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Coolant Cap seal hack.

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Murdertalon

Proven Member
396
82
Jul 7, 2016
East side, Washington
Would like some input on this, how you feel about this AND where this post should be. My coolant cap was not holding pressure properly I played with seeing how well the hoses once the engine was hot were holding or not holding pressure.

Turns out my engine running hot may have had less to do with my radiator (what I thought was the issue) and more to do with not enough pressure being held by the coolant cap. The regular oem coolant cap part seems to be the weak point in the cooling system of the 4g63. It doesn't seem to apply much force or really seal up incredibly well.

Grab your upper coolant line cold and see if its holding pressure or just pushing fluid/air around. Next drive your car till at operating temp, grab upper coolant line and see if its holding a little pressure or a lot. If your hose feels like its free to move cold and just sorta kinda pressured while hot..

This is how I solved the issue. I cut one of these master cylinder gaskets to form a new rubber gasket for my coolant cap. This gasket is THICK you have to use some force to get the cap on properly, this solved all of my temp issues.

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This is a picture of the parts used, I already made another one and its in my car. The engine temps went from 2/3 - 3/4 range down to below 1/2 on idle, full load one bar or notch below the image of the thermostat in the temp gauge.

This may help other DSM'ers I was wondering where this needs to be posted or if anyone wants to give feedback.

PS: I had already modded drivers side fan to always come on with ignition AND put a heat shield on my turbo to combat heat soak issues. It wasn't enough for me.
 
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just get a 15psi cap and make sure the seat is clean. rust and stuff likes to build up there because of the way coolant flows. these cooling systems should have came with a 15psi cap stock. (i run a 15psi cap, stock is 13psi)

you basically added some preload, accomplishing the same thing.
 
The same thing happened to me I bought two radiator caps one from advanced and one from auto zone. I ended up calling Mitsubishi and I was told those aftermarket radiator caps were junk so I decided to go to the dealer to buy the cap and they were right. The oem cap is alot better than the ones you find in your local auto parts store. The cap from Mitsubishi fixed my problem.
 
just get a 15psi cap and make sure the seat is clean. rust and stuff likes to build up there because of the way coolant flows. these cooling systems should have came with a 15psi cap stock. (i run a 15psi cap, stock is 13psi)

you basically added some preload, accomplishing the same thing.
It's not the pressure rating, the spring controls that. It's the flimsy 1mm rubber gasket that's on the cap itself. It doesn't seal anywhere near properly on my dsm.
 
The same thing happened to me I bought two radiator caps one from advanced and one from auto zone. I ended up calling Mitsubishi and I was told those aftermarket radiator caps were junk so I decided to go to the dealer to buy the cap and they were right. The oem cap is alot better than the ones you find in your local auto parts store. The cap from Mitsubishi fixed my problem.

Thanks I'll have to look at a mitsubishi one, the one at autozone is listed as the oem one but quite possibly it's inferior.
 
Year ago I had heat issues and notice the factory caps were much better the the parts store crap. I won't use anything but the factory caps from Mitsubishi. There are exceptions but pretty much applies to anything you want to last when buying parts for these cars.
 
Were any of you pushing coolant to the overflow (while at operating temperature) before this?
 
No. I never had any coolant pushed into the overflow bottle. I was losing coolant slowly, would run hot, etc. got a Stanton cap from AZ & would still run hot. Waited for the dealer to open, bought a Mitsubishi cap & problem was fixed.
 
No. I never had any coolant pushed into the overflow bottle. I was losing coolant slowly, would run hot, etc. got a Stanton cap from AZ & would still run hot. Waited for the dealer to open, bought a Mitsubishi cap & problem was fixed.

Yep basically same mate, my dsm wasn't pushing to the overflow just running hot (in the upper 4/6 temp range) I would shut it down if it got hotter above that I definitely saw about 5/6 ish under load cruising up hill and it worried me.

Last year I had a boil over or two (that did push to the overflow) and I had to replace a head gasket because of it.
 
get a 15psi cap. the extra psi will help heat transfer and the motor will run cooler. this is a common racing trick, bumping the cooling system PSI aids in cooling the engine. EVERY DSM on the planet should be using a 15psi cap.

if you are extreme road course racing and have head studs and a built motor i might even run a 17psi system.
 
Can someone please explain to me how your radiator cap PSI improves the efficiency of the cooling system when it is not actively releasing the pressure?

All of this sounds like malarkey and there are definitely variables that you guys aren't properly acknowledging. In fact, you are defying physics!

The radiator cap is sprung to maintain pressure a set pressure in the cooling system. As coolant temps rise, the liquid (or in a not properly burped system; air) the coolant (and/or air) expands. This creates pressure. Once the pressure of the system has reached the rated pressure of an accurately rated cap, no matter if it is rated 13psi, 15 or 17 psi, the cap will open and release pressure until it brings the cooling system pressure back into the caps sprung pressure range.

