93 XJ Heater/vacuum problems

[brainyass]

1043247

I can't get my heat to work. I tested the vacuum line going to the heater valve connected to the heater core and it pulls a vucuum. I figured it was the control unit. I replaced the control unit and it worked when I tested it. Now it doesn't work any more. No vacuum at the heater valve. I took off the vacuum lines at the back of the control unit and jumpered it again, presto I pull a vacuum and theoretically got the heater working. When it's hooked all back up, no heater. I just took off the piston at the heater valve to release the pressure on the valve so the hot water can go through the heater core. I now have heat but can't figure out why it doesn't work when it's hooked up. Everything else works, as far as the vacuums go. A/C works fine, all the vacuum lines close the vents when I change from A/C to heat on the control unit.....



Any help will be appreciated.[addsig]

 

[dingus]

1043253

lets just take a step back a second... your thermostat is working properly i assume?[addsig]

 

[brainyass]

1043319

The thermostat works, it keeps it real toasty now that I have the heater valve rigged so the hot water always goes through the heater core. Any way to fix this vacuum problem without just bypassing the control unit vacuum switch all together?[addsig]

 

[MudderChuck]

1043500

Had the same problem on an earlier model. Found that the hose had fallen off of the vacumn resivoir. Right after my wife sent a VW Golf to the junk yard by parking her XJ in the back seat. Didn´t really do much to the Jeep, a fender, a bumper end and heater problems. Heater seems to need alot of continious vacumn. Another time found that I´d lost a little gasket off of the heater control vacumn plug, removed when upgrading the radio to CD. Couple of things to look at.[addsig]

 

[MudderChuck]

1043501

Check your vacumn at the manifold and compare to the vacumn at the heater. [addsig]

 

[brainyass]

1043569

Thanks for the suggestions, but here is something I forgot. Sometimes when the engine is off, it pulls a vacuum at the heater valve but when the engine is running and the heater is on, it quits. Strange problem, that's why it's so hard to figure out....[addsig]

 

[MudderChuck]

1043570

I don´t know your vacumn system specifically. But from the symptoms it sounds like a good place to start looking for a vacumn leak, is somewhere after a vacumn switch. I often break a vacumn system down into sub systems and mark all the hoses of a sub system with different colored electrical tape. And if you have a vacumn schematic you can color the sub-systems with a colored pencil. Pack the whole thing away for reference, you´ll probably need it again sometime.

Could also be a ruptured diaphram in an actuator somewhere. I´ve often wanted to buy a small hand type vacumn tester. I now use the old rubber tube, suck and tounge over the hole method to test for vacumn leaks. Which is marginally effective, but definitly not tastey.

I´ve spent a whole bunch of hours chassing down vacumn leaks, good luck.[addsig]

 

[MudderChuck]

1043571

Another thing to keep in mind is that above the throttle plate connections and below the throttle plate (manifold) connections, when switched, can cause really strange problems. The above throttle plate vacumn and below throttle plate vacumn go in opposite directions with RPM increases.

[addsig]

 

[MudderChuck]

1043572

TBI or MPI? Does your model have a vacumn booster? An internal leak in the vacumn booster can cause the problems you´ve discribed.[addsig]