Check Engine Light - Vacuum Leak
 

I have a 1999 Cherokee. It has an intermittent Check Engine Light, and at the auto parts store, the reader said Large Evap Leak detected. I got a new gas cap, and thought it fixed it, because I drove it three days with no light. However, when I turned on the heater tonight, the light came back on. Also, sometimes when I turn on the defroster, I hear a loud pop and then the heater seems to fizzle out. But, if I turn it off and on again, then it usually is OK. I'm frustrated because I don't want to take it to a dealership, but I don't have any idea where a leak is occuring because I can't find it. Can anyone help a poor public school teacher with few mechanic skills? Thanks.

After the fuel filler cap was changed were the codes cleared?

Are you sure its the same code?

HVAC system controls depend on engine vacuum to operate, as does the evap. system. Might start by checking vacuum lines and vacuum operated stuff (like your HVAC controls) for problems. Hear any hissing? Any of the rubber or plastic vacuum lines look brittle or deteriorated?

As others have said, most likely a vacuum leak. The vacuum lines that run in the cab and up behind the dash to the HVAC controls are usually okay, as they are protected from heat, vibration, impacts, dirt, and chemicals.

Most vacuum leaks can be found under the hood or under body somewhere.
On Cherokees, there is a spherical vacuum canister mounted behind the front bumper on the passenger side. It can sometimes take a hit from a rock and crack, or it may just crack from age and fatigue. The lines going to it are also common points of failure, usually at their ends where they connect to the canister or something else.

Aside from that, check all of the vacuum lines that are attached to the intake manifold, and any that go to the break booster. Also check the lines that run to the cruise control mounted on the passenger side of the engine compartment.

Fortunately, vacuum line is very cheap, usually less than a dollar a foot for anything up to 3/8" diameter. Larger pre-molded specialty pieces can be sourced from a junk yard or the stealership.

The heater problem and the EVAP leak are unrelated.
The computer checks the EVAP system randomly, setting a code if needed. That's why it was off for a few days.
The EVAP system involves the hoses to/from the tank to the canister, and to the EVAP solenoid. Smoke test it for leaks.

The heater problem has nothing to do with the EVAP. Smoke test the manifold to find the leaks there.
The noise - sounds like something fell down a dash vent - a pen, a coin, a paper etc.
Remove a duct hose and look inside. The blend door is hitting it.

Thanks
 

Thanks everyone. After pulling the vacuum line from the intake, we discovered it does have a slight vacuum leak, but it doesn't seem to be the canister or any of the vacuum lines in the engine compartment. I guess I'll just keep hunting for it.