have 302 & owned for 19yrs 90k miles - fast idle problem occurred slowly over about 3yr period - now idles @1700rpm in park. I have checked all vacuum lines -replaced pcv valve - idle control valve - throttle plate sensor -I'm sure there must be a vacuum leak behind the throttle plates
Is it possible intake manifold gasket? Otherwise truck runs perfect - no
oil burning

 

Intake gasket would usually cause rough idle, but an easy way to check is to take carb cleaner and spray along the gasket while the engine is idling. If the idle changes, then there is a leak. Idle usually increases from the spraying.

Is there a chance the screw on the throttle body that controls the throttle cable idle position got turned in to cause the high idle?

 

Sometimes a simple thing like a sticking accelerator cable will result in the problem you are seeing. If you can remove the cable from the throttle body and the engine will idle down that would confirm a sticking cable.

 

the dual throttle plates are completely closed against the throttle body-
this fast idle problem did not occurr overnite - but gradually
has gotten faster year after year. As you can judge by the mileage
90k I do not drive the truck much - runs perfect otherwise

 

Found this on a web page. #1 is out because of EFI, but look into the others. I'd look into #2 and #3 first...



Symptom: The engine idles too fast. After the engine has run long enough to be warm, the idle speed does not come down to normal. You really notice it when you come to a stop and must have to push hard on the brake pedal to keep the car from moving.

Possible causes:

1.If you have a carburetor, you may have a bad accelerator pump or power circuit.
The Fix: Replace accelerator pump or replace carburetor.
2.The engine may be overheating.
The Fix: Check and repair cooling system.
3.The fuel pressure regulator may be operating at too low a pressure.
The Fix: Check fuel pressure with a fuel pressure gauge. Replace fuel pressure regulator. (Generally not a DIY job)
4.The ignition timing may be set wrong.
The Fix: Adjust ignition timing.
5.There may be some type of ignition problem.
The Fix: Check and replace distributor cap, rotor, ignition wires and spark plugs.
6.There may be a fault in the computerized engine control system.
The Fix: Check engine control systems with a scan tool. Test circuits and repair or replace components as required. (Generally not a DIY job)
7.There may be a vacuum leak.
The Fix: Check and replace vacuum lines as required.
8.You have a bad idle speed control unit.
The Fix: Replace idle speed control unit.
9.The alternator may not be working properly.
The Fix: Replace alternator.

 

Bullitt supplied the complete list. Two of the things that I have found with a problem like this was;the PCV hose or valve and just plain vacuum leak from a dry rotted ,cracked or broken hose resulting in a vacuum leak. Also the IAC valve may be getting tired and need to be replaced.
There are many,and I mean many, vacuum hoses on this engine going to many valves and controls that one way that may help is to carefully spray each vacuum hose with a small shot of carb and choke cleaner , just a small shot and if the engine speed changes you found it. Just be careful, because if you spray too much the fumes will get into the throttle body and cause a false indication. There are TAD (Thermal Air Divert) and TAB (Thermal Air Bypass) solenoids that could be leaking, so it could take some time to find the leak. Patience here pays !