Overheating at Idle Problem

[ya boy willz]

My 97 NA with 87k miles has been overheating while at idle since the beginning of summer now :fp: I've already researched the topic quite a bit and used the Cooling System Problems FAQ on m.net. The car will only overheat if I'm at a long red light or if I'm driving very slowly (<15mph) for a few minutes, it doesn't matter if the A/C is on or off. It will reach the 3/4 mark on the temp gauge and then I'll shut it off to prevent any overheating damage or start driving. As soon as I gain some speed and start driving it will drop back down to regular operating temperature (a little under 12 o clock) quickly and stay there until another prolonged idle.

The radiator was replaced about 10-11 months ago when I first bought the car. The cooling fan kicks on when it's supposed to and seems to be turning at the proper speed (I'm not really sure how to check what is the "proper" speed so I'm just eyeballing it), and if I remove the radiator cap and stare at the coolant as soon as the thermostat opens the fluid starts flowing. So I decided to replace the radiator hoses since they were old. I did all three hoses (using the Autozone Dayco ones) and it didn't change anything. So then I replaced the Heater Hoses (again with Dayco ones) and again nothing. So at that point I decided to just go ahead and throw an OEM thermostat and a new (autozone brand) radiator cap at it today and again to no avail.

Today while the car was heating up again I decided to shut off the A/C and put on the heat to make sure the heater core wasn't bad. It started blowing warm for about the first 30 secs but then immediately started to blow HOT. Something odd was that even with the heater on 2 the temp didn't really go down and would just fluctuate a bit between the 3/4 position and a bit more towards the 12 o clock position. I remember reading that turning on the heater core should help lower the temp since all that hot coolant is being used to heat the car.

At this point I'm just stumped and getting ready to call it quits and just take it somewhere to have someone else diagnose it. But before I do that I wanted to see if anyone on here could possibly have any ideas that could lead me to a solution. The car isn't blowing smoke out the tailpipe and when I checked the oil from the dipstick it didn't seem to be contaminated with coolant and the coolant in the radiator doesn't seem to have oil in it so I don't think my head gaskets blown. I also don't have any misfires or rough idles so I'm leaning towards my head gasket not being blown. I'm going to pull the plugs tomorrow just to be sure there's no coolant on them. I'm also going to be connect my OBD 2 w/ the torq app again tomorrow just to ensure the fans kicking on at the right time and the new thermostat is opening at the right time.

Could my water pump possibly be calling it quits or am I just overlooking something obvious? Any and all help is appreciated. And no I don't have a grill blocking air flow to the radiator/condenser. I do have a radiator cowl cover though, but that should help cool not heat.

[kung fu jesus]

Did this happen before you replaced the radiator?

Makes me think of three possible issues:

A - Air trapped in the system.

B - Bad water pump. That is low mileage for the year. If you replaced the radiator for sludge, the impeller on the waterpump *could* be rusted away to nubs.

C - Bad thermo sensor.

Are you losing coolant? Is the reservoir tank showing this? When you replaced the radiator, was the coolant sludge-y? If so, did you flush it? If not, your radiator may be clogged. Did you use the correct radiator cap (the pressure matters)? What coolant mixture did you use?

I don't mean to insult you, I am just covering the bases with those questions. You can test the thermostat by boiling it in water on the stove. Maybe at that time, leave it out and try driving the car without it and see if it starts to overheat again.

[maharaj]

I am guessing water pump.

[ya boy willz]

Did this happen before you replaced the radiator?

Makes me think of three possible issues:

A - Air trapped in the system.

B - Bad water pump. That is low mileage for the year. If you replaced the radiator for sludge, the impeller on the waterpump *could* be rusted away to nubs.

C - Bad thermo sensor.

Are you losing coolant? Is the reservoir tank showing this? When you replaced the radiator, was the coolant sludge-y? If so, did you flush it? If not, your radiator may be clogged. Did you use the correct radiator cap (the pressure matters)? What coolant mixture did you use?

I don't mean to insult you, I am just covering the bases with those questions. You can test the thermostat by boiling it in water on the stove. Maybe at that time, leave it out and try driving the car without it and see if it starts to overheat again.