If the cooling system reaches 13 psi on a 13psi cap, the cap will open and allow the additional pressure above 13 psi to bleed off. Once the cap reduces the system pressure to 13 psi, it closes and maintains that pressure until the system once again over pressurizes.

The cap does this by expelling coolant.

Now here's the great part and why you guys are defying physics...

IF YOU WEREN'T EXPELLING COOLANT, THE CAP WAS ADEQUATELY MAINTAINING THE SYSTEM PRESSURE JUST FINE.

By further increasing the pressure of a adequately working system, you aren't augmenting the coolants ability to absorb more heat. You most certainly aren't doing anything to improve the radiators ability to radiate heat away from the coolant.

The thermostats job is to bring the engine up to operating temperature and to maintain it. From there, your coolant temperature is based on the systems ability to transfer that heat away. This is done by the coolant absorbing the heat and a heat exchanger (the radiator) to radiate it back out of the coolant. This is why in time your car runs in the 210° area on a stock radiator despite plopping in a 170°, 180° or 190° thermostat. The cooling system is not regulated up top by the thermostat, it is regulated by the speed in which the coolant can absorb heat and the radiator can remove it.

Since a replacing a properly working radiator cap with another unit, or changing the pressure of the system neither changes the ability for the coolant to absorb heat better, or the systems ability to radiate the heat away, then it does NOTHING for your coolant temperatures.

Now, I will say that the boiling point of the coolant is increased when you increase the pressure of the system. By raising the boiling point, you increase the vapor pressure and allow for more coolant to contact the coolant passages versus the passages being contacted by voids left by non-coolant being knocked out of homeostasis. Pure water will boil at 212° at atmospheric pressure (at sea level). Your cooling system is running at atmospheric pressure (~14.6 psi @ sea level) PLUS the pressure of your radiator cap. AP@see level +13psi cap = 27.6psi. That puts pure water boiling in your cooling system at roughly 246° F. That's when the vapor pressure is met and your cooling system begins to loose efficiency due to air molecules being knocked out of homeostasis. Temperatures will certainly get that warm in pockets throughout the coolant passages and heat can not be removed because those pockets are in contact with vapor and not PURE WATER. HOWEVER, adding a 15 psi cap will raise that boiling point OF PURE WATER to 250°. Considering the heat of combustion is far greater then 250°, the change in a +2 PSI radiator cap is darn near insignificant. Additionally, this was for pure water, not antifreeze/coolant which has an even higher boiling point then pure water.

To be very blunt at this point- the ability of the cooling system to operate at a higher pressure is highly insignificant. In actuality, your heat exchanger is not radiating the heat away nearly as well as it should. A higher pressure cap is the equivalent of applying a cotton swab to a head wound caused by a M1 Abrams tank.

Again, I brought up that other variables had not been taken into consideration and I reflect once more on the fact that your prior radiator caps were holding system pressure due to the fact that they were not releasing coolant into the overflow jug. If they had to reduce pressure in the system, you'd have coolant pushed out. Now, if your system was not properly burped (which is likely the case given the level of technical understanding assumed by this thread), your cooling system efficiency was likely improved by the fact that you added coolant.

I am all for installing a higher pressure radiator cap to gain that very very slight edge and improve the resolution of time the cap survives as the spring weakens with age, but it is not a solution to overheating given the insignificance (~4°f difference for pure water between a 13 and 15 psi cap) in temperature that the coolant can sustain before boiling and forming vapor pockets. If you are running too hot, fix the ability for the system to exchange heat- get a better radiator or improve your current radiators efficiency as a heat exchanger by improving the airflow across the core. In a perfect world, if your thermostat is not working it's ass off to keep the cooling system at the thermostats rated temperature, your radiator lacks efficiency. Perfection is when the cooling system is so efficient it can radiate heat away to a degree that is lower then the thermostats rated temperature, and either completely closing the thermostat (too efficient) or partially closing to reduce flow (perfection). In that environment, a factory 190* thermostat is optimal.

You all need to upgrade the efficiency of your radiators.
 
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Can someone please explain to me how your radiator cap PSI improves the efficiency of the cooling system when it is not actively releasing the pressure?

You all need to upgrade the efficiency of your radiators.

Hey maybe I didn't explain this properly.. This hack does not change your psi hold setting. All it does is seal your cap better to allow proper function of the coolant system.

I didn't modify the spring rate, this is literally making a better seal of the upper seal on the cap only. The autozone caps don't actually hold pressure correctly to even allow 10psi to build up and no coolant will come out it will just slowly seep evaporate.

