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1800-2000rpm constant idle

This is a discussion on 1800-2000rpm constant idle within the General Help forums, part of the LSx Technical Help Section category; I'm about to pull my hair out with this one. I found this car knowing it had high idle problem ...

  1. #1
    Junior Member cammedta's Avatar
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    1800-2000rpm constant idle

    I'm about to pull my hair out with this one. I found this car knowing it had high idle problem but was reasonably cheap so I decided to chance it. 98 trans am, crate GM shortblock(receipt) 241 based heads, ls6 intake, vrx2 cam, th400 transmission w/transbrake and 3500 stall.

    I hooked the trailer up and went to pick this thing up last Friday, took my scanner and pulled up o2 sensor codes and obviously all of the transmission codes (factory a4 now th400), I give him cash and drive a little over an hour back home, get the car unloaded and put it on the lift. I started by changing the o2's with brand new ones, cleared the codes but the high idle stayed. Since I have researched and looked over this car everywhere trying to find the problem. I started with checking TPS and iac, TPS reads .7v at idle at 4.3 wot. I UNPLUGGED the TPS and idle fell to 1000 rpm, sounded good, drove it and at steady throttle you could feel the surge in it and it was pretty rich, plugged it back up and idle went straight back to 1800. Put new TPS in and same high idle. Ohm'd tps wires at PCM in case of a break but all tested fine. Put new iac in, didn't change. Checked every vacuum I could find. Just a couple hours ago I took the intake off to see if it was cracked or leaking somewhere abut couldn't find anything other than a gasket that looked a little stretched, it won't fit back in the grooves but I can't remember if it looked exactly like that when I pulled it out or not. Was supposed to have been tuned in May but I have no proof. Has no rev limiter before 6500 rpm and fans kick on at 160 so I assume some sort of tune. I am driving myself crazy with this thing, praying to god somebody has had this same thing happen and have an answer. Sorry so long of detail. All help will be appreciated. Thanks.

  2. #2
    Yeah baby! Yeah! silverWS6's Avatar
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    MAF? That can make these cars go crazy

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    Junior Member cammedta's Avatar
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    Quote Originally Posted by silverWS6 View Post
    MAF? That can make these cars go crazy
    I have read that they make them go crazy, but didn't read much about high idle. Anything I seen was bogging or popping, nothing 1800rpm related. Now that you mention it though I forgot to add my MAF has been ported and descreened, it looks old but I did clean it and no change. If I unplugged it at idle it dropped down but only around 1400-1500.

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    Senior Member Naaman's Avatar
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    Hmmm....

    I see you changed the IAC... did you swap out the O-ring as well? I had a high idle issue when I did a TB swap. I borke the O-ring, but figured I could reuse it by puting it back together just right... I spent a week or so trying to get my idle to stabilize. Finally swapped in a new O-ring and problem solved.

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    Junior Member cammedta's Avatar
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    Quote Originally Posted by Naaman View Post
    Hmmm....

    I see you changed the IAC... did you swap out the O-ring as well? I had a high idle issue when I did a TB swap. I borke the O-ring, but figured I could reuse it by puting it back together just right... I spent a week or so trying to get my idle to stabilize. Finally swapped in a new O-ring and problem solved.
    Well here's the thing, I've got several good buddies that work at advance auto parts, so they let me swap and try all my parts for free. But they couldn't find a IAC for my car in stock they were going to have to order it. We decided to look at one for my 5.3 truck, and it looked 100% identical. I put it in there with a new O-ring but nothing changed. Now that you mention it though my o-ring is pretty well shot and stretched out pretty bad but goes in there. But if it was the o-ring that was the problem, would it still choke out if I manually pull the iac out? Because if I do that it still chokes it out.

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    Welcome to the site!

    High idle is almost always extra air entering the engine from somewhere. Let's start with the basics: With the engine off, is the throttle blade fully closed. Start the engine (with the air duct work removed) and see what the blade does. Has the throttle blade been drilled? Has the throttle body been ported too much? If so, the IAC replacement isn't going to do a thing.

