Engine runs hot when decelerating

Tiny
CORNICUS
  • MEMBER
  • 1992 BUICK ROADMASTER
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 222,500 MILES
Car was just purchased for $800.00. Everything looks and sounds clean. Rides amazing no knocking, cracking, creaking, squeaking, or abnormal sounds or sensations. When driven around city everything behaves per usual. When driven on highway when decelerating the engine runs very hot. Comes right back down after a few seconds of either staying still or accelerating. I'm trying to use this as temporary means of transportation for a few months. I was wondering if by my not very thorough explanation of the problem I'm having you could maybe point me in the direction of remedy. I can answer questions if there's follow-up needed. Thank you so much. This is a wonderful act of generosity from all of you guys for giving free advise.
Tuesday, December 10th, 2019 AT 2:51 PM

4 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,573 POSTS
Hi,

The problem only happens when you slow down from highway speeds? Interestingly, one would thing the opposite would happen. Tell me. Have you checked engine coolant? Is it clean and full?

https://www.2carpros.com/articles/engine-cooling-system

Let me know. Also, let me know if it happens all the time.

Take care,
Joe
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Wednesday, December 11th, 2019 AT 7:44 PM
Tiny
CORNICUS
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It happens all the time. The coolant is at full and appears to be clean. Seems to happen all the time now. It goes up until it hits 260 then right back down after a few seconds to around 180-190.
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Thursday, December 19th, 2019 AT 1:43 PM
Tiny
CORNICUS
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It also only has one huge belt driven fan behind the radiator that is working from what I can tell.
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Thursday, December 19th, 2019 AT 1:44 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,573 POSTS
Hi,

If it is doing it all the time, and gets that high and then drops, it sounds like you have a thermostat sticking or maybe even the temp sensor is bad. I hate to ask this, but when it shows 260°F on the gauge, does it seem as though the engine is actually that hot? For example, is there any coolant sounds, steam, or coolant coming out of the system? I would hate to say replace the thermostat and find that isn't the issue and that the engine really isn't that hot.

The strange thing about this is that it happens at highway speeds and not in town. That is the opposite of the normal overheating issues.

If you want, here are directions on replacing a thermostat. You can check the old thermostat by dropping it into boiling water to see if it opens. However, once it's out, I would just replace it, but that would be your decision.

https://www.2carpros.com/articles/replace-thermostat

Since it drops as fast as you described, I suspect it is the thermostat sticking, but a faulty temp sensor should also be considered. I do have one other thought. Is the cooling system pressurizing when it is at operating temp? In other words, do the radiator hoses get hard from pressure?

Let me know.

Joe

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Thursday, December 19th, 2019 AT 7:39 PM

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