Yesterday’s weather was fabulous. The temperature was in the mid-70s with mostly clear skies. Today looks like more of the same. This won’t be the case this weekend though. A large storm system is supposed to hit southern California tonight. In San Diego, they’re forecasting more than two inches of rain this weekend. This storm system should reach us by Saturday. We may have a wet weekend ahead.

Yesterday, after I posted, I went to work on my project of the day. It was a fairly simple task. I needed to adjust the v-belt that drives the A/C compressor. When I replaced the belt, it seemed tight. I knew it would loosen up after the engine was run for a while. The 25-mile drive from Mesa to McDowell Mountain Regional Park (MMRP) was enough to seat the belt.

When a new belt is first installed, it’s almost impossible to put enough tension on it to fully seat it in the groove of the pulley. You need to run the engine so the belt seats in, then adjust it again. I didn’t want to fire up our big diesel while we were at the Apache Wells RV Resort just so I could adjust the belt. I knew the short drive to MMRP would seat it in. Also, since it’s the drive belt for the air conditioner compressor and I wasn’t running the A/C, having a little slack in it wasn’t going to be a problem.

After I completed that task, I poked around the engine compartment. I was trying to figure out why some coolant was spit out of the overflow hose when I was parking our coach here. I brainstormed a bit the other day with my friend and former colleague, Keith Burk, and we came up with a couple of possible causes. Then I studied the cooling system on our Cummins ISL engine.

The cooling systems in these diesel pusher coaches are unique to each brand. When Cummins supplies the engine to the chassis builder, they specify certain cooling system requirements. The engine comes with the water pump and hose fittings in place. It’s up to the chassis manufacturer to provide a suitable radiator and plumbing for the cooling system.

Back to the problem at hand, it made sense to start with the easiest thing to check – pressure testing the radiator cap. The cooling system in a liquid-cooled internal combustion engine is held under pressure once the engine is running. The system is sealed. As the coolant starts to warm up, it begins to expand. With a sealed system, this expansion creates pressure. The radiator cap is designed to relieve the pressure at a specific point.

Having the coolant under pressure raises the boiling point. For each pound per square inch (PSI), the boiling point is raised three degrees Fahrenheit. In most systems, when the pressure exceeds the specified value, a valve in the cap opens and allows coolant to flow into an external, vented container. When the coolant cools, it contracts, creating vacuum in the system. Another valve in the cap allows the coolant to be drawn back from the external container into the system.

On our coach, there isn’t an external container. The Peak chassis utilizes a surge tank with enough volume to accommodate expansion of the coolant. The radiator cap is on the surge tank.

Cooling system surge tank

The other thoughts on why the cooling system overflowed weren’t so simple. One possibility would be a leaking exhaust gas recirculation (EGR) cooler. This would be bad, however, our Cummins ISL doesn’t have an EGR system. So that’s not an issue.

Another possibility would be a leaking head gasket. This also would be very bad news. I didn’t think this was happening because leaking head gaskets leave other tell-tale signs. I didn’t have any other symptoms.

My thought was to take the radiator cap into town and have it pressure tested at the auto parts store. Most auto parts stores will test them for free, and this was true at the O’Reilly Auto Parts in Fountain Hills.

I removed the cap and saw there was no need to test it. The seal was visibly bad. It was distorted and had ruptured. There’s no way it was holding pressure in the system. Although the fluid in the surge tank wasn’t boiling, this may not be true of hot spots in the engine. With no pressure in the system, the coolant will boil at about 223 degrees Fahrenheit. Parts of the cylinder liners and cylinder head may have had localized coolant boiling, pushing the coolant out of the cap seal.

Old radiator cap with bad seal

My next task was to determine what pressure the system should be limited to. I searched online, posted questions on forums and went through my manuals. The only definitive specification I could find was in the Cummins engine manual. It specified seven PSI minimum pressure. That’s really low. Online I found anything from 10 PSI to 15 PSI. The old cap didn’t have a part number on it, but it was stamped with the number 14, leading me to believe it was probably rated at 14 PSI.

I didn’t want to over-pressurize the system and create a new problem. I found a 13 PSI cap that was dimensionally correct and bought it. Using the formula of a three degree rise in boiling point for every PSI, this would raise the boiling point by 39 degrees, making the coolant boiling point 262 degrees Fahrenheit instead of 223 degrees.

New radiator cap

This is a long-winded, technical explanation for what amounted to unscrewing the old cap and screwing a new one on. If you’ve read this far, it must have some value though.

Today, I’ll head back to town to look for more effective allergy medicine. Donna is going for a run to test her new cooling system. She just picked up an Osprey Rev 1.5 Hydration Pack – a small backpack with a 1.5-liter water reservoir with a hose to sip from while she runs and rides her bike. She used to have a Camelback hydration pack, but the Dirty, Rotten Thieves got that when they stole our cargo trailer back in San Diego.

I also need to talk to Mike Hall this afternoon and see when he wants me to return his Jeep. It would be good if we had it on Sunday so I could pick up Donna and her bicycle after the race. On the other hand, the Jeep is open and if it rains on Saturday I don’t have a covered parking area for it at MMRP.