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Got hot then cool real fast?

This is a discussion on Got hot then cool real fast? within the External Engine forums, part of the LSx Technical Help Section category; So I left work tonight and swung through a drive-thru for dinner. I didn't think about it, but I did ...

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    James Bond Spikito's Avatar
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    Got hot then cool real fast?

    So I left work tonight and swung through a drive-thru for dinner. I didn't think about it, but I did sit there in line for quite awhile, but it was dark, in the 50s, and I had JUST cranked my car, but sure enough, as I pull out, I see my "check gauges" light on, and see my temp all the way up at 260. I had just pulled out on the highway and was coming up to a redlight, I'm less than a mile from my house but I was gonna stop anyway in a lot just past the light.

    So as I'm sitting there at the light, eyes glued to the temp gauge, It suddenly drops to just under 210, right about my normal driving temp. I was again, sitting still, and had only driven about 300 yards since the light had come on.

    I figured once I got going itd creep down, but not a sudden drop, and I figured at 260 the thermostat would have been stuck wide open and wouldnt make a sudden shift from the hot water in the engine, to the cooler radiator water, plus, an instant 60+ degree drop, i doubt my radiator is THAT efficient in 300 yards of 30mph driving.

    Could it be that my thermostat was stuck shut and just happened to pop back open at the red light?

    Since it dropped back to normal, and I was so close to home I went ahead and came back, temp stayed the same all the way, and when I got out I could def tell some things got a bit warm under the hood.

    It was dark when I got home, and obviously under the hood was hot, Ill look tomorrow to see if Im low on fluid or anything.

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    Certainly sounds like a stuck thermostat. Were the fans running?

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    Member SuperSSguy's Avatar
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    Have you recently opened the system maybe got an air bubble in it?

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    My Monte Carlo's temp gauge would go to the red, then drop under half way in about 5 seconds time when the Coolant was low. It was one of its quirks I guess...

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    Member excrider121's Avatar
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    The temp sensor sits high in the coolant system. If you're more than 2 quarts low on antifreeze, the temp sensor will read the temperature of the super heated air, not the coolant itself.

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    expensive tires az gt eater's Avatar
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    Quote Originally Posted by excrider121 View Post
    The temp sensor sits high in the coolant system. If you're more than 2 quarts low on antifreeze, the temp sensor will read the temperature of the super heated air, not the coolant itself.
    It's true.

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    Senior Member Too Fast's Avatar
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    I'd first check to see if it's low on coolant. If that's ok, I would change the t-stat and watch for the cooling fans to come on, then shut the car down and let it cool off 30 minutes, then top off the coolant as needed. Then check for proper cooling fan operation, coming on about 225 and going off about 210.

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    I would change out the thermostat and flush the cooling system if it hasn't been serviced in a while. Your temp gauge is an estimator at best -- only '98s have a functional gauge. My dash gauge reads just below 210 degrees all the time when actual engine temps run from 185 to 196 degrees on my scanner.

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    James Bond Spikito's Avatar
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    lots of good info on here, thanks guys.

    I said I'd check on it "tomorrow" but forgot I had to get up early and head out of town for two days. just got back, and I really will check it out tomorrow.

    I have a 160 t-stat that Ive been meaning to put in, and intended to do a coolant flush before it got too cold, I may as well do it all now.

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    If you plan on installing that 160 degree t-stat, it's recommended to also get your radiator fans tuned to turn on sooner. Otherwise, it defeats the real objective of installing the lower temp t-stat.

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    James Bond Spikito's Avatar
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    they may be, my PCM was tuned by Frost awhile back, not sure exactly what all he did

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    expensive tires az gt eater's Avatar
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    Quote Originally Posted by Spikito View Post
    they may be, my PCM was tuned by Frost awhile back, not sure exactly what all he did
    I'm sure he keeps all that on record, call him up and ask.

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    James Bond Spikito's Avatar
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    So When I was leaving church yesterday I noticed that either at red lights or normal driving, the temp was cycling between 250 and 190, about once a minute. to me that seems like a steam pocket.

    Once cool I popped the radiator cap with the car idling, I had to look down into the radiator about 8 inches to see any coolant. I started adding water and had to put in about half a gallon. at that point, water started sputtering in from the throttle body return, I stopped and watched for a minute as lots and lots of bubbles came up to the surface, as the level dropped I added more water and it finally leveled off and quit bubbling.

    fluid was pretty murky, so I think a flush is in my near future....what does one of those normally run? cost wise?

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    It's easy to do. Just takes time.

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    James Bond Spikito's Avatar
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    yeah I did it on my 3.8....just not sure if I did it RIGHT, lol

    I did'nt know about freeze plugs at the time, so I just opened the stop cock on the bottom of the radiator and kept running fresh water until it ran clear, then drained as best i could, then refilled with coolant

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    Senior Member Too Fast's Avatar
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    Quote Originally Posted by Spikito View Post
    So When I was leaving church yesterday I noticed that either at red lights or normal driving, the temp was cycling between 250 and 190, about once a minute. to me that seems like a steam pocket.

    Once cool I popped the radiator cap with the car idling, I had to look down into the radiator about 8 inches to see any coolant. I started adding water and had to put in about half a gallon. at that point, water started sputtering in from the throttle body return, I stopped and watched for a minute as lots and lots of bubbles came up to the surface, as the level dropped I added more water and it finally leveled off and quit bubbling.

    fluid was pretty murky, so I think a flush is in my near future....what does one of those normally run? cost wise?
    First find out where the coolant went. It'd be a good idea to have your system pressure checked somewhere, to see if there are any leaks. If it checks ok, then do the flush. No sense flushing the coolant only to find out the water pump is seeping, then do another flush. 8'' or so down in the radiator is quite a bit low and explains the wild temp fluctuation. Good thing you have a temp gauge to get on top of the situation before just an idiot light comes on to tell you it is already overheating. It's true our temp gauge is not a direct connection to the sensor, but it is a feed from the PCM to the cluster, so it still indicates temp with a bit of a damper. One less wire to run.

    A pressure check is about $25 around here, and a flush for your car is like $130.

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    James Bond Spikito's Avatar
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    ok cool, ive never noticed a leak....i wonder if its been a slow evaporation via the overflow tank....ive never really checked out the coolant in the two years ive had the car

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    Senior Member Too Fast's Avatar
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    ^^^if you've never checked your coolant in the two years, then hold out your left hand and slap it with your right lol.

    Cars do use a bit of coolant over the years, so it's a good idea to check the level at least with every oil change. Hopefully that was you only problem, a bit low.

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    James Bond Spikito's Avatar
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    i know i know...im usually on top of my maintenance, that's just one i never messed with a lot. I also knew that as soon as i did, I'd want to do something about it, lol.

    When I had the cap off, i gave it a few little revs, hoping to spin the water pump faster and hasten this process, when i did, i noticed the coolant level in the radiator would rise, then fall...any ideas?

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    James Bond Spikito's Avatar
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    just passed a pressure test....

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