Adjust your Early C4 TPS and Idle Speed
by Lars Grimsrud
Musclecar, Collector & Exotic Auto Repair &
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
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
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.
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
Tools & Equipment
You will need the following tools
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
4. A small screwdriver.
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
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
� 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
� 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
� 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
� 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
� 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
Questions, Comments & Technical Assistance
have questions or comments regarding this article, or if you notice any
errors that need to be corrected (which is quite possible since I’m
writing this from memory…), please feel free to drop me an e-mail.
Also, if you need any technical assistance or advice regarding this
process, or other maintenance issues, feel free to contact