Question about 2003 Oldsmobile Silhouette

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2003 oldsmobile silhouette idles rough, starts to buck at around 2000 RPM wondering if my PCM is bad?

Transmission just rebuilt, new IAC, cleaned throttle body, new air filter, oil filter, new plugs and wire, new fuel filter. I'm questioning the PCM because I get no change to how the engine idles (idling to low right now) when I unplug the MAS and the IAC sensors while running.

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  • 5 more comments 
  • Robert Wood
    Robert Wood Jun 15, 2015

    Can you look at your O2s on a scan tool ?

  • Tony Raye Jun 15, 2015

    Tonight I can, I did also replace an ignition coil that had a bad resistance reading.

  • Robert Wood
    Robert Wood Jun 15, 2015

    Sometimes a bad coil can cause the coil driver to go bad in the PCM/ECM. Check the O2s and make sure the front O2s are fluctuating between .200 and .800 even .100 to .900 but in those ranges and the rear O2s should be steadier around .450. Also, if you can remove one or both of the O2s and see if it runs better, as the cats or exhaust can be stopping up.

  • Tony Raye Jun 15, 2015

    Great input I'll let you know what I find out tonight!

  • Tony Raye Jun 16, 2015

    Ok, I did not notice any difference while turning the wheel, the O2S seem to be off from what you said. I attached some data I got this morning. Car was at running temp when I recorded the data. I didnt have time to remove one of the O2s last night, but will try tonight. I have some other data if you'd like to see it I just have to put it in my excel sheet. You'll see where I started to rev the engine on the RPMs that right around when it starts bucking.

  • Tony Raye Jun 16, 2015

    if you can't read the image I can send a link.

  • Tony Raye Jun 17, 2015

    OK SO I got the post O2s out, did not seem to make much difference, pre O2s is a little harder to reach so I'm working on it still.

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  • Oldsmobile Master
  • 5,911 Answers

When you turn your wheels at idle in park does your rpms pick up ?

Posted on Jun 15, 2015

Testimonial: "Have not noticed, I'll check later today."

  • Robert Wood
    Robert Wood Jun 15, 2015

    That's one way to know if your IAC is controlling idle speed. It should idle up when you turn your AC on too.

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My 2000 olds silhouette will start and run, as long as you have your foot on the gas, but as soon as you take it off it dies. Will not idle. Any ideas??


First eliminate all vacuum or air leaks in the system. You may have a bad IAC ( idle air control) but try cleaning the throttle body first. Remove the intake snorkel, have someone hold the throttle wide open for you and scrub the back side of the throttle plate and surrounding bore with an old tooth brush and some carb cleaner. Be sure to spray some into the small holes next to the throttle plate. That should help stabilize the idle. If it still has a problem, replace the IAC.

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What causes 2000 taurus engine rough idle but no codes


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I have a 2001 Oldsmobile Aurora that has a high idle; had diagnostic done, says to replace PCM. Replaced PCM after having programmed by Chevy dealership in town, and it still has a high idle of 1100 RPMs....


Why wasn't the Throttle Position Sensor replaced? The first suspect should probably have been the IAC (but you would have gotten a code), the next logical step would have been checking for something mechanical like the throttle body being gummed up and needing cleaning, but I would replace the TPS before I'd EVER think the PCM would need replacing...I'd hate to see that bill. I have a 2001 Aurora with a 4.0 and I have replaced the IAC (got one at a salvage yard, out of a Caddy, same one)-it did fix mine, but the next culprit would have been the TPS in any case, PCM's rarely go bad.

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1 Answer

Will not idle, will run if you push on the gas and hold it. No engine light indicator


you need to clean your idle air control valve ( or replace it) Spray carb cleaner does a nice job of cleaning it.

This tutorial is from autozone.com


OPERATION



The Idle Air Control (IAC) is a bi-directional motor driven by two coils. The purpose of the IAC valve is to control engine idle speed, while preventing stalls due to changes in the engine load. The IAC valve, mounted in the throttle body, controls bypass air around the throttle plate. The PCM controls engine idle speed by adjusting the position of the IAC motor pintle. The PCM pulses current to the IAC coils in small steps (counts) to extend the pintle into a passage in the throttle body to decrease air flow. The PCM reverses the current pulses to retract the pintle, increasing air flow. This method allows highly accurate control of idle speed and quick response to changes in engine load. If RPM is too low, the PCM will retract the IAC pintle, resulting in more air being bypassed around the throttle plate to increase RPM. If the RPM is too high, the PCM will extend the IAC pintle, allowing less air to be bypassed around the throttle plate, decreasing RPM.



