I/m sure there are plenty of post about this issue, but I have not found one that matches my issues.
I have been having a higher than normal idle. When I first start the truck in the morning and let it warm up its around 1500rpms. After its warm the engine does a weird idle. It will fluctuate from 1500 down to 1000 then go up to about 1250 and back down to 1000. It will doe this continuously.
Also when slowing down or coming to a stop the it goes to about 2500 for a few seconds and then slowly drops downs to 1000. After that it goes back to the above mentions rpms.
As soon as the clutch is pushed in the rpms rise to 2000 and then go up to 2500. At times they go up 3000 before dropping very slowly down to 1000.
I have replaced the IAC and cleaned the throttle body and sprayed the throttle cable springs. Cleaned the air box and MAF.

 

What year and what engine?

After engine is warmed up and idling, unplug the 2 wire connector on IAC Valve, it will then close all the way and RPMs should drop down to 500 or engine may even stall, either is good.

If RPMs stay above 700 then you have a vacuum leak.
With IAC Valve still unplugged, remove 1 vacuum hose at a time from intake manifold and plug the port in intake with your finger, if RPMs drop then that hose/system has a leak.

Start with PCV hose first, then Brake booster hose, then move to smaller hoses

 

My truck is a 1998. 4.0L V6.
Replaced the pcv valve and checked the hose.
I am not sure what it is, but underneath the IAC there is a module that has two hoses and a plug in connector. One of those hoses has a rip in it.
I replaced that hose and in the process I broke the plastic end on one of the modules' hose end. What is this module so I can get a new one.
I am not sure if that is the hose that created the RPM issue, but its a start.

 

Find out where the hoses run to

DPFE sensor has two hoses that run to Exhaust Tube for EGR Valve

EGR solenoid has two vacuum hoses, one runs to EGR valve itself and the other to intake manifold

 

It was the DPFE sensor that broke. I have since gotten a new one and new hoses from there to the EGR tube. I have a new EGR, but have not gotten around to trying to get the old one off....seems to be a PITA. Anyway, I Got the DPFE installed and reset the battery cables. I then tried the tests you suggested. I unplugged the IAC and the truck did slow to 500 before stalling and shutting off. I will be driving it tomorrow and post more information as to how the RPMS are behaving.
Thanks for the help

 

Takes a few days for computer to relearn idle settings

 

I have been having the strange high rpm issue still.
When the heater is on, the idle will go from 1000 to 1500. back to 1000. up to 1100 then to 1000 and then 1500. it will keep repeating this cycle. if I turn off the temp controls, the idle will stay at 1000.
When driving, the rpms will still go up to 2500 at a stop light and then slowly drift down to 1000.
Don't hate me, but I am unsure as to how to properly check for vacuum leaks.
I have replace the DPFE sensor and both hoses, the PCV valve and checked the hose to it. I check the brake booster hose. and have as new IAC installed.
I have checked the IAC and unplugged it. not much change in the idle rpms.
Do I keep the IAC unplugged and proceed to check each hose or do I plug the IAC in and then check each hose?
At this point, I have had the IAC plugged in and then checked hoses.
When I unplug each one, the engine does drop in rpms and then plugging the port with my finger it goes back up.
just to get all the information to you....
unplugging each larger hose drops the rpms.
unplugging just the green tube from the top of the egr does nothing.
unplugging the the red and tan tube combo from the intake drops the rpms.
unplugging the tan and green combo from the sensor that is bolted to the side of the intake to the right of the DPFE drops the rpms a little.
unplugging the MAF changes the rpms and it seems to correct itself.
unplugging the sensor in the air tube just before the throttle plate does nothing.
ALSO unplugging the IAC does drop the rpms but after plugging it back in the rpms climb to about 1200. then drop back to 1000.

 

Not sure what any of that means???


After engine is warmed up(MUST BE WARMED UP) unplug the IAC valve wires
RPMs should drop to 500 or engine may even stall, either is good, no vacuum leaks

If idle is at or above 700 then you have a vacuum leak, which you will need to find.

leave IAC valve unplugged
Disconnect one hose from the intake manifold(NOT A device) at a time and PLUG the port in the intake with your finger, if idle drops then that hose or its device has the leak.

