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How to Adjust your Early C4 TPS and Idle Speed
05-10-2018, 03:34 PM
#101
Right now, I am testing ALDLDroid. Or you can get THIS
I think you will be ok. It is a stepper motor and moves in and out under the ECM command.
Maybe it might change things from the standpoint it tells the ECM there is more air coming in but the rest might not reflect it so hard to say. You are telling the ECM that the throttle plates are open but the MAF and the O2 sensor don't seem to know that and the ECM might not make a decision based on the TPS. I tell you I am depositing $1000 in your account. I don't. Your bank doesn't see that. Does it matter what I say?
I think you will be ok. It is a stepper motor and moves in and out under the ECM command.
Maybe it might change things from the standpoint it tells the ECM there is more air coming in but the rest might not reflect it so hard to say. You are telling the ECM that the throttle plates are open but the MAF and the O2 sensor don't seem to know that and the ECM might not make a decision based on the TPS. I tell you I am depositing $1000 in your account. I don't. Your bank doesn't see that. Does it matter what I say?
I Checked out the AutoXray. looks ok and assume it can do OBD1 good...but more interested in the ALDL Droid. I was able to find a 6E xdf and ads, but not an adx file as that program is looking for I think...I also found some bins, that I'm not sure those are needed. How is it working for you, and did you just need/find the ADX, or did others work? (I've got '89 TPI)
Updated... I went ahead and ordered the 1320 BT to 12 pin cable... seems there are a few apps including that one that should work with XDF too.
Last edited by Drewdude; 05-10-2018 at 04:02 PM. Reason: Updated...
05-23-2018, 03:38 PM
#102
Right now, I am testing ALDLDroid. Or you can get THIS
I think you will be ok. It is a stepper motor and moves in and out under the ECM command.
Maybe it might change things from the standpoint it tells the ECM there is more air coming in but the rest might not reflect it so hard to say. You are telling the ECM that the throttle plates are open but the MAF and the O2 sensor don't seem to know that and the ECM might not make a decision based on the TPS. I tell you I am depositing $1000 in your account. I don't. Your bank doesn't see that. Does it matter what I say?
I think you will be ok. It is a stepper motor and moves in and out under the ECM command.
Maybe it might change things from the standpoint it tells the ECM there is more air coming in but the rest might not reflect it so hard to say. You are telling the ECM that the throttle plates are open but the MAF and the O2 sensor don't seem to know that and the ECM might not make a decision based on the TPS. I tell you I am depositing $1000 in your account. I don't. Your bank doesn't see that. Does it matter what I say?
Thanks for posting the help and the procedure!! (I have and FSM now too
)
05-23-2018, 07:27 PM
#103
Team Owner
Fix update...I got the ALDL app and adapter (from 1320 Electronics) and was able to see TPS voltage was inconsistent. I got a Delco replacement, installed, and went thru the IAC and TPS adjustment procedure, and got it sorted out for the most part. It's down to about 700 now (was 11-1300). I had it around 600 which was the ECM desired RPM, but had a hard time adjusting TPS without affecting idle (even when adjusting with ignition on and engine not running as in the procedure...when I started it was higher..)
Thanks for posting the help and the procedure!! (I have and FSM now too)
Thanks for posting the help and the procedure!! (I have and FSM now too
)
05-25-2018, 10:05 AM
#104
Yes, it seems the ECM desired is at 600 most of the time I've seen it (ALDL Scan app). maybe it's diff at startup as it runs higher for a while then settles down (AC off). I got into a back and forth that every time I tried to lower the idle, I'd have to increase TPS and visa-versa..felt like I was chasing my tail. I did the entire procedure again and got the TPS at .54, but when I started, the idle was higher 700ish than where I set the min idle (was going for the 500 as in the instructions). I still need to get a timing light still to check timing next.. Thanks.
05-25-2018, 01:01 PM
#105
Team Owner
Set timing first.
Check IAC for about 20 counts after you get to closed loop and EVERYTHING but the motor off.
Adjust and Rev the motor and let it settle down and read again
Keep doing it till you have about 20 counts give or take.
After all is said and done adjust TPS.
