Just got back. That was the problem. The cap was missing.Hope it all works out for you/him. Photo in your link shows how easy it is for someone to fabricate a cap which will work, too.
Made a cap with a 1/4" ID fuel line and plugged it with a M8 x 1.25 socket head cap screw. It's very tight and will not fall off easily.
Erased the code and drove back home with no issues.
Happy New Year to you speleos!!! You saved me tons of aggravation.