i know but the system wouldn't go under pressure.