• throttle position sensor..etc...
  • throttle position sensor..etc...
  • throttle position sensor..etc...
  • throttle position sensor..etc...

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  1. #16
    Moderator vetteboy86's Avatar
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    What RPM does your car idle at now? I wouldn't be adjusting the idle with the throttle screw. It's not like a car. You start turning that screw than you are fighting the computer that is trying to compensate by using input from the TPS and also varying the IAC steps.

    I think I have read tech articles that say to set it at WOT then what it falls back to, just has to be it. Yes you are fighting to variables, and you need to do the best. The WOT setting i believe is more important, and it needs to be close to 4.5 volts.

  2. #17
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    Quote Originally Posted by vetteboy86 View Post
    What RPM does your car idle at now? I wouldn't be adjusting the idle with the throttle screw. It's not like a car. You start turning that screw than you are fighting the computer that is trying to compensate by using input from the TPS and also varying the IAC steps.

    I think I have read tech articles that say to set it at WOT then what it falls back to, just has to be it. Yes you are fighting to variables, and you need to do the best. The WOT setting i believe is more important, and it needs to be close to 4.5 volts.
    I don't remember the idle speed. I've only driven it 4-5 times in the last 6+ months. I'll figure the idle out when I put everything back on the engine that I have off, and when I change the coolant.

    Though, I think it idled at 700-750, but not sure. I know it is not a good idea to mess with the throttle screw. But the block off plate is missing, so the PO has already messed with it. I noticed the blades don't shut all the way. The blades are very slightly still open. I don't know if the blades are supposed to be like that, or straight up and down. I know there is a little square hole on top you can look in and see the top edge of one of the blades. I'm assuming that is there to tell you where the blade is stopping at.

  3. #18
    throttle position sensor..etc... TedC's Avatar
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    Minimal air (yes the screw that some think is idle speed adjustment) should be checked before setting the TPS, especially if one feels it may not be right. Search under my name for the detailed steps. Properly set, the throttle plates (blades) do not close 100%, this is why it is called minimal air. The ECM is calibrated to adjust the IAC position based on a minimal air speed of around 450 rpm. I've had better luck with idle quality by replacing IACs than trying to clean them. Just make sure (new or old) that it is properly seated before installing. IACs come with different shapes, so make sure you match up if replacing. The throttle body has a year on it. I worked on a 1988 that someone had installed a 1986 throttle body using a 1988 IAC. Once we figured this out and installed a new 1986 IAC, it idled perfectly.

  4. #19
    Technical Writer for Internet & Print Media
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    Quote Originally Posted by toptechx6 View Post
    A great book that explains a lot about the systems in our cars, should be in every Vette owners library along with the correct Factrory Service Manual for your car.

    Corvette Fuel Injection Electronic Engine Management 1982 Through 2001 : L83, L98, Lt1, Lt4, Ls1, Ls6, Zr-1 Charles O. Probst Paperback english 9780837608617 BENTLEY PUBLISHERS TRANSPORTATION BOOKS
    I disagree.

    I reviewed this book for the CAC right after it was printed. It's full of errors and inaccuracies.
    Corvette Fuel Injection & Electronic Engine Control: 1982 through 2001 - Corvette Product Reviews

    I agree with "toptech6" in that the Factory Service Manual is a good thing to have in a Vette related technical library, but the Probst Fuel Injection book is not worth even half of its cover price because of inaccuracies. If you're looking for books which explain electronic fuel injection there are many other better titles.

  5. #20
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    Quote Originally Posted by TedC View Post
    Minimal air (yes the screw that some think is idle speed adjustment) should be checked before setting the TPS, especially if one feels it may not be right. Search under my name for the detailed steps. Properly set, the throttle plates (blades) do not close 100%, this is why it is called minimal air. The ECM is calibrated to adjust the IAC position based on a minimal air speed of around 450 rpm. I've had better luck with idle quality by replacing IACs than trying to clean them. Just make sure (new or old) that it is properly seated before installing. IACs come with different shapes, so make sure you match up if replacing. The throttle body has a year on it. I worked on a 1988 that someone had installed a 1986 throttle body using a 1988 IAC. Once we figured this out and installed a new 1986 IAC, it idled perfectly.
    Thanks. I'll do that search and check the year of my throttle body. I was detailing the engine and plan on replacing the valve cover gaskets, and paint the valve covers. So, I figured I'd just pull and clean the throttle body while I had everything apart.

    Got any tips on cleaning the throttle body and smog pump? Can't seem to get those clean like the plenum.

