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Amedee, Former ASE Master Tech
Category: Car
Satisfied Customers: 25195
Experience:  ASE certified tech advanced level specialist. Wisconsin certified emissions state inspector
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How to correct engine idling at 2500rpm

Customer Question

99 Isuzu Rodeo 2WD 3.2 CEL was on error was throttle position sensor. Sensor was changed but idle still high. The CEL is not on now while engine is idling.
IAC valve is just over a year old (changed from previous troubling sensor problem). Only other sensor changed was the crank position sensor).

Cleaned the IAC ports and checked the O-ring-looks good. Cleaned the EGR valve ports, checked the piston doesn't stick. Tried the ecm fuse removal for the reset didn't work.

Checked the hoses and intake manifold gasket for leaks (sprayed water) no change in idle. Help needed, thanks.
Submitted: 7 years ago.
Category: Car
Expert:  Amedee replied 7 years ago.

Greetings! You might have an air or a vacuum leak. This is very common! Open the hood and listen for a hissing sound when the engine is running.
Unmetered air can enter the engine through a vacuum leak, a dirty airflow sensor that is not reading airflow accurately, an EGR valve is not closing and is leaking exhaust into the intake manifold, an EGR valve that is allowing too much flow.
If it is hard to pinpoint take some brake cleaner or starting fluid around the intake manifold and vacuum lines and see if the engine stumbles or if the idle is affected. Be extremely careful when doing this!

Also, your throttle body may be carboned up and need to be cleaned! This can cause all sorts of idle and hesitation problems. This is caused by the throttle plate not seating properly. The First thing i would do is clean out the throttle body with some throttle plate and intake cleaner and a small brush. Another common cause would be the Idle Air Control motor. This is very common on older cars. The IAC motor gets lazy and cant keep up with the fast idle changes. Also when the IAC motor is out, I rec to check the passages for carbon build up. If they are plugged they need to be cleaned out.


Check for the following conditions:


Poor connection at PCM or IAC motor. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
Damaged harness. Inspect the wiring harness for damage.
Restricted air intake system. Check for a possible collapsed air intake duct, restricted air filter element, or foreign objects blocking the air intake system.
Throttle body. Check for objects blocking the IAC passage or throttle bore, excessive deposits in the IAC passage and on the IAC pintle, and excessive deposits in the throttle bore and on the throttle plate. Check for a sticking throttle plate. Also inspect the IAC passage for deposits or objects which will not allow the IAC pintle to fully extend.
Vacuum leak. Check for a condition that causes a vacuum leak, such as disconnected or damaged hoses, leaks at EGR valve and EGR pipe to intake manifold, leaks at throttle body, faulty or incorrectly installed PCV valve, leaks at intake manifold brake booster hose disconnected, oil filler cap, oil level indicator loose or missing, etc..

Expert:  Amedee replied 7 years ago.

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Customer: replied 7 years ago.

As listed in my post I tried most of what you mentioned to no avail. The only "hissing"sound come from the throttle body even with the valve closed. I have pulled all of the hoses and temporarily plugged from the intake manifold to determine if there would be a difference and there was none. Again no CEL. As mention ed the IAC is about 1 year old, changes the TBP sensor also.

Expert:  Amedee replied 7 years ago.

What is the engine idling at?


Do you know the code number that was in the computer?


WHat about the egr vale.. has that been changed?



Customer: replied 7 years ago.
The code was a 0122 for the throttle position sensor-I changed the sensor and the coded cleared. I plan to check the area where the throttle shaft passes through the body. I read n a forum that there have been leaves in that are plus the the hissing suking sound appears to be coming from that area. I inspected the EGR valve and it was not sticking. I cleaned it of deposits and reinstalled it. The PCV valve si working-the check valve is working. It does sound like a vacuum leak the problem is exactly where?
Expert:  Amedee replied 7 years ago.

Ok... your best bet to find this vacuum leak if there is any is going to be to have the intake smoked with a smoke machine. This is the easiest way to find a vacuum leak if there is any.


There is also a Technical Service Bulletin for this problem. Here it is.



1998-2000 Rodeo (UE)/Amigo (UA) models with 3.2L V6 engines


Some owners may comment on high/erratic idle and/or the MIL light on. In some cases, vacuum leak noise may be heard as well.

Possible Cause:^ There may be poor contact or loss of contact at various main ground points or electrical connections.

^ Power steering pressure switch may be shorted with debris.

^ Inlet manifold gasket may be leaking.

^ Broadcast code and calibration part number may need to be updated.

To correct this condition, follow the procedures outlined in this bulletin to determine proper diagnosing, inspection and correction of ground contact points for proper electrical contact, power steering pressure switch cleaning or replacement and power steering system flush, replacement of inlet manifold gasket, and/or PCM reprogramming


ALso, here is some info about your code

Circuit Description
The throttle position (TP) sensor circuit provides a voltage signal that changes relative to throttle blade angle. The signal voltage will vary from below 0.6 volts at closed throttle to about 4.5 volts at wide open throttle (WOT). The TP signal is used by the powertrain control module (PCM) for fuel control and many of the PCM-controlled outputs.

Conditions for Setting the DTC

The ignition is ON .
TP sensor signal voltage is less than 0.22 volt for a total of 0.78 second over a 1.5-second period.
Action Taken When the DTC Sets

The PCM will illuminate the malfunction indicator lamp (MIL) the first time the fault is detected.
The PCM will store conditions which were present when the DTC was set as Freeze Frame and in the Failure Records data.
The PCM will use a default throttle position based on mass air flow and RPM.
Conditions for Clearing the MIL/DTC

The PCM will turn the MIL OFF on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.
A history DTC P0122 will clear after 40 consecutive warm-up cycles have occurred without a fault.
DTC P0122 can be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed.
Diagnostic Aids
Check for the following conditions:

Poor connection at PCM - Inspect harness connectors for backed-out terminals improper mating broken locks, improperly formed or damaged terminals, and poor terminal-to-wire connection.
The TP Sensor shares a 5 Volt reference with the EGR valve. If these codes are also set, it could indicate a problem with the 5 Volt reference circuit or components itself.
The TP Sensor shares a ground with the EGR Valve and the IAT Sensor
Damaged harness - Inspect the wiring harness for damage. If the harness appears to be OK, observe the throttle position display on the scan tool while moving connectors and wiring harnesses related to the TP sensor. A change in the display will indicate the location of the fault.
If DTC P0122 cannot be duplicated, the information included in the Failure Records data can be useful in determining vehicle mileage since the DTC was last set. If it is determined that the DTC occurs intermittently, performing the DTC P1122 Diagnostic Chart may isolate the cause of the fault.