Quick question on thermostat operation

I just put a scanner on my 99 sunfire 2.4L and read the engine temperature after a short trip. The temp rose to 89 Celcius, then hovered around 88 - 89 degrees... about 190 farenheit. The thermostat should be 180 for this engine.

Is it normal for the engine temperature to be 10*F higher than the thermostat rating at idle?

Thanks!

190 is not a scary number. You might even see it go up to 210. Anything over 210 is what i'd consider starting to overheat.

Thanks for the response.

The reason I ask it that I'm failing the emissions test on Oxides of Nitrogen, and I'm wondering if this might indicate that a 190* thermostat was installed instead of a 180* without actually draining the cooling system to check it.

A thermistat opens up typically at 190 degrees. Depends on the vehicle though they are all differant. The thermistat will stay closed untill that point so the engine will heat up quickly. Once the thermistat opens if you conitnue to watch you will see the temperature rise untill eventually the cooling fan kicks on. Once again depending on the vehicle this could be 220-235. Varies from car to car obviously. When the cooling fan kicks on it will bring the temp back down to around where the thermistat originally opened at. Then the fan will shut off and the temperature will rise again.
Thats the basic jist of it anyway. :sly:

High NOx most commonly would be caused by lean fuel mixture. High engine temps could cause this but you are not running "HOT". Maybe if you where running over 240 and actually overheating but that is not the case. Starts with having the computer scanned, see if there are any codes. That would be the first place to start. You could always pull some plugs and see what the look like but that will not tell you for sure what the problem is but it will tell you if your running lean....

Thanks for the response.

The reason I ask it that I'm failing the emissions test on Oxides of Nitrogen, and I'm wondering if this might indicate that a 190* thermostat was installed instead of a 180* without actually draining the cooling system to check it.

Your car should have a 195 deg thermostat in it. A 180 stat is actually incorrect. With it, the ECM will think the engine is running a bit too cold and will be trying to warm it up, in theory, causing a possible slightly rich mixture and wasted fuel.
Of course, this is in principle. Acutal fuel mixture depends on the particular ECM programming and the sensetivity of the sensors.

Also, excess NOX often indicates a malfunctioning EGR system.

Thanks for the replies guys.

That is odd... according to the info I've found it should be 180*. Also I do not have an EGR system...they canned it halfway through the 1999 year.

I guess I'll start the engine cold and look for the point where the temp drops to determine at what point the thermostat is opening.

As for the emissions test, it is indeed running lean. I think the main culprit is a bad B1S1 O2 sensor... it was reading up to .9V at times. However, since my NO levels were 3X the limit, (and no EGR to help out) I'm trying to do everything I can before taking it in to be re-tested to get the NOx down... Will be doing fuel filter, plugs, O2 sensor, even an oil change. Just figured that ensuring I had the correct thermostat in there would help keep things cool.

Check, double check and recheck your vacuum lines. Those things always have worn out vacuum lines, most notably (but probably not your problem in this case) the MAP sensor had a vacuum line going to it instead of being on the intake and the bend is usually always cracked on those.

Check, double check and recheck your vacuum lines. Those things always have worn out vacuum lines, most notably (but probably not your problem in this case) the MAP sensor had a vacuum line going to it instead of being on the intake and the bend is usually always cracked on those.

Good advice. I remember when I bought it the MAP vacuum line was corroded. I'll be sure to check all the vacuum lines tomorrow.

195-deg thermostats are the norm for most vehicles, but looking up your thermostat replacement, it does come up as 180 degrees.

If no EGR, then I'd suspect a bad O2 sensor as well or a partially clogged cat. Swap the upper and lower O2 sensors and recheck on the scanner for voltages and codes. If they remain the same it is in the wiring and/or the cat itself. If not change out the bad sensor.