Edit: 06/29/2017 - 10:28am - Forgot to mention that throttle body & IAC are on the bench.

1988 F150 Lariat - 302 V8 - FI, PS, PB, AC.

Engine starts fine, high temporary idle, dropping close to normal after a warm-up period. But after a small period of running around town, releasing the gas pedal results in the idle dropping so low that the engine actually stalls (No power steering etc). Reminded me of a TPS failure of a few years ago.

Cleaning the throttle body, etc. TPS was replaced two years ago. All readings conflict with the Haynes manual but are identical with a new TSP... going to reuse my two year old TSP.

The IAC readings, according to the Haynes manual.... wiring harness to solenoid, key ON, engine not running, I should have 10.5v at the wiring harness connector, This connector has two red wire terminals... I get 10.5v out of one terminal, nothing from the other terminal. Is this okay?

The IAC solenoid.... The Haynes manual states the resistance should be between 7 to 13 ohms. I'm assuming that would be between the two terminals of the solenoid? My meters (tried more than one) peg the needle like a dead short.

Need a litle guidance here. Some Haynes instructions conflict greatly with various areas of the Internet.

 

On idle, if you unplug the IAC any change in RPM?
IAC like to restrict as well, not just solenoid end issues. Same with the throttle body, they collect debris in the bore/plate and cause low idle numbers.
PCV, EGR checked? Might put a vacuum gauge on it and check idle values. Fuel pressure checked? Regulator for bleed?

 

Hayapower... I neglected to mention that the throttle body w/TPS any IAC are on the bench. I'll try your suggestions when I get it back together, thank you.

The engine is a few hundred miles over the 100,000 mark... apparently needs a valve job as carbon build up (hard soot) is present.

 

How did the engine comp out?

 

Haven't put the engine back together yet... waiting to see if I get a reply to the following of my initial post.

The IAC readings, according to the Haynes manual.... wiring harness to solenoid, key ON, engine not running, I should have 10.5v at the wiring harness connector, This connector has two red wire terminals... I get 10.5v out of one terminal, nothing from the other terminal. Is this okay?

The IAC solenoid.... The Haynes manual states the resistance should be between 7 to 13 ohms. I'm assuming that would be between the two terminals of the solenoid? My meters (tried more than one) peg the needle like a dead short. Is this solenoid shot?

 

Hi Jim,
Glad to see you are still fighting with that 88 !
I believe Hayapower gave you some good suggestions and in light of the "carbon build up" , it just might be holding the EGR valve open a bit, worth checking it for carbon holding the pintle slightly open enough to affect hot idle.
Are you working with a metal or black plastic solenoid for the IAC?

 

The IAC solenoid is metal.

Glad to see you're still here and active Hanky. BTW.... that's "Joe".

 

Sorry about that, Joe !
One of those wires should come from either pin 37 or 57 of the ECA and is supposed to be vehicle power (12 volts). The other is controlled by the ECA to determine throttle air by pass , idle speed. Confirm the 12 volt supply , could be poor connection, battery etc.
The metal solenoids can be cleaned of carbon if necessary , but not the plastic ones.

 

The name error... no problem, you know the saying "A rose by any other name."

No carbon problem on the IAC solenoid. My concern follows (copy/paste)......

"The Haynes manual states the resistance should be between 7 to 13 ohms. I'm assuming that would be between the two terminals of the solenoid? My meters (tried more than one) peg the needle like a dead short."

Should I consider the solenoid shorted out?

 

I believe in 89 they installed diodes within the solenoid assy. What you might try is to reverse the probes of the ohmmeter and see if you still get 0 ohms. If so,you could also check for shorted windings by checking for any reading between any one of the terminals and outer housing . Should be open if not replace it.
If there is another poor connection or shorted component TPS, coolant sensor,Air charge temp sensor, that could also pull down the available voltage to your IAC.
Don't forget to check the things Hayapower suggested !!