Heat suddenly starts blowing cool air after some time
Hi - I’m at my wits end with my Toyota service repair center. We bought our 2017 Sienna in September of 2017 and in February we began to notice the heat not working properly. As it is still under warranty, we are trying to deal with the dealership garage. They haven’t been able to replicate the problem, or so they say. So after driving the van for some time, sometimes as little as 10 minutes, the heat simply stops blowing out warm air. The air coming out then feels quite cool in comparison. It doesn’t matter at this moment whether we turn the gauge to warmer or not, there is no change, just the cool feeling air. Can someone please give me an idea of what this could be causing this problem? My guess is a faulty thermostat, as it doesn’t seem to happen until the car engine reaches a certain temperature(again, my guess.) Of course the technicians literally won’t do anything because the car is under warranty and they don’t want to waste money doing anything exploratory. They can’t “replicate” the problem because they haven’t taken it for a long test drive, all they’ve done is let the car idle for over an hour. I tried to explain to the service manager that a car engine idles much cooler than when it is driven, which could be why it’s not happening in idle. I want to identify the problem and fix it, even if it’s not with the Toyota garage. Please help!
So what is likely going wrong? One likely cause would be that the system is seeing the interior temperature too high and so it reduces the heat that is being put out. By default the system should go to full heat if commanded to, but anything less than that would result in lower heat output. Since the system is always trying to keep the interior at a comfortable level, it constantly adjusts the temperature blend door adding and taking away heat as required during normal operation, so if there is an issue with the temperature blend door control that could also cause a reduction in heat inside the car.
The main point here is that all of the diagnostics have to be performed while the problem is occurring. Scan data will show the engine temperature, which would rule in or out a thermostat issue. Plus the ambient and interior temperatures would also be observable in scan data. The easiest approach would be to get the car to act up and take it to them without shutting it off and get them to look at it right away since it is intermittent in nature.
Other possible issues such as low coolant flow can occur but are unlikely with a newer vehicle. That is an easy check to see if it is occurring, all you have to do is feel the heater hoses when the problem occurs. Under normal operation they both should be too hot to hold onto. If one of them is noticeably colder, (you will feel that it is warm instead of hot and can hold onto it) then you have a coolant flow issue.
updated info after getting my van back today. So a more accurate description of what is happening is that the cabin temperature is greatly influencing the air temperature coming from the vents, or vice verse, or some combination of the two. Who knows?! I tested this with a digital multimeter, so +/- 5% error is possible. I set the temp to 73 and drove around for a good twenty minutes. The air coming out felt a little cool to me, but as the air temperature outside is a little warmer recently, I could attribute it to that. The multimeter read between 70 and 74 during this time. Just to see what would happen from there, I raised the temp to 76. Definitely hotter air coming out now. I changed it back to 73, back to the warm air. Now this is where things got interesting. I set the heat to 70, and the multimeter started going down, and quick. Within 10 seconds it was at 57 degrees, and it stayed there for the next 5 minutes as I took it back to the dealership. I ran in, got the service manager, and just in about 45 seconds by the time we got back to the still running car, it now read 61, 62, and was rising. In about 2 minutes it was up to 70-72 and stabilized. So my question is, one...why is it regulating based on the cabin temperature so drastically, if that is indeed what’s happening.? And two, why is it more pronounced when the car is moving? Thanks.
When you changed the temperature from 73f to 70f, if the interior temperature was higher than 70f as measured by the climate control system then it should have kicked on the AC system to bring the cabin temperature down and it sounds like that is exactly what it did. Once the temperature approached what was commanded then the cooling should be reduced and again that sounds like exactly what the system did.
As far as why does something "feel" a certain way take air at 80f and blow it around at a moderate speed and it will feel cool simply because our bodies at 98f are warmer than that. So "feel" can be very subjective. You'll learn some day that what the techs have to deal with to try and be there to help you when a vehicle actually has a problem is important. The only question is will you learn the value of a great technician before or after you don't have any available to help when you have a vehicle problem?
The diagnostics can’t be checked because, as I said, within 45 seconds of idling it regulated the air temp being blown. For 5 solid minutes it was blowing cold air, then I put it in park and coincidentally it returns to normal? I don’t need diagnostics to tell me there’s a problem, nor should the MASTER technicians. I would not take my car to this garage if it wasn’t under warranty.