 

Thanks for all your input Ron, but I am not finding out what's wrong with the truck still.
definitely have a leak somewhere. unplugging the IAC leaves the engine at around 1000.
I proceeded to check every hose I could see. Most of them appear to be on the drivers side. I checked the brake booster, PCV, and EGR. There is also a hose coming out up front under the throttle body. I unplugged that and no change.
I'm not sure where to look next.

 

With engine warmed up and IAC valve unplugged spray some soapy water around upper and lower intake, engine should stumble if it sucks some in.

Some use carb cleaner or Quick Starter(ether) for this test, but........hot exhaust and flammable fluid can get out of hand pretty fast :)

Also check that someone hasn't miss adjusted the anti-diesel screw on the throttle linkage.
It looks like an "idle screw" but isn't, so make sure it isn't holding throttle open more than it should, i.e. unscrew it slightly to see if idle drops(IAC still unhooked), if it does then someone F*ed up, so keep unscrewing it until idle is down to 500RPM.
If idle doesn't change then screw it back to where it was

 

Not sure if this could be something, but when I have the heater (or A/C) in the cab on, the engine surges from 1000 to 1500 then back to 1000 in a continuous cycle.
When I turn the heater off, the engine rpms stay at 1000. Probably not that related, as the rpms hang or raise some when pushing in the clutch to change gears.

 

Could also be engine PCM temp sender .

 

Computer will raise RPMs when AC compressor is activated, to compensate for extra load on the engine.
If heater is in Defrost position then AC compressor is activated to Dry the air blowing onto the windshield.
But in just panel or floor position AC compressor would remain off and RPMs shouldn't change.

The up and down idle with AC compressor on reads like either compressor is cycling on and off or IAC Valve is loose, not able to hold steady position.
The computer pulses Ground to IAC Valve to set its position, but valve inside must be tightly connected to the motors shaft or it can float, slightly opening and closing , so computer also changes ground pulse to compensate for RPM changes and you get a variable idle.

Yes, on newer manual trans setups the RPMs should hang high for up to 5 seconds or so, this is to reduce emissions when shifting gears.
And idle will also stay high when coasting until speed drops below 5mph , then it should drop to regular warm engine idle, this is also to reduce emissions.

 

It's been a while, and I think I have the issue figured out. Have been dealing with the high hanging idle since day one of the post. Ill get to a stop light, and in first gear with the clutch in, the idle will hang at 2000rpms. I can tap the throttle and it will drop to about 900. I think it may be the brake booster. When I press the brake pedal, i can hear a whoosh of air. I am pretty sure the booster is going out, but I am not sure that is the cause of my vacuum issue.
Can anyone chime in here?

 

It's been a while, and I think I have the issue figured out. Have been dealing with the high hanging idle since day one of the post. Ill get to a stop light, and in first gear with the clutch in, the idle will hang at 2000rpms. I can tap the throttle and it will drop to about 900. I think it may be the brake booster. When I press the brake pedal, i can hear a whoosh of air. I am pretty sure the booster is going out, but I am not sure that is the cause of my vacuum issue.
Can anyone chime in here?

 

If you have Cruise control there are two "air valves" on the cab side of power booster.

The atmosphere valve is on all boosters, when you press down on the brake pedal it lets in the 15psi outside air pressure into the booster which gives you the "power assist" for brake pedal pressure.

Cruise valve releases Vacuum from cruise module in engine bay, it is a safety backup for the brake light switch cut off for the cruise.
Cruise is cut off electrically but just in case the vacuum is released so it can't pull on throttle cable in any case

The air sound could be a leaky diaphragm inside the booster, but some sound is normal.

Test if booster holds a vacuum by doing this test.
Start engine
Let it run for 30 seconds or so
Shut it down
Wait 30 seconds
Press brake pedal and release
Press it again and release
Press brake pedal and release

You should get 2 or 3 "power assisted" brake pedal pushes before all vacuum is lost and brake pedal gets hard to push

If vacuum is lost after shut down then either booster check valve is bad or diaphragm is leaking
Check valve is the plastic elbow on the booster the hose plugs into, it pulls out and can be checked by sucking and blowing on it