Hi
Truck idles around 875-925 rpm. when i up the RPM to 1500 and let it return the idle fluctuates between 600 (nearly stalls) and 1000 a few times before settling on 875-925. I have no codes, Checked Vacuum lines no problem found.
i unplugged the IAC and this made no difference either way, which i thought it should. The IAC reads 11 ohms on the pins. Any Ideas?.
Thanks
1994 4.9L F150 XLT

 

I wood check the throttle body gasket and clean the throttle body its self ! And check for wear on the throttle body plates .

 

Thanks for the reply, The T/B gasket is new and the body is clean. will check tomorrow for condition of T/B plates.

 

I checked the T/B plates and they look and feel good. The problem only happens when it warm. When i pull the IAC valve connector after start up the engine dies a bit ,and also coming down from acceleration it idles quicker than whens its warm.

 

Well, i unplugged my O2 sensor and it really improved. drove for 10 miles. only thing is why did i not get an CEL with the O2 unplugged. Also no pending Faults. In fact no codes at all. just fuel pumps and a flash of the CEL.

 

Well i changed, T/B gasket .IAC, TPS, O2 sensor ECT sensor and Air temp sensor. Checked for leaks still not good. Idles now about 1000 rpms, not really hunting anymore. My engine does not seem to have a purge solenoid and there is a large amount of air been sucked into top of throttle body from the fuel canister. Should i have a purge solenoid. I appear to have a connector for it.

 

Ok.
No EVAP solenoid on my model. so moving on , Replaced intake\ exhaust manifolds gaskets (upper Lower) at weekend. Vast improvement. Idles about 7-800 rpm. Hunts between these two values continuously.
IAC is still not operating when i remove the connector no difference to rpms. Tried 3 different IAC valves ,1 new and 2 used and the same results. All 3 IACs operate when connected to 12V source. Connector for IAC has 14V on one pin and 1M Ohms for the ground(considered an open).
When i slave in the other 2 IAC valves i get no valve movement. I tried this with KOEO and engine running at idle. There are no codes either KOEO or engine on, other than the pass codes. My understanding is that the ECU provides a ground to the IAC at Idle and full throttle open and maybe KOEO.
Open wire ?Bad ECU
Thanks

 

Replaced intake gasket (again) and now idles about 700 rpm. Have another idle problem but will start different post and kill this one.
Thanks