Hey All!

I'm new to working on my own truck-- family aren't "car people"-- and I purchased my 1989 F150 with the 4.9l 300 I6 with 68k original miles in attempts to teach myself how to work on it. That being said, I need a little help here.
When I initially got the truck it had a bad vacuum leak, it would start right up but then die until I warmed the engine to about 160 degrees. Once it was warm though it ran well, just had poor mileage. So far I've changed the oil, oil filter, fuel filter, thermostat, plugs, wires, distributor cap and rotor, egr control solenoid, pcv valve, iac valve, cleaned iac connection, replaced the ignition switch and ignition tumbler (failed on me on my way to a date), changed the brakes and rotors, replaced SEVERAL hoses that I believe were leaking, ran seafoam carb cleaner, and a seafoam motor treatment through the tank. After all this, the truck runs 100 times better. Only problem is now, I'm holding high rpms after the truck is warm. I'm talking 1500-2000.
I've read about the throttle body issues that trucks these years are prone to but I'm not sold that this is the issue, as it wasn't happening until I replaced the iac and egr solenoid (did it all at once). Plus-- remember I'm learning everything for the first time here-- I didn't disconnect the battery when replacing the iac and egr solenoid. Could this be the reason for the high idle when warm? Any way to correct this?
This forum has been a great resource for me, so thanks for any help!!

 

I replaced my OEM IAC with an aftermarket one because mine would rev high at startup to about 2000 rpm and take a few seconds to go back down to a normal idle. It won't do it every time though.

The aftermarket one had a constantly higher idle rpm to the point where the truck would slam into gear when shifting from park or reverse to drive.

I just put the OEM one back in after cleaning it and live with the initial rev when starting but normal idle afterwards.

 

 

I would be checking for air leaks in the intake and vacuum lines.

 

Unplug the connector from the IAC when this happens. The idle RPM should drop way below normal or the engine may stall. If it does, your problem is electronic. If not, meaning the idle RPM remains high, you have a vacuum leak.

 

side note, make sure you have the correct thermostat in it, i think these things go into open loop not far below 180* water temp, which = worse mpg

 

I'll definitely unplug the iac and check that next. I'll probably just go ahead and completely replace every vacuum line that I haven't already.