Stripped IAC Worm Gear?
08-29-2014, 01:24 AM
#41
Race Director
That is not true. The ECM uses what is known as "throttle follower mode". It opens the IAC as the throttle is opened (TPS reading goes up). The reason for doing this is so that the engine doesn't stall when you take your foot off the gas and the throttle valves slam shut. The ECM has a "decay rate" setting that determines how long it takes to decrease the IAC position back to idle RPM. This decrease to idle RPM is a lot more obvious if you have a manual trans and take your foot off the gas and put in the clutch.
08-29-2014, 01:39 PM
#42
Drifting
Thread Starter
That is not true. The ECM uses what is known as "throttle follower mode". It opens the IAC as the throttle is opened (TPS reading goes up). The reason for doing this is so that the engine doesn't stall when you take your foot off the gas and the throttle valves slam shut. The ECM has a "decay rate" setting that determines how long it takes to decrease the IAC position back to idle RPM. This decrease to idle RPM is a lot more obvious if you have a manual trans and take your foot off the gas and put in the clutch.
08-29-2014, 05:13 PM
#43
Burning Brakes
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EDIT: I tested for a short by unplugging the plug and testing resistance between the two wires. I got infinite resistance, so there's no short.
What? Checking for resistance on say 1 foot of wire. You would just put one lead on one end and the other lead on the other end of the wire.
Then u want to see a resistance or you want to see your meter beep.
That would tell you that the wire doesn't have a break in it somewhere in the length.
If u have no resistance, its bad. Broken somewhere.
When you pull the harness connector off of the CTS and check the connector by inserting one probe in to the end of the plug end and then move down about a foot from the connector and stab the same wire.
You want to see resistance or a value, and want resistance when you wiggle the end of the plug. If it goes open, then its broken and bad
What? Checking for resistance on say 1 foot of wire. You would just put one lead on one end and the other lead on the other end of the wire.
Then u want to see a resistance or you want to see your meter beep.
That would tell you that the wire doesn't have a break in it somewhere in the length.
If u have no resistance, its bad. Broken somewhere.
When you pull the harness connector off of the CTS and check the connector by inserting one probe in to the end of the plug end and then move down about a foot from the connector and stab the same wire.
You want to see resistance or a value, and want resistance when you wiggle the end of the plug. If it goes open, then its broken and bad
08-29-2014, 05:17 PM
#44
Burning Brakes
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You can not measure Infinite
in·fi·nite
ˈinfənit/Submit
adjective
1.
limitless or endless in space, extent, or size; impossible to measure or calculate.
"the infinite mercy of God"
synonyms: boundless, unbounded, unlimited, limitless, never-ending, interminable; More
in·fi·nite
ˈinfənit/Submit
adjective
1.
limitless or endless in space, extent, or size; impossible to measure or calculate.
"the infinite mercy of God"
synonyms: boundless, unbounded, unlimited, limitless, never-ending, interminable; More
08-29-2014, 05:19 PM
#45
Drifting
Thread Starter
EDIT: I tested for a short by unplugging the plug and testing resistance between the two wires. I got infinite resistance, so there's no short.
What? Checking for resistance on say 1 foot of wire. You would just put one lead on one end and the other lead on the other end of the wire.
Then u want to see a resistance or you want to see your meter beep.
That would tell you that the wire doesn't have a break in it somewhere in the length.
If u have no resistance, its bad. Broken somewhere.
When you pull the harness connector off of the CTS and check the connector by inserting one probe in to the end of the plug end and then move down about a foot from the connector and stab the same wire.
You want to see resistance or a value, and want resistance when you wiggle the end of the plug. If it goes open, then its broken and bad
What? Checking for resistance on say 1 foot of wire. You would just put one lead on one end and the other lead on the other end of the wire.
Then u want to see a resistance or you want to see your meter beep.
That would tell you that the wire doesn't have a break in it somewhere in the length.
If u have no resistance, its bad. Broken somewhere.
When you pull the harness connector off of the CTS and check the connector by inserting one probe in to the end of the plug end and then move down about a foot from the connector and stab the same wire.
You want to see resistance or a value, and want resistance when you wiggle the end of the plug. If it goes open, then its broken and bad
I tested resistance from one wire to another with it plugged in, but I didn't wiggle the plug when I did so. I'll do that today once I'm done with some school stuff I have to do.