    Unplugging the MAF forces the PCM into speed density and it estimates, rather than measures, air flow. Once you get things sorted out, do yourself a favor and toss the hacked up MAF into the trash -- it is misreporting airflow to the PCM. You can buy a used or new one for not a lot of $$.

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    Junior Member cammedta's Avatar
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    Quote Originally Posted by pajeff02 View Post
    Welcome to the site!

    High idle is almost always extra air entering the engine from somewhere. Let's start with the basics: With the engine off, is the throttle blade fully closed. Start the engine (with the air duct work removed) and see what the blade does. Has the throttle blade been drilled? Has the throttle body been ported too much? If so, the IAC replacement isn't going to do a thing.

    Unplugging the MAF forces the PCM into speed density and it estimates, rather than measures, air flow. Once you get things sorted out, do yourself a favor and toss the hacked up MAF into the trash -- it is misreporting airflow to the PCM. You can buy a used or new one for not a lot of $$.

    Thanks for the welcome! Already nicer and more responses than i could get on ls1tech. I made sure the blades were closed but I didn't check while it was running. I have read the factory hole is 3/16, but mine is 5/16. I have put tape over it and it won't idle at all, put tape over it and turned the idle screw up and it would still choke out real bad until I gave it a little steady throttle and slowly brought it back down then it idled but was still rough. Oh after it gets fixed that maf is getting thrown away.

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    Junior Member cammedta's Avatar
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    Another thing when I unplug the TPS, start it up the idle goes to 1500, and withing 10 seconds it goes to 1000 and stays there. Should it instantly start up and go to 1000? Also I just noticed last night that my hvac controls won't switch from defrost to vents. But I capped off the vacuums to see if that was my problem and nothing changed. Just trying to list every detail I can remember for you guys to see if anyone notices anything strange. Still unsure why my TPS will not drop below .7v idle.

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    Spaz is My Mentor SMWS6TA's Avatar
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    Did you check behind the intake? The HVAC connects to a a outlet just under the map sensor on the back of the intake. I've heard of ppl having map senors crack or not seating correctly causing air leaks. Might need to check while the intake is off.

    Try this, start up and unplug the MAF. See if idle changes better or worse. Post back.

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    Junior Member smokestack's Avatar
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    Find someone who can pull the tune and check it. Wonder if its not the wrong pcm or something

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    Junior Member cammedta's Avatar
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    Quote Originally Posted by SMWS6TA View Post
    Did you check behind the intake? The HVAC connects to a a outlet just under the map sensor on the back of the intake. I've heard of ppl having map senors crack or not seating correctly causing air leaks. Might need to check while the intake is off.

    Try this, start up and unplug the MAF. See if idle changes better or worse. Post back.
    I checked out the map sensor last night, didn't see any visual cracks. I can wiggle it around in there but I mean it takes a little force to get it out. I'm going to put it back together tomorrow when my new gaskets come in. Trying to play it safe and use new gaskets when I take something apart. All of the vacuums seem to be fine, although my controls won't switch from vent to defrost or floor, stays on defrost and floor all the time. But before I pulled the intake I blocked those vacuums off that run the hvac and nothing changed.

    As for the tune that is about my last resort. IF I had the PCM flashed to a 6-speed and stock tune. Would the car even run? Considering the cam. I'm new to all of this I'm really tempted to just buy the carb conversion kit and say to hell with it so I can tune it myself.

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    Junior Member cammedta's Avatar
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    I know the PCM has been out because the tabs that hold it in are broke and there is no cover over it. I assume it is supposed to have a cover. Is a dyno shop about the only place to pull the tune? I don't want to spend a fortune just checking a tune if I don't have to.

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    Junior Member cammedta's Avatar
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    Quote Originally Posted by smokestack View Post
    Find someone who can pull the tune and check it. Wonder if its not the wrong pcm or something
    I assume it is the right PCM because I pulled up a 1998 PCM pin diagram and all of the pins I checked where right where they are supposed to be.

