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High reving on cold start up

This is a discussion on High reving on cold start up within the General Help forums, part of the LSx Technical Help Section category; When I started my car the RPMs went up to around 3500 and stuck there.Then I put it in gear ...

  1. #1
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    High reving on cold start up

    When I started my car the RPMs went up to around 3500 and stuck there.Then I put it in gear and they went down to 2500. I drove it around for a minute thinking it just needed to warm up a little but it just would rev up and down on its own. The car has sat for about 5 weeks since the last time I drove it so the battery was weak when trying to start it. This happened last year when the outside temp was under 30 degrees but the RPMs weren't that high and it went away after the car warmed up. The car will go up to 60 mph without even pressing the gas pedal(automatic) because the high RPMs.Could this be a throttle position sensor or a bad intake gasket,the car sits a lot (19,000 miles ) and there are no engine lights on. If anyone could help on whats wrong I would appreciate it.

    Thanks

  2. #2
    Race cars have a clutch redss454's Avatar
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    First off, I would try to avoid putting it in Drive at 3500 rpm. That's usually a disaster waiting to happen. Does it happen all the time now? Or just for that one happening. If you start the car and leave it in park, after it warms up, will the RPM fall back to a normal idle? If it eventually comes back to idle, I wouldn't say it's a big deal, although 3500 sounds way high. If it's all the time and the fact that the car will go to 60mph without touching the gas, it's time to look at the TPS or the TB. Is there something in the way of the throttle cable that is causing it to bind? Is it closing all the way after the car starts?

    I would check the TB first. If the car sits for awhile, and then goes right up to 3500, I would be real concerned with my oil pressure. How is it when it starts? And you don't pump the gas when you start it, correct? (I ask because many of my friends pump a FI'ed car... )

    Chris

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    Yes it does happen every time but the RPMs drop to about 2000 after it warms up.Right at start up it it revs up and down for while.Nothing is in the way of the throttle its closed all the way too.

  4. #4
    Race cars have a clutch redss454's Avatar
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    I am definately not an expert on these new computer controlled cars, but I'll do my best. So theres nothing in the way of the throttle cable? and the TB's closed? I personally wouldn't try to drive it like that. Too much stress on the transmission.

    None of your idiot lights are lit? When it pulses up and down, is the throttle closed? Sounds to me like a computer problem. Any strange noises under the hood when it starts and is surging? And the idle is at 2000? I would take it to a mechanic and have them look at it. I don't know enough about the internals/computer on the LSx to be making any technical suggestions. I apologize, but I'm doing my best.

    Chris

  5. #5
    Veteran 0rion's Avatar
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    I would have to think on it a bit to see if I'm missing something but my initial impression is to look for a vacuum leak or leak between the MAF and throttle body.

  6. #6
    Grand Imperial Wizard Sarge's Avatar
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    Vacuum leak or sensor that has gone south or is unplugged. Sensors being the TPS/IAC/IAT/EGR( Don't remember if the 98 has a EGR )..Orion gives good advice to check all your tubes from your intake are tight and not leaking....TB may have a leak....Check your throttle cable is not kinked or bent...make sure it is closing all the way to the stop....if all that checks out... I would disconnect all the sensors...remove the air intake assembly completely....remove the TB and clean it with carb cleaner...get all the carbon out of it...many times carbon build up will not allow your blade to come all the way to close...this will cause the symptoms you are experiancing.....clean that TB good....make sure your blade is closing properly and there is no obstructions or warping......once your happy your TB is clean and the blade is unobstructed and closing all the way...reassemble everything...making sure there is no gaps/breaks/cracks that would allow unmetered air into your intake track.....now disconnect your battery...turn your key to the on position to discharge any volts being stored in the capacitors....leave your battery disconnected for 1 hour....now reconnect your battery...start it up....it may "search for idle" while the ECM is relearning....take it for a drive around the neighborhood....get in your driveway and let it idle for a minute....now turn on your A/C full blast....let it idle with your A/C on for a few minutes....turn off the A/C and let her idle for another couple of minutes.....Shut it off.
    If your still experiencing issues after this exercise it is time for some diagnostic equipment to read what the sensors are telling the ECM.

  7. #7
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    Thanks for all the helpful advice. I will try all those things as soon as it gets a little warmer outside because I really don't want the engine going to 3500 with a cold startup again.

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