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Running out of Options

This is a discussion on Running out of Options within the General Help forums, part of the LSx Technical Help Section category; Guys, two weeks ago my idle jumped up to 1200 rpm for no reason. I did all the normal things; ...

  1. #1
    Member Nowhereman's Avatar
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    Running out of Options

    Guys, two weeks ago my idle jumped up to 1200 rpm for no reason.
    I did all the normal things; changed TPS, IAC, changed plugs, checked for vac leaks cleaned TB and Maf with no luck.
    Took it into the dealer and he stated that the PCM could not talk to the TPS or IAC.
    He stated there was a 5 volt signal that was not dropping down to 0.
    They attempted to re-flash the computer but, it told them to contact GM for specifics.
    This WS6 has been a perfect dream for 12 years with little problems if any.
    It only has 112k miles on it and I treat it like a long lost family member.
    What other options do I have?
    Changing the PCM?
    I'm getting tired of throwing money at this issue and not getting it solved.

  2. #2
    Electrical Engineer KMdef9's Avatar
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    Have you inspected every inch of wire?

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    Member Nowhereman's Avatar
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    What wire?
    Be who you are and say what you feel 'cause people who mind don't matter, and people who matter don't mind.'
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    Veteran 0rion's Avatar
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    I don't understand why the dealership couldn't get it fixed. I would probably do whatever they told you to do.

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    Senior Member 98maro's Avatar
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    If you think it could be the PCM. Buy a used one. You already paid the dealership to check it out. And still don't know what the issue is.
    I would say get a used PCM and try it out. If the car gets fixed. There's nothing else to worry about. If it doesn't get fixed. Just sell the used PCM you got that way you get the money back.

    IMO could be the PCM.
    Or a messed up sensor, air sensor, or the sensor on the tb.
    I could be wrong tho.

    Keep us posted, and good luck

  6. #6
    Senior Member Naaman's Avatar
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    If the 5-volt reference isn't dropping down to zero, it means that there is a problem with one or both of those sensor circuits. The PCM is sending out the 5 volts, and it's getting 5 (or almost 5) volts back. This (in my opinion) means that you probably have an open ground somewhere in one or both of those circuits (TPS or IAC). Since you already changed these components, I'm guessing that the wire(s) or the connector would be the culprit. If it's not a wire/connector, the internal PCM ground could be bad.

    Is your check engine light on?

    I would think that if the TPS signal was funky, the check engine light would be on for sure. Are any symptoms present when you accelerate?

    If you have a volt meter or multi-meter, you can check the ground side of the circuit for voltage. If you get more than ~0.5 volts, you may have a bad/open ground. If you get exactly 0.000 volts, you almost certainly have an open circuit. I would check all the wires and connections (the dealership should have done this already, but... you never know).
    Last edited by Naaman; 02-27-2012 at 05:20 AM.

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    Veteran 0rion's Avatar
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    Quote Originally Posted by Naaman View Post
    If the 5-volt reference isn't dropping down to zero, it means that there is a problem with one or both of those sensor circuits. The PCM is sending out the 5 volts, and it's getting 5 (or almost 5) volts back. This (in my opinion) means that you probably have an open ground somewhere in one or both of those circuits (TPS or IAC). Since you already changed these components, I'm guessing that the wire(s) or the connector would be the culprit. If it's not a wire/connector, the internal PCM ground could be bad.

    Is your check engine light on?

    I would think that if the TPS signal was funky, the check engine light would be on for sure. Are any symptoms present when you accelerate?

    If you have a volt meter or multi-meter, you can check the ground side of the circuit for voltage. If you get more than ~0.5 volts, you may have a bad/open ground. If you get exactly 0.000 volts, you almost certainly have an open circuit. I would check all the wires and connections (the dealership should have done this already, but... you never know).
    that's kinda what I was getting at.....wouldn't the dealership have done this instead of trying to flash it? Also, with a tech 2 why wouldn't they be able to flash it? Something isn't adding up here.

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    Member Nowhereman's Avatar
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    This one has them stumped.
    I havde no error codes, nor check engine light on.
    I can get a used PCM flashed to my VIN number for around $100 bucks on Ebay.
    Dealer wants $300 for a new factory unit.
    The only thing I can think of is this but, you guys need to tell me if I am off base.
    I installed a chip set a while back.
    It splices into the ODBC connector.
    All was fine, then when this high idle thing came up, the dealer asked me to remove it because they could not talk to the IAC.
    I did unhook it and nothing changed.
    I by some weird thing, one of the spliced leads going into the ODBC connector is open, would that cause this issue?

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    Member Nowhereman's Avatar
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    Quote Originally Posted by 0rion View Post
    that's kinda what I was getting at.....wouldn't the dealership have done this instead of trying to flash it? Also, with a tech 2 why wouldn't they be able to flash it? Something isn't adding up here.
    Actually, it accelerates great.
    It does buck under steady hold of the throttle in 1st gear though.
    Never did that before.

