Engine Cooling reservoir questions


Active Member
May 26, 2020
Hello everyone!

Thank you everyone for all your help in the past. I cant tell you how much I've learned from here!

I had a cooling issue a few weeks ago. Temp would be a little more than half way on the stock gauge, then go up kind of quickly to the next line then back down and back up all while driving.. It never overheated but it was just running too hot for my liking and the engine just felt excessively hot. I was running a 195* stat , a Champion 2 core alum rad and stock fan. So I decided to replace everything! This passed week I replace the thermostat to a MotoRad UltraStat. Stainless steel 192* that came with a jiggle pin, Changed out the water pump, installed a HD fan clutch and replaced the rad cap to 16lb.. The cap that came with the Champion rad was 13lb according to the manufacturer. I'm running 50/50 premix.. I burped the system using the funnel system.

I ran the car yesterday (idle and revving only) and all the new parts seemed to do the trick. The temp climbed slowly to juuust under the halfway mark and stayed there. The upper rad hose became hot and pressurized so I know the stat opened. The HD fan clutch is definitely working and moving way more air than before.

My question and possible issue is: When the car was up to temp which was just at the half way mark or a little under the half way mark, the upper hose was pressurized for sure and hot, the car idling for about 20 minutes or so. Nothing went into the reservoir tank. The coolant just stayed on the cold line the entire time. The engine itself definitely now had a cooler feel to it than before I changed all the parts. The hvac heater puts out hot air too. It is ok or normal for the coolant not to expand into the res? I know this means the cap never opened due to it not reaching 16lbs of pressure? Now when I turned off the car and let it cool, I eventually noticed the radiator sucking some coolant out of the res but not much..

Is this normal or should I be concerned? Should I put the 13lb cap back on?

Thanks for all your help and ask any questions :)
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The difference between 13psi and 16psi is that the 16psi will allow for a higher boiling temp of the coolant. A 50/50 blend of coolant/water will boil at approximately 220 degrees. This will go up approx 2-3 degrees for every pound of pressure. So a 13# cap would boil around 250ish. A 16# cap will be around 260 degrees before it boils.

As your cooling system pressure builds, the excess pressure would be bled off in the form of excess coolant flowing into the overflow. Typically you should see the overflow rise 2-3" but that all depends on the volume of the overflow.

Since most Mustang guys break out in a sweat the minute they get over 180 degrees, those upper limits will likely not be seen, so a 13# cap will put less stress on your cooling system.

Given that NO coolant passed the 16# cap, and the hose seemed excessively swelled, i would suspect the cap is bad and try the 13# cap instead.
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Hey 5L5 - you are 100% correct on the excessive swelling and no coolant passing the 16# cap. The cap that came with the Champion Rad had no indications of poundage on it so I tried using the stock recommendation of 16#.. I just picked up a new 13# cap which was for a 1996 F150 5.0 Fit perfectly.
I'll give you the verdict shortly.
Ok I was able to take the car out this weekend. With all the changed parts that were done above, the car seemed to stay cool.. Drove for about 30 minutes. Took it on the highway, some side roads and a bunch of lights.. The stock temp gauge would stay a little under the half way mark most of the time. There were times the temp would creep up just past the half mark but then drop back down. Nothing like before though. I just assumed this was the stat opening and closing.

I ran a 13lb cap like suggested.. The res never moved off the cold mark again.. The upper hose was pressurized but nothing drastic. Could this be a normal thing? What I mean is, even with a 13lb cap, could it just be the cooling system is running efficiently and doesn't need to open the cap? This is not a stock radiator. There is a 2 core aluminum rad, 192 ultrastat and HD fan clutch in there. I'm just guessing that if the car "needs" to release pressure it will and if it doesn't, it won't?
After cool down I did notice the rad sucked a little coolant out of the res again. like an inch maybe. Could It be possible that I still have some air in the system? The stat does have a jiggle pin in the 12 o'clock position.

What does everyone think?
Without a aftermarket gauge with numbers one the face to point to can't make a comparison, but based on what you say now I'd be comfortable sending it.
My comparing stock to aftermarket gauge readings found that the stock gauge seemed to move a lot for a small temp rise.
During normal operation, the hose will be pressurized. It should max it around 13psi before bleeding off some coolant into the overflow to avoid over pressurizing. That is normal.
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During normal operation, the hose will be pressurized. It should max it around 13psi before bleeding off some coolant into the overflow to avoid over pressurizing. That is normal.
So if there is no bleed off at running temp, hose pressurized, but there is some coolant recovery at cool down, does this mean there is more air to bleed?
Usually if there is air in the system, that will gather up at the top need the cap and will be the first to bleed out. But, as the system cools, it will draw coolant back in.

So after filling the system, you want to check the overflow a couple times and top off each heat cycle until the levels stay consistent
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Took it for a long ride today. Stock temp gauge stays basically right at center or a little under. At lights, it goes up half way to the next line from center but comes right back down. once i was home i checked the res for as rise in coolant. it went up probably about and inch? once cooled it came down that inch.

Is that normal for such little movement? I know i’m running a larger than stock rad and HD fan clutch but just want to make sure all is ok. after doing all the cooling upgrades and replacements, i have noticed the engine bay itself is cooler than i remember. like the ambient air and just the engine components are cooler.

what do you guys think?
The actual distance depends on the volumn of the resevior. The actual act of coolant rising and falling is normal
Im using the stock reservoir. it’s hard to see in this pic but when it’s cold it’s under the cold line. at temp it’s right above the word cold. it never gets any higher than that.


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The marks are really meant for your extreme limits. In other words, cold engine should be above the bottom line to avoid sucking in air. When hot, it should be below the top line to avoid puking some out. Really movement anywhere between the two lines is fine provided your engine temp is fine.
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