It only leaks coolant when the heater is on..

[tas6691]

'02 Trailblazers do have a heater control valve. The heater hoses are a total PIA to change and the fittings at the firewall are no fun to get to either. Heater core is a big job as well. Really need to take it to a pro at this point. AutoCool in Smyrna will treat you right.

 

[hunterofopportunity]

Time to trade it in and step away.

 

[John Moses]

I've got a 2002 trailblazer...it's leaking coolant it seems from down the firewall maybe..but it only leaks when the heater is on...anyone have an idea what this could be..4.2 inline 6 motor...almost seems like it could be coming from around the reservoir tank...but only loses coolant when heater is on

Had same issue, back flushed the heater on my Camaro.
No more leaks or passing out from fumes.

 

[Comanche]

Here’s a good test that also dubs as a quick fix to get home if your heater core is puking.

Find the two hoses going from the water pump to the heater core.
If they’re the same size you can unplug one at the water pump a d unplug the other one at the heater core and plug the side that’s removed from the heater core in a single hose loop to the port you removed the other hose from arm the water pump so you just have one continuous loop of one hose on both sides of the water pump.
zoe totally eliminates the heater core and is a great way to save from getting towed or troubleshooting this issue.

My best guess is your heater core is leaking or possibly a heater valve inside the dash.
See if you can see anything from under the dash....most likely not the source but may see the coolant running.

 

[BDR1]

Hope I’ve found the problem.. looks like the hose from the heater core to the motor is leaking at the bottom clamp. Leaking at the clamp.. let’s keep our fingers crossed.. thanks for all the help from you guys.. climbed under with a flash light today and found that

 

[Comanche]

You got.....

 

[BDR1]

View attachment 2511799 You got.....

I hope this all that it is... I’ve been hit hard the last few weeks with our 3 vehichles

 

[Comanche]

I hope this all that it is... I’ve been hit hard the last few weeks with our 3 vehichles

Me too....always hits at once.
See my recent tick...tick thread

 

[BDR1]

Me too....always hits at once.
See my recent tick...tick thread

Yes sir it does.. I saw that and hate to hear that for you.. hopefully it won’t cost you too much money

 

[BDR1]

So far so good.. a hose coming out of heater core had a hole in it. Replaced that and seems to be the ticket.. thanks for all the help from you guys