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Just starting today, I had my engine overhear and had prestone coming out of the overflow bottle. I did notice that at times the cooling fans would not come on, but a few times it did come on. At times it seemed it should have shut off, but kept running. i was going to change my thermostat because I haven't had any heat most times for about a month. At other times it does seem warm, but not hot. I was going to install a 180 thermostat. I was also wondering if my main issue might not be the engine coolant temp sensor, since at times the fan does start up like it should. Is this located right next to the thermostat housing, or down under the throttle body? Is there two sensors that could be causing this issue or just one. I did notice also that my heater core had a slight leak coming inside, so I wasn't sure if this could be part of my no heat issue. I did notice the coolant has lots of crap in it, and I did flush it once using the prestone flush. Seems my coolant lines are circulating though, when i felt them. Is the thermostat sticking or what? Should I just start with changing these two items? I drove it home tonight and the fan was working fine and kept it under half way on the gauge. Any advice would be appreciated. Here is what they sold me today.
 

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Does your car have air conditioning that works. When temp in Montana gets high enough for ac to function, around 60 degrees, turn on ac. Fans should run with ac on.

Factory turn on temp is around 226 degrees, which to many people seems really high. Also temp gauges in GM cars are not known for their accuracy.

Yes 3.4 uses 2 coolant temp sensors. One for gauge mounted at left front of engine on the cyl head. Coolant sensor for pcm is mounted on front top of engine under plenum.http://www.mediafire.com/?40mfgeoe4ctti

94 service manual will be found at link I provided
 

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So is this correct? coolant sensor is by the thermostat. the air temp is on the driver side head behind the power steering pump. I thought the sensor by the head was for the inside gauge, and the sensor under the plenum was for the cooling fan itself.
 

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Go back and read my post again. There are 2 coolant temp sensors. One for gauge and one for pcm. So what you gauge is reading will not be what pcm is seeing for temp.
 

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I think you may have two issues. The dirty antifreeze could be the problem. If you only used the detergent to flush, you'll need to do better (flush and fill kit) in order to flush the radiator (probably your issue) and heater core completely. Did you or someone else use the radiator stop leak stuff. If so, your probably screwed and need to replace the radiator. The most common no heat situation is a clogged core. Bottom line, if your gauge works and the fans come on and you still can't get the temp down, it's probably not a sensor, but either 1) radiator 2) thermostat 3)waterpump. Using the stop leak stuff, stops the leaks but clogs the crap out of the radiator and heater core - in my experience permanently

Hope it helps.
 
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