Huge Idle Variation When Cold

[94RangerVa]

Greetings,

My 94 Ranger (4L, 6 cly, 5-Speed, 4x4, Super Cab, Non-ERG equipped) has 158K miles. It has recently been tuned and treated to new fluids, filters, o2 sensor, and complete exhaust/C-Converter system.

So I started it today and thought it started right away, it began a wide range of idle which varied between 500 rpm and 1500 rpm. This went on for several minutes until the truck warmed up, at which time it assumed its normal, very steady idle of 750 rpm.

I'm thinking its not the carb causing the issue because once it warms up, everything runs great. I don't believe it has a vacuum leak for the same reason.

Anyone have ideas?

Thanks.

 

[Spott]

I would also assume it's not the carb; mostly because that engine does not have a carburetor.

When exactly was this "recent" tune up and exhaust replacement? What error codes (if any) are the computer reporting?

My first guess would be a sticking Idle Air Control valve. This is a common and simple problem. Search for information on locating and cleaning the IAC, and see if that helps.

 

[RonD]

Yes, ^^^
Fuel injected engines can't use idle screws, the computer that runs the fuel injection needs to set the idle, so all fuel injected engines use an IAC(idle air control) valve, Ford calls theirs an IAC Valve, others call theirs other names but they all do the same thing, they are controlled ported vacuum leak.

IAC valve is on the upper intake next to throttle plate, it is not small, it has an electric motor and 3 wires, so easy to spot.
It is held on by 2 bolts, so easy to remove.
There is a gasket and if you are careful it can be reused.

Once it is off you will see the valve inside that is moved by the motor turning.

Google: cleaning IAC Valve

And 750rpm is too high for manual trans, should be under 650
Automatics can run 700-750

Once engine is warmed up unplug the wires on the IAC Valve while engine is idling, IAC valve should close and RPMs should drop to 500 or engine may even stall, either is good, if idle stays at 700 then IAC valve is stuck or you DO have a vacuum leak


4.0l engine has a common problem of the lower intake to head bolts getting lose, check a few