James Posted March 10, 2010 Share Posted March 10, 2010 Hey guys. New issue has arisen. I bypassed the heater matrix yesterday so that I have no air lock and, thus, the car doesn't overheat. What I found last night was that the car is "underheating" I guess you could call it. If I leave it idling, it starts to heat up to the correct temperature. However, when I start to drive (not even hard) it starts to go down again. If the bottom reading on the coolant temp gauge is say 50 degrees, with the top being 130 and the "usual" middle point being around 90, then it is getting to 85/90 at idle but then dropping down to 65/70 when I get going again. I'm confused as the oil temperature is spot on at 80 degrees driving normally and is heating at the typical rate. I'm guessing the first port of call would probably be the thermostat? I have no warning lights coming on and it boosts fine as well as idling spot on, it's just that I'm very conscious of that temperature and - essentially - I'm wondering what could be causing it and if I drive, how damaging will it be doing so? Thanks in advance! Quote Link to comment Share on other sites More sharing options...
SupraHuman Posted March 10, 2010 Share Posted March 10, 2010 Temprature will rise and fall depending on load. Quote Link to comment Share on other sites More sharing options...
James Posted March 10, 2010 Author Share Posted March 10, 2010 As drastically as that it'll continue to stay perfectly at what I'd deem "optimum" temperature as in at the middle of the gauge at idle but when I drive around it drops to a quarter of the way up the gauge though?! It never did that before. Normally stayed put. As mentioned, the oil is doing what I'd expect (rising if I plant it a bit, staying happy around 80 if I am just strolling). Quote Link to comment Share on other sites More sharing options...
Suprab1 Posted March 10, 2010 Share Posted March 10, 2010 Hi mate, i had this prob with another car and it was thermostat. Its worth checking it anyway. Quote Link to comment Share on other sites More sharing options...
Lee P Posted March 10, 2010 Share Posted March 10, 2010 Thermostat is stuck open by the sounds of it. Quote Link to comment Share on other sites More sharing options...
SupraHuman Posted March 10, 2010 Share Posted March 10, 2010 Or what Lee said Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted March 10, 2010 Share Posted March 10, 2010 Fit a new genuine Toyota thermostat. If it still overcools it would be very weird, but you could try plugging the heater hoses, which is only what effectively happens when the heater is turned to the cold setting. Quote Link to comment Share on other sites More sharing options...
dr_jekyll Posted March 10, 2010 Share Posted March 10, 2010 aslong as the oil temp is correct i would imagen this would be bad for the engine no? Quote Link to comment Share on other sites More sharing options...
James Posted March 10, 2010 Author Share Posted March 10, 2010 aslong as the oil temp is correct i would imagen this would be bad for the engine no? Would be? Or wouldn't be? I'm hoping it's not! How easy is it to fit a thermostat and where is it located? Quote Link to comment Share on other sites More sharing options...
dr_jekyll Posted March 10, 2010 Share Posted March 10, 2010 sorry wouldnt be. i have run my old bmw without a thermostat for a long time and its never caused a problem, my thinking is aslong as the oil is at optimum temp the cooler the the water the better. the thermostate only gets the water up to temp quickly for the in car heaters. or am i completely worng and just making things up again ? Quote Link to comment Share on other sites More sharing options...
James Posted March 10, 2010 Author Share Posted March 10, 2010 sorry wouldnt be. i have run my old bmw without a thermostat for a long time and its never caused a problem, my thinking is aslong as the oil is at optimum temp the cooler the the water the better. the thermostate only gets the water up to temp quickly for the in car heaters. or am i completely worng and just making things up again ? That was kinda my thoughts too! I hope it's the case anyway! Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted March 10, 2010 Share Posted March 10, 2010 Seeing as the engine management system reads the water temperature to decide on fuel and spark delivery, and a high water temp is needed to run correct bore clearances, as well as drive combustion water products form the oil, which is water cooled, I would say having the water temps the car was mapped and designed to achieve are pretty important. Quote Link to comment Share on other sites More sharing options...
