How to Adjust your Early C4 TPS and Idle Speed -- 1 of 1
Date Published: 2001-10-01
Submitter's Name: Lars Grimsrud
Email Address: lars.grimsrud@lmco.com
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 incredibly easy, there are a total of about 5 pieces in it), 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.
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.
Start the engine. It may take a few seconds for the car to catch on to its new settings.
Date Published: 2001-10-01
Submitter's Name: Lars Grimsrud
Email Address: lars.grimsrud@lmco.com
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 incredibly easy, there are a total of about 5 pieces in it), 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 the 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.
