Radiator Reservoir Cap question

Topic: Radiator Reservoir Cap question (Read 1722 times) previous topic - next topic

Radiator Reservoir Cap question

July 05, 2012, 07:35:29 pm

a last little detail...

Since we have had our coach, it dribbles coolant out the reservoir cap when it is hot. Not a lot, just drips. How is the cap supposed to work?

I see that the cap has a rubber seal for the inside of the reservoir fitting (by reservoir, I mean the black metal tank with the sight glass at the bottom edge). I guess that when the pressure is low (cool) that seals the reservoir. I also guess that when it gets hot, that seal lifts and allows the coolant go through the tube to the overflow tank (the translucent plastic tank).

Since the outside seal of the cap is brass, how is it supposed to keep the coolant from dribbling out? The brass of the cap seal would be against the brass of the reservoir fitting, without a good seal. The tube to the overflow tank is at the high side of the reservoir fitting.

Am I missing something?

Matt B

Re: Radiator Reservoir Cap question

Reply #1 – July 05, 2012, 07:40:08 pm

If I understand correctly, it sounds like you either need a new 15 psi cap or need to work on the seat for the cap. Sometimes the seat gets damaged. If so it can sometimes be repaired but occasionally you will have to have a new neck brazed onto the tank. It does need to be fixed.

Re: Radiator Reservoir Cap question

Reply #2 – July 05, 2012, 07:44:38 pm

Big difference between the RADIATOR cap and OVERFLOW cap. The radiator cap is pressurized. The overflow cap is not.

How full is your overflow reservoir? Is it near the cold mark when coolant is cold? If too high, when hot, it WILL overflow.

Let us know which cap you are talking about and what the coolant level is.

Brett

Re: Radiator Reservoir Cap question

Reply #3 – July 05, 2012, 08:30:31 pm

Existing cap is a 5psi. I have inspected the sealing surface of the fitting for the radiator cap (Brett's term - I guess it is a radiator cap even though it is not on the radiator) and it seems pretty pristine. The overflow reservoir (the translucent tank) is near the cold mark when the coolant is cold.

Jumping ahead a bit, but should the radiator (the black tank connected to the radiator) be full, or filled to the sight glass at the bottom edge?

Matt B

Re: Radiator Reservoir Cap question

Reply #4 – July 05, 2012, 09:11:14 pm

The radiator (metal part) itself should be totally full, with the reservoir coolant level falling when coolant is cold and rising when hot.

This is reasonably important, as oxygen in the cooling system is not a good idea.

Brett

Re: Radiator Reservoir Cap question

Reply #5 – July 05, 2012, 09:26:49 pm

So why does it go around the cap and dribble out rather than into the reservoir? The cap looks good, the sealing surface looks good. I still don't see how brass against brass would seal enough to push the coolant uphill to the tube at the top of the radiator cap fitting.

Matt B

Re: Radiator Reservoir Cap question

Reply #6 – July 05, 2012, 09:36:53 pm

Maybe I am missing something but there should not be Brass against Brass. Under the cap there should be a rubber gasket then another one to seal the inner rim.
John h

Re: Radiator Reservoir Cap question

Reply #7 – July 05, 2012, 10:07:55 pm

Quote from: John Haygarth – July 05, 2012, 09:36:53 pm

Maybe I am missing something but there should not be Brass against Brass. Under the cap there should be a rubber gasket then another one to seal the inner rim.
John h

that sure would explain the symptom. It has been a long time since I had something with a conventional-style cap - I have forgotten what they should look like. Photo below. There is the center brass button, the rubber sealing surface for the radiator neck, a brass backing plate, the spring, then a brass disc about the diameter of the radiator sealing surface.

Am I just missing a gasket?

Matt B

Re: Radiator Reservoir Cap question

Reply #8 – July 05, 2012, 10:21:43 pm

Below is an excellent explanation from a recent Barry and Cindy posting:

"Since the advent of cooling system overflow bottles, radiator caps have two different spring-loaded parts, each with their own gasket and spring. There is a third gasket that keeps the cap tightly sealed all the time on the top lip. Cap must be rotated closed all the way to the stop for this third seal to work.

An inner gasket with a strong spring opens to let expanded coolant out and the number on the cap represents the maximum internal PSI of the cooling system before the cap opens. This is the gasket that seals against the radiator housing inside flat 'shelf'. Over pressure moves coolant from coolant system to overflow bottle.

A small spring loaded gasket is in the center of the cap and is held tightly closed when the cooling system is under any pressure. When the cooling system cools it starts to draw a vacuum and the center will open at the slightest vacuum as it is held closed with a very light spring. Under vacuum, coolant moves from overflow bottle to coolant system."

Re: Radiator Reservoir Cap question

Reply #9 – July 05, 2012, 10:55:21 pm

That is a good explanation - I did not know some of that. It mentions the third gasket, but is not clear if it should be a rubber gasket.

Matt B

Re: Radiator Reservoir Cap question

Reply #10 – July 05, 2012, 10:56:17 pm

Quote from: oldmattb – July 05, 2012, 08:30:31 pm

Existing cap is a 5psi

Matt B

5psi seems a bit light for a radiator cap.

Roland

Re: Radiator Reservoir Cap question

Reply #11 – July 05, 2012, 11:07:34 pm

Quote from: Roland Begin – July 05, 2012, 10:56:17 pm

5psi seems a bit light for a radiator cap.