Check IAC for about 20 counts after you get to closed loop and EVERYTHING but the motor off.
Adjust and Rev the motor and let it settle down and read again
Keep doing it till you have about 20 counts give or take.
After all is said and done adjust TPS.
05-25-2018, 01:02 PM
#106
Team Owner
What the ECM does is to change desired idle based on conditions so it can change.
05-29-2018, 12:35 AM
#107
Burning Brakes
I know 6-650 is good for a stock engine but; its too low for my car with the cam.
thanks for any help
05-29-2018, 01:30 PM
#108
Team Owner
Is the 20 count for a stock engine? I have a cam, intake, heads, headers, etc. I had the car dynoed last year and a chip burned. This year the idle is lower than last year. It used to be 7-750 in gear. Now its 550-6 in gear. I have another tach that is more accurate. I have access to a scanner but; I'm not sure what is good for my IAC count??
I know 6-650 is good for a stock engine but; its too low for my car with the cam.
thanks for any help
I know 6-650 is good for a stock engine but; its too low for my car with the cam.
thanks for any help
If 600 is too low for your cam, and I don't know it either way since I don't know your car, that would be for your tuner to decide. If not, you can turn the screw all the way in and force it but now you lose adjustability. In short, idle speed is in the EPROM and not on the TB like it is for carb cars.
Last edited by aklim; 05-29-2018 at 01:32 PM.
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rrt898 (05-29-2018)
05-29-2018, 09:36 PM
#109
Burning Brakes
I have an idle of 900 at some points and still have about 20 to 30 counts. Reason being that you want to have some space for the ECM to adjust and not so much that it has to leap up to get to. So if you set it for 80 counts, it has to go that far out at startup so it might take a few seconds more.
If 600 is too low for your cam, and I don't know it either way since I don't know your car, that would be for your tuner to decide. If not, you can turn the screw all the way in and force it but now you lose adjustability. In short, idle speed is in the EPROM and not on the TB like it is for carb cars.
If 600 is too low for your cam, and I don't know it either way since I don't know your car, that would be for your tuner to decide. If not, you can turn the screw all the way in and force it but now you lose adjustability. In short, idle speed is in the EPROM and not on the TB like it is for carb cars.
thanks again
05-29-2018, 09:53 PM
#110
Team Owner
Yes. Unless there is some strange reason which the ECM programmer might tell you, when it is in closed loop, stable temperature. Everything but the motor off and check IAC. Your target is about 20-30 counts. If it is more, turn the screw in and rev and check. If it is lower, turn it out. Those counts should give the IAC enough space to adjust.
05-29-2018, 11:57 PM
#111
Burning Brakes
Nope. Cool idle. Hot idle is about 800.
Yes. Unless there is some strange reason which the ECM programmer might tell you, when it is in closed loop, stable temperature. Everything but the motor off and check IAC. Your target is about 20-30 counts. If it is more, turn the screw in and rev and check. If it is lower, turn it out. Those counts should give the IAC enough space to adjust.
Yes. Unless there is some strange reason which the ECM programmer might tell you, when it is in closed loop, stable temperature. Everything but the motor off and check IAC. Your target is about 20-30 counts. If it is more, turn the screw in and rev and check. If it is lower, turn it out. Those counts should give the IAC enough space to adjust.
Is the TPS still set at .54 volts? Does it have to be exactly .54 or is there a range?
thanks.
05-30-2018, 12:14 AM
#112
Team Owner
05-30-2018, 09:24 PM
#113
Burning Brakes
I finally got the car to 27 counts on the IAC. The closest I could get the TPS was .58 volts. The car hot idles 8-830 rpms and in gear 7-730. I know with my cam I need the idle higher than stock or the car stalls. I am happy with these numbers and the car runs great.
Thanks.
Thanks.
05-30-2018, 09:26 PM
#114
Team Owner
I finally got the car to 27 counts on the IAC. The closest I could get the TPS was .58 volts. The car hot idles 8-830 rpms and in gear 7-730. I know with my cam I need the idle higher than stock or the car stalls. I am happy with these numbers and the car runs great.
Thanks.
Thanks.
