I was able to get the round cap/plug back in the intake manifold today. I had to remove the wiper arms and plastic molding along the bottom of the windshield. Then remove the bracket that runs from side to side (this is so there is enough clearance to get your hand behind the intake manifold).
Tip - apply epoxy to the cap and attach some string so if you drop it you can retrieve it easily and you can tie the string to the bracket in case it happens again and you won't have to worry about losing it.
Make sure to clean all surfaces to be sure that the epoxy can bond better. I used JB-Weld 2 part epoxy and applied liberally to the cap and positioned (stem side in). A trial run without epoxy is suggested to you can be sure the parts will align properly. Let sit for a couple of hours to let the epoxy cure and start the car. Once tested OK you can reassemble everything!
As for the cause, my best guess is there was a backfire which caused a pressure build-up and the weakest part gave out. No clue what caused the backfire but a couple of days before this happened the check engine light came on and the code thrown was "Cylinder 1 misfire". Maybe this is related but I am not sure. All I know is the car runs as good as new now and my fingers are crossed that it holds :-)
Hope this helps.
Gary
Tuesday, February 26th, 2013 AT 2:04 AM