Air pushing though the radiator.. 3406e

Pete always used a 7lb cap, but CAT recommends a 9lb cap. The problem is nobody make a 9lb cap in the size of the Pete opening. The 7 lb cap will work fine

If your truck is pushing water.........if you listen you may hear it. Get the truck hot and let it cylce the thermostats few times. When you shut the truck off, if you listen to the tank you may hear bubbling afterwards from compression thats entered the cooling system and trying to get to the highest point.

 

If this is a Pete you won't be able to find a 9lb cap, you can get a 10lb in that neck size. I've run either of them with no problems

 

It won't allow it to build air, but it will not control the expansion of the coolant from heat and you're constantly loosing some then. It will also allow air to be sucked in when the system is cooling off after a hard pull and the water temperature is dropping

 

Put a coolant system test guage on your reservoir or radiator where that cap goes and run the motor until your thermostat opens. Watch the pressure after it opens. If it's overpressurizing, I'm sorry to say, you have either a blown head gasket or a cracked head. I had a DD 60-Series doing this. No other symptoms except losing coolant (pushing out the exhaust system) rough running and poor power. Mine ended up being a cracked head.

I hope it's just the cap, but it never hurts to check the running pressure on your cooling system.

(Add) also, I had a 3406E Cat blow a head gasket once. It was in a 1999 International. It was losing coolant and blowing air into the reservoir for months before the head gasket went fairly spectacularly on a trip to Oregon. The owner of the truck had no idea there was a problem with the head gasket and never went any further than continuing to add coolant. I was able to limp the truck back to Michigan, but once there, it never started again. If it is a blown head gasket, get it fixed now so you don't end up having to do a complete out-frame overhaul...or engine replacement. 3-5K sounds a lot better to me than 15K. Just sayin.

 

Supposing you find a head gasket leak. How do you intend on plugging it? Shove JB Weld in it?