Hello,
I have been searching everywhere for the answer to my high idle problem.

Here is the current condition:

The truck was working fine.
Next, during cold weather, it would idle very low at start up.. I tapped on the EGR valve and the rpm's would come up to normal.

Now, the engine idles very high (maybe 2K?) ALL the time.

So I searched the forum and the internet for possible solutions.

Here is what I have replaced:
EGR valve
EGR vacuum solenoid
PCV and hoses
TPS (Throttle Position Sensor)... (When I installed it, I set the minimum at 1.0 volt)
IAC (Idle Air Control) solenoid
Plenum Gasket
Throttle Body Gasket
Thermostat
Charcoal vacuum hoses

I have sprayed around the engine with water, but the idle never changes.

There are no trouble codes.

I have 15hg of vacuum.



If I manually pull some vacuum on the EGR valve, the idle will go down.



I would welcome any suggestions....




Here are some photos....

 

Here are some more photos....

 

Sounds like you might have a vacuum leak still. Start your engine up. Take some carb and choke cleaner and spray around the vacuum hoses and the EGR area. If the RPM's rev higher when you spray in a particular spot, replace the vacuum hoses. I had a similiar issue on a car of mine. Turned out it was a crack in the EGR tube.

 

If you unplug the IAC, does the idle decrease?

 

If it is a vacuum leak, what sensor(s) would tell the injectors to add more gas?

Is it in the exhaust? Is there some other intake sensor that is telling it to add gas? Or.... is extra gas being added?

The mass air flow sensor is upstream of the throttle body. The throttle body is not opening, so something is telling the engine it needs more gas.


And why would opening the the EGR valve cause the idle to go down?

From what I understand, the EGR valve start to open during cruising speeds.

 

I will try this and report back.
I understand the IAC operates when the AC (or some other power-robbing device) comes on.

 

Do you have an O2 sensor on this vehicle? Vacuum leaks, cracked EGR tube, etc. screw up the fuel mapping and the O2 sensor can see it. If it senses its too lean, more fuel is added to the system.

 

If the O2 sensors are operating in "closed loop", then the signal from the O2 sensors will tell the PCM to richen the A-F mixture to compensate for the additional air.

The EGR adding exhaust gases to the intake charge does not cause a lean condition, due to the lack of oxygen. Adding exhaust gases to the intake charge
lowers the combustion temp, which lowers NOX emissions.

The PCM uses the IAC to increase idle speed for various reasons, AC compressor being on is one of them as you mentioned. Another condition that will raise idle speed is the ECT sensor indicating a HOT condition. If the ECT sensor is defective, causing a false HOT condition, the idle speed will be increased.

 

The ECT (Engine Coolant Temperature) sensor... is it the same one that drives the instrument panel gauge? Or are there two different sensors?

The instrument panel gauge never gets above medium (normal) temp.



Update....

In digging around on the internet, it looks like there are three sensors. An air tem sensor mounted on the air box, a gauge temperature sensor mounted on the manifold, and the ECT mounted near the thermostat.

I'll have to verify that tomorrow. Hopefully, there is a test to determine if the ECT is good...

 

Separate sensors. The ECT sensor has a 2-wire connector, the temp gauge sensor has a single wire connector.