After committing the 'perfect murder', Mr Jones, 44, of Brockhampton, Gloucester, planned to take her body to the car to make her death look like suicide.
But his plot was foiled when his wife, Margaret, woke to find him crouching over the contraption at the end of their bed, Christopher Clark QC, for the prosecution, said.
He said the couple, who had three children, had problems with their marriage, and Mr Jones was having an affair with another woman. He killed the family cat during a trial run of his gas chamber.
In the early hours of 7 December 1991, Mrs Jones woke to hear a strange noise, Mr Clark said.
'There at the foot of the bed was her husband crouching over a wooden box, a box about two feet square and two feet high. From the box came a noise like the sound of a car engine.'
Mr Jones, who denies attempted murder, told police the reason his wife's car was running was because he was working on it, and a hole drilled in the wall was for a sink he intended to install.
However in a letter found later, Mr Jones made it clear there was no chance of a reconciliation.
The trial continues today.