Hello I have a 1991 olds eighty eight with a 3.8Litre engine..........went to replace the coolant thermostat because it would only heat to 150 maybe 160 degrees.........and stay there unless I was sitting still........well after I replaced it.........the temp guage now reads higher, but I notice a fluctuation from slightly over 220 down to 170, 180 maybe(factory guage with numbers).........debating on whether to leave it alone cause the heater inside the car works better now, or search for what causes this fluctuation........My 1986 s10 truck with a 2.8 does this too.....temp guage fluctuates............the gauge movement on both vehicles moves smoothly but like something is shutting off coolant capability so guage will rise to 220 on car and then let coolant do its thing till guage slowly drops down to 170.........I would sure like to know if this is normal or not
Cooling system problem or not?
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Had a 76 olds that did that when the fan clutch was on its way out. The 220� is high for it to start cooling itself back down again. If it has an electric fan check the leads for corrosion. Test the thermistor to the fan and make sure it is turning on and off where it is suppose to. Find where the factory sending unit is located and make sure the electrical lead is corrosion free. Make sure the ground strap to the engine block is clean too. It could also be a sign of an EGR valve not operating properly so you may want to check that too. Take particular note of how it behaves at various speeds. If the temp is normal while driving 35-40mph and spikes only when idling or when the vehicle speed exceeds that, then you can pretty much go straight to the electric fan. Fastford is right the factory gauges could be a little erratic but don't just assume that is the case. Test it and know for certain.
well the guage didnt fluctuate before I replaced the thermostat.......it would just read low like 150 to 170..........and it seemed like the cars heater didnt heat up as much as it should....................well now on the freeway it will go up to a hair over 220 and go back down to 180..it will go back and forth too......you can tell when something kicks in at 225.........you can see the movement in the needle.........I would think cruising speed would warrant a constant steady guage
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The change at 225 makes me suspect a hose problem. Any chance the pump inlet hose could be collapsing? Is this a viscous fan or electric set up? I agree with you. It should be relatively steady at a cruise speed. If you put a 190 thermostat in, it really shouldn't show above a 5 to 10 degree fluctuation curve. If the temperature sensor is in the head between the exhaust ports you might see a little more but it shouldn't be as high as 220 to 225. The fact that the low temperature reading was backed up by the lower heater output indicates that the gauge is working. What temperature thermostat came out of it? Did you flush the system when you changed the thermostat? Does the temperature come down relatively quick or slowly? Slowly could indicate a weak pump or a slipping water pump impeller. Olds typically used a two belt pulley system to drive their water pumps. Any chance one is loose or missing? Something is not right. Keep swinging until you find it. :boxing: Good luck with it.
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OK...sounds like your thermostat is working correctly and you got your heater working right again......but now I think you should replace your temp sending unit. Try that and see if you don't get a more accurate reading on your guage,
ok if the temp sending unit is malfunctioning, wouldnt it have the temp swing before I replaced the thermostat, or not...........yes dots again......this isnt a dam english class, its people helping people. So thanks again to the people who are genuine caring people offering assistance.....anyway I will check pump inlet hose......because the condition does indicate sonething is restricting coolant flow until it is overcomed by what ever. Well it is an olds front wheel drive 3.8L engine with 118,000 on it........I am curious on the dragging impellor thing.........one belt drives it all. Car sat for a year, and there is rust on the smooth water pump pulley surface, kinda pitted a little, but smooth over all. I am not sure if that would cause something or not. never experienced a colapsing hose before........well I gotta guess it has to be water pump, pulley, or hose....and btw the thermostat that came out said ac delco.........so that is a good one, just wore out
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A loose wire on your sending unit can make one fluctuate also. Just a thought. To find other threads or posts on cooling problems just go to the search function on the site........here is some dots ha ha......look up near the top of the page and click on search then key in cooling system problems or something like that and click submit or go.
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Sounds like you could have some air trapped in the system.
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Yep your right....he could try to burp the system to get it out... that may be all thats wrong with it because if the sending unit probe is not in the water constantly....when the water is away from it the reading will rise even the the actual water temp is not. Thats one reason that I suggested replaceing the sender....it has 116,000 miles on it so he really needs a new one...they do not cost much and when he removes the old one....as he looses a little coolant...the air might just escape also...then when he gets through puting the unit in and refills his radiator he might be good to go then.Jake_Dragon said:
Doc here, 
imp:
If you have rapid excursions of temp readings on your gauge, The prime suspect is Trapped air in the Cooling system.
To Burp the system, The best way is, place the front of the car on a slight incline, Fill the system AND The recovery tank.
Get a lower pressure than normal Radiator Cap, Install that, Start and warm the car..Monitor the Gauge, Do Not let it Overheat!!
Once warm, stand back a bit, and watch and listen as the system clears it self...It will expel coolant into the recovery tank, and draw back pure coolant replacing the airspace..
It looks terrible and sounds really bad while your doing it, but it is the best way to clear the air. Run until it no longer expels air..and DO Not let the recovery tank run dry.
When your satisfied, Shut the engine off, let it cool, and replace with the proper cap, lower the front and test drive it.
Air can become trapped in a water pump, and reduce or even stop cooling, Also, A Temp sender can not read air..Hence as water Flows past it the bubble moves a bit, as it settles back the bubble goes back surrounding the sender..High..Low.
Try that and let us know the result.
Docimp:
imp: If you have rapid excursions of temp readings on your gauge, The prime suspect is Trapped air in the Cooling system.
To Burp the system, The best way is, place the front of the car on a slight incline, Fill the system AND The recovery tank.
Get a lower pressure than normal Radiator Cap, Install that, Start and warm the car..Monitor the Gauge, Do Not let it Overheat!!
Once warm, stand back a bit, and watch and listen as the system clears it self...It will expel coolant into the recovery tank, and draw back pure coolant replacing the airspace..
It looks terrible and sounds really bad while your doing it, but it is the best way to clear the air. Run until it no longer expels air..and DO Not let the recovery tank run dry.
When your satisfied, Shut the engine off, let it cool, and replace with the proper cap, lower the front and test drive it.
Air can become trapped in a water pump, and reduce or even stop cooling, Also, A Temp sender can not read air..Hence as water Flows past it the bubble moves a bit, as it settles back the bubble goes back surrounding the sender..High..Low.
Try that and let us know the result.
Doc
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