Any suggestions to troubleshoot?

 

See HVAC FAQs. Link below.

 

Thanx projectSHO89. I'm posting for my cuz with no computer. He said this occurs cruising so don't think it's blend door. He's gonna check vacuum lines.
Does the hot water valve close when in AC mode or is hot water in heater core all time?

 

Print them out and give them to him.

You didn't specifically identify the vehicle in question, so we don't even know if it has a "hot water valve". Vehicles with a factory coolant valve shut the valve ONLY when in MAX AC.

In any event, it would not cause the outlet temps to go HOT, only 1) a loss of control of the temperature blend door or 2) a combination of hot coolant flow through the core combined with a drop-out of the compressor will cause that particular symptom.

Vacuum is not used for temperature control, it's used only for inlet and outlet airflow selection.

 

a shot in the dark, my 05 6.0 F350 did this to me when I first bought it, the air gap for the ac clutch was too wide, truck had 230K on the odometer, so it had cycled a few times. I used a thin stick to "touch" the clutch while is was running and not cooling. The clutch picked and worked like it should until it cycled off like it should. it would not pick sometimes, I pulled the bolt from the center of the clutch, removed a shim or 2(closed the air gap), put it back together, in 15 mins all was well, cost $0.00 still running that way today 54 years and 50K later.

 

Sorry, it's '02 F350 7.3 which he bought new, 164K mi.. He's in N.GA, I'm in W. TN. He said vacuum hoses look good. How would you find vacc leak(s) on diesel? He says it goes HOT suddenly, then slowly cools back down. Is there any way to hold blend door in position?

 

It is NOT a vacuum problem, see my previous post. It's a blend door problem and that is electrical.

If the blend door responds to the temperature control almost all the time, it probably isn't an issue with a broken blend door. More likely, it's an electrical intermittent in either the blend door controller or in the temperature control potentiometer. I had a similar issue three years ago with my 01 SD where the temp would intermittently loos heat (it was November). I found a cracked solder joint on one of the connector pins on the blend door controller. I re-soldered it and the thing is still working correctly. I had already ordered the part in from Rock Auto which is still in a box in my garage.

That's where I'd suggest starting.

 

Thanky projectSHO89. I passed your advice along. I'll report back on outcome.

 

Found out today it only does it in 3rd position, works fine in 2nd, he's never ran it in high. Could the faster air flow be forcing blend door open?

 

Its like the post above say's Compressor air gap issue diagnosed and repaired many of them.

 

If it were the clutch gap, the duct outlet temperature would go WARM (ambient plus a little) due to loss of cooling just like switching to VENT but it wouldn't go "HOT" as was described. It's truly a matter of degrees (pun intended). Blower speed would not affect the issue it were a clutch gap issue.

Plausible.

 

Does the blower on these work to blow open or to close blend door? Is it open hot or open cold?

 

The blend door "blends" both hot and cold so there is no "normal" position. If the blend door hinge breaks and there is no control over the blend door movement by the blend door actuator, then the blend door can flop around randomly.
Since the blend door's function is controlling the airflow across both the heater core and the AC, if it is flopping about randomly then the temperature of air coming out the vents will be a corresponding random temperature relative to the current flop position of the broken blend door. It will usually be one extreme or another as the blend door now broken loose freely swings to either the cold or hot side. It may be a bump in the road that adjusts its position, or cornering, hence the randomness in temperature felt coming out of the vents.

 

All airflow ALWAYS goes through the evaporator core. The blend door only determines how much passes through or bypasses the heater core.

 

Yes, one door controls both hot and cold temperatures.

When the blend door hinge fails the temperature will be random as the door flops around. And when I experienced this myself on my own truck, it was either hot or cold, not much in between.