Radiator Cap Question

[Hedgehog]

I just finished flushing and filling the cooling system on my 2006 Monaco Knight. While doing this, I noticed the radiator cap gasket was slightly dry rotted so I want to replace it. I also notice when the RV gets up to temperature and I've been driving it for a while that the small brass fittings on the coolant bottle are slightly bubbling, more like a hiss. I've taken these fittings out in the past, cleaned them up and applied several wraps of teflon tape to them. With that said, what would be the affects of dropping down to a 9# radiator cap? I think the stock one is 16#'s but I'm not sure.

[Mike Canter]

If you drop the radiator pressure cap down the anti-freeze will boil around the cylinder liners and create bubbles which will pit the outside of the cylinder liners and will cause major engine damage in time. You have to keep that radiator pressure where it is specified. Instead of using teflon tape try some silicone gasket sealer. Get the water below the level of the brass fitting and take them out and let both threads dry them coat them lighty with Permatex Silcone sealer and then let them set for 24 hours before you fill it back up.

[Diplomat Don]

Hedgehog....If you're talking about the two on the side of the tank, here is what I did......Coolant Overflow Tank Leak

If you're talking about one on top of the tank....90 degree fitting with hose, mine leaks too, I just haven't tackled that one yet.

[Hedgehog]

Don, I am referring to the two on the top of the tank. As for the radiator cap, I bought a 9# and a 13# cap, which one should I use?

[walt2137]

Well I run a 7# and have not had a problem and so does a lot of the other guys on another forum, I would suggest that you clean your radiator a couple times a year as I do and I have never had a heating problem.

[Mike Canter]

You will not see a problem because it is only overheating and turning to steam against the outside of the cylinder liners. These steam bubbles stop the anti-freeze from taking the heat away from the cylinder where that bubble formed so you get pitting on the metal wall of the liner at that spot. The correct term is cavitational pitting and the end result is a cracked cylinder liner. Believe me guys this is a common problem with diesels and that is why you also have to use special anti-freeze. Do what you want but it is going to be happening and after an unknown period of time you will have to have an in frame overhaul with new liners installed.

You can read about cavitational pitting here http://www.fleetguard.com/pdfs/produ...s/3300963A.pdf

and here
http://www.aeisndt.com/whitepaper/pd...r%20Liners.pdf

[Twomed]

Great articles explaining cavitation...no mention of pressure. The failure analysis does not conatin any info on how the coolant was maintained which is the nunber one cause for cavitation failures. Seeing it is broken but only speculating why is not very good science compared to failure analysis with comprehensive maint records.

Higher pressures are more to increase boiling points, which some manufs use rather than heavier cored radiators. Mostly to overcome lack of operator attention to proven maint practices of properly maintaining cooling systems, and proper monitoring of guages. Or even having accurate guages. Higher pressures help hold down warranty costs in the short run, and increase parts/labor profits on plastic coolant tanks sfter warranty, Planned obsolesence at it's finest.

But...you do have me searching for pressure/caviattion related hard facts. Also sending this thread to my FL engineer buddy for comment, realizing that he may not be able to make any remarks.

[Mike Canter]

I agree that one of the causes of cavitation is old coolant BUT that is because they don't expect people to change the radiator cap to a lower pressure. The higher pressure is to increase the temperature at which the coolant boils off into steam. So think about that a little. The temperature of the coolant directly against the cylinders liners is a lot higher than the temperature you are seeing on the gauge If you lower the pressure of the radiator then the coolant is going to boil and turn to steam against the cylinder. By keeping the pressure up the coolant does not boil and turn to steam bubbles and you do not get cavitation. It has nothing to do with the construction of the radiator or to over come neglect from the operator. I really find it hard to believe that people are arguing this fact and are prepared to lower the pressure and risk damaging or cracking the liners. If you lower the pressure enough you will have a wall of steam around the cylinders that will insulate the cylinders from the coolant and you will experience a thermal runaway and boil the coolant out of the motor. I really don't care what people do to there own engines you can argue this point day and night but it is a fact but I just don't think it is right to tell somebody it is OK to do something that they know nothing about and end up ruining their motor and costing $18-$20K to get it fixed.

[Twomed]

Back to diesel theory 101...caviatation is the creation of air bubbles in coolant caused by high freq vibrations inherent to diesel combustion. If you are creating steam then you probably have 100% water for coolant. SCA (supplemental coolant additive) was developed to overcome this bubbling. It actually makes coolant (proper ratio) slicker so it flows/adheres properly to cooled surfaces to provide proper heat transfer form metal to fluids.

Check it out: Cavitation,_SCAs_and_the_Proper_Maintenance_of Diesel_Engine_Cooling_Systems

I usually respect the comments of your posts so I will ignore the unnecessary snideness of your of your last sentence in keeping with the civil spirit of this forum; that being to help each other learn how to keep these magnificent beasts running down the road to adventure.

Per the Cummins Shop Manual

Specifications
Cooling System
Cooling System Data Automotive
Page E-10
Pressure Cap 220 degree systems B3.9 Engine 15 PSI
Pressure Cap 210 degree systems B3.9 Engine 7 PSI

These pressures can vary with the new generation EPA engines, some have added features that do require higher pressures due to higher operating temps.

[Mike Canter]

Coolant and SCAs are only one of the factors that control the bubbles of steam that can be created on the outside of the cylinder liners and can damage the cylinder liners. Lowering the pressure of the coolant system and keeping the radiator size and thermostat all the same can also result in steam bubbles forming on the cylinder liners at a lower temperature which will result in the same damage. You will not see this happen on the temperature gauge or by any other indication until the damage has been done. I would not expect at a owners manual level that they would go into this kind of detail about what happens if you deviate on things that have been specified for the motor. An owners manual is at a pretty high level.

[William Willard]

I was having trouble keeping the cap on my coolant tank -- Harry Martin (Happycarz) had an aluminum tank made for his & a friend`s Monaco with good results, so I had one made using Harry`s prints -- 2 years & no problems -- I use the 16# cap, because that`s what Monaco/Cummins put on as stock -- I`m now on my 3rd, inch & a quarter coolant hose, going from the tank to the engine -- They`re hard to find on the road, so get a spare --
Has anyone changed the coolant thermostat on the 400 ISL ? I need the part number if you have it -- Bill Willard

[Twomed]

Nice tank Bill, I am carrying one for "someday". I have replaced six of them on Moanacos and had no problems after with 10# caps. 155k on a parent bore 3126CAT with 10# and they are more prone to cavitation than sleeved engines.

For all of us that still have plastic tanks, those tanks are getting harder to find. If you can look down into your engine compartment and see the tank, so can the sun. I suggest painting wherever the sun hits so you can keep the UV rays off that plastic, it will help avoid the inevitable.

[William Willard]

Twomed -- I still have my plastic tank, & nothing is really wrong with it, but after I saw Harry`s tank, & how pretty it is, I had to have one -- If someone needs it, I`ll sell the plastic tank -- It`s from a 07 Camelot, & I live in the Phoenix area (Gilbert) -- Bill Willard

[walt2137]

Mike you are generally right on with good info with most of your post but I sure cant agree on this one, I do agree with Twomed and I dont see any proof of what you are stating.