CEL woes, 99 XJ

I have a 99 xj sport, 4.0 aw4/231. All stock truck.

About a year ago my CEL came on, my obdII scanner was telling me the CEL light should be on, but it had no codes. another lesser cheap obdII scanner told me the same thing. i had a shop check it, they said it was reading a code for the evap purge soleniod. I replaced the solenoid, cleared out the CEL, was good for about 8k miles. Same situation all over, light came on, no codes. Shop read same code for the purge solenoid, I cleared it with a different scanner, good for about 12k miles. Just came back on the other day!!

Any suggestions? other than taking it to a dealer.

Your evap monitors run every two warm up cycles, the malfunction indicator lamp
(MIL) must be off, barometric pressure exceeds 75 Kpa, at start-up, IAT and ECT is
between 45°F and 85°F, IAT is not more than 2°F greater than ECT, ECT is not more than 12°F greater than IAT, Fuel tank level is between 26 percent and 74 percent, the TPS is between 9 percent and 35 percent, the EVAP solenoid is at 50 percent pulse width PWM, within 65 seconds of engine run time. That sounds complicated but it goes to show you how strict the parameters are to run your evap monitor. The mileage is really irrelevant between lights coming on its just one or more conditions were not right to run the monitor. Also if it fails once it sets a pending code that won't show up on a lot of scanners and no light it must fail twice to set the light and if it passes the test another 30 times the light goes off.

World's best kept secret:

Don't replace parts based JUST on a DTC.

The DTC is simply pointing you in the direction of the problem--the PCM is interpreting data from sensors/monitors and is telling you that in such-and-such a circuit or sub-system something isn't right.

You most likely have NOT suffered the failure of three purge solenoids.

Did you try just clearing the code, noting the date/time/miles, and then waiting to see when it comes on again?