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Old 10-21-2011, 07:55 PM   #1
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Heats way up, then cools rapidly

So lately I've noticed my car has been running a little warm and would sometimes heat up to about 200, then cool to about 195 and stay there for a while. Starting about yesterday morning it decided it was going to heat way up, to about 230, sit there for a few seconds, and then cool back down to 195 within the course of about 10 seconds. This happened while driving on the interstate to work at about 70~75 mph. I finally had to pull of to a gas station when it decided to only cool down to about 210 or so, then heat back up, then cool down to about 220, then heat back up again.

So after letting the engine cool for a while and calling in to work I drove it home on the back roads at about 50 mph or so, and stoplights. Then set out to fixing this, but I'm really running out of ideas here.

This heating up/cooling off is happening at just about any speed. It happened driving on my way home. It happened at a stop light. It happened on the interstate. I could drive for miles at the same RPM/speed and it stays cool... then out of nowhere starts heating up quickly, sits there for a few seconds, then starts to cool and sometimes will cool all the way down to 195. It does not always heat up rapidly during acceleration. It does not always heat up at a stoplight. It also doesn't always heat up when I'm driving a constant speed, and can start this odd cycle at any time it seems (even idling).

Here's what I did yesterday and today to try and resolve this:
  • Full flush of the cooling system, including the heater core.
  • Replaced hoses.
  • Replaced thermostat (even set it in a pot and boiled it to make sure it opened and closed properly).
  • Replaced coolant
  • Replaced heater core divert valve (I thought it might be sticking - needed replacing anyway as something inside had broken).
  • Bypassed the heater core entirely (I wanted to see if the heater core had anything to do with it) - ran it with and without it hooked up.
None of this has solved the problem. A quick 15 minute road test and it was doing it again on a straight stretch of road going 45 mph from start to end, about 5 miles into the drive it suddenly decided to start this.

I had various cooling problems last year, including a hole in the radiator, so I took the opportunity to replace the radiator, water pump, thermostat, bottom radiator hose, top radiator hose, and fill up with coolant after flushing the system... so those parts are about 1yr old.

I just don't know what would cause this, or at this point how to address it. It doesn't have a problem cooling because for a good part of the drive it stays cool. It's just at random times it decides it's going to heat up and sometimes cool down, sometimes not. What else could it be? Any help on this is appreciated!
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Old 10-21-2011, 08:07 PM   #2
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Re: Heats way up, then cools rapidly

Im just going to speculate, tehre may be a more obvious, simpler answer.

My car seems to have a combo with a much more effective radiator than most. The thermostat is a 195 degree unit. The car warms up to 195, and then immediately sinks to 160 and stays there. Sometimes when it's REALLy cold out, it will actually go back up to 195 and drop to 145-150ish and slowly rise back to 160. I've determined this is my thermostat opening and unleashing cold water back into the engine. When it's super cold out it gets cold enough to close the thermostat a while and I get a similar, but much less dramatic temperature swing.

If I were you I would suspect the thermostat has something to do with it. Are your fans coming on when they should?
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Old 10-21-2011, 08:21 PM   #3
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Re: Heats way up, then cools rapidly

Yes, the fans are coming on properly, at least when it's idling and I'm able to stand over the engine bay and look. I can't really verify they're coming on when I'm driving but when I'm able to watch they do come on.

I've replaced the thermostat already and tested it. It appears to do what it's supposed to. I can definitely say it doesn't over-cool, it always just get hotter. The problem is it gets way too hot very rapidly (like it's not getting coolant) and will sometimes cool off to about 195 and may or may not heat back up again for a couple miles.
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Old 10-21-2011, 09:21 PM   #4
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Re: Heats way up, then cools rapidly

It sounds like the thermostat isnt activating till a certain temp (which is what it is supposed to do) Then when it does it drops down to a lower temp. What did your normal operating temperature used to be before you started having these problems?
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Old 10-21-2011, 09:24 PM   #5
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Re: Heats way up, then cools rapidly

Normal operating temp was about 195, it stayed there pretty well. When it's staying cool, it sits right there.

