1995 SOHC NEON, 5-speed. 198500 miles. Complete rebuild at 157K.

AC on (fan speed and recirc/fresh mode does not matter) - outside air temp over 90 - radiator fan running full out - reservoir shows full to hot level.
The temp guage slowly crawls to overtemp over a period of time especially if I am doing stop and go driving. Does it faster if I go up hills, accelerate, haul A$$, etc.

If I turn off AC, guage slowly returns to normal.

If I leave AC on, but switch to full fan/heat, heat pours out of system and guage starts to drop quicker.

If I turn off AC, and leave switch on full heat/full fan, heat pours out of system and guage drops to normal in seconds.

Turn AC back to cold (regardless of fan speed or recirc/fresh mode), guage starts creeping up again to overtemp.

When at home, after cool down, check fluid, seems to be full even overflow tank shows fluid in it - usually mid level.

Does not do this at all in winter, spring, or fall. Does not do this on cool days, or during my morning commute when temp is in 70's and lower 80's.

Does not seem to matter if I let the car get cooler by keeping the windows down for a while before I turn it on or not, and only does this after it cools down the interior of the car.

No coolent has been lost, none added, no water in oil, compression - great, oil pressure - great, car runs great, gas milage in mid 30's for normal commute. A damn fine car all around, just one quirk that could be a problem if not carefully monitored - which I am.




I have a guess what it may be, but I want to hear from some of you what you think. If you have experienced this before and fixed it, let me know. I don't want to "taint the pool" so to speak.

 

My thoughts are: bad "radiator cap", improper mixture of coolant/water or air in the system. Have you checked the level of coolant when the car is cool? Your coolant overflow may be full but may be low in the system.

 

I forgot to mention that I replaced the radiator cap with a new one last year when I noticed that it would not maintain proper pressure and would pop off and stay open too long allowing the overflow tank to overflow too easily. Now that does not mean the new one hasn't bit the dust, but I doubt it. Radiator cap is a 16lbs one.

Coolent mixture check shows that I am good to -20 deg. Which is overkill for AL. When I replaced the fluid last year, I used the 50/50 mix.

I had a big air bubble in the system (I checked and found it last night). Apparently, opening up the heater core allowed the coolent to flow in a different manner and it emptyed the overflow tank. This is the first time I have added fluid since I changed it out last year. Most likely, I did not properly bleed the system and air got trapped in the heater core and would not auto-bleed out.

Once cool, I filled it back up, took off the radiator cap, put my hand over the cap hole and hand pumped the upper radiator hose until I could not get the system to hold another drop or squeeze out any more bubbles. I topped off the overflow tank, sealed the system back up.

Today at lunch, it did rise quite a bit, but never all the way to the overheat line and I was driving all over Huntsville, AL in stop and go lunch traffic. I will check the overflow tank level when I go out to go home tonight and see if it is still full. I checked when I got back to work and the system is holding pressure, no leaks, and the overflow tank shows full hot.

Even with the system having a full fluid level, after tooling around at lunch, the system still showed a tendency to climb which it does not do unless it is very hot outside and I have the AC on.

Two things I think can help:

1) A lower temp thermostat value (say 165 or 180). However, that only controls the lowest temp it can be to open up once it gets to operating temp., so if it is already higher then that value, a lower temp themostat won't help much.

2) A larger capacity radiator. A larger radiator can handle more heat, but does anyone know a drop in replacement that will work without having to modify everthing under the hood on that side of the engine? A supplier and part number would be helpful.

 

Drove home yesterday after checking the system - full in overflow tank, full in line. Had the AC going full blast - nice cold air on a hot drive. Even hotter today and going to stay that way all week. Ewwww. Sticky heat! Makes me want to be in Canada until the end of August.....

It almost got to overtemp, but you could see when the thermostat opened because the guage would start to drop almost immediately. Never lost a drop. System is still full. So it appears that my first problem was the air bubble.

I think I am going to go to Autozone and get part # 4208. It is a 180deg thermostat. That is 15deg different from the OEM one. If having a 15deg quicker opening point means that more coolent can pass thru the radiator sooner, then maybe it will do the trick. Not expensive to try, quick to do too. I'll let you know. That should also be just fine for AL winters too.

 

Good idea.

Have you also ran the car on with the radiator cap off for a certain amount of time to let any remaining air out as well? It's a good way to get any residual air out of the system.

 

No. Good idea. Will do that.

Also, I decided before I spend $12 on a lower temp thermo., I will go without one to see if it makes any difference at all. If it does, THEN I'll get the lower temp one. If it does not, then no need to spend the $12. I have time tonight to give it a whirl.

I already have an old thermostat with the center knocked out and still have the outer ring and seal to put in its place.

I do have one question to ask. Has anyone ever experienced a problem with the temp coolent sensor ONLY giving inaccurate readings once it reaches "x" temperature (which for the sensor means 'x' voltage)? High side failure is what I would call it.

I might think it was a temp. guage readout problem, except it behaves perfectly fine all other times and only seems to give an 'incorrect readout' when hot outside with AC on. The voltage comes from the sensor, so the PCM gives the readout based on that voltage, and the guage is just doing its job (so to speak). If the guage were wonky any other time in any other scenario, that I would begin thinking "cold solder issue" and look harder at that.

I mean, as long as the outside air does not reach a certain level (usually 85+), I NEVER see the guage move above the 1/3 mark (AC on or off). When it gets cold outside, it NEVER goes above the 1/3 mark. On hot days (85+), if I run the AC, it quickly gets up to 1/3, stays there until the interior car is cool, then begins to climb. I can turn off the AC, switch to full heat, let it blow on full fan, and it will almost immediately drop back to 1/3 and stay there. I can then turn off the heat and just let outside air blow, and it does not rise back up!

 

i ran into the same problem here in flordia anything over 80 degrees and at highway speeds it would overheat with the ac. stop and go no problem. the problem i found was the air dam that mounts to the bottom of the core support was null and void. i gave up on finding one and made my own.

 

I'm sorry, but I don't understand. "Air dam that mounts to the bottom of the core". I assume you mean the radiator core here, but what do you mean "air dam" and to that what you do mean by "null and void"?

Sorry to be thick headed, but if I don't know, I ask.

 

I was wondering the same thing. I think he may be referring to the fan housing being cracked or broke and not circulating air properly because it wasn't directed through the cores. Maybe, idk.

 

Wanted to say air bubbles will work there way out over time on there own. Usually cause false temp readings or slows the circulation of fluid. If all the sudden the level of coolant was low I would be trying to figure out why. Does the vehicle warm up slow and then slowly climb into the red? Usually plugged fins on the radiator or stuck thermostat causes that. Heres the big question I have for you, do the fans work properly? Get a good scanner and check your high/low switch speed.