454 1993 idle surges 200 RPM when cold.

[WarWagon]

Ok another weird one. 1993 454 3/4 ton pickup. 170K on it.

Simply running lean. Starts up fine and runs for a a min or so. Then the RPM will start to go up and down almost like it has a cam in it. 200 RPM on tach at least. After warming up some more the idle stabilizes. (couple min of this at least.) WD 40 or other fuel in a can sprayed in the TBI steadies the idle. Serious attempt made to find vac leaks. Including clamping brake booster off. Some vac hoses replaced.

Runs fine otherwise.

New injectors, TBI rebuilt and new gaskets. New fuel pump. New ECM temp sensor. New complete distributor (Magnet shattered), coil, wires, and plugs. Grounds added and cleaned. All parts improved the running of a truck that was towed after it was bought due to not running well enough to move under it's own power.

I am thinking EGR leaking, vac leak somewhere, tune too lean, maybe bad timing chain? Something else cause this?

 

[THEFERMANATOR]

The TBI base gasket is the most common cause of problems. The aftermarket gaskets are just plain paper whereas I believe the OEM is a metal reinforced gasket. Also check the TPS. They can fail and cause all kinds of idle issues without tripping a code for it. Normally they will cause it to shift to high of an RPM, and mess with the idle.

 

[Will L.]

Idle air control valve is only other thing I can think of, but vacuum leak is the money shot.