Strange overheating problem
 

My daughter's 2008 CX-7 with 185k miles is overheating and I'm having trouble diagnosing the cause.
She had the CEL light come on while driving it. She pulled over immediately and found the coolant reservoir was empty. She got a ride to Autozone, bought coolant, and topped it off. I was out of town so my wife came to help her. My wife drove the car back to my daughter's place, ignoring the CEL and apparently ignoring the temp gauge as well so she says she does not know if it overheated or not. A few days later I was able to look at the car. It still had nice-looking oil but the coolant was down about 1/2 gallon. I topped it off with coolant and started the car. The car started normally and sounded normal. Here are the symptoms I noted:

Symptoms:
The car starts fine. I left the radiator cap off for a minute and did not see any bubbling of the coolant. I let the car idle for 10 minutes and it warms up and then holds the temperature right in the middle of the gauge as it should.
I took the car for a drive and it quickly overheated after 1 mile or less at city street speeds. I pulled over and let it cool down. Then drove it back to her place with the heater running full to give me some extra cooling.
I assumed that there may have been air trapped in the system. I added a bit more coolant but it did not need much.
I took it for another test drive and made it 3 or so miles that time and assumed it might be stable now. I decided to drive the car back to my house about 30 miles away so I could spend more time investigating it. The car overheated again after about 3 miles and then it would overheat about every mile after that. After the 3rd time, I decided to Google a high-rated shop in the area and take it to them to fix since it would have taken me all day to get the car 30 miles home at that rate.

The mechanic is stumped. He found one small hose that was leaking and he's replaced that hose. The car is still exhibiting the same symptoms. He says he does not think it is the head gasket. I did not ask what tests he has done. He's going to give me a quote on replacing the thermostat but I doubt it is the thermostat since it holds temperature at idle.

I'm thinking of having the car towed back to my house so I can investigate further but that will be an additional $275 spent on a car that's not worth much.

Does this sound similar to any overheating experiences other CX-7 owners have had? I'd appreciate any recommended steps.

Thanks.

First find a different "mechanic: There is no "stumped" in the vocabulary of diagnosing problems.
It is NOT a MAZDA or specifically a CX7 problem. This has happened to many platforms for , well since water cooled engines have been around. ;)
There are about a baker's dozen things to check starting with the condition of all the cooling system parts. This mean pressure testing the radiator and cap.
You should also consider a coolant chemical test to be sure you have no contaminates from a possible head or head gasket leak.
Then there is the thermostat.
Because you have overheated it at least twice you may have damaged the coolant temperature sensor so reading the gauge on the dash may be giving the wrong information.
Just with those alone should narrow it down to the cause of the overheating or at least eliminating them and moving on to the harder to test parts.

As for the CELs/ DTCs what were "ALL" the codes pending and permanent? And did you reset them before each test drive?

Thanks for the advice. I was planning to do the chemical test to see if I have exhaust contaminants in the coolant.
I think I may just pay to have it towed to my house so I can work on it myself and not have to pay for all the labor.
I checked the codes on the car the day that I went to retrieve it from my daughter. There were no codes. I should have mentioned that.

That is a sign of concern. If you did over heat you should have had at least 1 if not several DTCs???

I called the mechanic and asked him if he read any codes. He said it had a steering-angle sensor code and that was all.

The other odd thing is that when I drove the car and it began overheating on me, the CEL never came on. I did not think to plug my code reader in and check the codes at that time. I would pull over and shut the car down before the needle actually hit the red, but it would be one needle width away, so it was indicating that it was pretty hot and you'd think that be enough to set off the CEL.
Another odd thing is how quickly the temp goes up once it starts. The temp will move from 1/2 gauge to full scale in less than 30 seconds. I don't see how the water can change temperature that fast. During all of this, I've had the heater on blowing hot air, so that tends to tell me that the water pump is continuing to work.

A friend just said that he'd seen an engine act like that when it was not getting proper oil flow because the head was warped and causing blockage in the oil galleys. It seems like I'd be getting a big oil leak or oil in the coolant if that were happening.

Anyway, I'm going to retrieve the car tomorrow and will start looking at it this weekend.

Hmm I would like to know what engine that is? :rolleyes:
Tic tic tic very load if that were to happen long before the head would warp !
At any rate that is NOT your problem for so many reasons to list .... :cool:

ASE

The mechanic also told me that he had the car overheat on him after idling for about 20 min. I'd never let it idle that long before taking it out for a drive where it then started overheating. The mechanic said it holds temperature for quite a while then it just takes off and quickly pegs the temp gauge.

I'll have the car back to my house tonight or tomorrow. I'm not sure what to check first. First I'll replace the serpentine belt. Mechanic said it was in bad shape. Then I'll replace the temp sending unit and radiator cap. Then, before I spend any more money and time replacing parts, I may get one of the kits to check for exhaust gasses in the coolant to see if I may have a failed head gasket.
Any better ideas or plan of attack? I hate to just start shot-gunning it by replacing parts.

Maybe I should fix the car up so I can junk it :)

I seldom ever recommend to randomly replace any parts until they are confirmed bad. Often you can create new problems or make a current problem worse.
I will say that the last response how the heat suddenly went up suggests a sticky/faulty thermostat.

You can verify this by either using a handheld temperature gun or removing it examine it and testing is in water in the kitchen. lol

I will replace the thermostat this weekend.

I got the car towed back to my house about an hour ago. I did not find any DTC's set. I've found that my OBD reader and app show a different temperature than the temp gauge does. On my app (OBD Fusion) the temp started out normal and rose to 190F and to about 210F and held there for a bit. I stopped watching the app for a bit and was just watching the temp gauge on the dash since they seemed to be roughly tracking. Then when I looked at the app next (maybe a minute or so) the app was showing engine temp in the middle of the red-range at 230F. However, the temp gauge on the dash was still in the middle of the range and had not really moved.
Without a good service manual on this car I have no way to know if my OBD reader and app are pulling temp from the same place that the temp gauge is. It seems they are not. Perhaps the gauge on the dash is oil temp and the OBD reader and app are reading coolant temperature. Even though the app showed temp in red at 230F, there were no codes that got set.
When I had attempted to drive this car after the event happened with my daughter, the temp gauge on the dash would eventually go to the red and when it started it went quick. So now I know that whatever temp the OBD reader is reading had probably gone in the deep red long before the temp gauge did. But if I trust the readings from the scanner then it does seem that it may be as simple as thermostat stuck closed.

Update: I tried to read oil temp with the OBD reader and there is no data. I plotted a graph of the coolant temperature and it just steadily trends up until it is showing in the red zone for the OBD reader. The dash temp gauge shows middle range. I guess I'd have to get the car really hot before that gauge would hit the red. I'm not sure why the OBD seems to indicate differently from the dash gauge.