87 4kq cooling system problems

[Luke Rickert]

The good news first, I got a new exhaust system installed yesterday,
including new a high flow cat, the car now is relatively quiet and has
more power. The bad news, when I got home from the exhaust shop, (15
miles) there was coolant coming out of the cap of my expansion tank. I
just replaced the cap with one from an auto parts store because it was
hissing. The expansion tank is less than a year old, the fan was cycling
on and off normally. I think the temp gage is not functioning properly,
so that isn't much help but all is certainly not well. 

I have a few theories as to the problem: 

1. A blown head gasket adding extra pressure to the cooling system which
is overloading the cap. 

II. The cap I bought is defective and I need to go find the VW/Audi
part.

C. The car is actually overheating due to some obstruction such as a bad
thermostat or plugged radiator. 

I don't like option 1 very much but it should at least be easy to test. 
I can get a new cap or put to old one back on and then change the
thermostat. Does anyone had any insights into this problem? Are there
any other reliable ways to test for HC's other than getting a mechanic
to "sniff" the coolant? (some sort of test kit from the auto parts store
perhaps?)

thanks

Luke 

-- 
Luke Rickert
www.engr.orst.edu/~rickert