so in my idle adventure - on my 1985 after changing the iac -driving and still noticing a higher idle than what is prescribed - 450 manual - I am at closer to 7-800 at 150* - I checked the timing and it seems to be right at the 6* mark (2* every peak and valley on the timing ) - so if it is still there and there does not seem to be any noticeable vac leaks using a vac gauge - is it safe to do the iac reset with the idle screw? the cap is off from someone else - so I am going to assume at some point the po moved it for some reason

any thoughts?

thanks

Sean

 

85 is very sensitive to vac leaks.

im not sure you would be able to notice a small vac leak on a gauge of any type.

carry out the minimum idle rest procedure. aim for 500 minimum idle. you might even need 550.

Be sure to do the full minimum idle procedure! you will need a voltmeter.

 

thanks

i did re tighten throttle body bolts and screws

i am going to order some vac hose and start replacing that piece by piece so something may turn up - i sprayed the lines when i checked them before the vac gauge - i may try the propane torch method one time before and see

first timer here - so - voltmeter for the tps? or to check voltage to the iac?

 

...for re-positioning the TPS to proper voltage - after you set minimum idle air via the idle screw.

 

excellent thanks

 

just curious - as I have been looking at some of these used hand held scanners - would that be helpful with this as well?

 

Well, yes. You can read the TPS voltage that the ECM is seeing instead of reading it from the sensor itself at the source, and you can also see the IAC counts that the ECM is commanding to adjust the idle. You are supposed to shoot for 20-30 counts on the IAC value for the '89 TPI, so I would guess the '85 target is near that - but please confirm on your own for that year. Plus you get to see all the other data on your scan tool as well of course.

 

thanks - i think i will still use the source for now - but good to know for the future - keep tabs on things and in a few cases find what's right and wrong a little sooner

 

you will not be able to do this on an 85. the act of connecting to the 85 computer, forces 1000rpm by opening ip the iac, and various other things in “field test mode” i think its called. the only way on an 85 is to set minimum idle adjustment is with volt meter at tps.

 

Thank you very much Viking for stating the different operating mode of the ECM. Striking to see that much evolution in what is essentially a very similar motor '85-'89 physically, but obviously some changes along the way in the software and engine management. Thanks -

 

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.”


found this from black85 posted before - sound good

 

I did this procedure as written about two weeks ago (after top-end engine work) and it does work well. After you set everything it will take a bit of running for the ECM to 'learn' your new settings and optimize, but it does run really well after that - as long as you have no vacuum leaks or other basic problems.

 

One more thing - don't overtighten the TPS bolts. They are long, thin, and just when you think you're at final torque one of them will snap at the head if you overdo it. Ask me how I know.

 

easily strip too.

 

thanks - is there a torque for those - i would not think very much -

 

Yes, that's right, but the problem is that the TPS housing is plastic and at the right torque you think "that's never going to hold" so you tighten it further. Those crescent-shaped washers are not the most confidence-inspiring if you get my drift....

 

ok so when I put the paper clip in and wait a little then tried to start it - it would barely catch to start or stall immediately without the gas pedal depressed - (with iac unplugged). i should state that i re-checked for vac leaks with the propane torch and did not notice any fluctuations - but maybe because I did that fist the car was about 120* - should the iac test happen with a cold car? or am I doing something else wrong

 

keep in mimd this is supposed to be done on a warm engine.


when u put the paperclip in and wait......with fan running....u went over to unplug iac right?

now you have the iac stuck full closed. thats what u want.

now take the paperclip out.

try to start it.

if it wont start without slight pedal depression thats fine. grab your torx and DONT USE ONE SIZE TOO SMALL!!! turn the throttle body plate screw adjuster in one full turn.

try agian to start it. keep doing this until it starts. and idles. now back that screw off until it holds 550 rpm idle. the manual originally suggested 450. gm revised to 500. then to 550.

now if it holds the idle here (it may bounce from 500-550) then turn the car off. remove the paperclip

go over to the tps, position it so voltage so 0.53/0.54/0.55 and then snug the screws down without stripping.

What you now have achieved, is the throttle body plate adjusted so that even with the iac valve fully closed, the car will hold 550 idle. And, the 0 throttle position according to the tps has now been set properly at that 550 idle mark.

I typically restart the engine now, just to ensure its good to hold that 550 idle once more.

If all is still good, it should be, then i turn off the car. I unplug the negative battery cable to wipe the fuel tables, plug in the iac, reconmect the neg battery cable, and start the engine. It can/will take a bit for the computer to learn whats its doing again and i take it for an easy cruise and i lay into the pedal a few times.

report back

 

If it will not run without working the pedal at operating temp, turn the screw to open the throttle some. With IAC disconnected and paper clip removed at this point. Then back it down to 400-450. Just for clarity, the point of the paper clip and unplugging of the IAC is to fully extend the IAC and close the air bypass temporarily while setting minimum idle. Hope this helps.

 

thanks - i will give it another go