black85

Since our tech tips aren't working (and I just typed this to help out a new member) I thought I would post it somewhere where it could be found later. Luckily I printed it out in 2001. I had nothing to do with writing this, I just typed it up for a couple new 85 owners we have.

How to Adjust your Early C4 TPS and Idle Speed
By Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO

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 it general to other years. Later models do not have adjustable TPS's.

General:
Idle speed and off-idle 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 mechanice 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 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.

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 and 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” to “B”) in the connector.

1. 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.
2. 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.
3. Turn the ignition to the “ON” position without starting the engine. Wait 30 seconds.
4. Now, with the ignition still in the “ON” position, disconnect the IAC connector at the IAC.
5. Remove the paper clip from the diagnostic connector.
6. Start the engine and allow it to reach normal operation 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.
7. 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.
8. Adjust the idle speed screw to obtain 400 rpm in drive or 450 in neutral.
9. Shut off the engine and reconnect 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 Min 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.

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

Addition by CorvetteForum Member Charles Warner

As an addendum to “How to adjust your early C4 TPS and idle speed,” I would like to add that checking your WOT TPS voltage is also, potentially, a worthwhile endeavor. Frequently the TV cable that controls shifting pressure to the transmission binds the throttle linkage thereby not allowing for WOT. After measuring and, if necessary, adjustiong the idle voltage (nominally .54 volts) check the WOT voltage and ensure that approx. 4.5 volts is realized. A minimum of 4 volts at WOT are required for the ECM to go into fuel enrichment mode (FEM). If you are not seeing over 4 volts at WOT you are probably not opening the throttle to the stops and are not seeing FEM. A significant performance improvement is possible with this adjustment.
Charles Warner
Vette4cw

aklim

I have found that the easiest method is to use the scantool when the car is in closed loop and running. Set it so the IAC is at 15-20 counts and then set the TPS.

Crzyfrlss1

Same here...

But not everyone has that luxury.

So...a digital true rms voltmeter is the next best thing and does work very well.

I currently use this one and it's affordable.
http://buy1.snapon.com/catalog/item....re&dir=catalog

lawdog44

Great! Thank you guys for the info!!

lars

All my tech papers are available directly from me by e-mail request. Papers posted on the internet are usually outdated and obsolete, and should not be used. For current issues of papers, and for a complete tech paper listing, just drop me an e-mail:

V8Fastcars@msn.com

Lars

yesitsfat\st

I have a problem. I cant get my TPS lower then 1 volt. What can I do to fix this problem?

rodj

If it not the result of TB blades hanging up then you most likely need a new TPS.
Take TPS off TB altogether and measure Volts to confirm.

GREGGPENN

I used this method but the new motor still has a start-up issue.

When I restart it hot, the motor immediatly drops to hot idle speed and idles rough. Since it sounds kinda like it's going to die, I raised the stall saver to "catch" it fairly quicky. However, I'm sure this isn't the correct cure. (Plus, it's ANNOYING listening to it stumble until the stall saver kicks in.)

Tonight (for the first time), it didn't even hit "fast idle" when started from cold. It dropped to 800ish rpms though it ran fine. I have to assume the mixture was correct -- for a cold car, but clearly something (IAC?) didn't hold idle at 1150 where it's supposed to be when COLD.

My IAC seems to be working so I'm at a loss for what the problem is. Normally cold start works correctly. The only differnce tonight is my car was OUTSIDE in damp conditions.

If the IAC were bad/sticky/mis-adjusted, why would it work correctly to "save" the motor from dieing when it hits the stall-saver conditions? It also works correctly on cold start-up 99% of the time.

BTW: I'm getting an occasional code 34 but the FSM manual says this can happen with rough/unstable idle.

aklim

Gregg, what I suggested is for people who have had their PCM tuned. For instance, if you just cleaned your TB. The PCM would want to see a certain idle. If I had a stock chip, nothing I did would affect anything since it cannot run at 400 rpm of commanded idle.

Ray Quayle

Hey Lars, howya been buddy. Long time. You still with Lockheed? I go over 26 years with them next month! Damn I'm old.

GREGGPENN

I have (and continue to work on) a new tune for my 383. That's why I had to reset min idle stop, TPS, IAC, etc....

Blame

I did the paper clip trick a get one code 12. It does one flash stop then two flashes then stop for like 3 seconds then repeats. My question is (what is code 12?)

Calderone

is the default code

aklim

Code 12 is normal. It basically says life is good as the PCM sees it.

GREGGPENN

According to TunerPro, Code12 is "No reference pulses". I would take that to mean the ECM can't "see" where the dizzy is at. IOW, a wire/connector has lost connection to the distributor. Or... the ignition module has shot craps.

Pwnage1337

I thought the car is supposed to flash 12 when the aldl terminal is grounded? Its supposed to flash 3 times, then read codes.

Ghast

Help! I;ve followed all of the instructions properly, but the car will not stay running without manually giving it gas. gonna try redoing the IAC reset in the meantime.

1986 Late model with 4+3 trans.

tunedport85inject

This is an old thread (10/11/2010) why do you reset minimum idle?what your symptom?
do you perform it right?

Ghast

It's still relevant though, I fixed it, I had to replace my IAC and set my new TPS, but forgot to relearn the ecu by disconnecting the battery after.

Cliff Harris

You don't need to "relearn the ecu" after replacing the IAC or adjusting the TPS.

The ECM recalibrates the IAC every time you cycle the ignition from off to on.

The ECM looks at the TPS voltage when you first turn on the ignition. It remembers the value it sees and calls that "idle". That's why you shouldn't touch the gas pedal while turning on the ignition. For those times when you need to "give it some gas", do it AFTER the ignition is on.