Surging Idle 1985 98

Hello,

My car is a 1985 98 Regency Brougham sedan (3.8L V6, fuel injected). It had a recent tune-up (seafoam the motor, replaced plugs, wires, air filter, cap & rotor) and it still has a problem starting and running smooth (no check engine light, no trouble codes). It usually cranks for a long time, unaffected by temperature of the engine or environment, and when the motor is cold, the idle surges (rpm's rise & fall). Vacuum lines all appear in good shape. Gas mileage is 12-15, which is pretty bad for how gentle I drive it. I also replaced the fuel filter and several injectors, and adjusted the idle slightly higher to try to stabilize it a little. The car only has about 120,000 miles and there are no classic symptoms of a blown head gasket. The radiator was also just replaced, along with a new thermostat and fresh coolant. Anyone have any ideas? Thanks in advance.

You cannot adjust the base idle beyond factory specs, this throws the tps off, a vacuum leak can cause this, sticking egr, dirty injectors, dirty throttle, body misadjuisted ( it is now) tps are all the common issues.

Thanks for the reply,

I will adjust the idle back, to see if it makes a difference. Injectors are fine to my knowledge, I replaced the ones I couldn't hear functioning (old screwdriver to the ear trick) and I run a bottle of Lucas fuel injector cleaner every other fill-up. The throttle body was pretty dirty, but I cleaned it and it made no difference. I'm wondering if it's either the TPS or the idle air control valve... I'm unsure about the EGR as well. The valve covers seem to have been leaking oil for quite some time... Could needing gaskets be part of the problem too?

No, not the gaskets, the tps should be .45v at base idle, if you move the preset base you bring up the tps and the ecm thinks the throttle is open and the idle hunts for the proper setting. Also the vacuum leak issues.

Checked coil output?