Hi All,

1984 Duke.

I am getting a rhythmic "whopping" exhaust noise when decelerating especially over 40 mph. It is not a backfire sound. Car runs perfectly. Has OEM 40 year old muffler but catalytic convertor is new. If I just lightly touch the gas pedal it stops. Any thoughts? My timing as best I can tell is right on having been set with the paper clip method for base timing set to 8 degrees BTDC.

Thanks Fellas

I think that's pretty normal, called decel pop, done engines is more loud than others, backfires are through the intake, this is just unburnt gas exploding in your exhaust, and unless it's really loud, or you're doing it enough to heat anything red hot, it's nothing to worry about. My v6 does it between 4500 and 2750 when decelerating, I like the sound, not loud, but a little burbly etc. it's the only reason I downshift when stopping haha.

Has a stick trans often will engine brake until you push the clutch & make "weird" noises & do other things.
"weird" noise won't be a problem but things like control arm & cradle bushings can steer the car. see https://web.archive.org/web...ierocave/bumpbum.htm

Auto trans can do it too but less obvious most times & often only get "weird" noise unless those bushings are way bad.
More confusing can be early failure of torque converter lock solenoid taking more time to release when you hit the brakes etc.
To check, drive somewhere w/o people behind @ 40+ MPH & hit the brake just so taillight turn on. That kill power to the valve & should see Engine RPM change by ~ 200 higher/lower as you push & release the pedal.

Try doing same after driving on the highway as trans heat can matter for this problem.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Hi

That's car music. Rhythmic is good. Nasty, irregular snappy pops and bangs is saying that there is a problem.
The Duke decelerating sounds OK but the 4 pipes on the V6 seem to be tuned for it.

wait a sec... a "Whop whop whop" noise?
Engine braking likely shouldn't cause that.
check the control arm & cradle bushings, engine mounts, shocks/struts, etc for bad parts.
Iffy parts can allow axle loads to oscillate in turn engine loads can make weirder noises.

Could even be the muffler failing internally?

Can you get a decent sound clip?

quote Originally posted by theogre: wait a sec... a "Whop whop whop" noise? Engine braking likely shouldn't cause that. check the control arm & cradle bushings, engine mounts, shocks/struts, etc for bad parts. Iffy parts can allow axle loads to oscillate in turn engine loads can make weirder noises.

I think the exhaust sound would drown out any chassis noises.
Whop whop whop .... like the three stooges. (that dates me)