08-29-2014, 05:20 PM
#46
Burning Brakes
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Maybe go back and check that CTS harness connector for resistance and wiggle it while your checking it. To make sure its open/Infinite.
If it is. Its bad and not briging its readings from the CTS to the ECM and is a problem
If it is. Its bad and not briging its readings from the CTS to the ECM and is a problem
08-29-2014, 05:20 PM
#47
Drifting
Thread Starter
I do agree though that infinity is an entirely theoretical concept.
08-29-2014, 06:06 PM
#48
Burning Brakes
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If the/a meter can't read it, its broken/open and is Bad and is not doing its job sending input back to the ECM. Replace the CTS's harness connector pigtail
http://www.summitracing.com/parts/sm...FYVDMgodIV8A0g
If no readings are present at the end of the black or yellow wires in the above Summit Racing photo. You need to buy that and then your ECM will know the motor's water temp is anything but cold { you said the CTS itself checked out good} and then it will not send a signal to the IAC to bypass air to raise engine RPM
http://www.summitracing.com/parts/sm...FYVDMgodIV8A0g
If no readings are present at the end of the black or yellow wires in the above Summit Racing photo. You need to buy that and then your ECM will know the motor's water temp is anything but cold { you said the CTS itself checked out good} and then it will not send a signal to the IAC to bypass air to raise engine RPM
08-29-2014, 06:13 PM
#49
Drifting
Thread Starter
If the/a meter can't read it, its broken/open and is Bad and is not doing its job sending input back to the ECM. Replace the CTS's harness connector pigtail
http://www.summitracing.com/parts/sm...FYVDMgodIV8A0g
If no readings are present at the end of the black or yellow wires in the above Summit Racing photo. You need to buy that and then your ECM will know the motor's water temp is anything but cold { you said the CTS itself checked out good} and then it will not send a signal to the IAC to bypass air to raise engine RPM
http://www.summitracing.com/parts/sm...FYVDMgodIV8A0g
If no readings are present at the end of the black or yellow wires in the above Summit Racing photo. You need to buy that and then your ECM will know the motor's water temp is anything but cold { you said the CTS itself checked out good} and then it will not send a signal to the IAC to bypass air to raise engine RPM
When the plug was connected to the sensor the DMM was showing current flow from one wire to the other, which is correct since the circuit was closed. However, the resistance was lower than it should have been, so I suspected there could be a short between the two wires.
I unplugged the connector from the sensor, so the circuit was open. I then checked the resistance between one wire to the other, to see if there was a short between those two wires. If the meter got a reading, that would mean current was able to flow between the two wires indicating there was a short. On the other hand, if it indicated infinite (well, as we've established, not really) resistance - as it did - that would mean there was no short.
When I tested resistance of the sensor, it verified that the sensor was good.
I now believe that I got a lower resistance than I should have because of antifreeze in the plug causing a short.
08-29-2014, 07:27 PM
#50
Burning Brakes
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Gotcha, my bad. I really don't know at this point and would just have to say. After all this probing to just get a new one and put it on. Maybe its Sticking and make sure their is no build up of a carbon ledge on the bottom of the throttle body stopping the little amount of air that does need to pass from that point.
On another note. If u have a Auto trans, generally if its going into overdrive after a little of running time. CTS is ok, generally. The trans will not go into overdrive until it sees I think about 145 degrees of water temp and then just after that should go into closed loop
On another note. If u have a Auto trans, generally if its going into overdrive after a little of running time. CTS is ok, generally. The trans will not go into overdrive until it sees I think about 145 degrees of water temp and then just after that should go into closed loop
Last edited by cudamax; 08-29-2014 at 07:29 PM.
08-30-2014, 01:37 AM
#51
Drifting
Thread Starter
Gotcha, my bad. I really don't know at this point and would just have to say. After all this probing to just get a new one and put it on. Maybe its Sticking and make sure their is no build up of a carbon ledge on the bottom of the throttle body stopping the little amount of air that does need to pass from that point.
