bad radiator cap?

I was checking my truck over this morning to get it ready for the week and noticed the coolant level was just a bit under the max level(added less than 1/2qt). I went to add some and noticed that there was some coolant residue on the outside of the expansion tank and some around the threads on the top of the tank. Could it be that the cap is bad? There are no visable leaks anywhere else.

I've only had the truck for 15k and it hasn't used any coolant up until the last run I made this week. I have had the collant drained a few times while replacing hoses and shutoff valves that went bad, so it could be that I wore the cap out removing it so many times. The cap is a 15lb plastic cap like most of the newer trucks have. I just got my oil sample back from the last change and there was no coolant in the oil, but that really doesn't mean much since the problem just started.

oh, engine is a C15 acert with 300k on it. There was no white smoke on the last trip and no coolant in the oil.

 

one more question, when you look for bubbles caused be a bad head gasket , is a small amount of bubbles normal. When I was warming the truck up this morning I took the cap off the expansion tank and looked inside and there was a small stream of bubbles going from where the coolant dropped out of the return hose to the main part of the tank then the bubbbles disappeared. The bubbles were small, kinda like you get when you pour one liquid into another.

I've had so many small issues with this truck that I keep looking for something to go wrong, so hopefully it's just something simple like the cap and not something serious.

 

part of my problem could be that I keep the coolant to the top of the max line. I noticed today when the engine is at operating temp the coolant is only a couple inches from the top of the expansion tank when the cold fill spot is at the max line. I had to order a cap since my local guy didn't have the right one so won't have that until Wed.

I checked when I got home tonight and there was a slight trace of residue on the side of the tank. I checked everything over when running and the discharge lines are flowing properly and there are no visable leaks anywhere and no smells that would indicate that coolant is seeping out and burning off.

 

update: I replaced the cap and sucked a little bit of coolant out of the expansion tank after I originally posted this(2 weeks ago) and haven't had any coolant loss problems until today. I changed the oil tonight and noticed that there was a small amount of dried crusty coolant on the lip of the tank and a faint trail where it ran down the side of the tank. The cap was tight. I think that I might still need to drain some more coolant out since it's still kinda high up on the tank when hot, but I would think it should've leaked a little before now. Only thing I've done different was I plugged the block heater in last night since it was going to get down to 28*F and I wanted to see if the heater worked. Could that have maybe caused a pressure build up to cause the leak? I checked the truck over last night before dark and the coolant level was where it has been for 2 weeks. I also had the cap on the oil fill pop off today and some oil came out and made a small mess down the side of the engine compartment, I put the cap back on tight and it stayed put on the trip home. I added some oil 1k ago and probably didn't get the cap on tight enough so this probably isn't related to the coolant issue.