Temperature swinging

My daughter's '92 Grand Am with a SOHC Quad Four is exhibiting some perplexing behavior from the coolant temperature gauge. At steady, flat highway speeds the temperature continuously cycles between about 160 and 200
over a forty second or so time span. The swing from 200 down to 160 is about three times faster than the swing from 160 up to 200. Don't know how long this has been going on. Just drove the machine back from Delaware to give it a thorough brake inspection when I noticed the gauge fluctuation. No fuel/ignition/drivability issues are present.
Didn't really think that the thermostat was causing this but went ahead and replaced it anyway. No difference afterward. By the way this is a 195 degree thermostat.
Is the gauge's behavior telling me that there is a problem needing found and corrected? What are the possible causes of something like this?
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Could be an early sign of head gasket blow-by.
............ Jonny

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Silver Surfer wrote:

possibly air in the system?
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Thank you,


CL Gilbert
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Temperature sensor is the first suspect. They are cheap but most always hard to replace. Do it. It will control the electric cooling fan(s). Next, on level of cost, is the radiator hoses. A collapsed hose could cause this. Lastly, replace the radiator, given this is a '92 model and the original, it is corroded. I doubt it is air in the system, from a head gasket failure or any other means. That would cause immediate overheating and you would see other symptoms from what you describe.
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Nope. Early signs are only visible when the engine is revved for a long time like highway driving. Or just a dribble around town. The cooling system can take care of the additional heat, but will show from time to time on the temp gauge. Simple test is to remove the radiator cap, and manually rev the engine while under the hood. Bubbles that correspond to the revving are a pretty good sign.
Now, let's get down to your input. Radiator that works then doesn't work, IE cools then doesn't cool. I'll buy that for a dollar and a song if you throw in the Brooklyn bridge too. They usually clog, and fail to some extent to remove some heat. Depends on the clogging conditions. Collapsed radiator hose, that means there's a high/low pressure difference someplace. Not the hose that's the problem. Temp sensor. Finally, yes that can happen, but they usually fail or work. No wish washy stuff, unless its female of course. ............ Jonny

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Silver Surfer wrote:

I agree with Jim on this one. Possible head gasket going bad. See if you can get an automotive HC (hyrdocarbon sniffer) to use in the radiator reservoir. If you know someone in environmental business you could use that type, also. Don't contaminate the probe though. Run compression and leakdown tests.
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Some good points have been made. Thanks to all who have responded.
In all honesty fear of another head gasket failure was lurking way back in the dark recesses of my mind. Maybe it was just wishful thinking that this wasn't a possibility.
I'll try the bubble test first as it is easiest and cheapest.
By the way if there is a slight head gasket leak into the coolant what effect, if any, would the combustion gases have on the coolant? When I drained it to change the Tstat it looked to be a much darker green than usual with little transparency. Of course the cooling system hasn't been drained and flushed for a number of years. Maybe that's the explanation. Maybe not.
" Paul " <"=?x-user-defined?Q??= Paul

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Silver Surfer wrote:

My observations are that the coolant gets murky... less transparent and more translucent, but not really darker. Probably due to suspended HC particles.
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Filled up the cooling system to the brim of the expansion tank today and raced the engine for a while with the cap off. Saw a medium bubble of something exit on two or three random occasions. My thinking is that a very small quantity of air might have been left in the system after the recent draining and refill for the Tstat replacement.
Never did see a stream of bubbles of any kind even at high RPM.
Took the machine for a spin with the "radiator" cap loosened. Pretty much the same temperature swing as before, but maybe a little less perhaps because ambient was about 15 degrees colder today. After 5 or 6 miles tightened the cap and headed back home. Noticed the swing seemed to be lessening so drove across the bridge to Ohio and continued driving another 5 or 6 miles during which movement of the temperature gauge needle became barely perceptible. Took the exit and swung back for the trip home. This time the needle movement was noticeable but by no means as much as it had been at the beginning of the drive.
The behavior today put me in mind of a control system taking a long time to reach equilibrium after a step change in conditions. It didn't get much of a chance to reach a steady state on the drive from Delaware because of the ever changing terrain between here and there.
My next move is to track down a sniffer or other tester to find out if there really is combustion gas in the coolant, although it's hard for me to believe there is a head gasket leak after today's experience. Still, an explanation for what is happening would be preferred even if it turns out to be bad news.
" Paul " <"=?x-user-defined?Q??= Paul

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Thanks for the update. Paul
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<big ol' snip>
If you can get more bubbles to come up through the radiator neck, you can also use your own 'sniffer' to see if the bubble(s) smells like fuel and exhaust fumes.
Worked for me several times, but I don't know how good of a sniff you'll get from a bubble or two.
I believe there are test kits available to test your coolant for the presence of dissolved combustion chamber gasses. I've never used or looked for one though. I don't know how much they cost.
Good luck.
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