My purge spring rate will still purge the system at whatever autozone deems nesisary, probably 13psi idk, the point is that the water before wasn't absorbing the heat because it wasn't under proper pressure and your coolant does a better job under pressure (stock factory pressure) water boils at a higher point under pressure.

I know it was cavitation or doing something weird because my heater was barley working before the seal mod.

I can't make this clearer but my system was maybe holding 5psi before* maybe probably not even that, yes the water was still circulating yes it was still cooling better than not being there but it was not working properly. It was not holding more than 2-3 maybe 5? psi it is kinda crazy that I didn't have coolant leaking and I can't tell you exactly why other than I had steam evap going on.
 
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technically true, its not exactly what is happening here.

think of the cap as a BOV for your coolant system, and we know that the OEM 2g BOV basically leaks at stock boost levels because it was designed to open very easily and had no headroom built in. the radiator cap does the same thing, it is a 13psi cap on top of a 13psi system so it is like having a pin hole leak all the time and it causes the efficiency to be bad.

unknown to the kid that posted this, adding the thick gasket did make a better sealing area sure, but it also preloaded the 13psi cap and made it function like crushing a 1g BOV. it holds more psi now.

so, adding a 15psi cap does only gain a tiny bit of cooling capacity in theory, sure. BUT in reality it will net a fairly large cooling improvement due to not allowing the 13psi system to continually crack open, this will prevent a ton of small hard to track down deficiencies in an already average cooling system.

IDENTICAL to how replacing the 2g BOV with even a nearly identical looking evo BOV will gain some performance and more importantly, allow some headroom for that 120 degree day stuck in traffic.

and the stock radiator is actually good enough for anything this car could do. a more efficient radiator will make the deficient cap less noticeable, the cap will still bleed pressure when it should HOLD and make even a high end radiator seem to only be adequate.

how in the wide world of automotive has this issue not been beaten to death on this forum and the common ground is only "get a better radiator" crap kids this has been covered on basically every other automotive forum on the web. a ton of vehicles have this same issue and the fix is "3.99$ for a different cap, or replace the radiator if you want to look cool"
 
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technically true, its not exactly what is happening here.think of the cap as a BOV for your coolant system, and we know that the OEM 2g BOV basically leaks at stock boost levels because it was designed to open very easily and had no headroom built in. the radiator cap does the same thing, it is a 13psi cap on top of a 13psi system so it is like having a pin hole leak all the time and it causes the efficiency to be bad.unknown to the kid that posted this, adding the thick gasket did make a better sealing area sure, but it also preloaded the 13psi cap and made it function like crushing a 1g BOV. it holds more psi now. how in the wide world of automotive has this issue not been beaten to death on this forum and the common ground is only "get a better radiator" crap kids this has been covered on basically every other automotive forum on the web. a ton of vehicles have this same issue and the fix is "3.99$ for a different cap, or replace the radiator if you want to look cool"

I think we are somewhat on the same page, I didn't think that it would add pressure to the system per se over what it should hold properly functioning, it might do that.. I didn't design the cap. The reasoning for not thinking that it would add additional pressure is that I didn't modify the spring rate or anything so if it exceeds whatever the spring holds it will just dump into the overflow. It may actually build system pressure over what stock could be if that cap is more of a pressure regulator on the top than I thought. My thoughts were the cap just holds X pressure and releases into dump tank if exceeds. This whole situation is probably brought about by my not using direct mitsubishi replacement cap/ **and the housing the cap seal mates to on my dsm has a chip in the mating surface. The normal seal on these caps seemed to seal around it anyway but I was worried about leakage. I should post a picture of that later too. My temps are so much better now and my heat works now! That makes me excited because before I would only get mild warm air out and just so you know I didn't add coolant to my system I literally just popped this seal into the cap and it likes it.

I should tap a pressure gauge into my coolant system because now I am curious what it runs at. I will say though the upper radiator hose was too easy to squeeze before once the car was warm. It is also winter so I dont doubt that the temps I was seeing now would have translated to in summer would have been pegging my temp gauge to hot. I did experience an overheating condition last year which required a head gasket replacement to remedy. Maybe it was my coolant cap all along that caused my engine to over heat and warp my head causing loss of pressure in 2 cylinders last year.

I posted this because if your engine is running hot blooded like mine was maybe its these inferior radiator caps and it could cause you much pain and loss of time and money.
 
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I do this with the cheapo parts store caps for my 1g and mighty max. I use a rubber flat washer as an extra gasket. You can get them from the plumbing section or where they have the drawers of bolts in hardware at lowers/home depot. They are only like a buck and fit right out of the package. You just need to make sure its not too thick or else the system will hold too much pressure and find an alternate route to relieve itself.
 
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