05-22-2019, 10:56 PM
#115
6th Gear
Member Since: May 2019
Posts: 6
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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
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
05-22-2019, 11:12 PM
#116
I have a question. My 1985 the car has poor idle and the spark plugs are black. I want to see if the FIs are leaking. They are rebuilt.
If I connect a fuel pressure gauge to the fuel rail.
Turn on the key the rail will pressurize with fuel
If I let the car sit without trying to start it.
If the fuel injectors are working correctly will fuel pressure stay constant? Or will it eventually go to zero.
I am going to check the resistance. But, if that checks out. I was wondering if this would be a good test to see if the injectors are leaking.
Thank you
If I connect a fuel pressure gauge to the fuel rail.
Turn on the key the rail will pressurize with fuel
If I let the car sit without trying to start it.
If the fuel injectors are working correctly will fuel pressure stay constant? Or will it eventually go to zero.
I am going to check the resistance. But, if that checks out. I was wondering if this would be a good test to see if the injectors are leaking.
Thank you
12-12-2021, 12:07 PM
#117
Instructor
jseremba & gmtuners.com
01-31-2022, 10:41 AM
#118
Advanced
C4 Cross Fire Rough Off-idle
I am keeping my friend's 84 Cross Fire for the winter. I own a '71, -this is first exposure to a C4 and especially 'intrigued' (can't think of a nicer way to put it) by the cross fire injection. This is a very nice 3-owner 69000 mi car; an automatic. So my problem is....The idle is just unhappy, ...that's the best way to describe it. It starts fine, idle speed seems to be what it should be as it warms up, but just a lot of stumbling and roughness. I cleaned the IACs and that seemed to help quite a lot. Also adjusted the TPS targeting .54v, but that in itself was pretty fussy. It kept changing when I tightened it down so I aimed at it, got it pretty close, but not confident it stayed put.
Anyhow, as I write, it starts good and runs well as it warms up, and idles good. Higher RPM is great. But off-idle, about 1000-1300 RPM it doesn't know what to do; erratic up and down varying probably 400-600 RPM. I suspect the TPS. Why would it change so much as I snug the screws? Any thoughts much appreciated. Or this this something else altogether?
Anyhow, as I write, it starts good and runs well as it warms up, and idles good. Higher RPM is great. But off-idle, about 1000-1300 RPM it doesn't know what to do; erratic up and down varying probably 400-600 RPM. I suspect the TPS. Why would it change so much as I snug the screws? Any thoughts much appreciated. Or this this something else altogether?
12-04-2022, 02:24 PM
#119
Instructor
Lars Grimsrud posted by black 85
The Lars Grimsrud procedure to set base idle and TPS is dead-on the money. Follow it and you will succeed. If you are unable to follow this simple basic procedure you should not be trying tuning your motor... find a reliable mechanic.
A few pointers... 1. set the base idle at an RPM that will give you a good IAC- commanded idle. If you're comfortable with an idle higher than the normal IAC commanded 650 rpm (in gear for auto trans) say for example 750, then go for it. 650 is not the Bible. 2. Target TPS setting is .54 volts but it does not have to be exactly .54. .54 is mid-point in a range of acceptable voltage say .52 to .56. 3. disconnecting the single timing wire to set base idle is not necessary. 4. do NOT EVER use the stop crew in the throttle body to adjust idle after you have followed the proper procedure to set base idle. It's tempting because that is the way it was done on carb motors.. avoid the temptation.. if you're not happy with the IAC-commanded idle then repeat the procedure.
A few pointers... 1. set the base idle at an RPM that will give you a good IAC- commanded idle. If you're comfortable with an idle higher than the normal IAC commanded 650 rpm (in gear for auto trans) say for example 750, then go for it. 650 is not the Bible. 2. Target TPS setting is .54 volts but it does not have to be exactly .54. .54 is mid-point in a range of acceptable voltage say .52 to .56. 3. disconnecting the single timing wire to set base idle is not necessary. 4. do NOT EVER use the stop crew in the throttle body to adjust idle after you have followed the proper procedure to set base idle. It's tempting because that is the way it was done on carb motors.. avoid the temptation.. if you're not happy with the IAC-commanded idle then repeat the procedure.