Now let’s talk a bit about how to solve an overheating issue.
The #1 cause of overheating is air in the cooling system. I say this from 20 years of experience as a technician. There are many causes of air getting into the cooling system, and finding the cause will be the key to solving the issue for good.
What I see most often when it comes to air in the cooling system is an overheat that occurs after the vehicle has had a bunch of work done that involved opening up the cooling system. Some common symptoms include:
- You or a garage just finished replacing a head gasket, water pump, intake gasket, radiator, heater core, or coolant hose, and now that the work is done, you have an overheat condition.
- You have a condition where the vehicle starts to overheat, then suddenly drops to normal.
- You have issues getting consistent heat from the vents.
- Your idle fluctuates up and down and will not level out.
These are all classic symptoms of air in the cooling system. What happens is an air pocket gets trapped somewhere in the system, and does not allow the coolant to flow properly. When this happens, it creates a steam pocket that does not allow coolant flow; hence, the overheat. When the air pocket moves and things begin to cool down, things go back to normal for a while until the air accumulates again. The process continues until you purge the air from the system.
Any time you open a cooling system for service, you must purge the air from the system. If you don’t, you get an air pocket and the symptoms described above.
Bleeding a cooling system is fairly simple. It’s handled differently depending on whether you have a system with an overflow or expansion tank. I’ll first cover a system that does not use an expansion tank and just has an overflow bottle.
I made this video some time back on the procedure for bleeding a cooling system on a 90s Honda Accord. This engine has a bleeder valve. Not all vehicles have this, and that’s OK. You don’t need the bleeder valve to purge the air from the system. Service procedures vary between makes and models, but in my experience, the bleeder valve is there to make filling the system easier, not necessarily to bleed the air from the system. You’ll notice in the video that I open the bleeder when first filling the system, and once that’s done, I close it off and I don’t open it again after that point. If I didn’t open the bleeder valve as I filled the system, it would take longer to fill and you’d notice that the air would work its way past the incoming coolant by bubbling out as you filled the system. Bloop, bloop… bloop. Instead, you hear the hissing noise of the air escaping from the bleeder valve as it’s displaced by the incoming coolant.
If you don’t have a bleeder valve and you wish you did, you can remove one of the small coolant lines going to the idle control valve or other small hose toward the top of the engine. The key here is to find a line placed high on the engine. This will maximize the effect. Removing a small coolant line will have the same effect as an open bleeder valve would. This video I made about bleeding the air out of a cooling system should shed some light on this. Remember that you don’t need a bleeder valve to do this. If you don’t have a bleeder valve, don’t worry; just follow all the other steps outlined in the video and you should be able to purge the air from the system.
If the system has an expansion tank, you do things a little differently. Instead of using the spill-free funnel as I did in the video, use the expansion tank itself as the funnel. First, don’t fill it up all the way; this will allow the coolant to expand during the bleeding process. The coolant level will rise as the engine gets hot, so be prepared for that. Do everything else the same. The only difference in purging the air in an expansion tank system is that you have an expansion tank instead of a radiator cap on a radiator. Watch this video to see how I handled bleeding a cooling system on a vehicle with an expansion tank.
An important note about the bleeding procedure: If the engine starts to overheat during the procedure and the coolant starts bubbling out everywhere, stop. Shut the engine down and let it sit for about 10 minutes. You’ll notice that during that time the coolant level might suddenly drop. This is because the thermostat opened up and is now allowing coolant flow. You can then resume the procedure as outlined in the video.
Lastly, if you have a vehicle that does not use electric fans, it can be difficult to tell when the air has been purged from the system. For these, I normally run the engine for about 10 minutes or so and check the heater output. If the heat is consistent and I have good circulation through the system, I call it a day and move on. Here is a video that shows that procedure.
The first thing I do if I don’t seen an obvious coolant leak on a vehicle with an overheat condition is bleed the cooling system. As pointed out earlier, many times the air is the result of improper service, or sometimes the coolant just gets low enough to develop an air pocket. Bleeding that air out often cures the problem. If you see an obvious coolant leak, then by all means, repair it, bleed the system, and recheck for the overheat condition. If it’s not obvious where the leak is, there are methods you can employ to find it. These methods are outlined in this video, along with many of the topics covered in this article.
Much of what I’m about to cover in this next section is covered in the video, but I’ll go over it again just to be thorough.
The next step after you’ve purged any air from the system is to check for leaks. The best way to do this is with a pressure tester.
This tool pressurizes the cooling system and forces the coolant to leak if there is a place for it to escape. Often you can find those pesky small leaks that are difficult to spot with this method. Another method is to just put your eyes on the engine and surrounding area. Often when coolant leaks, it leaves a nice stain around the area where it leaks, which can be easy to spot if you’re lucky. Once you’ve found the leak, repair it, bleed the system, and recheck for the overheat condition.
If you don’t find any external leaks, it’s time to get creative. Also, don’t forget to check under the dash for coolant leaks when pressure testing. A leaking heater core can cause this, and since it’s buried in the HVAC system, it can be hard to spot a leak from this area. If you notice wet carpet on the passenger-side floorboard, or your windshield fogs up and is perhaps coated with a greasy residue, you could have a heater core failure. I’ll cover this more in the HVAC article.