On another note. If u have a Auto trans, generally if its going into overdrive after a little of running time. CTS is ok, generally. The trans will not go into overdrive until it sees I think about 145 degrees of water temp and then just after that should go into closed loop
On another note. If u have a Auto trans, generally if its going into overdrive after a little of running time. CTS is ok, generally. The trans will not go into overdrive until it sees I think about 145 degrees of water temp and then just after that should go into closed loop
My dad thinks the tach could be off; says the idle doesn't sound like 1000 RPMs. I thought of a way to (roughly) check - what is the lowest RPM an L98 will run at before it dies? I can compare that to how low my gauge goes before it dies to see if the gauge is accurate.
Mine gets reads about 475 before it does.
08-30-2014, 08:20 AM
#52
Thanks for the info. I don't have an automatic, but thanks anyways.
My dad thinks the tach could be off; says the idle doesn't sound like 1000 RPMs. I thought of a way to (roughly) check - what is the lowest RPM an L98 will run at before it dies? I can compare that to how low my gauge goes before it dies to see if the gauge is accurate.
Mine gets reads about 475 before it does.
My dad thinks the tach could be off; says the idle doesn't sound like 1000 RPMs. I thought of a way to (roughly) check - what is the lowest RPM an L98 will run at before it dies? I can compare that to how low my gauge goes before it dies to see if the gauge is accurate.
Mine gets reads about 475 before it does.
08-30-2014, 01:08 PM
#53
Drifting
Thread Starter
When it started, it idled lower than normal and the tach read about 800 RPM, but when after running for a minute the gauge increased to 1000 RPM.
The numbers from the C68 display indicate that it initially started at 675 RPM (just about where it should be) but when it increased it went up to 800 RPM.
Long story short, it looks like my tach reads a bit high, which is good and bad. Good because my idle isn't as high as I thought, but bad because that's one more thing to fix.
I repeated the test and had the same results. Idle started at 675, then increased to 800-850. I noticed the idle seemed to increase as soon as the coolant temp started to rise. Hopefully this correlation has some significance.
Last edited by C4ProjectCar; 08-30-2014 at 01:15 PM.
08-31-2014, 01:51 AM
#54
Race Director
As the coolant temperature rises the RPM is lowered by the ECM.
08-31-2014, 04:24 PM
#55
Drifting
Thread Starter
Usually it's the other way around. The ECM increases the idle speed when the engine is first started and the coolant temperature is low (it's the same as the old fast idle cam on carburetors). On my car the target RPM at startup is 1200 RPM and the ECM adjusts the RPM downward depending on the startup coolant temperature.
As the coolant temperature rises the RPM is lowered by the ECM.
As the coolant temperature rises the RPM is lowered by the ECM.
I noticed that my cruise control vacuum line had a hole in it, so I wrapped it with electrical tape for a patch. I was looking at it to figure out its dimensions to replace it, and I noticed that the line had split where it went onto the fittings on each end, causing it to hiss if I bent it at all. I replaced the line, and now my idle is higher again. When it starts, it will read at about 1000 (which is actually 800), but then after running a while it will creep up to reading 1400 or so. I can sit in the driveway with it still running when I get home, but the idle remains at 1400. If I turn it off and immediately restart it, it will go back down to reading 1000.
08-31-2014, 08:36 PM
#56
Burning Brakes
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Check and replacing every single vacuum hose on that motor that might be leaking.
Replace sticking IAC and make sure it's fully extended/in its seated position by following this. Its a 1989 but should be the same
I purchased a 1989 corvette last year, and decided it was time to check the timing and clean the throttle body. Throttle body was removed, cleaned, and reinstalled with new gaskets. Replaced the Idle Air Control motor while I had it apart. Prior to starting this project, I noticed the TPS voltage was within spec but on the high side, and timing was not set to spec. Removed minimal air cap to adjust. Net results; improved get up and go, smoother running engine from idle to WOT, and better MPG (+3 highway, +2 around town). Provided are the documented steps I used. Some of the other posts on this topic are not complete (or incorrect), and the directions in the factory manual required a little refinement. I used an external tach that had a low setting with 20 rpm increments (Update: scanner works even better).
