I've recently replaced the belt tension pulley and a little after a week the engine had trouble running. The next day I tried to start it it wouldn't run without constantly pressing on the gas while in neutral. I started removing components that I had removed to change the pulley; the exhaust intake hose and the fan. Low and behold, the engine runs without the fan attached! The fan seems to be the Achilles heel. The engine runs fine without the fan, but as soon as it is attached (the only alteration to configuration), the engine quickly sputters and dies. I've had an ex-mechanic look at it and he was completely stumped. The belt and connected system all seem to operate fine as well. Any help would be appreciated.
If the fan is installed
Check if it has any play by placing one of the blades at about 12-o'clock and applying light pressure in and out. (top goes toward firewall and the blade at roughly 6pm would push outward.)
You should have very little play here. If the fan isn't installed then you can just grab the water-pump shaft _(where the pulley should be bolted to)_ and give it a good wiggle. If there is bearing play you should hear it clicking or knocking when doing this. If there's is a significant amount of play in the pump then replace it.
Reasoning behind condemning the water-pump
With the fan clutch installed, the pump will have added weight to the very end of the shaft. This in turn will cause an excess of pressure to be put on the bearings inside the pump. If the bearing are worn it may cause the pump to seize at this point. If you and your friend check it together this could be tested with one person in the vehicle keeping it running while the other observes the water-pump pulley to make sure that it's turning.
Things to check:
Fan could be bent, causing it to be off balance.
Check and see that you didn't somehow bump the timing. If the distributor is lose, you could have knocked it out of timing when replacing the belt, then bumped it back into the same area while pulling the fan off.
Otherwise, I'm as stumped as your mechanic! :)