With ctmccloskeyVery,,Very first thing I would check is Coolant Temperature Sensor the one for your ECM not the one for your gauge
C4 Corvette Start and Idle issues
Drifting
Joined: Mar 2013
Posts: 1,952
Likes: 239
From: Venice Cali.
You need to do a minimum idle adjust, this will have you set your TPS sensor correct. On 85-90 models it does not set itself.
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.
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.
Start the engine. It may take a few seconds for the car to catch on to its new settings.
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. ยท 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. ยท 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.
Team Owner
Joined: Jul 2004
Posts: 28,473
Likes: 3,291
From: Hartford WI
That above is the FSM method. It works for the factory. Your problem is it assumes everything is in good shape. Timing is right, tach is right, no air leaks. 100% stock, etc. IF you are certain, go for it.
OTOH, this isn't your Daddy's 1960 (insert car here) where you can listen and feel it to be somewhat right. The ECM has been programmed with a certain speed in mind. I would get a scanner so you can see what the ECM sees. Lets say you get away with it this time. Sooner or later, something might crop up where you either throw parts till you get lucky or have to buy a scanner and see. Might as well be now. You can also set the TPS afterwards using the scanner. This will check for air leaks if you cannot adjust it and it reads 0. It also can easily check (one man operation) the smoothness of the TPS when you slowly depress it in the car.
OTOH, this isn't your Daddy's 1960 (insert car here) where you can listen and feel it to be somewhat right. The ECM has been programmed with a certain speed in mind. I would get a scanner so you can see what the ECM sees. Lets say you get away with it this time. Sooner or later, something might crop up where you either throw parts till you get lucky or have to buy a scanner and see. Might as well be now. You can also set the TPS afterwards using the scanner. This will check for air leaks if you cannot adjust it and it reads 0. It also can easily check (one man operation) the smoothness of the TPS when you slowly depress it in the car.
Thread Starter
Heel & Toe
Joined: Oct 2019
Posts: 20
Likes: 1
Ok great. I will look into getting a scanner. What do you recommend I get? I have a buddy with a very expensive one but thats for obd2 cars, if i were to buy an adapter from obd1 to obd2 would that work? Also, if this helps, the car seems to start very easily when it sits for a week, more or less. Starts instantly actually. Probably the CTS then?
Team Owner
Joined: Jul 2004
Posts: 28,473
Likes: 3,291
From: Hartford WI
Ok great. I will look into getting a scanner. What do you recommend I get? I have a buddy with a very expensive one but thats for obd2 cars, if i were to buy an adapter from obd1 to obd2 would that work? Also, if this helps, the car seems to start very easily when it sits for a week, more or less. Starts instantly actually. Probably the CTS then?
Advanced
Joined: Feb 2016
Posts: 68
Likes: 14
From: MotorCity
For the '84- end of '93, 12 pins:
Pin "B" is the diagnostic enable pin and pin "A" is ground. Grounding pin "B" to enable the diagnostic readout of the ECM/PCM.
To recover the codes, short pins "A" and "B" together using a small section of electrical wire or paper clip.
With the ignition turned OFF, short pins "A" and "B" on the ALDL.
Turn IGN ON (but not to RUN).
The "Check Engine" light (early C4s) or "SYS" light (later C4s) will flash a Code 12 (a single flash followed by two flashes) and will repeat three times (Flash (pause) Flash Flash (long pause), Flash (pause) Flash Flash (long pause), Flash (pause) Flash Flash (long pause).
Code 12 is a delimiter or marker code to show where the error code string begins and ends.
After the three Code 12 flashes, you will either get an error code (or codes) or you will get another string of Code 12 flashes if there are no trouble codes stored.
All codes are repeated three times with a long pause between each code group (36 or Flash Flash Flash pause followed by six flashes, repeated three times, folowed by a long pause with any additional codes stored then flashed).
The "Check Engine/System" light on with engine running means the condition(s) are currently present. If the light is not on during RUN operation, the limits were exceeded at some point in time and the event was recorded in memory, but the reading has since returned to the normal operating range.