Start and run engine until it reaches operating temperature (closed loop)
Check and set ignition timing to 6 degrees before dead center (BDC) with tan EST wire disconnected
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Jumper terminals "A" and "B" on the ALDL
Turn ignition key on and do not start engine
Wait 60 seconds so that the idle air control (IAC) motor fully extends
Without turning ignition key off, remove connector from IAC motor
Turn ignition key off and disconnect ALDL jumper
Attach external RPM meter using Tach port or scanner using ALDL port
Start and run engine until it reaches operating temperature (closed loop)
Remove minimum idle air cap using awl if required
Adjust idle speed to 425 (+/- 25) rpm in either in park or neutral
Turn ignition key off and reconnect EST wire and IAC motor
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Reset ECM by disconnecting and reconnecting power at battery terminal
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds
Drive vehicle to assist in ECM reprogramming
If you used a scanner, you should see around 20 IAC steps
Make sure TPS IS GOOD
Replace sticking IAC and make sure it's fully extended/in its seated position by following this. Its a 1989 but should be the same
I purchased a 1989 corvette last year, and decided it was time to check the timing and clean the throttle body. Throttle body was removed, cleaned, and reinstalled with new gaskets. Replaced the Idle Air Control motor while I had it apart. Prior to starting this project, I noticed the TPS voltage was within spec but on the high side, and timing was not set to spec. Removed minimal air cap to adjust. Net results; improved get up and go, smoother running engine from idle to WOT, and better MPG (+3 highway, +2 around town). Provided are the documented steps I used. Some of the other posts on this topic are not complete (or incorrect), and the directions in the factory manual required a little refinement. I used an external tach that had a low setting with 20 rpm increments (Update: scanner works even better).
Start and run engine until it reaches operating temperature (closed loop)
Check and set ignition timing to 6 degrees before dead center (BDC) with tan EST wire disconnected
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Jumper terminals "A" and "B" on the ALDL
Turn ignition key on and do not start engine
Wait 60 seconds so that the idle air control (IAC) motor fully extends
Without turning ignition key off, remove connector from IAC motor
Turn ignition key off and disconnect ALDL jumper
Attach external RPM meter using Tach port or scanner using ALDL port
Start and run engine until it reaches operating temperature (closed loop)
Remove minimum idle air cap using awl if required
Adjust idle speed to 425 (+/- 25) rpm in either in park or neutral
Turn ignition key off and reconnect EST wire and IAC motor
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Reset ECM by disconnecting and reconnecting power at battery terminal
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds
Drive vehicle to assist in ECM reprogramming
If you used a scanner, you should see around 20 IAC steps
Make sure TPS IS GOOD
08-31-2014, 10:21 PM
#57
Drifting
Thread Starter
Check and replacing every single vacuum hose on that motor that might be leaking.
Replace sticking IAC and make sure it's fully extended/in its seated position by following this. Its a 1989 but should be the same
I purchased a 1989 corvette last year, and decided it was time to check the timing and clean the throttle body. Throttle body was removed, cleaned, and reinstalled with new gaskets. Replaced the Idle Air Control motor while I had it apart. Prior to starting this project, I noticed the TPS voltage was within spec but on the high side, and timing was not set to spec. Removed minimal air cap to adjust. Net results; improved get up and go, smoother running engine from idle to WOT, and better MPG (+3 highway, +2 around town). Provided are the documented steps I used. Some of the other posts on this topic are not complete (or incorrect), and the directions in the factory manual required a little refinement. I used an external tach that had a low setting with 20 rpm increments (Update: scanner works even better).
Start and run engine until it reaches operating temperature (closed loop)
Check and set ignition timing to 6 degrees before dead center (BDC) with tan EST wire disconnected
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Jumper terminals "A" and "B" on the ALDL
Turn ignition key on and do not start engine
Wait 60 seconds so that the idle air control (IAC) motor fully extends
Without turning ignition key off, remove connector from IAC motor
Turn ignition key off and disconnect ALDL jumper
Attach external RPM meter using Tach port or scanner using ALDL port
Start and run engine until it reaches operating temperature (closed loop)
Remove minimum idle air cap using awl if required
Adjust idle speed to 425 (+/- 25) rpm in either in park or neutral
Turn ignition key off and reconnect EST wire and IAC motor
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Reset ECM by disconnecting and reconnecting power at battery terminal
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds
Drive vehicle to assist in ECM reprogramming
If you used a scanner, you should see around 20 IAC steps
Make sure TPS IS GOOD
Replace sticking IAC and make sure it's fully extended/in its seated position by following this. Its a 1989 but should be the same
I purchased a 1989 corvette last year, and decided it was time to check the timing and clean the throttle body. Throttle body was removed, cleaned, and reinstalled with new gaskets. Replaced the Idle Air Control motor while I had it apart. Prior to starting this project, I noticed the TPS voltage was within spec but on the high side, and timing was not set to spec. Removed minimal air cap to adjust. Net results; improved get up and go, smoother running engine from idle to WOT, and better MPG (+3 highway, +2 around town). Provided are the documented steps I used. Some of the other posts on this topic are not complete (or incorrect), and the directions in the factory manual required a little refinement. I used an external tach that had a low setting with 20 rpm increments (Update: scanner works even better).
