Coolant overflow problems

[2001gold]

I've been having some peculiar issues with the cooling system on my 2001 (135k miles). The history/symptoms:

Right after I got the truck I noticed there was no heat from the heater. I suspected the dealer I bought from had flushed the cooling system but hadn't bled it properly afterwards. I attempted to bleed it two times with no improvement. Around this time I started to notice that after driving enough to get the engine up to temperature, stopping and looking under the hood, coolant was getting sprayed under pressure out of what looked like the expansion tank overflow hole. The liquid was most concentrated at that point, with the spray pattern continuing up over the air filter box and intake manifold. Taking off the expansion tank cap I could see bubbling coming up out of the coolant. Took it to a shop. They said they found a hairline crack in the radiator and replaced the radiator and cap and bled the system. I got heat from the heater, things seemed good.

Then last week I took a full-day drive. After a good portion of it I stopped at a rest stop and noticed a coolant smell. So I opened the hood and lo and behold, coolant had again been sprayed over the engine compartment, originating from the expansion tank overflow. Apparently this overflow condition continued to pump coolant out until it the level was too low to continue cooling the engine: the temperature started fluctuating quickly, climbing up to the top of the indicated "normal" range for a minute or two, then back down to the middle, back up again, etc. Luckily this was right at the end of my 9 hour drive, so I was able to stop shortly after this started happening. The next day I refilled the coolant before driving, which took care of the overheating.

The spraying coolant issue continues. I replaced the radiator cap again just to be sure, but that hasn't had any effect. As long as I keep the coolant level up there doesn't seem to be an overheating issue.

I don't know if this is related to the above issue, but the "service engine soon" light is now on, and once or twice the engine has missed badly on one or more cylinders when cold. Currently, it's running smoothly though.

Has anyone seen a similar issue? Ideas about what could be causing this?

Thanks for any help.

[NMR50Pathfinder03]

I wish I could tell you just what is wrong, but no such luck. I have an '03 Pathfinder and a few months ago I popped my hood, only to be greeted by a large dried spray of coolant emanating from the radiator cap. I had never seen this before, my R50 is at approximately 95,000 miles. This occurred at about 2 years since my last coolant flush. Two weeks later I flushed the system and have not seen any problems since. I do not know if this is a side-effect of old/low coolant or just some freak occurance. Also I did not notice any increase in running temps before the coolant spray. Good luck bro.