    Oh, and here is a copy and paste of that article on adjusting the TPS. I have on order that TPS adapter that lets you hook up your multimeter. So, what screw is he talking about to adjust for the minimum idle speed? The screw for the minimum air?

    How to Adjust your Early C4 TPS and Idle Speed
    by Lars Grimsrud
    SVE Automotive Restoration
    Musclecar, Collector & Exotic Auto Repair & Restoration
    Broomfield, CO Rev. New 6-15-00


    This tech paper will discuss the procedure for correct adjustment of the Minimum Idle Speed and for adjustment of the Throttle Position Switch (TPS) on the early C4 Corvette TPI systems. These steps apply specifically to the 1985 model year, and in general to other years. Later model years do not have adjustable TPS's.
    General


    Idle speed and off-idle throttle response on the early TPI systems is determined by correct adjustment of the minimum idle speed screw combined with a correct setting of the TPS. I've seen many of these cars that have had their idle speed "corrected" by well-intentioned mechanics and owners by simply screwing the minimum idle speed screw in a few turns. This really messes up the settings, and will not make your car perform properly. Doing a correct setup of the TPS is one of the easiest ways to make your car feel and respond better. To maximize the benefit of this procedure, I recommend that you first remove your Throttle Body (TB), disassemble it (it's incredible easy - there are a total of about 5 pieces in itů), clean the TB up really good with some spray carb cleaner, and put it back together. A nice clean TB will really put an edge on the performance improvement you will get by doing this procedure.


    The Service Manual has instructions for doing these operations, but the directions are scattered through several sections of the Manual. Here is the complete, step-by-step process for doing this (not including TB rebuild). All specs and steps are taken directly from the Manual (all 3 different sections), and this process is absolutely correct.


    Tools & Equipment


    You will need the following tools and equipment:


    1. A set of Torx wrenches. You can buy a complete set in a nice, genuine plastic pouch at Sears.
    2. A good digital voltmeter that will read voltages less than 1 volt.
    3. A paper clip.
    4. A small screwdriver.


    Procedure


    There are two electrical components on the TB that you will be working with: The TPS and the Idle Air Control Valve (IAC). Make sure that the connectors for these two components are easily accessible and that you can easily disconnect the IAC.


    You will also be playing with the diagnostic connector under the dash. Remove the cover (if it's still in place). Bend your paper clip into a "U" shape. You will be playing with the two top right hand terminals ("A" and "B") in the connector.


    First step is to set the minimum idle speed. If nobody has messed with this on your car before, the set screw will be covered by a pressed-in plug. It's located on the driver's side of the TB. Remove this plug if it's there.


    With the IAC connected and the ignition "OFF," stick the paper clip into the diagnostic connector from "A" to "B." This grounds the diagnostic lead.
    Turn the ignition to the "ON" position without starting the engine. Wait 30 seconds.
    Now, with the ignition still in the "ON" position, disconnect the IAC connector at the IAC.


    Remove the paper clip from the diagnostic connector.


    Start the engine and allow it to reach normal operating temperature. The idle speed will probably be really low, and you may have to coax the engine a bit with the gas pedal to keep it running for a while.


    If your car is an automatic, set the parking brake and put the transmission in "DRIVE." If your car is a manual, leave it in neutral.


    Adjust the idle speed screw to obtain 400 rpm in drive or 450 in neutral.


    Shut off the engine and re-connect the IAC.


    That's it for idle speed. Now on to the TPS.


    There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires. You can either buy a special harness connector that breaks these wires out (from Mid America), or gently pierce the insulation of the wires with the pointy prongs on your volt meter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. Whatever is easiest for you.


    Turn the ignition to the "ON" position without starting the engine.
    Loosen the TPS Torx adjustment screws.
    Set your volt meter to a low scale DC volt setting that will accurately read less than 1 volt.
    Measure the voltage between the two top TPS wires.
    Adjust the TPS by rotating its position until you get a reading of .54 volts.
    Tighten the Torx screws and recheck the voltage. Re-adjust if necessary to make sure voltage is right at .54.
    Turn the ignition "OFF."

    You are now in perfect adjustment on idle speed and TPS output. Start the engine. It may take a few seconds for the car to "catch on" to its new settings.