  10. #10
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    Throw the chip away. Start by disconnecting your battery and then remove the damn thing. You may want to check the PCM connectors while you have the battery unhooked. Unbolt them and pop each one off in turn. Check the pins and connectors for corrosion or damage. Do not overtighten the connectors when you reinstall them. Once everything is back together, start it up. Since the battery was disconnected it is going to run like crap at first and may even stall. Give it time and things should settle in.

    If the problem persists, it is time to play with wires. Do you have a DVOM and know how to use one?

  11. #11
    Veteran 0rion's Avatar
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    ok....now it makes sense. The dealership didn't want to mess with it because of the mods you did. Like Jeff said.....toss the chip. They're the worst way possible to try and make more power. Fix that wiring and make sure there are no open leads.

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    Member Nowhereman's Avatar
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    The chip has been out for a week or more.
    If anything its worse.
    You guys didnt tell me one important thing.
    If there was an open at the connector. would it show in error codes because there are no codes.

  13. #13
    Senior Member Naaman's Avatar
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    The OBD2 connecter doesn't have individual leads/pins for each component. Pin #2 and #9 (counting from top row across left to right) are the communication pins. If the communication pins are bad, you will get NO data from the PCM (so they wouldn't have even known that the 5-volt reference was not getting to ground). The scan tool would simply indicate that it cannot communicate at all. If data isn't coming back from a specific component, that is where you need to start looking.

    As for a lack of codes, the PCM sometimes doesn't display a code/SES light right off the bat. It will use other data (MAF/MAP, crank sensor, cam sensor etc) to calculate algorithms and make decisions. In this case, the MAF sensor has the highest priority in terms of the air/fuel prep. If the MAF is good, the PCM may not get too worried about it, though it may have pending codes stored. Also, the PCM will not set the SES light until certain conditions are met. It uses drive cycles, which have very specific requirements, to make decisions. After some time of driving (and not driving), once all the parameters have been met by the driver, the PCM will compare the numbers between cycles and if they are consistent enough, it will set the code. Basically, your driving habits determine how long it takes for a code to set.
    Last edited by Naaman; 02-28-2012 at 07:43 AM.

  14. #14
    Senior Member Naaman's Avatar
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    Quote Originally Posted by Nowhereman View Post
    It does buck under steady hold of the throttle in 1st gear though.
    Never did that before.
    Can you elaborate on this more? Is it at a specific rpm/road speed? Does it happen off and on, or only as you go past a certain rpm? What details can you give?
    Last edited by Naaman; 02-28-2012 at 07:55 AM.

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    Member Nowhereman's Avatar
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    Quote Originally Posted by Naaman View Post
    Can you elaborate on this more? Is it at a specific rpm/road speed? Does it happen off and on, or only as you go past a certain rpm? What details can you give?
    I've noticed that sometimes in 2nd gear, and not under acceleration, it bucks like someone who is just learning to drive a clutch.
    That's the best way I can explain it.
    To me, that makes it a TPS issue but, I changed the damn thing.
    Tonight I tore apart the whole intake system, checking for leaking hoses, etc.
    Everything was perfect and new.
    Even theTP looked like new since it was cleaned 3 times in the last two weeks.
    Im now up against it.
    Pop e3 bills for a new factory PCM flashed by the dealer or $100 bucks for a used unit flashed to the vin by the firm who sells em on Ebay.
    Plugs and wires are new.
    This thing is giving me the red ass.

  16. #16
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    Time to get out a DVOM and take some readings. Let's see what is actually going on. Do you have shop manuals with schematics?

  17. #17
    Member Nowhereman's Avatar
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    Quote Originally Posted by pajeff02 View Post
    Time to get out a DVOM and take some readings. Let's see what is actually going on. Do you have shop manuals with schematics?
    No but, it looks like I'm going to have to get em if I want to keep this car in shape.

  18. #18
    Smiles for 9.5 Years cammed goat's Avatar
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    Gotta love electrical gremlins. Even the smallest one leaves us stumped. I am thinking shorted wire as well.

  19. #19
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    I like electrical issues. OP: I have the books for the '02 models, hopefully not much difference so let me scan and post some schematics for you later tonight to look at and I'll walk you through a diagnostic. Most will be simply continuity and voltage tests, so you don't need a super fancy meter.

  20. #20
    Member Nowhereman's Avatar
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    It happened.
    Had to go into San Jose for a med visit for my mother.
    Car started to idle at 2500 and this is in pouring rain.
    On the way back, the engine light came on.
    It gave me a Po121 code and some crap about the throttle position sensor.
    Since I put a new one in two days ago, it now tells me that the computer has finally crapped out.
    I have one on order; (factory replacement)
    This better phuuucking work.

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