Im finishing up a timing belt tstat water pump change. i just filled up with 13 coolant. all of the bow tos say to turn on the heat to bleed the system. but modern cars dont have a heater valve, they run coolant thru all the time and shut off airflow when heat isnt desired. so whats the point of turning on the heat? it wont flow any different. am i missing something?

 

I've always wondered about this too. I built in a cap at the outlet of my heater core so I can bleed it better and I noticed no difference in coolant flow with heat on or off. I don't think it makes a difference honestly

 

It opens up the upper radiator hose allowing coolant to flow/fill the system back up. I've turned my heat on full blast the 2 times I've had to drain the coolant, and both times the coolant in the overflow went down because the system was not yet full.

This was on 2 different cars as well, one being an A4.

I think this is what you're asking but not sure, your post was a little hard to follow.

 

+1

Your heater uses your (hot) coolant as it's heat source, which makes it part of the cooling system. If you don't leave the heat on when flushing, you won't get any flow through the heater core and therefore will not truly flush your system (since coolant won't be flowing through the hose that feeds the heater core).

 

First off, no. That is the job of the thermostat. The coolant heats up in the head. It is stopped from going down through the block by the tstat. Once it is hot enough, the tstat opens, allowing the coolant to exit out and travel up through the radiator, then through the coolant crossover pipe and into the back of the head to start its journey again.

secondly, I'm like 99% positive there is no valve that controls coolant flow to the heater core. I believe the flow of air from the core is controlled by a series of flaps. Don't want heat? it shuts the fan off and closes the door so heat doesn't make its way to the vents.

Last, if you think you have air trapped in the heater core, simply pull back the left heater hose at the fire wall to expose the bleeder hole at the top of the cooling system.

 

Thanks for the confirmation

 

 

+1 on the flap/fan combo. That's always been the (relative) industry standard (if I'm not mistaken).

Nice to know about the bleeder valve location. Thanks for that.

My pleasure!

 

My original question was whether it made sense to turn on the heater during system bleed, IF there was no heater valve controlling the flow of coolant to the heater core.

Essentially I'm disagreeing with the majority of how-to's, that I think are based on practices from older cars that had heater valves controlling flow to the heater core.

Summarizing:

As there is no heater valve, whether the heat is turned on or off makes absolutely no difference to flow through the heater core. The heater fan and hot/cold control only moving air through the core; it does not affect flow through the core.

The only way to bleed the heater core, if you think you need to, is by pulling the hose back to expose the vent hole.

Sound right? If you disagree, then show me where the heater valve is.

 

there is no valve. All I do when I need to fill up coolant is top the reservoir off, start the car, and let the t-stat warm up until it opens. you will need to periodically add coolant. once the car is up to operating temp, place the cap on so it can build pressure. if it overheats after that, you most likely have another issue. However, you can still try to remove the heater hose as shown above to expose the bleeder hole