I did a search and read for about an hour, but I can't find enough information to put the pieces of this particular problem together myself.

THE PROBLEM: Out of nowhere I noticed that the temp needle on my 98 EX is about 3/4's at a stoplight. Goes back down when I start driving. Since about 2 weeks ago, my engine will get about that hot at a stop, but will cool off once I start driving. Only 3/4's doesn't sound like an immediate emergency, but it's not 115 in Vegas yet.

THE FACTS:

1) Water pump replaced last year when I did my timing belt. (Probably not water pump)
2) I have not changed anything within the system since that change last year, so I don't believe the system needs bleeding. (Probably not air bubbles in system)
3) The fan DOES turn on when I'm at a stop. (Probably not fan)
4) All hoses are warm, so that makes me think that coolant IS flowing? (Probably not a clog?)
5) Coolant is filled and clean. No leaks. (Not lack of coolant)
6) While driving, the temp is right where it needs to be. No fluctuations. (Probably not thermostat?)
7) At a stop, while the temp is rising, I can stop the rise with about 2500 rpm. Make it go back down to proper temp at 3000 rpm.
8) During winter this year I was having a hard time getting the air out of the ducts warm unless at constant power (highway speeds). However, engine temp was always perfect.
9) The radiator has performed flawlessly in St. Louis 95+ degrees, so it should be able to handle 75 degrees.

I am at a loss with this. I'm staying with my sis out in LV because I lost my job...soooooo, I'm without much money and definitely without a garage. So, I'm trying to get this right the first time because I no longer have the luxury of just throwing in a different part to experiment with what this may be.

Any ideas guys?

 

There was someone else on here with a similar issue, and in their case it turned out to be a new (dual-core I think) radiator that solved the problem.

If there isn't air in the system, and the cap is holding proper pressure, I would think it sounds like a stuck thermostat (maybe not fully closed, or just doesn't want to open fully) or the water pump - I'd assume and hope not since it was replaced, but that would be my next guess.
If it isn't running lean or something, and the ignition-timing is correct or close to it, the only thing else I can think of is maybe running a flush product to remove the build-up from the rad. etc.

 

That was my 1st thought stuck thermostat or water pump.

To see if the thermostat is stuck stick in in a pot and boil the water. The thermostat should pop open.

 

How difficult is it to get to the thermostat? Looks pretty complicated from pictures that i've seen.

 

Actually easy to do. Drain the coolant.
Follow the lower radiator hose to the back of the engine block. There should be 2 10mm bolts holding the housing that holds the thermostat.
Remove the lower radiator hose.
Remove those bolts, some coolant may rush out so I'd advise putting some rags under there and under the car.
Pull the thermostat out, put new one in and make sure spring pin is up (also I think the word "up" is on the thermostat). The spring (from memory) should be facing away from you.
Reinstall the housing and torque down the 2 10mm bolts only to about 7-10ft/lbs! Good thing to buy a torque wrench.
Attach lower radiator hose, refill with fresh coolant.

 

Alright, I'm gonna give this a try today. Sounds like I don't need a crazy amount of tools for this. I hate that my garage and tools are in a different part of the country.

 

Alright, so I decided that without a garage I should go with the path of least resistance (easiest/cheapest first and so on). So I took another peek under that radiator cap and it looked like it wasn't full all the way.

1) Is it possible to look one day and have it look full and the next and its not? Of course there could be a leak, but I've looked all over, dont smell antifreeze, etc. Would an air bubble support the fluid level so that it looked filled?

So I went and bought a bottle of Water Wetter and poured that whole thing in there. After that, it looked full. On the way home, the symptoms changed a bit. Now instead of heating up at a stop, it was perfect. Then it would raise when I started off the line and stabilize hotter than normal, but cooler than it used to go when it was at a stop previously. However, this was a short trip and the time between stop and go ~could~ have coincided with the period of time it took for the temp to start to raise at a stop...does that explanation make sense? Also, the temp was much more erratic this time. It was not a steady rise then a quick fall, it went up and down a bit more.

2) Could this have been due to the Water Wetter mixing in the system?
3) Could the Water Wetter have lubed up the thermostat or is it possible that the additive is just good enough to keep the system cool at a stop?

I'm at a loss guys. I'm gonna go out now that its cooler and add some distilled water if I can fit it in there. I'll keep updating on the progress.

 

Ok, it looks like Fact #5 was, in fact, untrue. I was able to fit a little less than a quart of water in the radiator. What I believe happened was the black plastic lip down inside the filler hole had coolant on top of it, giving the illusion that the radiator was full. So far the temp has stayed perfect doing a little rough driving and stopping so I believe at this point the problem is fixed. Now I hope I did not cause any damage to the water pump, etc by not having enough fluid in the system. I guess I may have a very slow leak because everything I can see is dry and I have never smelled antifreeze.

 

I would actually be inclined to go with a stuck open thermostat also check your radiator cap, a malfunctioning radiator cap won't be able to pressurize the system and can allow coolant to evaporate. Also if you are going to pull the thermostat out to test it, you mite as well put in a new one for the trouble. Thing costs like 7 bucks.

 

Wouldn't a stuck open thermostat cause it to run cool, not hot?

The radiator cap seems to be providing pressure (if I release it slowly while hot, steam comes out as I release the spring).

I was also thinking that about testing the thermostat. Why go through the trouble for such a cheap part? Is there any difference between the $7 and $13 one? I'm not looking to save money on such a cheap part, but I'm just curious.