Engine Is Idling High On A 00' 5.3l

[ZBrink]

I have a 2000 Yukon with the 5.3L with about 170k on the clock. The engine is idling at around 1200 to 1500 rpm. Where should I start? Any ideas?

 

Thanks,

Zack

[az'03Silverado]

You might want to clean the throttle body if you haven't already. Or, check the idle air control valve. It should be on the throttle body somewhere. It controls the idle speed. Those are the only two guesses I have.

[AgDoctor]

mine did that when my throttlebody needed cleaning, and even afterwards until i unhooked the battery for 5 minutes. after that it idled at about 750 rpm.

[nickvrebel]

check for vacuum leaks, leaks in the intake system and clean the IAC

[ZBrink]

check for vacuum leaks, leaks in the intake system and clean the IAC

 

What is the IAC? Anyone have a schematic of the vacuum lines? Photos?

[ZBrink]

I cleaned the throttle body out (it was fairly dirty) but nothing changed. It only idles high AFTER I have been driving and the engine. In other words, even instances where the engine is warm at first start up, it will not idle high until after I have put it through the load of driving.

check for vacuum leaks, leaks in the intake system and clean the IAC

[rmck490]

This may help.

 

The purpose of the IAC valve is to control engine idle speed, while preventing stalls due to changes in engine load. The IAC valve, mounted in the throttle body, controls a portion of the bypass air.

 

An orifice located between the throttle valves also supplies a constant amount of bypass air. By moving a conical valve known as a pintle (3) in toward the seat, to decrease air flow, or out away from the seat, to increase air flow, a controlled amount of air can be bypassed. If engine speed is too low, more air is bypassed in order to increase RPM. If engine speed is too high; less air is bypassed in order to decrease RPM. The PCM moves the IAC valve in small steps, called counts. These can be measured and displayed with a scan tool, which plugs into the Data Link Connector (DLC) . The PCM calculates the proper position of the IAC valve during idle based upon the battery voltage, the coolant temperature, the engine load, and the engine RPM. If the RPM drops below specification and the throttle valve is closed, the PCM senses a near stall condition and calculates a new valve position in or der to prevent stalling.

 

 

• Engine idle speed is a function of total air flow into the engine. Idle speed is based on IAC valve pintle position, crankcase ventilation valve flow, throttle valve opening, bypass orifice airflow, and calibrated vacuum loss through accessories.
  
• Controlled idle speed is programmed into the PCM, which determines the correct IAC valve pintle position to maintain the desired idle speed for all engine operating conditions and loads.
  
• The minimum idle air rate is set at the factory with a stop screw. This setting allows enough air flow by the throttle valves to cause the IAC valve pintle to be positioned a calibrated number of steps, counts, from the seat, during controlled idle operation.
  
• If the IAC valve is disconnected and reconnected with the engine running, the idle speed may be wrong. If this occurs, reset the IAC valve by depressing the accelerator pedal slightly, starting and running the engine for 5 seconds , and then turning the ignition OFF for 10 seconds .
  

 

[kchasse]

Just for fun check to see if it only idles high when you have your foot on the brake or when you pump the brakes a couple times. You could have a leaky vacuum booster that causes a leak only when you apply the brakes

[ZBrink]

I tried the trick with the brake pedal and nothing changed.

Just for fun check to see if it only idles high when you have your foot on the brake or when you pump the brakes a couple times. You could have a leaky vacuum booster that causes a leak only when you apply the brakes

[ZBrink]

Thank you rmck490!! I am going to attempt to remove the IAC today and clean it.

 

What about the MAF? Could that be the culprit? Is there a way to clean the MAF?

 

This may help.

 

The purpose of the IAC valve is to control engine idle speed, while preventing stalls due to changes in engine load. The IAC valve, mounted in the throttle body, controls a portion of the bypass air.

