The tps on my 87 was 'accidently' moved quite a while back, i never got any codes, but when i went to take off from standstill really quickly, the car would hesitate bigtime, like no fuel was getting into the engine.
so on saturday i decieded to adjust it to get the correct .54 reading but found it was impossible.
in order to get that reading, the little screw that adjusts the spacing near the throttle cable needed to be wound right out.... and even then it still was short of length.
the tps was angled almost upright (at least as far as it would go) and the closest i could get to that reading (.54) was .45
i readjusted to what the car was set to before and it is working as before.

so im hoping someone can help here, does this mean my tps is stuffed? or is there like a spacer i may be missing??

thanks

 

That sounds like the min. air (idle scew)if so that is not the way to adjust the TPS.
On the side of the TPS there are 2 screws that allow it to move in an up and down fashion. Attach a vohm meter to the 2 upper wires of the TPS turn key to 'on' postion (not start) and turn the TPS till it shows the desired volts. retighten and done
Gathering from the earlier statement you will also be re-adjusting your min.air (idle) setting.

 

idle screw, yes, thats what i was refering to... sorry... and i know that is not the way to adjust the tps (via idle screw), but what i meant was, even with the tps adjusted to its max (via the two screws) and the idle screw wound right out, i still could not get anywere close to .54

with the idle screw in its normal position, the best i could adjust the tps (via the two screws) was to .38

with the idle screw wound fully out and the tps adjusted to its max (via two screws) the best i could get was .45

it seams like the tps is either busted or missing some sort of spacer between the little tps arm and tb arm

 

Theres a full procedure on how to set the (1) low idle, (2) TPS, (3) timing, etc... you will need to follow that procedure if you want to get all the settings right. If you email me, i'll send you the procedure... easy to follow and do...

 

Gahhhh, don't touch the idle screw my friend! You'll toss the ECM all outta whack (ok maybe not ALL out, but some). Replace that bad boy TPS and see what you can do first. It should be really cheap.

 

i measured the distance with a micrometre before moving it, so it should be almost where it was to start with.
thanks for the imput, ill try and find a known working tps, to see if mines suspect.
thanks

 

Yeah it definitely seems messed up. What voltage do you get if you open the throttle all the way up? If you have an analog multimeter slowly move the TPS and watch the needle. You want a nice smooth movement without any "jumps" up or down along the way. A digital multimeter makes it harder to pick up on this.

 

Damn, im sorry to say i never checked the wot reading, i hear its meant to be about 4-4.5 ? but im 100% sure it would not have gotten anywhere near that.
to get .54 the throtle was almost half open.

 

Yikes. Me thinks you need a new one. The reference line for the TPS is 5.0v (well.... supposed to be). I can tell you that in 1987 the ECM was programmed to read 0.69% of reference as full throttle. So anything over 3.49v and your car is going to kick into WOT (aka Power Enrichment) mode. You usually see anywhere from 4.5-4.8 at full throttle though.

*edit*

You may want to test your reference line too. I'd have to dig my shop manual out to tell you which one it is though.

 

yeah youre best off with a new one

 

Ok, well having heard what you guys think, ill just go for the new one. it definitely seems way outa whack, i just wasnt sure whether it was missing a spacer (which i now know isnt the case) or actually stuffed.
thanks

 

Is this a procedure out of the FSM, or something that a Forum member has generated. If it's something from a Forum member, could you send it to me?

 

Remove the TPS from the TB and manually run the lever through the full range with a voltmeter connected. Does the voltmeter follow from ~.50 volts to ~4.5 volts, or close? If so the TPS is good and the tang on the TPS wasn't properly indexed into the TB lever. Reinstall the TPS, correctly, reconfirm it runs through the full range, and then reset the minimum air adjustment you've messed up.

These cars do not have an idle screw. The idle speeds are burned into the prom, and through it, the ECM controls the idle speed by manipulating the IAC.

RACE ON!!!

 

Technically they do on the driver side of the throttle body covered with a rubber cap, we just should never have to mess with it.

 

you likely will need to adjust the idle screw if you do any mods...MUST follow the service manual procedure to drive the IAC to min and disconnect the ''set timing'' BEFORE adjusting the idle screw...if yours still has the factory plug over the screw (a few still do) and you want to keep it looking orig, it is possible to turn the screw by the end that should contact the throttle arm, without prying out the oem plug.

btw, a couple weeks ago, i had a bud stop in with a just installed new TPS from napa that would not go near .55v, MIL on, no operator error...a used acdelco TPS cured it

 

That sounds like the minimum air adjustment screw. Or have you found another screw I haven't noticed? Technically, they don't. What would the point be to have an idle adjustment screw on a car with no idle adjustment?

RACE ON!!!

 

Measure the full range voltage ability of the TPS, if it works in that test let me ask this....are the mounting holes on the TPS body round or elongated? I had to elongate the round holes on mine to allow it to move and VIOLA! I got full voltage range.

 

Me too, please. 1995 I know. I know. Too much money spent on mods lately. The FSM is way high on the list though.

 

Tested member setup and FSM combined... Very good instructions and process. I use it all the time and if followed, works.
email me to my address listed and I'll send

 

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Tech Tips >> C4 Related Tech Tips >> How to Adjust your Early C4 TPS and Idle Speed


Tech Tip

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
Corvetteforum.com Member Alias:

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 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 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. Questions, Comments & Technical Assistance If you 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 me: lars.grimsrud@lmco.com

This is in our tech tips section under "other" in the maroon header.
BTW, I bought a "New" TPS that was way off, got a Delco, and it worked perfectly. If yours can't be adjusted to .54, it is junk, get a new one.