Radiator Surge Tank

[Denny]

My surge tank has two caps on it. The top one is a 14 lb cap and there is no way any coolant could be added through this opening since there is just enough room to remove the cap. The fill cap is a 7 lb cap. Why are there two pressure rated caps on the surge tank? It seems that the 7 lb cap would release the pressure and make the 14 lb non functional other than to plug a hole. Is this correct? If not, what should the caps be?

[sawdust_128]

I have the two caps. I am not at the coach, but looking at a picture, the top cap is not pressure release. Did you check what it should be in Catbase?

Checking in Catbase, the top cap is the fill cap and it is not a pressure cap. the lower cap is a 14# cap.

Here's a picture of my tank top left of engine compartment.

6032

[BrianE]

Denny, On an XL the upper cap is the 14lb cap. The fill cap (#3) is PN530191 and is not a pressure cap. I tried to find a non-pressure cap at NAPA and local auto parts stores and couldn't find one. My guess is the cap was replaced with what was available. Sorry Sawdust but not all buses are H3's.

[Jon Wehrenberg]

Brian and Ed,

I think the fill cap holds pressure. It will not release pressure as the top one does. If it did the hot coolant could spray and potentially injure someone.

The top cap is the pressure release and it vents through a tube that discharges excess coolant to the ground.

I cannot define if there is a difference between the XL and H3.

[Denny]

Brian, Ed and Jon,
Thanks for the replies. I also found a post by Flyu2there where he quoted Bill Jensen and Bill talked about the 14 lb pressure cap and the non-releasing sealed cap for the fill tube to prevent scalding by the hot liquid. My top cap is a 14 and the filler is a 7. Makes no sense to me why someone would put two pressure caps on the same tank and completely defeat the safety that is build into the cooling system.

I tried to find the caps locally and no luck so Monday a phone call to Prevost parts is in order.

[JIM CHALOUPKA]

Brian, Ed and Jon,
Thanks for the replies. I also found a post by Flyu2there where he quoted Bill Jensen and Bill talked about the 14 lb pressure cap and the non-releasing sealed cap for the fill tube to prevent scalding by the hot liquid. My top cap is a 14 and the filler is a 7. Makes no sense to me why someone would put two pressure caps on the same tank and completely defeat the safety that is build into the cooling system.

I tried to find the caps locally and no luck so Monday a phone call to Prevost parts is in order.

Denny, looking at the situation from a theoretical standpoint, the two pressure cap logic is:

The cap with the higher pressure is acting the same (in this situation) as a non pressure releasing cap (the non releasing cap is apparently hard to obtain).

So, the higher pressure cap is sealing the tank up to its 14# rating and the second cap is sealing to whatever you have under 14#, say 7#.
When everything works as planned the 14# cap seals and the 7# cap works open and closed to maintain 7# pressure in the coolant tank and system.
The 14# cap will never open 9in this situation) unless you open/remove it manually.

HOT TIP OF THE DAY:
When you can not obtain a totally sealing cap then get one to take its place that is substantially higher in poundage over the cap whose pressure you want the coolant system to be maintained at!



:)

[Jon Wehrenberg]

Unlike cars in which expansion of the coolant is not so substantial our coaches can easily have a full system expand and contract by a lot. If I were to guess by 1/2 gallon or more.

If we look at the expansion tank design we would see the fill tube is not at the top, but it leaves a dead space above the coolant level when full for the coolant to expand. Even with the empty space above the bottom of the fill tube I have seen coolant from a hot engine drip from the pressure release cap drain tube.

The 14 PSI is important to be maintained. Our buses can exceed the normal boiling point of water so to increase the boiling point (resist boiling) the system pressure must be increased. If someone has modified the caps such as by using a lower pressure release cap on the fill tube the engine can boil coolant prematurely and that can lead to serious and expensive problems.

I have learned to limit the coolant level (cold) to just above the sight glass.

[Will Garner]

Denny,

I recently refurbished my Surge Tank. I went to our local NAPA since they usually either have or can get whatever part you need. No caps. In fact they had not even seen the simply sealing cap before. So, I like you made a call to my friendly local (not) Prevost Parts store and ordered one of each kind. They got here within three days from Jacksonville and are now on the bus and working.

Some parts you just can't settle for being "close" enough. These two fit that bill. Total cost from Prevost $ $26.10. Intrestingly, the filler cap was the most expensive of the two at $16.67.

[sawdust_128]

Follow up. On my coach, top cap is the pressure cap and it is mounted remotely from the tank. It appears to be south of the tank, but is functionally the top cap. Makes sense as you can relieve pressure releassing vapor and not liquid.

[Denny]

This explains why I have to occasionally add coolant to the system. It is not leaking anywhere and upon oil analysis after oil changes there is no coolant in the oil. Will call Prevost Monday am.

As a side note, I just finished replacing the air belt tensioner, yes the nasty one 6 inches out of arms length reach, and it has a slight leak. I could hear the air leak on the old one so I replaced it. When I started the aux air compressor and checked for leaks the soap bubbles were there. Have to get a replacement from Prevost and start the project all over again. At least this time it should not take so long since I now know how to do it.

[Jon Wehrenberg]

Denny,

As I stated earlier I keep the coolant just above the sight glass. When we lived in NY the temperature extremes from our barn on a January day and our destination in Florida were such that without the bus engine running or being warm we would have about 2" of coolant in the tank in NY, and with a cool engine in a Florida RV park we would have about 4" of coolant. If your bus is not in heated storage I suspect the temperatures this time of year are making the coolant contract, and as soon as your get on the highway and the entire system comes up to temperature if you have filled it to the bottom of the filler neck, you are blowing coolant out on the highway.

But remember...if the coolant is below the level sensor that is one of the three conditions that will shut your engine down, so when cold just be sure the sensor is covered.

[Denny]

Jon,

I keep the bus in my heated (50 degrees) barn so I do not have the extreme temp changes that you were referring to. I try to keep the tank level at about half of the sight glass at barn temp to allow room for expansion. After returning from a trip and allowing the temp to adjust, the level is well below the sight glass. I have never allowed it to get to the point where there is engine shutdown. I think the 7 lb cap is allowing an excess amount of coolant vapor to escape and causing the change in level. I will replace the two surge tank caps and watch it.

[Jon Wehrenberg]

If you are running it at the sight glass and it is going down you are losing it, and it is possible a 7 pound cap may be the culprit. I am not so sure of that because unless you are running hard in how weather you like most of us are not approaching the boiling point so some air with a little moisture may vent through the cap, but I cannot imagine a measurable loss.

If you can get under the coach look up around the driver's side near the forward bulkhead by the drive axle and see if there is any leakage where the copper lines going forward have a rubber hose joint between pipes. I don't know if your coach is set up that way, but on both my coaches those pipe joints on mine were leaking there. Doubling the hose clamps cured the problem, but the hoses have a 10 year life and then they get near failure.

Yours may not have this particular coolant line configuration because of different converters and no OTR, but it is likely you have these types of joints and connections on your coach. After 10 years I am afraid of all such type coolant hoses because they look good but are really on their last legs.