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'94 5.7L coolant temp fluctuation

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by rdoty, Dec 29, 2012.

  1. rdoty

    rdoty New Member

    The coolant temp on my 1994 5.7L Silverado varies widely. When I first begin to drive it gradually rises up to 210° then suddenly drops to about 150°. I changed the thermostat (old one was 195° too) but no change. I've also topped off the coolant several times....about a pint each time. Heater return hose is hot. It's obvious that the thermostat is doing its job but why does it go so low? Is this common? If not what is the cause?
  2. Conlan Rose

    Conlan Rose Active Member 2 Years 1000 Posts

    I was having a similar problem and replacing the thermostat fixed it. It is common as the trucks age and usually replacing the thermostat or flushing the coolant fixes it. Have you checked to see if you are leaking any coolant? Also when you replaced the thermostat did you flush the coolant? Contaminated coolant can damage the new thermostat.
  3. RayVoy

    RayVoy Well-Known Member 2 Years 1000 Posts

    The rise would indicate a closed t-stat, and the sudden drop would indicate an opening t-stat. The condition is normal, the amount of drop seems excessive.

    You didn't show your location on your profile; if your in Alaska, that drop might be normal; Florida, not so much.

    If it is cold where you are, the outside air is cooling the rad down too fast. Your fan should stop when the rad cools down, maybe it's turning all the time.

    Covering part of the rad will probably stop it.
  4. Conlan Rose

    Conlan Rose Active Member 2 Years 1000 Posts

    He has a 94' that year has a mechanical fan that is always running.
  5. rdoty

    rdoty New Member

    It just seems odd to me. I thought it might affect the ECM control since it looks for a minimum of 170°. I guess I won't worry about it.
  6. RayVoy

    RayVoy Well-Known Member 2 Years 1000 Posts

    Your good, thanks again :great:
  7. Conlan Rose

    Conlan Rose Active Member 2 Years 1000 Posts

    The ECM only cares about the temp of the O2 sensors and MAF. Only when engine temp goes to high does the ECM change fan speed, even though usually this is caused by the A/C running so really the fan is adjusting for that.. The fan has a clutch that allows it to change speeds, but it still runs at all times. The fans on these trucks are electro-mechanical and usually just go at engine rpm but can speed up and slow down a bit. The fan can run without the engine running but only when the A/C is on and the ignition is set to on.

    - - - Updated - - -

    Your welcome, I'm very well educated in my trucks generation.
  8. RayVoy

    RayVoy Well-Known Member 2 Years 1000 Posts

    Self taught, no doubt :great:
  9. Conlan Rose

    Conlan Rose Active Member 2 Years 1000 Posts

    Its amazing what Google, Wiki, and other sites can tell you. Also it says it in the owners manual :lol:
    Last edited: Dec 31, 2012
  10. j cat

    j cat Rockstar 4 Years 1000 Posts

    with the 1994 engine the temp should slowly rise and not over shoot then not drop too low.

    the temp should slowly rise to about 180-190 deg area.

    if the hoses are hot also both heater hoses then this can be the temp sender . I had this issue this year on a S10 truck. the connector to this may also be corroded.

    the pcm does look at the engine temp to vary fuel mixtures when cold. colder the engine the injectors open longer.

    other temp erratic operation may mean head gasket. as the exhaust gets to the coolant it does mess up the coolant flow. water pump does not pump air well.

    pressure testing the coolant system is recommended.

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