I know it's going to heat way up whenever I see it go higher than that. It's like if it starts heating up at all, it's going all the way.

Should I try running without a thermostat completely? That would rule out a faulty one, even the one that's only a day old.
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Old 10-21-2011, 09:30 PM   #6
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Re: Heats way up, then cools rapidly

well for me my engine will hit about 220 (without fans) then drop to like 180. which tells me my thermostat is working. The thermostat is set up so that when the spring inside hits a certain temperature it expands to the point where it opens up allowing more coolant to flow through (that is why you see it drop so quick)
That is my understanding of it. I might be totally wrong but i dont think i am
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Old 10-21-2011, 10:15 PM   #7
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Re: Heats way up, then cools rapidly

I don't get it. It just seems to have a mind of its own. I drive just down the road to the stop light and everything is fine, even stopped. I turn the corner and get to driving, once I get up to speed it just starts heating up, stops at about 220 then cools down.

Sitting in the parking lot here, I rev it up to 3000 rpms and leave it there for about a minute. It didn't heat up. I let off the gas and let it idle, it heats up. I rev it up again and it cools. Then I let it idle again, it heats up, then cools on its own without me revving the engine again. The temperature gauge just swings from 220 to 195 and sometimes 230, close to 240.

What the hell?
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Old 10-21-2011, 10:58 PM   #8
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Re: Heats way up, then cools rapidly

that does sound odd. Hmmmm ill look into what it could be. Do you have any leaks anywhere?
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Old 10-21-2011, 11:23 PM   #9
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Re: Heats way up, then cools rapidly

There was a minor leak in the hose that goes from the intake to the engine block, but I replaced it and the problem is still happening. I do want to note that there is plenty of coolant in the radiator, so I'm not running it dry.
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Old 10-22-2011, 08:52 AM   #10
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Re: Heats way up, then cools rapidly

im reading this and its really weird the way your temp swings are you sure your guage is right
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Old 10-22-2011, 09:32 AM   #11
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Re: Heats way up, then cools rapidly

Pressure test the radiator cap. (or just replace it)

you didnt accidentally put the thermostat in backwards? This could cause the thermostat to open much later than its supposed too.
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Old 10-23-2011, 02:09 AM   #12
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Re: Heats way up, then cools rapidly

My temp spikes too, but only around town. It is normally around 165, then BOOM! 220! It will eventually come down, but sometimes not til i get on the highway. New coolant, hoses, thermostat, rad cap, fan works. I just think mine is tempermental. 20 years... old and pi$$y.
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Old 10-23-2011, 05:14 AM   #13
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Re: Heats way up, then cools rapidly

X2 on system pressure, with no pressure it will slowly heat up, pressure test the system, Running without a thermostat doesn't keep coolant in the rad long enough to effectively cool it.

Last edited by Hipster; 10-23-2011 at 05:20 AM. Reason: added info
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Old 10-23-2011, 02:02 PM   #14
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Re: Heats way up, then cools rapidly

Ok, I'll see if I can get it pressure tested. What am I looking for in the way of readings on it? What's a good reading and what's a bad reading?

Also, what would a bad reading indicate? I don't see any leaks anywhere, what are some other ways it would be losing pressure besides a leaking hose/radiator?

Thanks!

