I thought I'd post this in case anyone else encounters this issue.
I was heading out of town yesterday morning on a day trip and decided to peek under the hood before I hit the road. I noticed both the upper and lower hoses were collapsed (engine was running). Turns out the radiator cap was bad. A new cap solved the problem. The FoMoCo cap that I removed seemed faulty...the cap would not make the click sound when fully tightened.

I think I am done with OEM degas bottle caps, I've had too much trouble with them over the years. I'm going to try the Standt brand for a while and see if they last longer.

 

I had this happen twice with the oem caps then got a different brand at auto zone and haven't had a problem since!

 

Same here - though I thought at the time the diesel I was getting from the bad cups was causing it. Same collapsing hose issue was solved with a cap from rockauto.

 

Thanks for the very-descriptive photo, Greg!

At least this condition should only occur while the coolant is cold. The hoses should "fill out" when the cooling system comes up to temp and pressure.

However, it is a bit disconcerting to know they will do this.

If it's going to happen during operation, it is more likely to occur on the "suction side", the lower one.

Remember when lower hoses had big coil "springs" in them to prevent that?

Pop

 

Correct.

I think I may have manifested the problem when I was camping earlier this month. The elevation there was almost 7700 ft and I took the opportunity to pressure test the system one day (tested fine...). The engine was slightly warm when I re-installed the (defective) cap. I then later drove back down to the valley where barometric pressure is much greater. No problems during the week (that I knew about) since the engine coolant temp never gets below 110 during the summertime (even while parked overnight...). However, we had a serious monsoon storm come through on Saturday evening and Sunday morning temps were very cool. The coolant must have cooled enough to build a vacuum in the system and the upper hose collapsed. Then when I started the engine the lower hose collapsed (I presume due to restricted flow from the upper hose...)

I also had the degas bottle cap off of my wife's truck when up in the mountains so I decided to check her truck as well. The hoses were fine so I presume there must be some sort of vacuum purge valve in the cap that accounts for static pressure balance when cold. I went back over to my truck and loosened the cap (engine running but coolant was still cold)....the hoses popped right back to normal shape.

 

I had the same problem awhile back but I didn't know what would cause it so I changed the upper rad hose to the wrap around serpentine belt . That seemed to fix the problem . Then someone a was talking to said I should change the cap when the truck wasn't heating up enough in the winter and voila , the truck started heating properly . I never knew that the bad cap was causing the hose to collapse . NOTE to self for future incidents . Thanks for posting this fix !