stumped....blown hg?clogged catalytic conv? runs fine, but.....

[lovinthedeez]

so before I hear the usual, let me explain what I have, and what I know.
swapped 1.6 n/a from 81 rabbit into a 86 jetta
still have the gasser exhaust from the manifold back
still using the gasser radiator from the jetta
never had a problem until i put in a new ( which turned out to be bad) water pump

car runs fine, even when getting hot.  no smoke or anything
seems to run a little cooler when I leave the exp tank cap off.  this makes me think head gasket.  but if it is, shouldn't it be smoking, or not running right.  the thing is that it runs FINE.  no noticeable difference other than the temp guage.  I live in a small town, and have been taking it all over the valley without a single snafu.  however lately, it seems at the guage level to be overheating.  I guess what I'm asking here is, is a couple of things

what would be the symptoms of a clogged gasser cc?
if blown hg, why running fine and no smoke: esp the cap on/off thing.
radiator plugged?

just getting real frustated.  gonna drop the oil in the next couple of days.
a link to what seems to be causing all my recent headaches.

http://www.vwdiesel.net/forum/index.php?topic=20720.0

I'm blown away by the fact it runs fine sometimes, and sometimes the temp guage will shoot straight to the top after only a couple of miles, but still run fine.

[clbanman]

Do you have secondary confirmation that it's really overheating or are you just relying on the guage?   When it shows hot, (if really overheating) see how hot the return hose from the rad to the water pump is, if it's cool, you probably have a thermostat problem.   I've only ever had luck with genuine VW thermostats.   FWIW, I think with a good thermostat and functional cooling system it's really hard to overheat these things.  Mine has had a non-functional rad fan all summer, and still doesn't overheat even in stop and go driving (although my stop and go driving only lasts about 20  minutes at a time).

[Doug]

Generally a failed head gasket will have combustion gases entering the cooling system which will lead to excessive pressurisation of the cooling system. This means boiling over at the reservoir as well as high temperatures. You may be just in the early stages where it is happening only eratically.
I am not sure why you have a catalytic converter in there but if it is plugged, and it will with a diesel sooner than later, then maybe excessive exhaust back pressure is your problem. Get rid of it before it overheats assuming that it is still functioning trying to burn up those hydrocarbon emissions.
 

[lovinthedeez]

it is boiling like a motherfarker and puking out the expansion tank like a thirteen year old after two crown shots.  .  I'm pretty sure its the hg.  the catalytic converter is there only because I haven't had the loot to replace the pipe after the gas-> diesel swap.  It will be replaced when I do the head gasket.  The exhaust is pretty much the only thing I didn't do properly when I did the swap, which I'm kicking myself silly for now, as well as finding a td radiator.  here's a link to my swap if anyones interested in looking at it. 
http://www.vwdiesel.net/forum/index.php?topic=19324.0
thanks to all for looking and trying to help

[smutts]

I would be careful in running an engine without a pressure cap on the cooling system, unless a really gentle 30mph get you home measure. At atmospheric pressure at sea level water boils at 100c , less at altitude. So if for any reason a surface inside the head gets to 100c (prechamber area, lots of go pedal?), bingo! steam bubble. This will now keep the cooling water away from the soon to be destroyed metal. Well that is my theory. The 15psi pressure cap is an attempt to keep the coolant liquid, well, liquid.