Start and run engine until it reaches operating temperature (closed loop)
Check and set ignition timing to 6 degrees before dead center (BDC) with tan EST wire disconnected
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Jumper terminals "A" and "B" on the ALDL
Turn ignition key on and do not start engine
Wait 60 seconds so that the idle air control (IAC) motor fully extends
Without turning ignition key off, remove connector from IAC motor
Turn ignition key off and disconnect ALDL jumper
Attach external RPM meter using Tach port or scanner using ALDL port
Start and run engine until it reaches operating temperature (closed loop)
Remove minimum idle air cap using awl if required
Adjust idle speed to 425 (+/- 25) rpm in either in park or neutral
Turn ignition key off and reconnect EST wire and IAC motor
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Reset ECM by disconnecting and reconnecting power at battery terminal
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds
Drive vehicle to assist in ECM reprogramming
If you used a scanner, you should see around 20 IAC steps
Make sure TPS IS GOOD
Wow, those are big improvements! I'll try that as soon as I can, but it might be a few days since my motor isn't running.
For checking the RPMs, I've found the electronic climate control display is pretty easy. For those of you that don't know, if you have the RPO C68 (automatic electronic air conditioning) you can check the RPMs via the following: hold both the fan up and the fan down buttons for a few seconds until a number (I think it's 00) comes on the screen. Use the arrows to get to 6, then press the automatic fan button to select that. You'll get a readout on the screen of the engine's RPMs divided by 25 (a little math involved here - just multiply the number on the screen by 25 to get the RPMs). Quick and easy.
09-06-2014, 09:13 PM
#58
Drifting
Thread Starter
Okay, I got it running. I went through the procedure detailed by Cuda to set the idle, although I haven't set the timing yet (don't have a timing light).
The idle creeps up to 1800 RPM when I'm driving it, but if I restart it instantly goes down to 800 or so. This makes me think it is in fact the IAC valve because I know that the IAC valve resets when the engine restarts.
EDIT:
I went through the procedure AGAIN (must be the fifth time) just for kicks, and I got the idle down. It's now idling about 550-600 RPM. Is it bad to have an idle that's too low as long as it still runs?
On a less positive note, I noticed the throttle body sticks slightly open. Is there a fix for this? I'm guessing cleaning the pivot point, or replacing the spring?
The idle creeps up to 1800 RPM when I'm driving it, but if I restart it instantly goes down to 800 or so. This makes me think it is in fact the IAC valve because I know that the IAC valve resets when the engine restarts.
EDIT:
I went through the procedure AGAIN (must be the fifth time) just for kicks, and I got the idle down. It's now idling about 550-600 RPM. Is it bad to have an idle that's too low as long as it still runs?
On a less positive note, I noticed the throttle body sticks slightly open. Is there a fix for this? I'm guessing cleaning the pivot point, or replacing the spring?
Last edited by C4ProjectCar; 09-06-2014 at 09:32 PM.
09-06-2014, 10:34 PM
#59
Team Owner
I'd set timing first, check for air leaks next. I normally set the idle speed with a scanner.
09-06-2014, 11:22 PM
#60
Race Director
The IAC is controlled by the ECM. That means that something is telling the ECM to raise the idle that high. Check the IAC counts on the scanner. They should be near or below 50 at idle.
A bad TPS can cause a high idle. Check the TPS voltage on the scanner. It might be wrong or erratic (bad connector pin or wire). The ECM defaults the TPS voltage to zero if something goes wrong, so the problem is outside the ECM
A bad TPS can cause a high idle. Check the TPS voltage on the scanner. It might be wrong or erratic (bad connector pin or wire). The ECM defaults the TPS voltage to zero if something goes wrong, so the problem is outside the ECM
