1999 Chevy Malibu Overheating only when driving

Tiny
KHAAN
  • MEMBER
  • 1999 CHEVROLET MALIBU
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 146,000 MILES
Hello, this is my moms car, and recently she started having overheating issues. The antifreeze had been disappearing for a couple months, and there were no leaks anywhere below the car. We decided to replace the lower intake gaskets, hoping that was where the antifreeze was going. Now, the antifreeze is always full, so one problem solved. However, now we have another. We replaced the thermostat for her as well, and I think I may have gotten a bad one, or one for a different temperature. Ever since replacing, she has had practically no heat. And the car is still overheating. I can idle the car in the drive way for as long as I want, and it will not overheat. The fans kick on, and the temperature holds steady. I can drive the car for about a mile or a little over and it overheats. It's like clockwork. If I take the same route whenever I test drive it, it overheats at the same street everytime. The coolant is forced out of the reservoir, so I have to refill it. If I turn around, and head towards her house, its downhill most of the way, so I can coast a lot, and it wont overheat as quickly. And I have noticed that when it starts to get hot, if I let off the gas and coast, the temperature will come back down after a minute. And everytime the temp comes down, I will have hot heat for a few minutes. As soon as the temp starts to get hotter, the heat gets colder. I was wondering if a thermostat could cause all of this, or if it might be something else entirely. I replaced the lower radiator hose because it was brittle on the backside and I know they can be sucked shut by the water pump when they are soft. I also replaced the serpentine belt because it was worn. Any suggestions would be greatly appreciated.
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Wednesday, January 7th, 2009 AT 6:41 AM

5 Replies

Tiny
ZACKMAN
  • MEMBER
It can be as simple as replacing your coolant reservoir cap. If and only if it is not maintaining the 15psi the cooling system requires.

If that doesn't work, you need to have the vehicle pressure-tested leak down tested to see where the coolant is leaking too.

Check your oil. Is it "milky"? If it is, along with your description, you have a tell tale sign of bad head gasket.
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Sunday, January 11th, 2009 AT 1:37 AM
Tiny
ZACKMAN
  • MEMBER
There is a Technical Service Bulletin for your situation.

ENGINE RUNNING HOT/OVERHEATING &/OR LOSS OF COOLANT
TECHNICAL SERVICE BULLETIN
Reference Number(s): 00-06-02-001, Date of Issue: January, 2000
Related Ref Number(s): 00-06-02-001
ARTICLE BEGINNING
ENGINE RUNNING HOT, OVERHEATING AND/OR LOSS OF
COOLANT (POLISH RADIATOR FILLER NECK AND REPLACE
RADIATOR CAP)
Model(s): 1999-2000 GM Passenger Cars and Trucks with Composite Radiator End Tank
Section: 06 - Engine/Propulsion System
Bulletin No: 00-06-02-001
Date: January, 2000
CONDITION
Some customers may comment on one or more of the following conditions:
Engine running hot
Engine overheating, and/or
Loss of coolant/low coolant message

CAUSE
The radiator filler neck may have an imperfection in the sealing surface.

CORRECTION

NOTE: Do not replace the radiator.

Using a piece of 400 grit wet/dry sandpaper backed with a flat piece of wood, polish the filler neck
sealing surface using a circular motion.
Replace the radiator pressure cap with a cap of the same part number as shown in the GM Service
Parts Catalog.
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Monday, January 12th, 2009 AT 9:58 PM
Tiny
ZACKMAN
  • MEMBER
It is. You do not have a filler neck on the reservoir itself. Your only access is through the coolant cap. What they are saying is that the cap cannot seal tight due to the imperfection around the filler neck. Let me know.

Zack
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Tuesday, January 13th, 2009 AT 9:07 PM
Tiny
AUSTIN94
  • MEMBER
Zack the Malibu is a plastic cap and neck I am having the same problem as him I have replaced my water pump thermostat and reservoir.
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Saturday, November 10th, 2018 AT 4:09 PM
Tiny
STEVE W.
  • EXPERT
Overheating while driving can be from various issues. A small head gasket leak that gets worse under load, bad radiator that cannot circulate enough water to keep the engine cool under load, blocked exhaust system that traps heat and does not let the engine cool down.
A quick test for the radiator is to crank the heater to max and see if the temperature stops rising or maybe goes down. If it does then it is likely an airflow or coolant flow issue through the radiator. On cars with AC the condenser can trap a lot of stuff between it and the radiator and block the air. Another test is to run the car until it gets warm, shut it off and use your hand or an IR thermometer to check that the radiator is the same temperature without any colder areas that would result from blockage.
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Sunday, November 11th, 2018 AT 8:23 PM

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