0900c15280217cd2.jpg enlarge_icon.gifenlarge_tooltip.gif

Fig. Typical Idle Air Control (IAC) valve components
During idle, the proper position of the of the IAC pintle is calculated by the PCM based on the battery voltage, coolant temperature, engine load and engine RPM. If the RPM drops below a specified value, and the throttle plate is closed (TP sensor voltage is between 0.20-0.74), the PCM senses a near stall condition. The PCM will then calculate a new IAC pintle position to prevent stalls.
If the IAC valve is disconnected and reconnected with the engine running, the idle RPM will be wrong. In this case, the IAC has to be reset. The IAC resets when the ignition switch is cycled ON and then OFF . When servicing the IAC, it should be disconnected or connected with the ignition switch OFF in order to keep from having to reset the IAC valve.
The position of the IAC pintle affects engine start up and the idle characteristics of the vehicle. If the IAC pintle is fully open, too much air will be allowed into the manifold. This results in high idle speed, along with possible hard starting and a lean air/fuel ratio. A Diagnostic Trouble Code (DTC) may set. If the IAC pintle is stuck closed, too little air will be allowed in the manifold. This results in a low idle speed, along with hard starting and a rich air/fuel ratio. Again, a Diagnostic Trouble Code (DTC) may set. If the IAC pintle is stuck part way open, the idle may be high or low and will not respond to changes in engine load.


TESTING



As with most all engine control sensors used in your vehicle, a thorough and proper test can only be performed by a qualified technician using a scan tool to read the data stream from the PCM. There are a few items a non-professional should check before taking the vehicle to a qualified technician for diagnosis and repair.



Check for a poor connection at the PCM or IAC motor. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connection. Inspect for a damaged wiring harness, especially if heavy work has recently been performed (intake manifold removal, engine removal, etc.) where the throttle body has been disturbed. Inspect for a restricted air intake system. Check for a possible collapsed air intake duct, restricted air filter element or foreign objects blocking the air intake system. Inspect the throttle body. Check for objects blocking the IAC passage or throttle bore. Excessive deposits, especially on high-mileage engines, can build up in the IAC passage and on the IAC pintle. Excessive deposits can also build up on the throttle plate, so check for a sticking throttle plate. Check for vacuum leaks such as disconnected hoses, leaks at the EGR valve and EGR pipe to the intake manifold, leaks at the throttle body, faulty or incorrectly installed PCV valve, leaks at the intake manifold brake booster hose connection, etc.



REMOVAL & INSTALLATION



  1. Disconnect the negative battery cable.
  2. Detach the electrical connector from the IAC valve.
  3. Remove the 2 attaching screws and remove the IAC valve from the engine. Remove the O-ring from the housing.

To install:



0900c15280217cd3.jpg enlarge_icon.gifenlarge_tooltip.gif

Fig. The Idle Air Control (IAC) valve is mounted on the throttle body-3.8L (VIN K) engine shown, others similar


NOTE Before installing a new IAC valve, measure the distance that the valve is extended. The measurement should be made from the motor housing to the end of the valve cone (pintle). The distance should be no greater than 1 1 / 8 inch (28mm). If the pintle is extended too far, adjustment is required, otherwise damage may occur when the IAC valve is installed. Adjust the IAC valve by compressing the pintle to achieve the correct length.


  1. Use a new O-ring, install the IAC valve and tighten the retaining screws to 18 inch lbs. (2 Nm).
  2. Attach the electrical connector.
  3. Connect the negative battery cable.
  4. The PCM will reset the IAC valve the next time the ignition switch is turned ON and then OFF .
  5. Start the engine and allow to warm to operating temperature to check idle quality.

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1 Answer

When started idels high about 2000 rpm then drops when hot cold ?


the iac probably has carbon built up around it this controls the idle speed try cleaning it first,if not it could be a bad iac or a vaccum leak that the iac takes some time to compensate for to control idle.

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2 Answers

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You want to check your charging system. Take a reading at the battery and see what you get. it should be 13.5-14 volts. Check the volts at the cables. with the battery hook up take a reading at the cables. if there is a big difference between the cables and the battery then you need to clean and tighten your cables. If the volts are less then I said before then you need to have your alternator checked.

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1 Answer

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you can't adjust the idle it's all done by the pcm.you might have a IAC going bad or sticking(idle air control solenoid)

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