 

An orifice located between the throttle valves also supplies a constant amount of bypass air. By moving a conical valve known as a pintle (3) in toward the seat, to decrease air flow, or out away from the seat, to increase air flow, a controlled amount of air can be bypassed. If engine speed is too low, more air is bypassed in order to increase RPM. If engine speed is too high; less air is bypassed in order to decrease RPM. The PCM moves the IAC valve in small steps, called counts. These can be measured and displayed with a scan tool, which plugs into the Data Link Connector (DLC) . The PCM calculates the proper position of the IAC valve during idle based upon the battery voltage, the coolant temperature, the engine load, and the engine RPM. If the RPM drops below specification and the throttle valve is closed, the PCM senses a near stall condition and calculates a new valve position in or der to prevent stalling.

 

 

• Engine idle speed is a function of total air flow into the engine. Idle speed is based on IAC valve pintle position, crankcase ventilation valve flow, throttle valve opening, bypass orifice airflow, and calibrated vacuum loss through accessories.
  
• Controlled idle speed is programmed into the PCM, which determines the correct IAC valve pintle position to maintain the desired idle speed for all engine operating conditions and loads.
  
• The minimum idle air rate is set at the factory with a stop screw. This setting allows enough air flow by the throttle valves to cause the IAC valve pintle to be positioned a calibrated number of steps, counts, from the seat, during controlled idle operation.
  
• If the IAC valve is disconnected and reconnected with the engine running, the idle speed may be wrong. If this occurs, reset the IAC valve by depressing the accelerator pedal slightly, starting and running the engine for 5 seconds , and then turning the ignition OFF for 10 seconds .
  

 

[ZBrink]

I removed the IAC and cleaned it and the area in which it sits the best that I could with card cleaner. I also sprayed the MAF screen and sensor with MAF cleaner. The engine however is still idling high after being driven.

 

Any other ideas?

 

Zack

[rmck490]

Did you disconnect the battery for five minutes like pineland suggested? If not, do that so you reset the PCM stored values to the default values. If you did, then my thoughts are:

 

I'm not an expert at all of this but if you are having this problem after it's been driven/warmed-up, my next best guess is it may have something to do with it being in closed loop. During warm-up, the system operates in open loop and the PCM makes idle adjustments based on table values stored in memory pretty much ignoring sensor inputs. Once the engine is warmed-up, the PCM reverts to sensor inputs in the closed loop system to determine idle speed and maintain an air-fuel ratio of 14.7. Your IAC may be functioning correctly and the PCM is adjusting idle speed based on other sensor inputs during closed loop.

 

Closed loop sensors include the MAF, MAP, throttle position sensor (TBS), and the O2 sensors (sensor 1 on both banks). One of these could be bad or are being influenced by some condition like an air leak or additional fuel. Like others have stated, a vaacum leak is a posibillity. Another possibility may be a leaking injector(s); too much fuel causing the PCM to add air through the IAC. The MAF tells the PCM how much air is getting in. The MAP sensor sort of verifies this and also provides information about engine load. The O2 sensors provide information about the efficiency of the ignition. The TBS tells the PCM whether the throttle is at idle, part throttle, or wide open throttle (WOT). From all this information, at idle (TBS = 0%), the PCM calculates the air fuel ratio and adjusts the air input (through the IAC) or adds/subtracts fuel as necessary. In order to do this correctly, sensor inputs need to be accurate.

 

A problem within the closed loop system sensors or some outside factor influencing sensor outputs are areas that should be investigated. Don't just throw parts at this. Start with checking for vaacum leaks and checking/cleaning the PCV. Anything connected to the intake manifold is a suspect so check all hoses for cracks or being disconnected. Intake gasket could be next. When the engine is cold and running, spray starter fluid around the intake where it bolts to the heads. If your RPMs increase, you've found the problem area. After all that, and the problem is still there, you'll need to start checking the sensors. Unfortunately, if you don't have a decent scan tool that can read all the sensors and diagnose the problem, you are stuck taking it to a technician. If you go that far, have them check the injector balance as well.

 

That's my thoughts but I may off base a little. Anyone else out there got other ideas or corrections?