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    Hmmm. How about the bore on the throttle body - does that appear to be hogged out? I haven't had to mess with my TPS or IAC on our car, so no way to compare with what you are seeing for voltage. Sounds like with the cam they tried the "drill the hole bigger" method to tweak idle. Know anyone with a good throttle body that you can swap on for a comparison? How about the PCV hoses -- are they all in place? The connection at the throttle body likes to disintegrate, and they also go bad back behind the passenger side rocker cover. I agree too that the MAP sensor grommet could possibly be an issue. Check that throttle blade to ensure that it isn't cracking open when you start the car. It doesn't take much to get to 1,000 rpm.

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    Junior Member cammedta's Avatar
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    Quote Originally Posted by pajeff02 View Post
    Hmmm. How about the bore on the throttle body - does that appear to be hogged out? I haven't had to mess with my TPS or IAC on our car, so no way to compare with what you are seeing for voltage. Sounds like with the cam they tried the "drill the hole bigger" method to tweak idle. Know anyone with a good throttle body that you can swap on for a comparison? How about the PCV hoses -- are they all in place? The connection at the throttle body likes to disintegrate, and they also go bad back behind the passenger side rocker cover. I agree too that the MAP sensor grommet could possibly be an issue. Check that throttle blade to ensure that it isn't cracking open when you start the car. It doesn't take much to get to 1,000 rpm.
    You don't know how much I appreciate you all trying to help because it's driving me nuts. My throttle body appears to be stock, can't see any sign of it being touched other than the hole. My pcv has been removed and breathers put in both valve covers and the passenger side pcv port is capped off. Under the throttle body has a hose ran from one side to the other where the pcv is supposed to go they just connected both side with a hose(no cracks) I've got the intake off and no cracks behind at the bend or the hose. I've got the throttle body completely off and cleaned and they blades are 100% closed. I talked to somebody earlier that mentioned something about the temperature. I assumed the fans were tuned to kick on at 150-160, the temp never gets over that. But since it's rich it makes sense that the coolant temp sensor may be failing right?

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    A bad temp sensor can cause issues for the PCM. Do you have access to a scanner that can show you live sensor data?

    I assume you checked the bolts on the throttle body to ensure there are no air leaks when you played with the intake. Could easily be something in the tune that is fighting you too -- being a cammed engine there was probably some form of tuning done. How about a pic of the "PCV hose under the throttle body" - not sure what they did there. Stock PCV hose setup looks like this (nothing runs under the throttle body):


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    Junior Member cammedta's Avatar
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    Quote Originally Posted by pajeff02 View Post
    A bad temp sensor can cause issues for the PCM. Do you have access to a scanner that can show you live sensor data?

    I assume you checked the bolts on the throttle body to ensure there are no air leaks when you played with the intake. Could easily be something in the tune that is fighting you too -- being a cammed engine there was probably some form of tuning done. How about a pic of the "PCV hose under the throttle body" - not sure what they did there. Stock PCV hose setup looks like this (nothing runs under the throttle body):

    Click for full size
    My apology, I guess the hose I'm talking about is where the coolant is supposed to go through. I sure don't have a scanner to show live, just one to pull codes. I'm going to try the coolant temp sensor tomorrow and that's about al I know left to do before I order a victor jr intake and 6ls box to put my quick fuel carb on lol.

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    That makes sense -- I couldn't figure out what else it could be under the throttle body. Do you know anyone with HP Tuners? I don't believe that you need to license the VIN in order to run the scanner function.

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    Junior Member cammedta's Avatar
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    Quote Originally Posted by pajeff02 View Post
    That makes sense -- I couldn't figure out what else it could be under the throttle body. Do you know anyone with HP Tuners? I don't believe that you need to license the VIN in order to run the scanner function.
    I wish I knew someone with hp tuners, haha. Most everyone around me has carb setup and do not mess with fuel injection.

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    Spaz is My Mentor SMWS6TA's Avatar
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    For some reason when my 98 worked the idle was around 900-1000 rpms even with a Frost tune. I have a LS6 intake and P&P S2 Throttle body.

    1500 I agree is too high. On the throttle body how high is the screw?


    Have you tried using some carb cleaner to check for vacuum leaks?

    Have you tried the disconnecting the MAF yet?

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