It is thought a squirrel had stashed its winter supply in David Bowler's engine while the car was in his garage.
Mr Bowler took the Rover 45 to a garage in Fareham, Hampshire, after his wife Jill told him it was having problems picking up speed.
When he went to fetch the vehicle, Mr Bowler was presented with a bag full of nuts weighing 1.5kg (3.3lb).
Mr Bowler, 69, from Fareham, told the BBC News website: "I phoned up and asked whether the problem was the exhaust and the receptionist said 'no'.
"She said there was a problem and I could tell she was smiling.
"When I went in to collect the car I was presented with this carrier bag half full with peanuts.
"I was amazed actually. Everybody seems to think squirrels have taken the nuts from their box in the garden.
"But it's a bit of a mystery really as to how the nuts got there because I do not think a squirrel could get into the garage. To me the evidence looks like mice."
Richard Lawman, after-sales manager at Pooles MG Rover in Wickham Road, said staff were "astounded" by the discovery.
He said: "What we think has happened is that some friendly squirrel had come up the air intake pipe and was storing his winter food in the air filter assembly and causing a blockage.
"It was a very unusual thing to see."
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