Heat goes cold on acceleration

[nxne75]

The truck: 1999 Ranger XLT, 3.0L V6, automatic, 212,000 miles.

Recently I've noticed that the truck's heater will go cold here and there - it'll be blowing really hot (as it always has), and then in the next instant (for say, 1-5 minutes or so) it'll blow nothing but cold air, and then back to hot again. I've noticed that it seems to do it on hard acceleration more often than not, but maybe it's just coincidence.

I've been thinking that maybe it's related to a malfunctioning thermostat (which has never been changed), but maybe that's a wrong guess? Any other thoughts/suggestions greatly appreciated. Thanks!

 

[srteach]

The heater box on a ranger has a bypass path for cold air and a heat path for the hot air. There is blend door that regulates the airflow mix (and therefore temperature) between the two. This door is vacuum actuated on the 98-up Rangers. You probably have a vacuum leak in the temperature control system.

Yes, it could be a thermostat. It is not likely, unless the temp goes down when the truck is moving and warms when the truck is not moving.

 

[nxne75]

Thanks much for the response! I hadn't even considered this possibility, but I'll do some more reading about it. Thanks again!

 

[AllanD]

Actually the blend door is vacuum actuated starting with the 1989's

tthe problem is most likely a bad/stuck check valve in the vacuum line
that feeds the HVAC controls.



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