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Old 10-01-2008, 17:08   #1
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Heat exchanger pressure cap size ?

Hi Everyone,

I have a 1977 Volvo MD11-C 23 HP Diesel. I think it may need a new pressure cap, I'm loosing coolant. The existing cap has engraved on it
7 lbs, this sounds low to me so can anyone give me some feedback before I buy a new one ????

Thanks again - Bob
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Old 10-01-2008, 17:40   #2
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Don't know about Volvos, but the Perkins has a 7 lbs cap..Not on the heat-exchanger, but on the reservoir tank.
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Old 10-01-2008, 18:53   #3
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7 to 10 pounds is average for most cooling systems. What it does is raise the boiling point of the coolant to around 250º F.

Without a pressure cap and recovery tank it allows the coolant to vaporize and dissipate.
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Old 17-10-2008, 18:13   #4
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Question Volvo Pressure Cap

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Originally Posted by delmarrey View Post
7 to 10 pounds is average for most cooling systems. What it does is raise the boiling point of the coolant to around 250º F.

Without a pressure cap and recovery tank it allows the coolant to vaporize and dissipate.
I have a Volvo 3cylinder diesel (MD2030B) and I need a new pressure cap. The existing cap has "09" stamped on one of the ears. Can I assume that means 9 lbs and not some metric unit of pressure?

Also, I found caps in the auto parts store that fit, but their pressure ratings are 16, 18 & 20lbs. I wonder why autos would use higher pressures than marine diesels? Mostly because I can get them for 4 bucks and no shipping.
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Old 17-10-2008, 18:25   #5
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I have a Volvo 3cylinder diesel (MD2030B) and I need a new pressure cap. The existing cap has "09" stamped on one of the ears. Can I assume that means 9 lbs and not some metric unit of pressure?

Also, I found caps in the auto parts store that fit, but their pressure ratings are 16, 18 & 20lbs. I wonder why autos would use higher pressures than marine diesels? Mostly because I can get them for 4 bucks and no shipping.
probably 0.9 Bar
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Old 17-10-2008, 18:51   #6
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probably 0.9 Bar
Thanks. Converting to PSI, that should be a little over 13. Would the auto parts store 16psi cap work or could I do damage to heat exchanger? (I don't have an expansion tank. Cap fits on top of heat exchanger)
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Old 17-10-2008, 20:16   #7
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Shipping a pressure cap by USPS should cost much more than a buck and a half
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Old 18-10-2008, 05:20   #8
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Thanks. Converting to PSI, that should be a little over 13. Would the auto parts store 16psi cap work or could I do damage to heat exchanger? (I don't have an expansion tank. Cap fits on top of heat exchanger)
you should either have a tube from the rad cap neck going to an overflow tank or the bilge. I wouldn't run a 16# cap.
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Old 18-10-2008, 06:47   #9
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you should either have a tube from the rad cap neck going to an overflow tank or the bilge. I wouldn't run a 16# cap.
Thanks for the advice.
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Old 18-10-2008, 08:22   #10
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Originally Posted by bob_deb View Post
Hi Everyone,

I have a 1977 Volvo MD11-C 23 HP Diesel. I think it may need a new pressure cap, I'm loosing coolant. The existing cap has engraved on it
7 lbs, this sounds low to me so can anyone give me some feedback before I buy a new one ????

Thanks again - Bob
Bob, My first question would be, are you sure you are loosing coolant from the pressure cap? If you can physically see it then OK, otherwise look at all possibilities also.
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Old 18-10-2008, 21:53   #11
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What is the fluid level when engine is cold?

A lot of people fill their heat exchangers to the top.

If you don't have an overflow, expansion will cause it to drain out.

When the engine cools....you will have air there.
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