GMan Posted March 10, 2010 Share Posted March 10, 2010 Seeing as the engine management system reads the water temperature to decide on fuel and spark delivery, and a high water temp is needed to run correct bore clearances, as well as drive combustion water products form the oil, which is water cooled, I would say having the water temps the car was mapped and designed to achieve are pretty important. Even more important on built engines with forged pistons as they have a higher coefficient of expansion Quote Link to comment Share on other sites More sharing options...
dr_jekyll Posted March 10, 2010 Share Posted March 10, 2010 Seeing as the engine management system reads the water temperature to decide on fuel and spark delivery, and a high water temp is needed to run correct bore clearances, as well as drive combustion water products form the oil, which is water cooled, I would say having the water temps the car was mapped and designed to achieve are pretty important. wow never new the water temp is really that important. so in that case when i fit my new bigger rad should i have the map adjusted to compensate for it, or will the ecu read that the car is running cooler and self adjust? Quote Link to comment Share on other sites More sharing options...
GMan Posted March 10, 2010 Share Posted March 10, 2010 If you use a stock thermostat the temperature should stay about the same since the thermostat will start to close when the temp drops bringing you back to the normal temp but you will have much more cooling reserve when you are running hard. A larger Rad makes your thermostat work alot more so should always be accompanied by a new thermostat as an old one might be done in by all of the extra opening and closing Quote Link to comment Share on other sites More sharing options...
dr_jekyll Posted March 10, 2010 Share Posted March 10, 2010 If you use a stock thermostat the temperature should stay about the same since the thermostat will start to close when the temp drops bringing you back to the normal temp but you will have much more cooling reserve when you are running hard. A larger Rad makes your thermostat work alot more so should always be accompanied by a new thermostat as an old one might be done in by all of the extra opening and closing thanks, i was having this converstaion on the weekend funnily enough, my car runs verry cool allways. it gets just past the quarter verry fast and stays there whaer as other supras i know go to the half, mine will only get up to half if you leave it to idle for a while but on the move it comes back and sits at just over the quarter again.?? Quote Link to comment Share on other sites More sharing options...
GMan Posted March 10, 2010 Share Posted March 10, 2010 thanks, i was having this converstaion on the weekend funnily enough, my car runs verry cool allways. it gets just past the quarter verry fast and stays there whaer as other supras i know go to the half, mine will only get up to half if you leave it to idle for a while but on the move it comes back and sits at just over the quarter again.?? You might have a low temp thermostat fitted. I have a nismo low temp thermostat on my skyline and it does the exact same thing, quarter way up real fast then stops, will only go to half if i leave it idling a very long time, will drop down to a quarter almost instantly when i start driving Quote Link to comment Share on other sites More sharing options...
loks Posted February 17, 2012 Share Posted February 17, 2012 Sorry to ressurrect this thread but I have the same "underheating" issue since getting my car back on the road. It now takes an eternity (5-10mins?)to heat up to the normal temperature reading on the gauge (around 30-40% up) and usually it stays quite stable. However today I did a longer run on the highway and during that higher speed run the gauge actually dropped to around 10% on the gauge :-() and I got a little concerned. As this is HK I was soon back in traffic and the reading went back up again to normal. I have invested in a new Toyota thermostat, seal and even radiator cap in an attempt to fit at the weekend. My question to the original poster - did fitting a new thermostat rectify the issue completely? Quote Link to comment Share on other sites More sharing options...
James Posted February 17, 2012 Author Share Posted February 17, 2012 I'm not sure what was causing it in the end. I ended up just taking on the big task of changing the heater matrix and all has been well since! No underheating or overheating! I never did change the thermostat. Quote Link to comment Share on other sites More sharing options...
Tricky-Ricky Posted February 17, 2012 Share Posted February 17, 2012 I have seen this happen with air locks, the temperature fluctuation seen is down to the sensor not always being in contact with the water, when it is the temp will rise, but when in air, it will drop, owing to it needing a physical connection with water in order to read, i also disagree with the earlier comment's about fueling being directly influenced by water temps, other that the ECU only registers water temp as cold start IE it uses cold below 20-30C as a trigger to run higher injector DC for stat up, once over this it ignores water temp, any further fuel trim is influenced by IAT inlet air temperature, and the trim is minor. Quote Link to comment Share on other sites More sharing options...
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