Struggling to find the reason for the high idle speed on the 5L V8. I've completed the following:

1. removed the throttle air bypass valve and cleaned.
2. removed the EGR valve and cleaned, tested OK with vacuum applied to diaphram port. Measured the voltage on the EGR position sensor (5 volts reference, voltage output increases from 3.3 volts to ~ 5 volts when vacuum is applied. When vacuum is applied while engine is running, causes engine to slow down and sputter.
3. with engine running, have blocked the air intake ports, the EGR intake port, the vacuum to the brake diaphram, but the engine keeps running at the same speed. Is there another port from which the engine is getting air?? If so, it must be out of normal sight. WHAT NEXT - NEED HELP !!!!!

 

How many RPM is your idle speed when you first start your truck?

Does it stay at that RPM for over 5 minutes?

What is the idle RPM that you consider high?

 

I haven't measured the RPM, but I'd say its 1500 to 1800. On initial start up the speed is maybe 2000 RPM and then slow slightly within 30 seconds.

 

The speed stays at 1500 to 1800 no matter how long the engine runs.

 

Thanks for your timely response and any insight your may have. I'll try to get the RPM readings tomorrow after work to respond to your question more precisely.

Gotta go now and get some sleep for the work day.

 

Have you checked the voltage on the IAC valve harness?

Turn the ignition Key On (engine not running).

The voltage should be 10.5 to 12.5 volts on the harness side from the PCM.

Then check the terminals on the IAC valve and see if it is 6.0 to 13.0 ohms.

Then measure the resistance from either terminal on the IAC to the IAC body. It should be 10,000 ohms or greater.

 

Might want to check Throttle Position sensor also. That particular year has a procedure for adjusting the minimum idle air rate ( adjusting the throttle butterfly for proper idle speed.) Might want to search the threads or look into getting a manual that covers it

 

vacuum leak between the upper/lower intake manifold

 

Vac leak is easy to spot, I use a foaming carb cleaner and spray it around the suspect areas. Vac leak sucks the foam in and changes RPM.... Bingo, leak found.

 

Insert flame here...

 

Temp 1,

I checked the speed as you asked details on. The engine starts at 1900 RPM and during the next 20 seconds it decreases and settles at 1400 RPM.

Per your guidance, I checked the following

The voltage measures 12.2 volts on the harness side from the PCM.

Then check the terminals on the IAC valve and see if it is 6.0 to 13.0 ohms. >>> Measures 10.7 <<<<<<

Then measure the resistance from either terminal on the IAC to the IAC body. It should be 10,000 ohms or greater.
>>>>> Measures ~10,500 ohms <<<<<<

 

Sounds like the IAC is ok.

TPS (Throttle Position Sensor).

On the back of the electrical connector, install meter probes into GND(-) and SIG(+).

Turn the ignition switch ON (engine not running).

Sensor should read .5 to 1 volts at closed throttle and increase to 4 to 5 volts at full throttle (You can open up the throttle in the engine compartment, I will produce pictures if needed).

Larry

If this checks out ok, then it is a PCM, injector or Vacuum leak problem...

Pestco says the butterfly has an adjustment for Idle so I will see if I can find an article on it. I kind of hate to adjust Idle if something else put the Idle out of whack though...

By the way, I now see your truck is 1990 and my manual is for 1997 thru 2002.

 

Ok, here is some information about checking Idle adjustment...

Clean the IAC valve (idle air control) and the throttle body butterfly. Also with the engine idling, pull the connector off the IAC. The truck should slowly die. If it keeps running, the throttle butterfly is adjusted too far open. If the truck dies quickly, the throttle butterfly is adjusted too far closed. If you adjust the throttle stopm you might want to re-check your idle volts from the TPS.

I would try pulling the IAC connector and see if the engine dies...

 

Its not as flamable as your posts. There is a real way to adjust Min. Idle air rate. I would suggest looking it up in a man. instead of hoping Temp1 has come down off what ever he is on long enough to give you some reliable advice.

Temp1 FYI You twist a wrench for 15+ years and then come back with something we can use

 

Im sorry that you had a bad day, pestco. I will not kid around with you anymore as I did not know you were so touchy.
I apologize as I did not intend to flame.

Flamable as my posts, thats a good one!

Im sure you are quite willing to help this fellow and he will be in good hands with your advice! Its all yours...