Yes I do have underdrive pullies also. The air temp outside was 55 degrees. I've been running underdrive pulleys for 5 years on the ford water pump from 89.. I never had this type of problem..I've been reading a few threads that stated some were having the same problem after their cobra intake install. Wonder if this might have anything to do with it..jyanna said:Are you running underdrive pulleys? Also, what was the air temp? I have a problem running hot with my pullies when it is hot outside.
When I had the headwork, they bypassed my heater coreEldor-stang said:from what it sounds like you having the same problem i did
check your passenger floor board and see if you are leaking coolant, it could be your heater core, but i was having the same problem with the spuraticly jump up and down
first i replaced the sending unit, its wel worth your money to spend the three bucks on this lil feller and if that fixes the problem then good, if not, i would check your heater core, i might be clogging up
hope i've helped
also, yes three core is probably the best you can get
it will help lots
i dunno about labor i changed mine my self, and sugest you do the same, you'll save your self a lot