Roland

Sorry, just looked at it again - seven pound. Getting a little off my topic, but I would guess that the cap pressure relates to the operating temperature - a higher pressure would allow the coolant to reach a higher temperature before boiling? High or low pressure cap, the coolant should go to the overflow bottle.

I just looked a a hundred or so stock photos of radiator caps. It looks like some have rubber gaskets and some brass for the primary seal. One diagram referred to the brass sealing surface as a "seal" and the smaller rubber part as a "gasket."

Matt B

Re: Radiator Reservoir Cap question

Reply #12 – July 05, 2012, 11:12:02 pm

I was at napa just yesterday to find a replacement radiator cap for my M11 . Strangely the new 10 # cap had no outer gasket but my old cap did, although it was deterioating. A 7# new cap also had no outer gasket. They finally stole a gasket from a 15# cap since they only had 1 10# cap. Glad to find out that a gasket belongs there, I hope. Someone must have borrowed the gaskets from the new caps.

Re: Radiator Reservoir Cap question

Reply #13 – July 05, 2012, 11:22:28 pm

After reading matt B last post, I wonder if there is really supposed to be an outer gasket on the cap I just bought.

Re: Radiator Reservoir Cap question

Reply #14 – July 06, 2012, 09:24:03 am

I just went through the radiator cap search and for my coach and I believe several other models around it's vintage (03) this is the replacement,

Stant Part 10294

Click on "part interchange" to see other replacement #'s.

Dean

Re: Radiator Reservoir Cap question

Reply #15 – July 06, 2012, 04:12:55 pm

I ordered a cap from NAPA, based on the number provided by FT. Balkamp 7031427, sells for $7.69. I will report on the presence of a seal or gasket when it arrives. It seems that many caps rely on a brass-to-brass seal to keep the coolant from leaking out.

Matt B

Re: Radiator Reservoir Cap question

Reply #16 – July 06, 2012, 05:48:34 pm

In order for the recovery from the overflow tank there must be a ruber seal around the top of the neck otherwise there is not a good enough seal to draw the coolant back from the overflow tank into the radiator

Re: Radiator Reservoir Cap question

Reply #17 – July 06, 2012, 06:10:38 pm

Quote from: fjm502 – July 05, 2012, 11:22:28 pm

After reading matt B last post, I wonder if there is really supposed to be an outer gasket on the cap I just bought.

I was told that you need both gaskets if you have a coolant recovery system. (Overflow) See previous thread.
Cummins M-11 450 Engine Overheating

Re: Radiator Reservoir Cap question

Reply #18 – July 06, 2012, 06:38:13 pm

The Stant 10294 is apparently the same as the Balkamp (NAPA) 7031427. The photo of the Stant from the post above clearly shows the rubber primary gasket. The cap on my coach and the one that NAPA showed me today did not have a rubber gasket.

Matt B

Re: Radiator Reservoir Cap question

Reply #19 – July 06, 2012, 06:45:31 pm

Quote from: oldmattb – July 06, 2012, 06:38:13 pm

The Stant 10294 is apparently the same as the Balkamp (NAPA) 7031427. The photo of the Stant from the post above clearly shows the rubber primary gasket. The cap on my coach and the one that NAPA showed me today did not have a rubber gasket.

Matt B

When I went to Napa to get my #703-1427 radiator cap, some had gaskets and some did not.
No one there had an explanation since it was the same part number. It was very confusing.

Re: Radiator Reservoir Cap question

Reply #20 – July 06, 2012, 07:33:51 pm

Quote from: Barry Beam – July 06, 2012, 06:45:31 pm

When I went to Napa to get my #703-1427 radiator cap, some had gaskets and some did not.
No one there had an explanation since it was the same part number. It was very confusing.

Just recieved my Stant cap #10294 today which works with the Napa part # Barry mentions and it does have two gaskets. Not sure how it could work otherwise to get coolent to and from the overflow tank.

D&D

Re: Radiator Reservoir Cap question

Reply #21 – July 06, 2012, 08:36:39 pm

napa cap that I bought is a 703-1422 a 10# psi and did not have a outer gasket but one was borrowed from another cap of the same diameter. I noticed that these gaskets fit loose and tend to fall off while handling. Some may have fallen off during packaging. I noticed that Barry uses a 15# cap, and the cap that was on my radiator is 10 #. I assume that this is the correct psi for my 1999 u320.

Re: Radiator Reservoir Cap question

Reply #22 – July 06, 2012, 08:47:50 pm

I was under the impression that Barry "Beam" has a 10# one that he got while in Cody a few weeks ago. I replaced mine and it is 10 as well.
John

Re: Radiator Reservoir Cap question

Reply #23 – July 07, 2012, 09:08:15 am

Quote from: John Haygarth – July 06, 2012, 08:47:50 pm

I was under the impression that Barry "Beam" has a 10# one that he got while in Cody a few weeks ago. I replaced mine and it is 10 as well.
John

Hmmm, The cap that was on my 03 u270 is a 15 lb and that's what I replaced it with. The difference between 10 and 15lb is considerable. It may warrent a call to FT.

D&D

Re: Radiator Reservoir Cap question

Reply #24 – July 07, 2012, 10:05:33 am

Felix verified that the cap recommended by FT is a 10 pound cap.

I had no idea this thread would get so deep and interesting. This gathering of knowledge will help someone later!

Matt B

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