Heat Only Works When Accelerating

[Drew Lonker]

I have an 01 4x4 ST, 122,000 on it and my heat only works when I am accelerating. When I am idling it will just blow cold air. Ive had the problem for years now but it never bothered me to much to do anything about it. Im in NC so winter isn't so bad.



I have looked all over the internet and after talking with some friends think that it could be a vacuum leak. Looking at all of my hoses, everything seems okay (given I couldn't really see the bottom of any hose). Any common vacuum leaks for an 01 or other ideas?



It is NOT coolant level, air in the cooling system, thermostat, and I wouldn't think heater core because I still have heat, but only when pressing on the gas. Thanks for any help!

 

[EddieS'04]

It has to do with vacuum. It wont be the hose on the door under the dash. since it does do something.

Have you pulled the right front inner fender liner down.? Check the vacuum reserve ball for cracks and tubing connections...

 

[Dan Long]

It definitely sounds vacuum related, and the cause is likely the heater core bypass valve that's installed in one of the lines near the firewall. This valve stops the flow of coolant into the core to help the A/C work better when on the Max A/C setting, hence the reason you can't get warm air on that setting regardless of the temperature selector position.



I've never investigated the exact operation of that valve, but my guess is that when it sees vacuum, it closes off. You likely do have a faulty vacuum reserve ball, because when accelerating you lose vacuum. Typically the other thing you'll notice when it goes bad is that when the engine is under heavy, prolonged load that the dash vents will change to the defroster vents until you let off the gas and the engine builds vacuum again, because the vent selector door is also vacuum operated.



Options are either fix the vacuum leak or reserve ball, or just remove that bypass valve and cap off its vacuum line connection.

 

[Richard L]

I agree with Eddie and Dan. Your problems is related to vacuum and it is probably not related to the vacuum line or vacuum motors controlling the HVAC blend doors. I also don't think it would be the heater bypass valve, and would be the last thing to check and I don't know how you would check it? You also need to check the operation of all the other HVAC control setting when accelerating and idling to see if it effects their operation.



Your problem appears to be a with the vacuum reserve canister (Ball) It's probably cracked and leaking, If you don't want to pay too much you should be able to pick one up in a junk yard prertty cheap.



...Richard

 

[Drew Lonker]

I just pulled off the fender and out came the vacuum reserve canister. It seems to look okay. No cracks or any noticeable leaking noise when the truck is running. I followed the hoses and they all seem to be in decent shape.



I don't have any other HVAC issues, just no heat at idle. The only thing I could think of with the vacuum canister is that the seal where the two pieces of plastic meet could be leaking ever so slightly. May just replace it anyway since I already have it off.

 

[Drew Lonker]

I just pulled off the fender and out came the vacuum reserve canister. It seems to look okay. No cracks or any noticeable leaking noise when the truck is running. I followed the hoses and they all seem to be in decent shape.



I don't have any other HVAC issues, just no heat at idle. The only thing I could think of with the vacuum canister is that the seal where the two pieces of plastic meet could be leaking ever so slightly. May just replace it anyway since I already have it off.

 

[Scott Tanner]

A vacuum leak would yield heat only, as it defaults to heat. The symptoms presented point to low fluid or air in system

 

[Gavin Allan]

Bad water pump.

 

[Richard L]

stanner,

If the vacuum canister is leaking it would default to Heat only



The reason why the heat shuts off at idle is because that is when the engine is generating the most vacuum. Wider throttle settings reduce available vacuum, and that is what the reserve vacuum canister is for...to hold vacuum during periods of low intake vacuum.



So at Idle the engine is creating enough vacuum to overcome the canister leak and still hold the heat door closed. When accelerating, the throttle is opened and engine vacuum drops to it's lowest level. If the canister is leaking, there is little or no reserve vacuum and the heater door opens to it's default setting.





PS: gopack,

There is also a check-ball valve where the engine vacuum line attaches to the canister (ball) to hold vacuum when the engine is not producing much vacuum. If the check-ball sticks or fail, you can lose vacuum even though there is no cracks or leaks in the canister. You usually cannot fix or replace the check-ball, so its best to just replace the canister.



...Rich