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Discussion Starter #1 (Edited)
I have a 2004 Chevrolet Optra/Forenza and it only overheats on the highway on days over 30 degrees celcius and when I drive at speeds that usually start at around 120 km/h and an rpm of around 3500. When it starts to overheat and gets roughly 3/4's on the temp gauge I turn on the heat during these very hot days and there is very little heat that comes out of the vents. I recently replaced the heater core and when replacing the coolant I ran the car with the heat on for roughly 20 minutes to burp the system of air. While doing this I also noticed the elecrtric fan started when the thermostat opened and shut off when it closed. I observed coolant flowing in the overflow container so I do believe that the coolant is cycling, however i'm not sure if the thermostat could be partially stuck closed. I am getting extremely frustrated with this becuase I had to take the long way home on hot days (off the highway) so the car will run cooler. This is the only time it ever overheats and I am beside myself trying to figure out this problem. Any help would be great with this issue. Thanks
 

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hi. sounds like it might be a thermostat problem or a fan problem. if you say the fans are turning on i would start with the thermostat. its a fairly inexpensive part i think. let us know what happens. any other questions just ask
 

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Fans shouldn't come into play at highway speeds. If you barely have heat it sounds like a t-stat not opening (I. E. coolant can't flow), a blockage, or possibly even a water pump that isn't pumping (broken off impeller blades?). Could also be low coolant since if there's no coolant reaching the heater core there will be no heat.
 

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With minimal heat coming from the vents at highway speeds, it sounds like you have a coolant circulation issue or low coolant level as was mentioned above. Considering that the heater core circulation happens whether the thermostat is open or not, a stuck thermostat should still allow for plenty of hot air out the vents.

I've actually had my water pump impeller crack right at the shaft so that the shaft could spin semi-freely within the impeller. Granted that was not an OEM water pump, it was a Gates aftermarket replacement with a plastic impeller and metal shaft. You may want to have a closer look at this... in my situation when the engine was cold, the impeller would spin OK, but would just slip more and more as it heated up.

At highway speeds you should have plenty of frontal airflow, so if you saw the fans working, I don't really see that being the issue.
 

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one thing i seem to be confused about. before you changed the heater core you had no heat coming out of the vents or after changing the heater core theres no heat coming out of the vents?? if now you have no heat coming out of the vens then yes you should also check the water pump. it might not be handling the higher RPM's of the highway driving. so check the thermostat and pump. thanks guys atc250r exist3nce for chiming in!!
 

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Discussion Starter #6
Hey guys..... This is why it's so confusing. The car only overheats on the highway on very hot days and driving at speeds of approximately 130km/h. On days with average heat I have no problems other then the fact I still dont get hardly any hot air out of my vents with the blower cranked on full, the blower is working as it it shoul. My fluid levels are where they should be. Upper and lower rad hoses and heater hoses are all getting hot. When I put the heater core back in I had great heat but since I overheated it and a bunch.of antifreeze was expelled from my system it just hasn't worked right.
 

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How badly overheated? Did / does the temp needle go up into the red? If so, you should immediately change your driving conditions and / or shut the car off to avoid damage. If it's been overheated and u kept driving you could have caused the headgasket to start to blow..

How does the car start up after its been sitting? Does it seem hard to start or rough at first and then clears up ? Any usual smells of coolant or oil getting burned?
 

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Discussion Starter #8
When it originally overheated it was hot enough it was expelling coolant out of tge system and yes it does get to the red line. I have since put in new coolant and bled the system of air. I also paid special attention for constant bubbling while it was closed to see if what you would normally see if the head gasket was blown and there were none. The car also starts fine and I now pay close attention to the temp gauge so I can slow down to take necessary action to prevent damage. I really just wish I could put a finger on exactly whats going on. Only get warm air, not hot and the overheating is really getting annoying. Every hose in the cooling system is hot and while bleeding the air I put my hand over tge overflow resevoir and coolant is hot. Can't figure it out for the life of me.
 

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Discussion Starter #10
I went out again and tried burping system of air and was paying close attention to my heater hoses. While doing this I did notice that only one of my hoses was gettng hot which leads me to believe that it is indeed the thermostat. I called around and of course this is a dealership only part and it comes with the housing as well. I really hopethis is a correct diagnosis as the thermostat is 85 and the gasket is 15 bucks lol.
 

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Thermostat + housing + gasket for the Forenza '04-'08 is about $20 at rockauto.com

Couldn't find anything for the Optra, but I believe it's the same engine.

Could any Optra owners chime in?
 

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yeah $85 is a pretty high price. its definitely not a dealer only item. its been available for quite a while now. you can even get them on eBay. id go with rock auto theyre a really good company. and yes its the same part for the forenza/reno/optra
 

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Discussion Starter #13
Yeah i'm not 100% certain they are the same motors or not. Rockauto has forenza thermostats but no listing at all for the Optra which makes me leary on purchasing an item that should realistically have the same part number if they are in fact the same. Any help on this would be great.
 

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Like I said above, They're the same car. The optra isn't listed because its not sold in America. They both have the 2.0L u20sed motor.
 

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I'm still skeptical that the thermostat is the problem.. but if its like only $20 or so, might as well give it a try - it won't hurt to have a new stat in there if you are still on the original factory piece. I was wondering, if you just idle the car and let it warm up, do both fans cycle on at a certain point? Hopefully they do, and then can you confirm that the air that they are blowing is actually all hot? What I mean is stick your hand down in front of each fan to feel the full spectrum of air blowing - top, bottom, sides, etc - ALL of the air should be hot. If just some of it is hot and the rest is cold, it means the hot coolant isn't circulating through your rad.

The reason I'm doubting the stat is the main problem is if both your heater hoses are not getting hot it means one or more of the following:

- coolant is not circulating properly in the system (failing water pump - I've had this happen)
- blockage in the system or heater core
- air in the system

The way the cooling system is designed is that the water will flow through the heater core long before the thermostat opens, so even if it was stuck closed you should still be able to get hot air from the heater vents REGARDLESS. However, one of the above points could cause this not to happen.
 

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Discussion Starter #16
Hey guys thanks for your help... Got thinking about it and a tgermostat made no sense. Why would one heater hose get hot with a punched thermostat... I flushed the heater core and wammo... Blistering heat and im sure that was my.overheating culprit. Thanks again!
 
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