Remember to remove the shorting device from the connector after you have read the codes.
Clearing the Codes
To clear the codes from memory, remove the negative battery cable for a minimum of 10 seconds.
Disconnecting the battery will clear all stored codes and and any stored memory (radio button presets, clock, trip odometer, average gas mileage memory, power seats). Your ECM/PCM computer will also have to relearn timing/mixture/exhaust emissions.
Make absolutely certain the ignition key is turned OFF. If you connect the battery with the ignition switch ON, you can destroy the ECM/PCM module.
Pin "B" is the diagnostic enable pin and pin "A" is ground. Grounding pin "B" to enable the diagnostic readout of the ECM/PCM.
To recover the codes, short pins "A" and "B" together using a small section of electrical wire or paper clip.
With the ignition turned OFF, short pins "A" and "B" on the ALDL.
Turn IGN ON (but not to RUN).
The "Check Engine" light (early C4s) or "SYS" light (later C4s) will flash a Code 12 (a single flash followed by two flashes) and will repeat three times (Flash (pause) Flash Flash (long pause), Flash (pause) Flash Flash (long pause), Flash (pause) Flash Flash (long pause).
Code 12 is a delimiter or marker code to show where the error code string begins and ends.
After the three Code 12 flashes, you will either get an error code (or codes) or you will get another string of Code 12 flashes if there are no trouble codes stored.
All codes are repeated three times with a long pause between each code group (36 or Flash Flash Flash pause followed by six flashes, repeated three times, folowed by a long pause with any additional codes stored then flashed).
The "Check Engine/System" light on with engine running means the condition(s) are currently present. If the light is not on during RUN operation, the limits were exceeded at some point in time and the event was recorded in memory, but the reading has since returned to the normal operating range.
Remember to remove the shorting device from the connector after you have read the codes.
Clearing the Codes
To clear the codes from memory, remove the negative battery cable for a minimum of 10 seconds.
Disconnecting the battery will clear all stored codes and and any stored memory (radio button presets, clock, trip odometer, average gas mileage memory, power seats). Your ECM/PCM computer will also have to relearn timing/mixture/exhaust emissions.
Make absolutely certain the ignition key is turned OFF. If you connect the battery with the ignition switch ON, you can destroy the ECM/PCM module.
Advanced
Joined: Feb 2016
Posts: 68
Likes: 14
From: MotorCity
1985-1991: ECM Codes
Code #12: Normal No Codes.
Code #13: Open Oxygen Sensor Circuit.
Code #14: Coolant Sensor Circuit Low.
Code #15: Coolant Sensor Circuit High.
Code #21: Throttle Position Sensor High.
Code #22: Throttle Position Sensor Low.
Code #23: Manifold Air Temperature Circuit High.
Code #24: Vehicle Speed Sensor.
Code #25: Manifold Air Temperature Circuit Low.
Code #32: EGR System Failure.
Code #33: Mass Air Flow Sensor High.
Code #34: Mass Air Flow Sensor Low.
Code #36: Mass Air Flow Sensor Burn-Off Function Fault.
Code #41: Cylinder Select Error.
Code #42: Electronic Spark Timing.
Code #43: Electronic Spark Control.
Code #44: Lean Exhaust indication.
Code #45: Rich Exhaust Indication.
Code #46: Vehicle Anti Theft Fault.
Code #51: Faulty Mem-Cal.
Code #52: Fuel Calpak Missing.
Code #52: (1990-91 Corvette Only): Engine Oil Temperature Sensor Low.
Code #53: System Over Voltage.
Code #54: Fuel Pump Circuit Low Voltage.
Code #55: Defective ECM.
Code #62: Engine Oil Temperature
Code #12: Normal No Codes.
Code #13: Open Oxygen Sensor Circuit.
Code #14: Coolant Sensor Circuit Low.
Code #15: Coolant Sensor Circuit High.
Code #21: Throttle Position Sensor High.
Code #22: Throttle Position Sensor Low.
Code #23: Manifold Air Temperature Circuit High.
Code #24: Vehicle Speed Sensor.
