My son has bought a 1980 240 DL wagon. Cool car but rough idle. Drives fine but that rough idle. He is concerned and wants to fix. He is somewhat handy and has buddies that all work on cars. He could really use a check list of things to inspect and replace. Oh spark plugs replaced. Thank you.

 

When buying an older car, I'd start by doing a "stage 0" tune up: plugs, wire, cap, rotor. Stick with copper core plugs - these are low voltage coil cars - fancy triple iridium etc don't help, they just cost more. Next is to check for intake air leaks - vaccuum lines, rubber air boots, injector seals etc may just be turning 42 years old??? Not sure about the fuel delivery system on the car but if its an early FI system like K-Jet, there's an idle air valve that has a small motor controlled valve which manages the idle mixture. when working you can feel it buzz. Pop that off and clean the ports and make sure it has a good install (ie no air leaks). Again if its a fuel injected car, pop off the intake air boot and clean the air metering plate. Finally check for any header or exhaust leaks. Sometimes theres an EGR tube that can break off and should be fixed. On older pre-smog cars, some people just remove the entire EGR system and plug up the port n the exhaust pipe. Last is to look for the fuel filter - should be in back under the car. care needs to be taken during the removal since the connections can be stuck, plus you'd need to avoid spilling gas. Note on finding air leaks - some can be hard to spot. I've had to resort to having a shop do a smoke test to find an evap system leak that was in a hose right about under the driver's seat... If the tune up / clean up doesn't resolve the idle, that's what I'd do next.