1995 gms sierra 1/2 ton 305 engine...truck overheated..let cool down and replace water...continued driving and noticed temp was getting hot again..turned on heater and was blowing cool air...let engine cool down again and replaced thermostat...refilled with water and replace radi. cap.. engine would start but rough and just die...replace spark plugs and now engine will stay running at idle...took it around block and its like it has no power...real slow take off... and at idle if you turn on ac motor will die...checked oil and no water in oil...and no water leaks any where...and suggestions on what it could be?

SOURCE: Engine will start and rough idle but dies when either the brakes are applied or the lights are turned on!

have you checked out your fuel filter? I had my 2003 denali do the same type of problem and it turned out to be a very dirty fuel filter.

SOURCE: 04 GMC Denali Overheating

check fluid level if full remove rad cap when car is cold and run while looking at fluid in rad if it is circulating you should see it moving if it does'nt move after car warms up thermostat needs to be replaced also check to see if fan is working properly and check the reputation of your mechanic some are not always honest

SOURCE: 2005 GMC Sierra Overheating

If your sure there is no trapped air in system after refill and air bleed off, is fan clutch pulling good air thru radiator? Even if it is, the heater should have coolant as hot as what temp gage is showing! Try backwards flushing heater core (dex cool is notorious for plugging these up). There is a TSB for cracked cylinder heads from GM for unexplained coolant loss.Remove drivers side valve cover, and look at center three head bolts for coolant around them, if coolant has been getting in thru this suspect area, there will be a milky/pudding like goo under valve cover and on head. GM had a company called Castec cast these and the areas near the head bolts were found to be too porous (NO recall YET!) Ya, you guessed it, replace heads! Unfortunately, if its gotten hot to many times, it has probably hurt the heads/gaskets. Hope this helps. PS: The castec logo is under the rearmost pair of rocker arms,may have to remove them to see.

SOURCE: AC blows hot air

try this website fix & troubleshoot heating & A/C www.automedia.com and another car A/C repair www.directgardening.com

SOURCE: GMC Sonoma SLE 4.3L 1991- Coolant Temp Gauge Not reading

there are 2 coolant sensors one for gauge and one for ecm