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Hagerty
Hagerty Employee

Piston Slap: Ride-Engineered By Lincoln-Mercury? | Hagerty Media

Hello! This is a great resource I wasn't aware I had! My question is this: My '78 Cougar came from factory with some kind of air ride system in the rear. At some point in my car's lifetime, the previous owner made a budget-based decision to swap "Mustang coils" into the rear of the car, which removed the air system.
https://www.hagerty.com/media/advice/piston-slap/piston-slap-ride-engineered-by-lincoln-mercury/
5 REPLIES 5
All_Caps_Crazy
New Driver

or the tank fuel filter
Swamibob
Instructor

Good analysis Sajeev;

I do agree with a previous reply regarding a filter, but I doubt it would be the tank filter. I'd change the fuel pump and the fuel filter, give it a general tune up with plugs, wires, cap, rotor and air filter. Double check the timing and make sure the distributors centrifugal and vacuum advance mechanisms are working properly. Then, after that works and the end user really wants to make the lovely smog era machine run with smog era parts, give it a heart transplant with a 460 and an overdrive tranny out of a Lincoln. 🙂 Sure the mileage would suffer, but I think the constant ear to ear grin would make up for that.
mhealy1
Detailer

Look for a plugged up catalytic converter. This problem was more common on GM products, from my experience, but any vehicle with a cat can experience it. A common cause was running an over-rich fuel mixture causing excessive temperatures in the cat, leading to a meltdown (literally). This will cause a significant loss of power, especially under heavy throttle.
FYI-the most common over-rich condition in that era was choke operation on a cold start. You’ve probably resolved it with the carb rebuild (assuming you adjusted the choke correctly), but if you have a loping idle and/or black exhaust after a cold start you’re looking at a likely choke issue. Look for a bad choke pull-off, or a failed choke spring. If I remember right, your engine had an electric choke spring heater. If that fails, the choke spring never relaxes, or relaxes too slowly, keeping the choke closed or mostly closed long after it’s no longer needed.
mhealy1
Detailer

Another possibility is a collapsed pipe in the exhaust. Very rare but I saw it occasionally on cars with double-walled y-pipes (the crossover pipe off the manifolds leading to the single exhaust). In any case I’m pretty sure you’re looking at a restricted exhaust problem.
Sajeev
Community Manager

A bad catalytic convertor also makes a lot of sense. Getting a cheap replacement converter will also net you a bit more performance over the original, because oh boy, those original converters were very restrictive. It won't be a rocket ship, but it will give a slight benefit.