Has anybody noticed pressure in the plastic jug coolant reservoir, even when the engine hasn’t been started for a few days? I’m not loosing coolant and the engine seems to be running OK, it just seems unusual for the cooling system to remain pressurized for days after the engine was last started. It’s an ’01 Duramax with 75k on it with a new set of injectors just installed. Would like to hear from some LB7 owners about what their rigs are doing. Thanks……….

Backfeeding the cooling system with air, or another method is a good way to check for leaks. This would include the whole cooling system, injector cups, etc. I have found the system will hold pressure for a long time after shut down or testing. I'd say you've got a good system, that's leak free. If you had a leak the pressure would normally leak back to where it came from originally. Do keep an eye on the coolant level.

Thanks for the info 02FREIGHTTRAIN, it sounds like you know your stuff. I know a little bit about the truck (not a lot) and always thought if you released the pressure from the cooling system when the engine was cold (take the radiator cap off) then you could expect pressure to build back up when the engine is warmed up, but I thought after the engine set overnight the cooling system would return to zero pressure unless something else was introducing pressure into the cooling system. My first thought was head gasket, but like I said I'm not noticing coolant loss. Is there anything else that could introduce pressure into the cooling system besides heat?

Headgasket...

The 01s had a problem with them. Hopefully it is a loose cup from the injector job and will be covered. I had the same problem about 8mo. ago and it turned out to be the HG.

I just checked mine and I have some pressure in the system . IIRC it always did have a little pressure. When I had a head gasket leak it would pressurize the coolant reservoir till the overflow leaked when running . Especially at idle !

IMO, there probably should be some residual pressure in the cooling system if its in good working order. But, since its an 01, you might want to have the resivior cap pressure-checked. They do fail with time.

Head gaskets are a trouble-spot on the 01's, and CAP describes the symptoms of blown gaskets.