Former owner let the exhaust rot......truck was loud. No tail pipe and leaking exhaust.

Had exhaust fixed......truck is quiet.

Then I noticed a whistle at idle because truck is quiet. Looked at the rpm at idle......it was 1000 warm
Disconnected the IAC valve. And the idle went to 700 where I want it. Whis long stopped.

The former owner also said the throttle was sticking. This was traced to the fast idle screw that sets base idle in case the IAC dies.
.......The screw was all the way out and letting the butterfly's sit all the way in the bore. I spent some time getting it to the point where it was not sticking. Not to far.....just right past the non sticking point. I did not do this with the IAC disconnected as I should have. I probably have it set to factory at this point.

I'm going to reset the computer by disconnecting the battery for 20 minutes to relearn the idle. Hopefully. I'm starting here first.

The intake system is very clean. If there was a Teflon ring inside the throttle body it is long gone. There is an air gap around the butterfly's....the ring of light. I'm thinking this is normal. I've seen it on old school carburetors. I've heard this Teflon ring seals it but who really knows. There is no Teflon ring on mine....maybe that's the problem.

I don't know if this truck ever idled fine after I rebuilt it. I doubt it. I just didn't notice because of all the noise.

So now it's idling high and whistling. I'm not throwing out it could have a vacuum leak but it's unlikely. All of it is new. Every line, gasket, and seal.

The whistling sound is getting to me though. Air leak that I couldn't hear before.
Fuel trims on bank 2 climb into the +7 as throttle is applied just for kicks.

Where is my problem? The whistling is driving me nuts

 

Warm idle with IAC unplugged should be 500.

The screw you mentioned is an anti-dieseling screw, and can also be used for minor adjustments for the TPS(throttle position sensor) closed throttle voltage, .69-.99volts

Fuel injection can't use an idle screw, no Jets, so uses an IAC Valve to set idle.
"Gas pedal" is now an "air pedal"

So after engine is warmed up unplug IAC valve and adjust that screw so idle is below 550, engine can even stall that is fine, it is not an idle screw, but it does need to be set "low" enough to stay out of the way of the IAC Valves ability to set "target" warm idle, 625 manual trans, 725 automatic

If idle won't go below 700 then there is extra air coming in to intake, could be vacuum leak or throttle plate leaking too much air.

Whistling can come from the black cap on IAC Valve, that cap covers a vent that allows valve to move in and out without pressure building up behind it.
If valve seal is leaking then air can be sucked in thru that vent causing a whistling sound, and this would be a vacuum leak.
Black cap just snaps off and on, it has an air filter under it.

Fuel trims are normally above +5 at idle, computer doesn't use O2 sensor feedback at idle or at WOT(wide open throttle), idle must be Rich or engine will overheat, and at WOT MPG is not a factor, lol.

 

Lowest base idle I can get is 570
Tps voltage at idle is .8
IAC % is 36%

The fuel trims are +7 at 2500rpm cruise

The o2 data shows .2-.7
Out of spec in my opinion.

I would expect with a vac leak to be lower than 36

 

So
.......the IAC shows 36% on a good running engine.

In engine with high idle due to excess air would **** the IAC down.....if it's metered?
Like turning up the idle screw.....that air would be coming in past the butterfly's. But it would be metered air. And shut down the IAC....thinking that it was off idle?

With a vac leak.....that's un metered air. Would that affect the IAC the same way? Would it still shut off or would it stay normal.
I'm thinking it would stay normal....at 36%.

Either way.....idle is high
Tips is normal .8
Base idle is set right but still above spec you listed at 500

 

570 RPM should be fine, it allows IAC valve to work

.45v is the sweet spot for O2 voltage
.2-.7 fluctuation is fine

-15 to +15 is OK for fuel trims.
0 fuel trim is the computers calculated amount of fuel to add based only on MAF sensor and air temp sensor data
Computer doesn't have a fuel pressure monitor so bases fuel flow out of an injector on spec pressure, 1998 should be 65psi

So as an engine system ages the sensor data and fuel pressure will change.
+ fuel trim means computer is opening injectors longer than calculated 0 open time

.2-.7 O2 voltage means oxygen content in exhaust is not showing Lean or Rich, so burn is good.

MAF sensor tends to under report air flow as it ages, so there is more air than the computer expects, when the calculated added fuel is burned it shows Lean and computer increases fuel trims to +1, +2, +3, ect....... until it gets correct voltage from O2 sensors.
O2 sensors work using a chemical reaction to detect oxygen, much like a car battery uses a chemical reaction to store electrical charge.
Both use up the chemicals over time so both wear out.
Battery after 5 to 7 years, O2 sensors 100-120k miles

O2 sensors, upstream, tend to get sluggish as they age, and show lean voltage so can add to higher + number fuel trims.

Computer won't set fuel trim codes unless they are above +15 or below -15 for more than 30-60seconds at a time, because it is not a concern unless it is a long term effect.

+ number does not mean engine is running Rich or Lean is is the Open Time added to fuel injectors to balance voltage at O2 sensors

 

The engine should pretty close to die when the IAC is unhooked and go to base idle. It goes to 700 rpm.

With base idle still higher than spec. It leads me to believe that extra air is making that happen.

With the IAC still operating at close to base idle. I would expect the IAC to close up some and the %value go into the lower ranges. But it's not.

The only thing I do know is the throttle blades are closed and the tps voltage is good. One could see that if the tps were higher than 1 volt it could cause fast idle. That is ruled out.

The fuel trims will be problematic at +15 over time and lean enough to trash the cats. Anything over 4-5 on a consistent basis is leaned out. My bank 2 trims being at 7-10 just on one bank mean that excess air is coming in on one side of the engine. The high idle could be the cause of that and probably is somehow.

I will check the IAC cap and see what I can do there.
The whistling sound.....I'm going to try propane and a smoke test finally to see if I can spot a leak somewhere. If my fuel trims go above 5 I'm looking for problems. I'm not getting any codes yet....just looking at a bunch of data that seems off.

When looking at the o2 at idle in closed loop I usually expect close to .1-.8 bounce.
.2-.7 seems ok. But I just glanced at it. I will replace it again as it might be slightly lazy. I will also check bank 1 o2 data. If it's actually .3-.6 I'll replace it.....maybe a data log will show better on a graph.

Just want to get through these problems before they become codes.
Changing out the bad injector helped the fuel trims go down from 10 to 7 but it's not enough.