I just bought a 1985 Mercury Grand Marquis with the carbureted 5.0 V8, and it has a shaky idle when under load (in drive and reverse, not in neutral or park). It rarely stalls out, but it shakes the car and causes the engine warning light to turn on when it drops to a really low rpm.
What could be causing it and what should I do to fix it?
1985 Grand Marquis
104,000 miles
Bought from private vendor
It might be time for a new carburetor, if it's the original one. If not, spray it down with carburetor cleaner with the engine running. There's a vent hole in the middle of the carburetor, spray that down. It leads directly to the jets.
"I like your car!"
Does it shake out of idle, when driving, too?
When 'no' it could be a vacuumleak between throttle valve and intake manifold. On mounting flanges or....
Spraying very locally with brake cleaner or carb cleaner on a leak will let disappear the shakiness for that moment.
Real mechanics -like our Scotty is- have a smoke generating machine which discloses the leak, without the danger of setting the car in fire... (which would solve the problem, but not in a preferred way 😀 ),
After finding the leak you could repair it quite easily -when's a broken gasket e.g.
