Vent Fan Shuts Off Under Load

[AF Comanche]

Recently, the vent fan in my 88 Laredo with the 4.0 shut offs when the engine RPMs go up, like when going up a steep hill or passing. It will come right back on when the RPMs drop back down and it doesn't make any strange noises before shutting off or when it comes back on. Any ideas on what I should check?  

[ComancheKid45]

Sounds like a vacuum leak, next time you notice it acting up place your hand up by the windshield and check to see if air is coming out of the defrost vents.

[Bobolink]

From my own experience, the most likely place for the leak is where the small hard black plastic vacuum line passes beneath the tray holding your coolant presure bottle, just prior to entering the firewall. I have had two leaks there, within 6-8" of the firewall. It is easiest to access if you move the pressure bottle aside and remove the tray. Thoroughly feel all along the vacuum tube for any roughness, and if you find any roughness, look closely there for abrasion and a leak. Fix it with a piece of appropriately sized rubber vacuum hose.

[Blue88Comanche]

From my own experience, the most likely place for the leak is where the small hard black plastic vacuum line passes beneath the tray holding your coolant presure bottle, just prior to entering the firewall. I have had two leaks there, within 6-8" of the firewall. It is easiest to access if you move the pressure bottle aside and remove the tray. Thoroughly feel all along the vacuum tube for any roughness, and if you find any roughness, look closely there for abrasion and a leak. Fix it with a piece of appropriately sized rubber vacuum hose.

 

+1 :agree:   Same leak i had, I finally found it after i converted my cooling from closed to open.

 

I think its the hose that goes to the heater valve. I capped mine when i removed the heater valve.