On and off heating

[Performance nut]

I think I know the reason for this but want to ask to see if there is another reason. When driving, the heater will blow cold air while idling at a light and then hot when driving. OSA is 38F, air conditioner is off, recirculation is on, full heat, vents set to blow into cabin (not feet or window).

My thought is it is my radiator. I have an aftermarket radiator that is more effective than the stock GT radiator. With the really cold outside air temperature, the cooling system is bypassing the radiator when idled (heat load is very small) and then goes through the radiator when driving (larger heat load). The only problem with that theory is that when the radiator is bypassed by the thermostat, the heater core shouldn't be bypassed as well should it? If it isn't bypassed, I would think there should be some latent heat from the idling engine enough to heat the cabin.

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[KingKona]

Ford over-engineers their cooling systems. Hence, you never hear of a Fords over-heating. The down-side is heat in the winter. It sucks.

Try it on a 5 degree or below 0 day.......you basically only get real heat when accelerating hard. Otherwise, it's luke-warm.

And then you went and added a larger radiator.

 

[Basspro302]

You potentially have low coolant, air pocket or restricted heater core. Typically the water pump or quick connects on the hoses leak and the low coolant.
The heat should be hot even on super cold days.
Ive had 3 5.0 mustangs driven in northern winters never had an issue due to OEM cooling systems if working correctly give me a heat issue.

 

[Performance nut]

Ford over-engineers their cooling systems. Hence, you never hear of a Fords over-heating. The down-side is heat in the winter. It sucks.

Try it on a 5 degree or below 0 day.......you basically only get real heat when accelerating hard. Otherwise, it's luke-warm.

And then you went and added a larger radiator.

Texas heat definitely caused the system to struggle. On the track, forget about it. Once we started hitting 90F at the track, I had to end sessions early. The new radiator rejects so much heat.

You potentially have low coolant, air pocket or restricted heater core. Typically the water pump or quick connects on the hoses leak and the low coolant.
The heat should be hot even on super cold days.
Ive had 3 5.0 mustangs driven in northern winters never had an issue due to OEM cooling systems if working correctly give me a heat issue.

You might have something there with air in the coolant issue. I had to add coolant during the summer and thought I had burped the heater core sufficiently. Have to check that. Thank you.

 

[Basspro302]

Try only jacking the front of the car up with the cap off and very engine rpms

 

[Performance nut]

Good call on coolant. About 1/2 gallon shy from minimum. Think it is time to start checking for leaks. Been putting too much coolant in to be air bubbles. This is about two gallons in almost two years.

 

[Basspro302]

Check the water pump and plastic quick connectors on the rad hoses.

 

[Whitedevil95]

Water pumps tend to leak or have weeping issues on these cars from what ive seen.

 

[Performance nut]

Ok, quick update. I'm not seeing where this is leaking. I even idled for over 10 minutes with the AC off and heater on without a drop on the ground. Though I can smell coolant through the vents. My first thought was that the reservoir cap is leaking by but I removed it while coolant has been warm (45 minutes after running) and there is a pressure release 2/3 of the way removed. Not sure if that is a sure sign that the cap doesn't leak by but its my only theory at this time.

The other weird thing is that the coolant loss stops at the very bottom of the reservoir (like the bottom neck area of the reservoir). After that it doesn't drop any more. If it were a leak, I would think it would keep going until it was bone dry.

 

[Bluelightning]

If you can smell coolant through the vents, then your heater core might be the issue.

 

[ORRadtech]

If you can smell coolant through the vents, then your heater core might be the issue.

That was my first thought as well. I don't know what year your car is but if it's an earlier model you might consider changing out the evaporator at the same time.
The other place that you can lose coolant without any outward sign is through a headgasket leak. That will often show up as white exhaust smoke on startup and a sweet smell in the exhaust.

 

[Performance nut]

If you can smell coolant through the vents, then your heater core might be the issue.

That was my first thought as well. I don't know what year your car is but if it's an earlier model you might consider changing out the evaporator at the same time.
The other place that you can lose coolant without any outward sign is through a headgasket leak. That will often show up as white exhaust smoke on startup and a sweet smell in the exhaust.

Ok, you both are confirming my thoughts. I'm not thinking head gasket as I haven't seen the white smoke. I see steam on colder days but on warmer days it isn't there. I'm running E85 exclusively so I'm not sure what coolant and E85 would smell like. 93 and E85 smell different already.

If it was the heater core though, wouldn't you see leakage under the car close to the firewall? I idled for 10 minutes one time and not a drop.

 

[Hack]

Ok, you both are confirming my thoughts. I'm not thinking head gasket as I haven't seen the white smoke. I see steam on colder days but on warmer days it isn't there. I'm running E85 exclusively so I'm not sure what coolant and E85 would smell like. 93 and E85 smell different already.

If it was the heater core though, wouldn't you see leakage under the car close to the firewall? I idled for 10 minutes one time and not a drop.

No, not necessarily outside the car. Probably inside the car on the passenger side in the footwell is where the coolant might end up.

Do you get the windshield fogging up?

 

[Performance nut]

No, not necessarily outside the car. Probably inside the car on the passenger side in the footwell is where the coolant might end up.

Do you get the windshield fogging up?

Yes but when weather conditions warrant it. Defrosts very quick

 

[Avispa]

I think I know the reason for this but want to ask to see if there is another reason. When driving, the heater will blow cold air while idling at a light and then hot when driving. OSA is 38F, air conditioner is off, recirculation is on, full heat, vents set to blow into cabin (not feet or window).

My thought is it is my radiator. I have an aftermarket radiator that is more effective than the stock GT radiator. With the really cold outside air temperature, the cooling system is bypassing the radiator when idled (heat load is very small) and then goes through the radiator when driving (larger heat load). The only problem with that theory is that when the radiator is bypassed by the thermostat, the heater core shouldn't be bypassed as well should it? If it isn't bypassed, I would think there should be some latent heat from the idling engine enough to heat the cabin.

Leave the fresh air open (recirc off) when using heat. Its a lot harder to keep windows from fogging when recirculating on heat. The AC compressor doesn't run enough to pull humidity out of the cabin air. I can't seem to make my wife understand this as she cruises around Maine in January with ice forming on the inside of the glass.

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