AC blows hot... sometimes

[nick_d_4.0]

I have been having this issue for a while on a 1998 ford ranger xlt 4wd. As I am driving on the highway with the AC off, very very hot air blows though the duct that gets directed to the driver and passenger feet. It does this for 10 minutes or so, then stops. It them just comes back on when it wants to periodically. I have removed the dash a few days ago and saw every vacuum line and heater part, it all looked fine! The air directors all work, blower speed works. I am confused as to why this happens only sometime and obviously why in general. Any insight please!!

Whether this is associated or not, the truck has a terrible start up, runs at about 70% power, and has lean codes. Suspect of a vacuum leak... could this vacuum leak be AC involved and effecting both things? New MAF, IAC, TPS, this bad start and code are not because of those.

 

[Spott]

It's quite likely a vacuum leak. I'm not familiar with the 98's, but many vehicles use engine vacuum to move little doors in the climate control system that control the hot/cold air mix, and which vents the air is directed to. If there's a lack of vacuum, those doors return to a "default" position, regardless of what the controls are set to.

A vacuum leak can also make your engine run poorly.

A quick way to search for vacuum leaks is to spray starting fluid or carb cleaner around the engine block while the engine is idling. If you spray near a leak, the engine will suck it in and the idle will immediately change. You can then inspect around that area and pinpoint the leak.

 

[Big Jim M]

Yes a Vac leak sounds plausible. You will have a vac canister that holds vac for reserve to be used under full power and other low vac times. I think yours is on the passenger side near the headlight.
In Jeeps, using an old battery causes the plastic vac lines to the canister to be degraded and the exact same problem arises.
Look at the firewall under the hood and find the vac line going thru the firewall. Then follow THAT line and see where the leak is.
Big Jim

 

[nick_d_4.0]

Yes the climate controls change and adjust to what position it is set, they all visibly move. I've done the spray test and a smoke test, no sign of the leak location sadly.

 

[nick_d_4.0]

Yes a Vac leak sounds plausible. You will have a vac canister that holds vac for reserve to be used under full power and other low vac times. I think yours is on the passenger side near the headlight.
In Jeeps, using an old battery causes the plastic vac lines to the canister to be degraded and the exact same problem arises.
Look at the firewall under the hood and find the vac line going thru the firewall. Then follow THAT line and see where the leak is.
Big Jim

will try that in the morning, thanks!

 

[enjr44]

Aren't the vents supposed to go to defrost when you get low vacuum or a vacuum leak?

Sort of a safety feature. Don't need heat. Need to see. I had a bad vacuum check valve in an old Ford I used to have and in the mountains the vents would go to defrost on a steep climb.

 

[kimcrwbr1]

When manifold vacuum is low yes it uses the vacuum reservor to control the doors. It is possible the check valve is defective it keeps the cannister from drawing air when engine vacuum is low (heavy throttle pressure).

 

[kimcrwbr1]

http://www.therangerstation.com/tech_library/diagrams/91-94_Ranger_vacuum_hose_diagram.gif
This diagram shows the check valve

 

[nick_d_4.0]

http://www.therangerstation.com/tech_library/diagrams/91-94_Ranger_vacuum_hose_diagram.gif
This diagram shows the check valve

thanks for the idea, aside from looking for damage, how will I know that is a bad sensor? voltage check?

 

[nick_d_4.0]

Aren't the vents supposed to go to defrost when you get low vacuum or a vacuum leak?

Sort of a safety feature. Don't need heat. Need to see. I had a bad vacuum check valve in an old Ford I used to have and in the mountains the vents would go to defrost on a steep climb.

yeah I have heard when there is a vacuum leak it blows through defrosters, but this air always blows through the floor vent and that only.

side note the air is not always hot, today it happened again and it was ice cold air, again AC was off and had air blowing through foot vents for about 5-10 minutes

 

[Big Jim M]

It is not a sensor it is a one-way valve. I check them by sucking on both sides if I get air thru both ways then the damn thing is broke!
Big Jim

 

[kimcrwbr1]

When the HVAC is set to defrost it should cycle the A/C to remove moisture from the cab. If you open the hood the compresser should cycle regularly when the controls are on defrost.

 

[enjr44]

Ok, so what is happening is that you are going down the road with the control set to say vents (one place to the left of off) and air is coming out of the dash. And at some point it stops blowing thru the vents and goes to the floor (like you had the control set two places to the right of off)?? And after some time it goes back to where you had it set??

Is the above correct? And sometimes the temp of air changes to either hot or cold; but, not always?? Is that right?

Well, you also said it runs bad for some reason, you mentioned lean codes. So cel is on. Lean codes are probably unmetered air getting into engine intake (vacuum leak). The cabin vent system runs on vacuum..... So,

Disconnect and plug the vacuum lines going into the cab, reset computer and see if your lean codes go away. If not fix the lean code problem first, engine vacuum will not be right with vacuum leaks and so maybe the vent system will not work right either.

 

[azvacs123]

Check the heater valve and see if it move when you increase rpm's. Then remove and cap the vacuum line that goes to it. See if that changes it. If so, it may be an internal leak in that valve. https://www.google.com/search?biw=1...ved=0ahUKEwiOm-O9s8XKAhUlvYMKHfNiAJwQwg4IGigA