  6. #21
    throttle position sensor..etc... TedC's Avatar
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    Yes, it is one in the same. It is not for setting minimal idle speed!!! The ECM controls idle speed. This is the minimal air flow setting that produces around 450 rpm. Some with higher lift after market cams need to set it higher, but don't do that with a stock cam. The ECM adds additional air via the IAC valve. I've watched with a scanner and it does a pretty decent job considering the vintage of the L98 ECM. You can turn that screw in and out and the ECM will hold the idle speed steady. However, you messed up the calibration between the IAC and the ECM. I wire brushed my throttle body. My car is not for show, so I'm sure I could spend hours detailing it. I see you found my old post. A scanner can also be used instead of the tach. I've found that a clean and properly adjusted minimal air and TPS can significantly improve how well the L98 runs.

  7. #22
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    Quote Originally Posted by TedC View Post
    Yes, it is one in the same. It is not for setting minimal idle speed!!! The ECM controls idle speed. This is the minimal air flow setting that produces around 450 rpm. Some with higher lift after market cams need to set it higher, but don't do that with a stock cam. The ECM adds additional air via the IAC valve. I've watched with a scanner and it does a pretty decent job considering the vintage of the L98 ECM. You can turn that screw in and out and the ECM will hold the idle speed steady. However, you messed up the calibration between the IAC and the ECM. I wire brushed my throttle body. My car is not for show, so I'm sure I could spend hours detailing it. I see you found my old post. A scanner can also be used instead of the tach. I've found that a clean and properly adjusted minimal air and TPS can significantly improve how well the L98 runs.
    Thanks, yeah I don't have a scanner, and I think those are expensive. I'm mainly wanting to get it to spec as best as I can with a volt meter. As when I pulled it to clean the throttle body, I didn't pay attention to where it was at. But then again, the PO could have messed with it and it was off anyways.

    I just found out my smog pump is seized. I sprayed water all over the engine cleaning all the grime. I pulled the alternator, battery, cap, and rotor. And I seran wraped and tin foiled the distributor top. Tin foiled the spark plugs on top and all open electrical connections.

    I've never read anywhere the need to pull the smog pump when power washing the engine, but I guess I should have. I noticed either later in the day or the next day I did it, the smog pump was stuck a little when I turned it, but then turned freely. A few days later, which is today, the pulley doesn't even turn. Got any idea if the smog pump from autozone would be decent? I guess I could get one from a parts car, but then again, it would be 20 years old and I figured a remanu one from autozone for $50 might be a little better. I might get it priced at Napa. Though in the past, I've had to replace Napa parts 2-3 times over, but have had good luck with Autozone for some reason.

  8. #23
    throttle position sensor..etc... TedC's Avatar
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    Never had to replace one. Eckler's shows one in their catalog for $90 (remanufactured with new bearings and filters). Corvette A.I.R. Pump, Rebuilt, 1986-1991

  9. #24
    Gone but not forgotten John Robinson's Avatar
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    Default Hope this sheds some light on the tps for you

    Broken Record
    I know that some of you may be getting tired of my posting this information on the TPS but after buying a new ECM only to find out how the TPS works and it was why my car would not start. I offer this in the hopes someone will be able to fix their car by looking at something most people do not understand what it's function is.

    Throttle Position Sensor (TPS)
    Copied from 1993 Service Manual
    The Throttle Position sensor (TPS) is a potentiometer connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts
    from the ECM and the other to ECM ground. A third wire is connected to the ECM to measure the voltage from the TP sensor. As the throttle valve angle is changed (accelerator pedal moved), the voltage output of the TP sensor also changes. At a closed throttle position, the voltage output of the TP sensor is low (approximately .5 volt). As the throttle valve opens, the output voltage should be approximately 5 volts.


    By monitoring the output voltage from the TP sensor, the ECM can determine fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor can cause intermittent burst of fuel from the injectors and cause an unstable idle, because the ECM detects the throttle is moving.

    If the TP sensor circuit is open, the ECM will set a DTC 22. If the TP sensor circuit is shorted a DTC 21 will be set. A problem in any of the TP sensor circuits will set either a DTC 21 or 22. Once a DTC is set, the ECM will use a default value for TP sensor, and some vehicle performance will return.

    A personal note, when my TPS failed it DID NOT set a code other than to tell me the ECM was bad.

    I used a couple of straight pins through the wires to hook the meter on

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  10. #25
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    Quote Originally Posted by TedC View Post
    Never had to replace one. Eckler's shows one in their catalog for $90 (remanufactured with new bearings and filters). Corvette A.I.R. Pump, Rebuilt, 1986-1991
    Thanks. That is cheaper than Corvette Central of $150 for some reason. I think I'll get the Autozone one. It is $50 and has a 1 year warranty. It is probably the same thing anyways as I'm sure there probably isn't alot of companies making remanu smog pumps compared to alternators or other items.

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