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Car overheats at highway speed only! 60 and above!

This is a discussion on Car overheats at highway speed only! 60 and above! within the External Engine forums, part of the LSx Technical Help Section category; Started having this problem where I would take my ride on the highway, and after about driving a few minutes ...

  1. #1
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    Car overheats at highway speed only! 60 and above!

    Started having this problem where I would take my ride on the highway, and after about driving a few minutes over 60mph, the temp gauge would start to rise and hit the red. After slowing down to about 45mph and driving like that the rest of the way, it would cool off and be fine. So what's the deal? Is it the waterpump?
    Thought so too... Just replaced it today, took it for a test drive and it still does it! What could it be? Do I need to flush the system? Do I need to check something else? Before I poured that dex-cool into the engine and waste it, I just used regular water to see if it would still overheat, and it did... so now what? Anyone else have this problem? Thanks

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    Senior Member Too Fast's Avatar
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    My first thought is to make sure the air dam under the car is there. It directs the air up into the radiator at highway speeds. I know the 3rd gen cars were picky about this problem exactly, I bet our 4th gens are the same.

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    Air dam, trash bag on on it, maybe even hose out the radiator from under the car. I cleaned mine out and it dumped all kinds of black crap out.(Cleaning from outside)

    Also did you change your thermostat?

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    3-Time F-Body Offender doozjamann's Avatar
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    Have you flushed your radiator? Trash can cause your thermostat to clog and overheat your car.

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    I remember like 1-2 years ago, I bought that 160 degree Thermostat. How do I test it? Also, I cleaned the radiator's outside and there was little debris, but it wasn't major. No plastic bags stuck to it, and as for the air dam under it. Well, I took mine off a few years ago. I was tried of hearing it scrape everywhere I went. BUT...... Here's how I know it's not that. I changed my hood, and added a Volant intake, the opening of the new hood, matches the volant really good. I mean, I could put the air dam on and test it out... but like I said, I started having problems a few years down the road, but will try that out. Thanks.... Anything else?
    Also, when it's cooler out... or night time, I could probably stay above 60 and the car won't over heat! WTF?

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    put the thermostat in a pot of water and put it on the stove. as the water heats up the thremostat will open.

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    Senior Member karpetcm's Avatar
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    Also if your car overheats just one time your coolant is useless and your car will continue overheating as it was for my case. Could be your thermostat, also make sure your fans are working properly.

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    Senior Member JaycenK's Avatar
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    I had that problem in an 85 IROC and it was 20 degrees out side the thing would go up to near red and then open for a minute and then close again and go back up to the red while I was driving 65 on the Hwy. Changed the thermo and problem solved

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    Quote Originally Posted by ItalianSS View Post
    I remember like 1-2 years ago, I bought that 160 degree Thermostat. How do I test it? Also, I cleaned the radiator's outside and there was little debris, but it wasn't major. No plastic bags stuck to it, and as for the air dam under it. Well, I took mine off a few years ago. I was tried of hearing it scrape everywhere I went. BUT...... Here's how I know it's not that. I changed my hood, and added a Volant intake, the opening of the new hood, matches the volant really good. I mean, I could put the air dam on and test it out... but like I said, I started having problems a few years down the road, but will try that out. Thanks.... Anything else?
    Also, when it's cooler out... or night time, I could probably stay above 60 and the car won't over heat! WTF?
    Its the Air Dam. Put it back on. Without it, air will not reach the radiator.
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    I can't believe you have run it that long without an air dam. As far as I know, the temp gauge in your '99 is an approximation at best. Have someone check your temp with a scan tool or use a laser temp gauge on the thermostat housing to get an idea. Did you re-program your fans for the 160 degree stat?

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    Royal pain in the.... Phantom2kz's Avatar
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    Quote Originally Posted by Ed Blown Vert View Post
    Its the Air Dam. Put it back on. Without it, air will not reach the radiator.
    + 1

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    Quote Originally Posted by pajeff02 View Post
    I can't believe you have run it that long without an air dam. As far as I know, the temp gauge in your '99 is an approximation at best. Have someone check your temp with a scan tool or use a laser temp gauge on the thermostat housing to get an idea. Did you re-program your fans for the 160 degree stat?
    I'll put the air dam back on... As for tuning it, I did.
    I'll check to make sure the thermostat is working. I'll pull it out and test it. Thanks for all your help. I really hope it's only the air dam or the thermostat.

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    Quote Originally Posted by JaycenK View Post
    I had that problem in an 85 IROC and it was 20 degrees out side the thing would go up to near red and then open for a minute and then close again and go back up to the red while I was driving 65 on the Hwy. Changed the thermo and problem solved
    Thank you... I really hope that solves the problem.


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