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Engine coolant tank cap


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I think our engine coolant tank cap needs replacing. We have the big plastic coolant expansion tank. The engine on our 2003 Holiday Rambler Imperial is a Cummins ISL 400. What cap should we get for this replacement? Thanks for your help!

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I'm asking about this because I'm concerned that the cap that's on it may not be the right one. We bought the motorhome used, and several pieces of engine equipment that were on it were wrong and/or broken. I'd like to make sure it's replaced with the right cap for this installation. Thanks again!

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The plastic fill cap on mine leaks. The coolant in the tank will slosh up against the cap during braking or whatever and then it drips down onto the frame. 

Is that normal?  Can I get a new plastic cap for it?  Doesn't seem to leak much, but still kind of annoying...

 

Screen Shot 2021-11-16 at 10.01.38 AM.png

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My ISC 350 has the large square tank and uses a 7 lb cap (Stant 10228).

Whatever you get I would highly recommend a cap with a pressure relief lever. They are typically easier to remove/install and the square plastic tanks become brittle with time. You really don't want to find and replace the tank after you break it installing or removing the cap!

-Jamie

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On 11/16/2021 at 12:17 PM, OhReally said:

My ISC 350 has the large square tank and uses a 7 lb cap (Stant 10228).

Whatever you get I would highly recommend a cap with a pressure relief lever. They are typically easier to remove/install and the square plastic tanks become brittle with time. You really don't want to find and replace the tank after you break it installing or removing the cap!

-Jamie

My plastic tank was getting brittle, and I found the identical Volvo tank online.  Came with new caps, and was fairly easy to install. 
Good for another 20+ years. 😄

On 11/16/2021 at 12:03 PM, RoadTripper2084 said:

The plastic fill cap on mine leaks. The coolant in the tank will slosh up against the cap during braking or whatever and then it drips down onto the frame. 

Is that normal?  Can I get a new plastic cap for it?  Doesn't seem to leak much, but still kind of annoying...

 

Screen Shot 2021-11-16 at 10.01.38 AM.png

That cap should hold pressure.  If your tank and cap are in good condition, you can just replace the O-ring inside the cap.
You can measure it with a caliper, and lookup the correct O-ring online.  I'd use silicone, as they seal better without having to overtighten the plastic cap.

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Most of our engines came from the factory with 16 lb caps. I myself run a 12 lb cap. The reason being is I rarely head into high elevations during hot summer months towing a heavy load therefore I am not worried about over heating. I personally feel as our radiators get older a little less pressure can possibly help in preventing leaks. While 16 #'s to 12 #'s sounds minuscule, it really is not when considering a pressurized vessel. I have seen tests performed on pressurized vessels where a few pounds created leaks.

JMHO

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1 hour ago, Bobbyboy said:

Chris:  Not being a physics wiz kid, higher pressure cap causes engine to run cooler???  Correct or not??

 

Bob L

Pressure caps raise the boiling point of the coolant. The higher you go on the pressure rating on the cap the higher the temperature required to over heat the coolant. The OEM 16 lb cap was a bit aggressive. I have customers running 7 lb caps without issues though I recommend a minimum of 10 lbs.  It should be noted that dirty radiators contribute more to over heating issues then radiator caps. 

 

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On 11/16/2021 at 10:03 AM, RoadTripper2084 said:

The plastic fill cap on mine leaks. The coolant in the tank will slosh up against the cap during braking or whatever and then it drips down onto the frame. 

Is that normal?  Can I get a new plastic cap for it?  Doesn't seem to leak much, but still kind of annoying...

 

Screen Shot 2021-11-16 at 10.01.38 AM.png

 

On 11/17/2021 at 10:25 AM, dl_racing427 said:

My plastic tank was getting brittle, and I found the identical Volvo tank online.  Came with new caps, and was fairly easy to install. 
Good for another 20+ years. 😄

That cap should hold pressure.  If your tank and cap are in good condition, you can just replace the O-ring inside the cap.
You can measure it with a caliper, and lookup the correct O-ring online.  I'd use silicone, as they seal better without having to overtighten the plastic cap.

So I found that I buy a new gasket for the filler cap online for about $24 + shipping (https://class8truckparts.com/volvo-truck-6771322-gasket-for-expansion-tank-cap.html), or an entire new cap with gasket for $33 + shipping (https://class8truckparts.com/volvo-truck-6794968-filler-cap-for-expansion-tank.html).

Do you have a suggestion as to where I can purchase the gasket online based on the caliper measurement alone? 

(Also surprised that nobody pointed out that my expansion tank is held on by a small white zip-tie connected to a wiring loom.  🙂

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I see that the description for the cap sold on Veurink's says "16# with vacuum relief". Do all radiator caps include vacuum relief? Also, Chris T says he runs a 12 LB cap, but I've been unable to find one of those, anyone know where I could find one? Thanks for your help!

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On 11/18/2021 at 8:52 AM, Bobbyboy said:

Chris:  Not being a physics wiz kid, higher pressure cap causes engine to run cooler???  Correct or not??

 

Bob L

The rating of the cap does not change the pressure or temperature in the system... unless of course the pressure exceeds the cap rating, which no one here has reported.  The answer I think you're after is whether running using say a 16psi cap will run cooler that with a 10psi one.  They will both run at the same temperature. 

If you do want your engine to run cooler (or hotter) it would involve changing the coolant thermostat.

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