I had the radiator replaced before I bought the car by the dealership I bought it from. It had the brown death and was leaking a bit after we test drove it so I had it replaced. Due to this I don't really know the condition of the coolant/cooling system before the radiator replacement. The thermostat housing did have some buildup on it though when I replaced the thermostat today so it could be possible that the water pump be rusted. I don't think I'm losing coolant but I'll refill my overflow tank to full and drive around until warm to then check and see if I'm losing coolant after it cools back down. I drove the car for about 6 months before it started overheating though with no problems.

The radiator cap is correct (0.9 bar, which is like 13 psi or so). I flushed the coolant when I replaced the radiator hoses and the coolant that came out was clean, but again this was probably due to the radiator having recently been replaced. I filled it with a 40% Peak Antifreeze 60% Distilled water mixture. The thermostat is brand new so it should work properly but I'll test it as suggested.

Is there anyway I can test the thermosensor? I'd imagine read the resistance of the sensor w/ it unplugged and then make sure the plug is getting 12v correct? Also a bit of a noob question but I could just jack up the car, remove the radiator cap and then burp the system with the current coolant in there correct? I just want to try the basic/cheaper stuff first before I tear down into the motor and replace a Water Pump just to find out that wasn't the cause of my problems.

[Agent☣Orange]

Before you do something drastic, just remove the radiator cap after the motor has sat all night and check your fluid level. KFJ made a good point about burping the system but you don't have to jack up anything to see if you're low.

You can indeed run the car with the radiator cap off provided you start from cold but really, if it's sat all night and the fluid level isn't right at the cap, top it off first. Burping the system takes a little time, sometimes overflowing a bit then all of a sudden a bunch of air bubbles out and you have to add a little more fluid.

Any time something was recently done gives cause to suspect the problem is related. The reason I agree with KFJ about burping first, besides the fact that he's smart, is that it takes a while and I don't rely on shop techs sticking around staring at your radiator for an extended period of time..

By the way, thank you for an excellent first post with thorough information.

[kung fu jesus]

The problem I have with the thermo sensor as the issue is the ECU will throw a CEL if it isn't reading correctly on OBD1 miatas. The OBD2 cars are more sensitive to this.

I would do as you said, try to burp the car. I like to suggest the easiest solitions first, too. I have seen cars take longer than others to accomplish that.

I am throwing this out there, too...perhaps the dealer replaced the bad radiator with a defective or maybe even a used radiator?

The junk collecting at the thermostat housing isn't uncommon.

[ya boy willz]

I'll burp the system tomorrow morning and report back. It is possible that the dealer replaced it with a used radiator but it was a small private owned dealer and the guy seemed honest and the radiator seemed new (no bent fins or browning side tanks) so I'll take his word on it being a new one.

Another noob question as for the proper level is it supposed to be all the way up to the radiator cap inside the radiator or just up to the fins? I've been running it with the level right at the fins so perhaps that's what I've been overlooking. Should I just keep filling the radiator until its way above the fins then?

Sorry for some of the noob questions and I appreciate the help guys!

[Agent☣Orange]

No need to keep apologizing. Feel free to ask anything. If your radiator appears new, no need to question it. Yes, a full radiator should show a water/anti-freeze level exactly at the level of the overflow tank drain hole. In other words, remove the cap and you should see green fluid right at the neck. If not, then you have to add a little fluid and burp the system.

[ya boy willz]

Added some more fluid and burped the system, then topped it off. I let it idle for a while in my garage and it got up to about 223 degrees (using an ELM 327 Bluetooth Adapter & the torq app to read temps) and then the secondary (A/C) fan would kick on and then temps would drop to 212. I let it do this 3 times then shut the car off and let it cool down. Re checked the level and both the overflow tank and radiator were still full. So then I cruised around the neighborhood and making sure to take a long time at stop signs and no overheating at all. Even after sitting for about 10 minutes it just reached 223 and then dropped down again once the secondary fan kicked on.

So far everything seems good but I'll have to test it tomorrow during a nice 85-90 degree day and not a cool 75 degree night. Hopefully the combination of it having some air in it and being low on coolant was the cause of my problems. I did pull all the plugs and no coolant or weird wear at all actually so that's a great sign.

[kung fu jesus]

Great news! Let us know how she holds up!