high mileage 1999 4.6 4wd 5 spd: Needed to replace the rear ABS sensor and decided to clean the throttle body too while the battery was unplugged. Cleaned it around the butterfly and not much else. Truck idled at 750 BEFORE cleaning the TB. Now it fluctuates between 820 and 950 rpms. Also tends to stay at a higher rpm after pushing in the clutch and taking it out of gear. Did a thorough check for vacuum leaks, nothing. After research, I cleaned the IAC and re installed (battery was disconnected then too). No improvement. Unplugged the IAC while running- ZERO change in idle. Installed new IAC, no change whatsoever. Unplugged NEW IAC, no change whatsoever.
Is there an issue with the PCM needing time to fully re learn? Am I missing something (besides the 40$ for the IAC)?

 

Tracked down Bluegrass 7's quick learn recommendations. Will try that tomorrow....

 

Powering down the PCM reboots the program.
All tables revert back to unaltered state.
Be sure you allow enough time for the motor to fully warm to operating temperature before concluding there is a problem.
Drive several miles then do several restarts to allow the PCM to learn the extra loads presented by the alternator charging the battery, lights on and blower motor running.
Under cold start the idle can normally go higher until the Ox sensors take over fuel control and temp comes up.
If the idle is still high with the IAC disconnected, look at the throttle plate for being stuck open some small amount more than is normal.
Look at the hoses around the area you worked on for seating or a fault letting excess air in to the motor.
Good luck.

 

Throttle plate and hoses have been thoroughly inspected. Engine coolant temp is-was at 192 F. It's just not stepping down that 100 or so rpms.... Thanks for the input. Pretty damned disappointed, I really didn't need to disconnect the battery and now this....

 

If you removed the IAC electrical connector and idle did not drop there is something causing excess air into the motor.
Either the IAC is stuck, the throttle plate is not closed down from the throttle cable sticking, the TPS is not set into the throttle shaft and holding it open.
At this point it is not a PCM issue when the IAC is disconnected.
The base idle should be in the 500/550 range with the IAC disconnected on a running engine.
Good luck.

 

Thanks for your help, I do appreciate it. I'll keep looking. The throttle cable was inspected and lubed the first time this happened. The throttle plate was cleaned and inspected yesterday. The only vacuum hoses that got moved at all in this process are the ones directly on the right side of the throttle body. Does the IAC get power directly from the PCM?

 

The IAC gets 12 volt power through the PCM relay when the key is turned to Run.
It's ground is pulse controlled through the PCM as the way bypass air controls idle mainly under unloaded or very light engine loads, cold starts and Deceleration idle control.
You can see from this that removing the IAC connector while the motor is at idle should drop the Idle to base RPM determined by the throttle stop.
If the throttle stop has been changed the base idle is no longer proper.
Air flow is the key to engine RPM and power at any time.
Fuel added is only done to expand cylinder pressure under ignition so power can be extracted from the airflow through the cylinders.
Your missing the problem somewhere.
Good luck.

 

I just got thru cleaning my throttle body for about the 5th time over the years. After I'm done I use compressed air to blow everything out to clean it again. Never a issue.
Go back to the T.B. and spray some wd40 at the plate and then wipe it clean. Work it several times before you reassemble it.

 

I appreciate the input. I cleaned and lubed the throttle plate yesterday and even checked it's position while operating at idle. It's fully closed.

 

My next check will be to see if the IAC is actually getting power. Seems like the idle would be rough if it isn't getting any, but it should also get rough if I unplug the damn thing and it isn't. Spent a few hours messing with vacuum hoses yesterday, with zero results.

 

I was told on another forum that the ecu makes changes so rapidly in the trucks idle that spraying the vacuum hoses with carb cleaner doesn't really make the idle change enogh to be noticeable to the untrained ear. Everybody agree with that? Makes sense that I couldn't find a vacuum leak with that method.

 

Maybe I missed it but did you run some/many miles since you pulled the batter Cable? Because it does take time to relearn

I pulled my battery Cable to work on my "blackout" odometer problem. The battery Cable was off for about an hour. Truck ran like crap for a trip to work. On the way home from work it shaped up. Takes a bit, yup. As mentioned above . I wouldnt panic unless you get some miles on it

 

Dan - thanks for the response. Yep, put a few miles on it, enough to re set everything. I ended up ordering the PVC hose assembly that replaces the PVC valve all the way to the rubber elbow at the back of the intake. Coincidentally, picked up a p1506 (IAC over speed) code which indicates a vacuum leak somewhere. Part comes tomorrow. Frustrated but determined.

 

Fixed. Got the new PVC line installed, doesn't actually fit quite right, but close enough to get a good seal for now. Truck idles at 650. Not sure if it was the actual PCV valve or the soft elbow at the back of the intake manifold. This was problematic because of there not being any initial codes. When it threw the p1506 the other day, research says it was the PCV set up. Never did get any lean codes, pending or otherwise. Just glad it's fixed. Thanks to everyone who chimed in and special thanks to Bluegrass 7 who must have the patience of Job.

 

Great to hear.
This thread is a text book example of the difference between just a bad rubber elbow and a PCV valve in the same line.
The system is so good it can pick out the difference.
I just did my rubber Elbow job a couple months ago on a motor that went 215,000 before it went bad..
It caused the 171/174 codes, poor idle and stalling as opposed to your high idle.
I did detect something else I think offers a significant fuel mileage gain but that's best left until I get around to testing the theory.
Forgive me for keeping on the track of your missing something because the effect you were seeing suggested a different cause, especially after finally seeing 1506 code to further support it.
Good luck.