Code #25: Manifold Air Temperature Circuit Low.
Code #32: EGR System Failure.
Code #33: Mass Air Flow Sensor High.
Code #34: Mass Air Flow Sensor Low.
Code #36: Mass Air Flow Sensor Burn-Off Function Fault.
Code #41: Cylinder Select Error.
Code #42: Electronic Spark Timing.
Code #43: Electronic Spark Control.
Code #44: Lean Exhaust indication.
Code #45: Rich Exhaust Indication.
Code #46: Vehicle Anti Theft Fault.
Code #51: Faulty Mem-Cal.
Code #52: Fuel Calpak Missing.
Code #52: (1990-91 Corvette Only): Engine Oil Temperature Sensor Low.
Code #53: System Over Voltage.
Code #54: Fuel Pump Circuit Low Voltage.
Code #55: Defective ECM.
Code #62: Engine Oil Temperature
Team Owner
Joined: Jul 2004
Posts: 28,473
Likes: 3,291
From: Hartford WI
Found the link. THIS and you can save the paper clips for holding bits of paper together for those who need it. I never understood why people bother with a paper clip to retrieve codes when you can do that and get data to explain what is going on so someone can interpret it.
Last edited by aklim; Nov 20, 2019 at 11:04 PM.
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Advanced
Joined: Feb 2016
Posts: 68
Likes: 14
From: MotorCity
The paper clip costs next to nothing and can retrieve codes that are what the ECM is storing as problems with any certain number of sensors that can be tested with a voltmeter. It's a OBD1 and pretty basic. As far as buying a expensive hand held device to connect and decode your ECM to let you know what sensor might be bad and then hope a new one repairs it your just going to end up with an expensive paper weight.
http://www.corvettephotographs.com/c4vettes/codes.htm
http://www.corvettephotographs.com/c4vettes/codes.htm
Team Owner
Joined: Jul 2004
Posts: 28,473
Likes: 3,291
From: Hartford WI
The paper clip costs next to nothing and can retrieve codes that are what the ECM is storing as problems with any certain number of sensors
that can be tested with a voltmeter. It's a OBD1 and pretty basic.
As far as buying a expensive hand held device to connect and decode your ECM to let you know what sensor might be bad and then hope a new one repairs it your just going to end up with an expensive paper weight.
http://www.corvettephotographs.com/c4vettes/codes.htm
that can be tested with a voltmeter. It's a OBD1 and pretty basic.
As far as buying a expensive hand held device to connect and decode your ECM to let you know what sensor might be bad and then hope a new one repairs it your just going to end up with an expensive paper weight.
http://www.corvettephotographs.com/c4vettes/codes.htm
Yes you can. More complicated but yes.
Absolutely. If that is all you are using it for, you are correct. So if you are referring to the code readers. whether it is OBD1 or OBD2, I totally concur. However, when you talk about scanners that can read live data to help you understand what is actually being seen and figure what your time is worth, taking longer with a DVOM, a scanner might not be as expensive as you think. Did you see the link I posted? A whopping $90. Buy yourself a decent DVOM with auto ranging and you are probably half way there. I can read the sensor data probably faster than you can go find out what the range is and disconnect it for measurement, etc. If the wire is broke, the sensor will test out good but the value would not be right (ask me how I know this. Hint: Spent a lot of time chasing a sensor problem instead of wiring problem). My time is worth something so for me, $90 is a bargain. YMMV
Thread Starter
Heel & Toe
Joined: Oct 2019
Posts: 20
Likes: 1
Hey yall. Its been a while. Schools keepin me away from the vette. Noticed it had a coolant leak recently and decided to take a look into it. Looks like its was pooling up right under the plenum so I decided to take all that off to get a better look. While Im under there, do you guys recommend I do anything to try and figure out the fuel pressure problem? I got the whole intake plenum off. Just not the pipes yet. Also, does anyone know where I can get those t40 bit screws for the intake plenum. Specifically the ones closest to the top that hold the pipes to the plenum. Stripped one out and gotta replace it. Thanks!