7/8 full fuel engine surge - possible evap issue?
 

I have some issue that causes the idle to fluctuate down several hundred rpm and almost stall the car. You can also feel it surging slightly between 800-1500 rpm while driving in traffic.

1. this only happens when the tank is 7/8 full. So roughly 50-80 miles after a fillup. It can be days after a fill up so if there was anything in the tank it would re settle. After this mileage range the issue goes away and does not come back until the next fill up

2. if I drive a normal freeway speeds it clears up

3. this happens in town or traffic

4. It is repeatable

5. I have no codes

6. I put on a brand new factory fuel cap - still does it

7. vent lines at filler are clear

8. I do not smell gas inside or around the vehicle

9. I have read the TSB’s and don’t see one that relates

I am wonder if this has something to do with the cap system?

2000 xkr convertible #A07485