Low pressure radiator cap ok? 7 lbs?

[Nebraskan]

I have a 1996 Buick Park Avenue that I recently changed the radiator cap to 7 lbs from the original. Back in Nebraska my cousin and I always run the lowest pressure cap we could and increased the coolant glycol to make the coolant less prone to boil. Example, at 70/30 mix the coolant will not boil until 276 degrees F at sea level normal atmosphere. So at 7 lbs it has to be much higher than that to boil.

I've read that the higher the pressure is of a closed coolant system, the more efficient it is (faster) to transfer heat away from the engine and heads. Is this true? Doesn't really seem to make any logic. If the coolant is not at 200 degrees and not boiling, what difference does it make as it moves though the radiator and lets air exchange the heat.

We run the lower pressure to make it easier on the heater hoses and radiator hoses. It always seemed to give them more life. Rick has always run all his vehicles on the farm with the lowest pressure cap he can.

Are there any reasons NOT to do this?

 

[MN Driver]

You are making your system more capable of boiling for this following reasons that I can think of:

1. 70% coolant will cool your engine much less efficiently than a 50% mix. Water conducts heat better than glycol.

2. A 7 psi cap will lower that boiling temperature as pressure is a major part of keeping the boiling from happening. The more pressure that a fluid is kept in the higher the temperature can be before boiling.

3. If you do this and your engine gets hot it will sent more coolant in the direction of the reservoir and if the reservoir overfills from the release of coolant because of internal pressure you could lose your coolant and once your engine cools down it will suck the coolant back into the engine and if there is not enough coolant in that reservoir it will suck in air which will make the problem worse.

The 7 PSI cap might be something that you could get away with but I wouldn't run anything over a 50/50 ratio to try to keep temperatures as low as possible inside the engine.


A 70/30 mix is a very coolant-rich mix, I personally wouldn't recommend over 60% unless it was for freezing protection such as below say -40 degrees.

 

[another Todd]

50/50 mix and 7psi cap. Done it for years without problems. Be aware however your cooling system is now capable of boiling over at about 20 degrees lower temp than with a 15 psi cap. If your radiater is marginal you may need that extra psi to keep it from boiling.

 

[punisher]

As it was explained to me by an old racer/engine builder who likely has forgotten more than I will ever know:

It is not uncommon for localized "hot spots" to form in coolant passages. Coolant flow is imperfect, eddies and low pressure areas form when coolant velocity is too high (a la Bernoulli), likewise temps go up where coolant flow stalls for whatever reason in a head.

As soon as you have any/combination of the above conditions that causes a coolant to go over it's boiling point you get vapor pockets. Vapor doesn't cool. What starts out as a small hotspot without coolant continues to grow as the temp rises and eventually you have a failure.
Add to that higher pressures cut down water pump cavitation dramatically.

Likely in a street driven car, nothing bad will happen. You might have a modest reduction in cooling efficiency that won't be noticed. If you have a race car, with as small/light a radiator as possible, running pure water with some "Water Wetter" in it, then things change.

This old racer ran 28 PSI reliefs on his coolant tanks which translated to ~250F boiling point. Many professional classes run in excess of 50-60 PSI.

 

[Nebraskan]

Interesting replies. For what it's worth, on my 1996 Buick PA, the temp gage never seems to show much more than 180 F. I've never seen it close to the 200 degree mark, even with the AC on and it's been pretty hot our here in SW Michigan, in the upper 80's and low 90s.

So the trade off is low pressure, less stress on hoses, but more cause of castration in the pump and perhaps some dead spots in the coolant line. More pressure, and hoses have to be "top" condition, else risk rupture. However, the temp of boiling is higher and the flow slower (? if this is right).

 

[alreadygone]

Hoses are so much better than in the good old days (whenever that was) I almost never see a swelled or ruptured hose anymore. Matter of fact, don't remember the last one!

Just how long do you Want them to last?

Bob

 

[mechtech]

You have reduced the margin for error for boilover.
Your incorrect mix is hurting, not helping, and the lower pressure makes it worse.
However, your car may run cool and not need the extra margin that the proper mix and pressure would provide.
In other words, you may get lucky.

 

[surfstar]

I just replaced a heater hose that swelled/bulged and had a tiny leak - the hose was the original from 1993 with 196k miles on it.

Use the correct cap. The Auto engineers went to school for a reason.

 

[Nebraskan]

Out here in Michigan I have a 50/50 mix of glycol. Back in Nebraska we get to -45 F sometimes and it was somewhat prudent to have at LEAST 50/50 mixture if not more glycol for the winter.

Our 1993 Chevy Lumina developed a leak in the heater hose when I was out to Nebraska this early June on vacation. I replaced the hose fairly easily, but it was on a 7 lb cap on the Lumina. So I don't think the hoses are made better since "the good ol' days".

New hoses do hold up well on higher pressures, so caps of 18 lbs or better don't seem to bother them. Engineers alway choose the maximum zone of compliance. An engineer would never put a 7 lb cap on a new car. Why? It' give him(them) greater chance that the new owner will not have difficulties even if the water pump doesn't work as well, or if in the near future the new owner changes mixture and/or ratios. 18 lbs is a safety net for the engineers. Makes a difference where the car is going to be driven as well. Who would want to run a 7 lb cap on a car in southern Texas? But in upper Michigan, or in other cooler climates it might work, but the engineer doesn't "know" where the car will end up and therefore designs for the extreme.

So, yes I might be going the wrong way, but the pros and cons on this are intersting.

 

[alreadygone]

When were the hoses last changed on a '93 Lumina? Or are they the original 13-14 year old OEM hoses?

Bob

 

[Nebraskan]

We bought the Lumina used a few years ago, around 130,000 miles (or so), so I couldn't swear for sure if they had been replaced or are original.

My cousin Rick has original heater hoses on his 1955 Ford pickup though.

 

[m2200b]

You're going the wrong way. Either stay stock pressure cap or go up to 18 Lbs or higher to make the system work better. Prestone might like your mix percentage but your Buick won't.

 

[John78]

I know this is an really old thread but I have been wondering about this for years. My dad always told me that coolant systems really didn't need the pressurization caps and that if you ever have a leak, you could always relieve the pressure on the system and it leaks a lot slower. Plus, it will be easier on the system.

Well, about 10 years ago I had a leaky radiator on a Jeep Wrangler. It is the Aluminium/plastic crimped type. Instead of replacing it, I just took out the inner rubber seal on the cap. Well it has never overheated and I just add about 1/2 a gallon of antifreeze at every year and it works fine. Its not boil-off just the leaky radiator. Yeah I know I need to just replace it but the question is, is this a safe practice. As a test I went ahead and did it to a 2004 sienna, 1990 Maxima and a couple others. All did fine and never overheated and never boiled over including the Jeep towing a 20ft trailer across the country in the summer with the a/c on.

Somebody told me about the hotspot theory so I went ahead and put a new cap on the sienna but the maxima and jeep have been running with no pressure for 10 years. I can't pressurize the Jeep until I get a radiator on it so this has turned into a long term experiment.

Would anyone like to comment on this practice and bet on the negative long term effects. I have not torn down any of the engines but the Maxima is probably the closest with 225K miles.

John

 

[Yah-Tah-Hey]

The thing you have to remember about Nebraska is that there is not much to do except watch the corn and beans grow and ponder Nebraska football without out that red faced, screaming idiot of a coach we had the last seasons. With that in mind, I understand how pondering one's cooling system pressure can be big doings down on the farm. You'll figure it out Nebraskan--or not.