Last edited by gulf_coast_228; 10-23-2011 at 02:21 PM.
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Old 10-23-2011, 08:26 PM   #15
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Re: Heats way up, then cools rapidly

Ok, got a pressure testing kit and here's my results:

  • The system appears to be holding pressure. I pumped it to 15psi and it went down very slowly, which I'm guessing means I have a small leak.
  • With the engine cool and the pressure tester in the radiator I started the engine. It built up to about 7psi and stayed there for a while. The engine temp was around 190 according to the gauge.
  • Then the engine kept heating up. When it got to around 220 the pressure gauge started raising again. At some point it got to around 240 (just near the red on the temp gauge). I checked the pressure and it was at about 10psi.
  • When I checked the temp gauge it had cooled back down to about 195 (this happened very quickly). The pressure gauge was still raising and was at about 13psi now.
  • The engine started to heat back up, to around 220 or 230 on the temp gauge. I checked the pressure and it was at 15 psi now.
  • The fan came on and the engine started to cool, hovering around 200 on the temp gauge. At this point the pressure dropped to 13psi.
I'm still not sure what's going on. According to this site, http://www.ehow.com/about_5332294_au...eck-tools.html, if the pressure raises quickly to 15psi it means I'm encountering vapor lock, but this didn't raise quickly at all. It stayed around 7 for a good while and even when it reached 15psi it went down to 13 before I shut the engine off.
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Old 10-23-2011, 10:13 PM   #16
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Re: Heats way up, then cools rapidly

make sure you soak down the tester and all connections with soapy water. Small pressure leaks are often caused by the tester itself.

~7psi seems really low to me.... not sure what kind of car that ehow article is basing those numbers on. 15PSI is a little high, but not crazy.

at this point, I might suspect your gauge and/or temp sender is flaky.
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Old 10-23-2011, 10:19 PM   #17
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Re: Heats way up, then cools rapidly

Well, the oil looks good at least, it's not milky looking.

I was following this and it might get me somewhere:
http://www.youtube.com/watch?v=zUpXg...feature=relmfu

How to bleed a cooling system. I'm wondering if there's air trapped in the system. Would that cause these symptoms?
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Old 10-23-2011, 10:32 PM   #18
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Re: Heats way up, then cools rapidly

Quote:
Originally Posted by gulf_coast_228 View Post
Well, the oil looks good at least, it's not milky looking.

I was following this and it might get me somewhere:
http://www.youtube.com/watch?v=zUpXg...feature=relmfu

How to bleed a cooling system. I'm wondering if there's air trapped in the system. Would that cause these symptoms?
I red that and said "Huh? dont you just need to run the engine for a while with the cap off?"

then I watched the video.. because maybe there is a "Trick"..

Theres no trick.... 5 minutes.. gone forever...

If you have not done that, its worth a shot....


Still suspect the gauge or sender...
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Old 10-23-2011, 10:57 PM   #19
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Re: Heats way up, then cools rapidly

That didn't work. Seems fine at idle now but as soon as I start down the road it heats up again, then cools rapidly, and repeats. If it was the gauge, wouldn't it do this at idle? That's my thinking at least.
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Old 10-23-2011, 11:49 PM   #20
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Re: Heats way up, then cools rapidly

This sounds like a cyclitic overshoot issue with the t-stat. It opens, and allows the engine to cool below the thermostat's setting, then closes, and doesnt open until its beyond its normal set point. How is the t-stat mounted in the housing? Is the mechanism portion facing down into the intake coolant passage? Did you drill two small holes in the t-stat to allow small amounts of coolant to bypass it when its closed? Sometimes that can help eliminate overshoot as it prevents stagnation when the t-stat is closed, or nearly closed.

From my experience, its normal to get a little overshoot after the engine is started, but usually just going down the road, the engine will settle out at the thermostats set-point, and not oscillate.

Its also worthwhile to check the gauge as well by manually inserting a thermometer into the coolant at the rad neck (engine cold, of course), and comparing it to the gauge as the engine warms up. But, from the pressure readings, it sounds like its more or less reporting the correct temperature.

It is also possible that it could be an electrical issue. The gauge is just a moving coil meter type deal, so if the electrical system voltage fluctuates, or there are bad grounds that cause ground-loops in the electrical system, the gauge will act whacky.
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Old 10-23-2011, 11:49 PM
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