Radiator Cap Tech.

[mgmshar]

I just got a very valuable lesson in radiator cap technology that I thought I would pass along. Turns out these things are more technical then I thought.

There are two types of radiator caps for cooling systems that use an overflow bottle (like ours). There are "partial pressure" (a.k.a. "vented") caps, and "full pressure" caps. The difference lies in the little one-way vent valve that is used to allow coolant back into the radiator as the vehicle cools. On a partial pressure cap, the vent valve is NOT spring loaded to close. If you pick-up a partial-pressure cap and shake it a little bit, the vent will flop open. On a full pressure cap, the vent is spring loaded to stay shut with a light spring.

So, what's the difference? According to the Chrysler cooling engineer with whom I spoke, the partial pressure cap is used for one reason - to allow air to more easily escape from the cooling system during the first few times the engine is run after a coolant refill. If you think about it, when the radiator has air in it, the air can escape through the "flopped open" vent valve on a partial pressure cap. The idea is that once the radiator is full, and coolant gets to the vent valve, the vent valve will snap shut. This type of characterisitic is good if you mass produce vehicles, and you don't have the time to properly fill the radiator completely.

HOWEVER, Chrysler's test data has shown that the partial pressure caps do NOT always allow the system to adequately pressurize. If the coolant is exiting slowly from the cap as it expands, then the vent valve may not close properly. Therefore, the coolant will just go to the overflow bottle, and the system will not pressurize.

Case in point: The radiator cap that is specified for our cars is a PARTIAL PRESSURE CAP. If you look at the vent valve on the bottom of the cap, you will see that it is not spring loaded. I have had four of these caps on my car (Stant and CST brands). Most of the time, once the car was warmed-up, the cooling system was NOT pressurized. Not having a cooling system that is pressurized can cause several issues, including water pump cavitation, increased boiling in hot spots around the head, and increased risk of boil-over. I have just replaced the partial pressure cap on my car with a full pressure cap (vent valve is spring loaded closed). Upon the next start-up, the cooling system pressurized very nicely, which is the first time that I've seen it pressurized in months.

According to the cooling system engineer, the ONLY drawback to a full pressure cap (with the spring loaded vent valve) is that you need to be careful when you fill your cooling system. You must fill it completely with a minimal amount of air. A full pressure cap WILL eventually purge all of the air from the cooling system, but it will take many more warm-up cycles to do it. Since most of us fill our cooling systems very carefully, there is no reason NOT to run the full pressure cap.

The CST lever-style radiator cap (specified partial pressure) for our cars will show "ST-16V" on the cap. I would highly recommed that you don't use this cap. Instead, I would recommend that you use the cap with the part number "ST-16" on it. That is the full pressure cap equivalent (16 psi) that fits our cars. As far as the CST line goes, the "V" in the part number on the cap signifies a partial pressure cap. As far as the Stant lever-style line, I would recommend that you use the 10330 or 11330 cap, rather than the 10331 or 11331. I would post the numbers for the non-lever style caps, but I can't find the numbers on Stant's web site.

Hope this helps...

 

[salvageV6]

Anything wrong with the cap that came with the car from the factory?

 

[waell]

That's good info thanks. I did a search on caps today and this is just what I was looking for. So where is a good place the get a cap. Local auto stores???

 

[Nick Micale]

Originally posted by salvageV6
Anything wrong with the cap that came with the car from the factory?

We have always used the Delco cap when replacing one on any GM car.

 

[chevyII]

cooling 101

The patial vent or full vent cap is for old style cooling systems that you didn't totaly fill to the top. As the engine warmed the water level rises and finds its level at this point you can add coolant to fill the radiator. The overflow will be left empty. The other style is a full presssure or non vented. This cap will push out coolant into the overflow when needed but after shut down or cool down the cap actually creates a suction effect on the overflow and refills the radiator. Also the stock cap is 15lbs cap,every pound is worth 3 degrees of temp raisng the boiling point.
water boils 212,+45= 257 degrees berfore the water can boil and create air pockets in the block. The vented cap is 212 becaues its vented. Now you dont run a engine at 212 but you can get there pretty fast if the pump stop turning and the water boils thair rise in the block and thats were the coolant sensor reads from and hot air doesnt give a true reading till its to late. I have lost an 383 chevy this way overheated so bad it keeped running without ignition,scaped it . Run a 15-18 Lbs cap large truck caps also will work for higher pressure 20 max,and I recomend putting the sensor in the head instead of the intake to keep it covered in coolant at all times.

 

[Nick Micale]

Re: cooling 101

Originally posted by chevyII
............ . Run a 15-18 Lbs cap large truck caps also will work for higher pressure 20 max,and I recomend putting the sensor in the head instead of the intake to keep it covered in coolant at all times.

You need to remember this is a "turbo Buick" tech forum, not a Chevy one.

There is no provision to install a temp sender in a Buick head. If it is a 109 block, it can go in the block drain hole below the head.

As Salvage and I said, the stock cap, or its equivalent replacement work fine even in our 110+ deg. temps. If it does not do the job properly, there is something serious wrong.

 

[FlyinGN]

hmm, good info but I don't ever recall seeing a cap without a spring.. ??
Anyone have a closeup pic of one??

 

[chevyII]

The pressure cap will have the pressure rating and or a metal ring on the bottom. Nick all I was try to say was dont put the sensor in the highest part of the engine. no wonder I couldn't find any chevy parts,buicks....hummm

 

[mgmshar]

Nick, my cooling system will hold pressure with a cooling system tester, and the cooling system would build-up pressure every time if I left the cooling system tester installed and started the engine. So, I am very sure that the caps I was using were the root cause of my lack of cooling system pressurization. I never tried a GM cap, so maybe that's the issue. Are the GM caps partial pressure or full pressure type?

See link below for a photo and description of the difference...

RADIATOR CAP LINK

Hopefully the link above will clarify some things...

 

[tom h]

interesting ...

I just looked at a stock replacement, Delco RC27 cap have.

The vent valve is NOT spring loaded to close, so it's a partial pressure/vented style cap.

 

[Two Lane]

Does the plastic-end-tank F-bod rad require a particular cap?

TIA!!

 

[mgmshar]

I just looked at a stock replacement, Delco RC27 cap have. The vent valve is NOT spring loaded to close, so it's a partial pressure/vented style cap.

...which makes sense. I am sure the original factory-installed cap was a partial pressure cap. Since it was used on the car when it was originally built, it makes sense to just keep it the same in the replacement part world.

Again, the only purpose of the partial pressure cap is to more easily allow air to escape from the cooling system after it has been filled, which is why many OEM's use them. If you do a good job filling your own cooling system, then you don't need it. And yes, a full pressure cap will eventually purge all of the air. It will just take longer - more warm-up cycles. I would rather have consistent system pressurization, but that's just me.

Am I the only one who has seen their cooling system NOT be pressurized when at operating temperature? A simply squeeze test of the upper hose will tell you if your system is pressurized or not. Maybe my system is just weird.

 

[FlyinGN]

Thanks for the info. Never knew that there was a difference..