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Weird overheating issue

I drive a 1999 Dodge Neon (SOHC, if it matters). In the past three weeks, it has gone into the red twice, and nearly so one other time. On one other occasion, the temp needle bounced around between it’s normal position and about 5/8th up.



Two of the two times when it went into the red and the time it almost did, I had been driving around, I parked to run in somewhere, and came back out a few minutes later and started the car. That’s when it was suddenly hot.



But if I turn the car off, and back on, the needle falls to normal.



Mechanic said it was the thermostat - third time happened after replacement, so it’s not.



My first thought was that the cooling temp sensor was off. I had some issues with cold starts in the winter (I’m in texas) so that seems to fit. I told the mechanic that, but he didn’t listen.



Thoughts? I don’t want to waste more money needlessly. Thanks in advance…

Whoops that should be one of the two times…

Much of Texas is very hot this time of year, so I’m assuming your car has AC and is operating in a hot climate. It is not unusual for the car’s motor temp indicator to spike after you turn off the motor. There is heat in the motor and there is no coolant circulating to cool it. When you turn the motor back on the water pump circulates the coolant and the temp drops.

When you run the AC there should be an electric fan motor that turns on to cool the radiator. There is another electric fan motor that should turn on whenever the motor’s coolant reaaches a specified temperature. These fans may not be operating correctly. With the car idling with the AC on you should hear one fan all the time and the other one will kick on after a few minutes to keep the motor cool. Check out these fans to see if they are operating properly.

The coolant temp sensor is easy and inexpensive to replace. Install a new one and see what happens. This is a DIY project if you have even minimal tools.

When you turn the engine off the coolant temp within the engine will go up because the coolant is not longer circulating. This is called “heat soak,” and it’s normal. As soon as you start the engine again the coolant circulates through the radiator and returns to normal temperature.

So I guess my followup question is - Is this something I need to address right now? I ask because I need to drive eight hours tomorrow, primarily highway, for a family event. And I don’t have time to mess with the cooling sensor before then. I’m trying to figure out if it’s safe to drive or if I should rent a car. My fiance’s car is in worse shape than mine so that’s not an option.

My big worry was that this could be the water pump not kicking on. But that would seem like an odd problem to me.

I did check to make sure both the fans were running once while it was in park. But I am not willing to risk keeping the car on when it’s overheating, so there hasn’t been a way to check if that’s what’s causing the issue when the issue is actually happening.

Thanks for the info about the “heat soak”. I didn’t see that happen last summer, but last summer just wasn’t as hot either.

Oh yeah - just remembered (sorry, still a little early here) - I’ve also occasionally been getting a check engine light. Owner’s manual just says that’s an emissions issue. Does that put another point in the coolant sensor column?

Engine temperature does have an impact on emissions, but rather than just dismiss the illuminated CEL as being connected to a defective coolant sensor, you need to determine what trouble codes have been stored by the car’s OBD II system. There could be more than one code, and more than one issue that needs to be attended to.

If you go to Auto Zone, Advance Auto, O’Reilly, and some other parts retailers, they will scan your car’s system gratis and provide you with the codes that they find. Then, come back to this thread and post those codes for more detailed help.

Just for future reference, it is important to mention an illuminated CEL in your first post, rather than mentioning it later. This makes for a more rapid consensus on the problem.

Water pumps don’t cycle on and off. The water pump is working whenever the engine is running.

Whether or not it’s safe to drive the car depends on the definition of “overheating.” You told us the temperature gauge climbs near or into the red zone, but you never said whether or not any coolant is lost. Does the radiator boil over and lose coolant, or does the gauge just read high?

So far you say this only happened after the engine had been shut off for a short time and then restarted. You didn’t say you’ve had any problems with overheating while you were driving the car. How does the temp gauge look while you’re driving?

The CEL is an early waring system, and the only way to find out why it is coming on is to have the car’s computer scanned. A mechanic can do it or you can stop by your local auto parts store and let them have a look.

Yeah, I know I should have mentioned it earlier… I honestly forgot about it. It was on the list of things that need to be fixed before my car gets inspected in August. Which, since I’m getting married in three weeks, isn’t at the forefront of my attention right now. Then I realized they might be connected whilst I was googling some stuff.

I’ll try to get to an advanced auto or auto zone tonight. There’s at least one 24 hour that I know of, so even if I can’t swap the sensor out, I might be able to stop in late for that.

The other time I went into the red I was driving… I was turning into our church parking lot after driving for about 15 minutes, and the warning beep went off. I immediately shut off the car. I turned it back on and it was normal again. Other than the three times I’ve mentioned, and the one time it was bouncing around, the gauge is at the same spot it’s always at - about 3/8 of the way up the scale.

On that occasion, we opened up the hood in the church parking lot after getting it to a spot, and poked around a bit. I didn’t see any indications that anything was actually wrong - no smoke, no steam, no weird sounds. I checked the top and bottom hose and they were both warm (which is why I wasn’t completely happy about the repair shop swapping the thermostat out - but I didn’t have the time or energy to argue). Both were warm enough that you wouldn’t want to hold your fingers on it too long, but not so hot as to burn on contact. We didn’t open up the radiator cap until after church, but there was definitely fluid and it seemed normal. I topped off the reservoir just in case.

I’ll try to get to a parts store tonight.

I previously had a water pump lose all it’s blades, which was misdiagnosed by a repair place, which is why I’m a little paranoid about it. They didn’t check it because “it looked new”. That’s an angry story for another time.

Well, the engine code was P0140… O2 sensor, according to the auto zone employee. So a totally separate issue that I’ll address later on. That problem has been intermittent as well, and the check engine light is off for the moment, so that’s fine.

If I make it on the trip and back with the car still functioning, I’m going to replace the coolant temp sensor next week. They said they’d have to get it from another store anyway.

Make sure your fluid is at capacity on the holding tank before you go. I have heard of some oddities designed into gauge operation, a radiator pressure test might be a good idea too.

I’ll double check tomorrow morning. You’d think the mechanic would have done that test when I took it in on Monday. I’m starting to think they gave me the blow off easy answer on account of being a chick.

Well, I made the trip. Fine all the way there on Friday. Fine driving around on Saturday to do errands and attend the family event. NOT fine yesterday. After the first hour of driving or so, the car spiked into the red, and we used the heater to bleed off the heat from the radiator. Pulled over, and checked everything. We spent the three hours after that alternating between A/C and heat. Fantastic. Fans are running during overheating, by the way. I no longer think this is a sensor issue, though, since blasting the heat did cool the system down. Still no outward signs of overheating - no boiling or bubbling in overflow, or steam or smoke or anything anywhere. We tried everything and eventually stumbled on the fact that turning the A/C completely off reduced the frequency of overheating. It still bounced upwards a few times, but much less than without A/C, so we did that for the last four hours of the trip. I’m starting to notice a correlation between left turns and overheating too, but that could be coincidence. If anyone has any thoughts, please respond. Right now I’m just trying to figure out the best local mechanic.

What comes to my mind is:

  1. Bad thermostat
  2. Bad temp sensor
  3. Air in coolant
  4. Blown (or partially blown) head gasket

Glad you made it home safe and sound. The pressure test and radiator cap is where I would start. If those pass a onto fitjeep below suggestions.