idle too low on cold start
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
idle too low on cold start
We'll the weather has slowly improved here in the midwest so I finally fired up my 1982 for the first time this year. Unfortunately for the first time, I have a problem right out of the gate. The car fired right up but the idle is very low almost to the point of stalling +/- 600 rpm. I also noticed the engine light is on. My first thought was the battery might be low so I let it run for about 15 to 20 minutes, rev'd the engine a couple of times but there was no improvement in the idle rpm. The car runs well with my foot on the pedal but when I take my foot off then the rpms drops back down almost to a stall again. I had the car on a maintanence charge all winter. The battery seemed plenty strong to start the car but is showing low on the voltage gage. Can low voltage cause the problem that I am experiencing?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The car is an '82 and has a computer. If the computer/controls system hasn't been removed or disabled, connect a diagnostic reader to it and determine the trouble code that is causing the "Check Engine" light to come on. It is most likely a sensor which has malfunctioned and needs to be replaced.
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
The car is an '82 and has a computer. If the computer/controls system hasn't been removed or disabled, connect a diagnostic reader to it and determine the trouble code that is causing the "Check Engine" light to come on. It is most likely a sensor which has malfunctioned and needs to be replaced.
Here's what I have done so far:
1. Checked the ALDL and no engine trouble codes are stored.
2. Took off the aircleaner assembly (I did note that the damper door in the air cleaner snorkel was not closed. It stays open all the time).
3 Checked all the vacuum hoses that connect to the the tbi, and air cleaner assembly. Everything was in very good shape.
4. Started car with air cleaner assembly off. Idle now at 700 rpm instead of 500 plus. Checked for vacuum on hose leading from tbi to base of air cleaner assembly (noted that when I plugged hose, idle rpm's dropped back to 500) seemed to have good vacuum although I have no way of testing the amount of vacuum.
Ok, any ideas what the problem might be?
Last edited by toddrob; Jun 13, 2009 at 01:05 PM. Reason: wrong information provided
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Yes you can connect term A to B and the check engine will start shooting you the codes. First 3 codes should be 12...... 1 flash followed by 2 flashes = 12 ............it will flash 3 times then you start getting the real codes.
Have you tried driving the car,the idle solenoids sometimes dont move until some miles have been put on the odometer ?
Have you tried driving the car,the idle solenoids sometimes dont move until some miles have been put on the odometer ?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
When first starting and with low idle [when battery light is ON], will the battery light go out if you give it a little throttle? If so, then there is nothing wrong with your alternator/regulation system...the engine is just running too slowly to get adequate charge from the system. It sounds like you only have a "low idle speed" problem at startup. I suspect that a temperature sensor is not working properly...the engine 'thinks' that it is at normal operating temps when it is really cold. That would not cause the computer to send an error code, because the sensor signal is not seen as "improper" by the computer; it is merely reading a 'valid' temperature, just not the correct temperature. When the engine is cold, the computer should set a higher than normal engine speed until it warms up.
Find the offending temp sensor and install a good one. That should fix your problem. {Others on this Forum may be able to guide you in how to diagnose which sensor is defective.}
Find the offending temp sensor and install a good one. That should fix your problem. {Others on this Forum may be able to guide you in how to diagnose which sensor is defective.}
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
When first starting and with low idle [when battery light is ON], will the battery light go out if you give it a little throttle? If so, then there is nothing wrong with your alternator/regulation system...the engine is just running too slowly to get adequate charge from the system. It sounds like you only have a "low idle speed" problem at startup. I suspect that a temperature sensor is not working properly...the engine 'thinks' that it is at normal operating temps when it is really cold. That would not cause the computer to send an error code, because the sensor signal is not seen as "improper" by the computer; it is merely reading a 'valid' temperature, just not the correct temperature. When the engine is cold, the computer should set a higher than normal engine speed until it warms up.
Find the offending temp sensor and install a good one. That should fix your problem. {Others on this Forum may be able to guide you in how to diagnose which sensor is defective.}
Find the offending temp sensor and install a good one. That should fix your problem. {Others on this Forum may be able to guide you in how to diagnose which sensor is defective.}
Does the damper door on the air cleaner snorkel have anything to do with this or does that just preheat the air intake to the tbi's when the engine is cold?
Thanks for your help.
TR
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
When first starting and with low idle [when battery light is ON], will the battery light go out if you give it a little throttle? If so, then there is nothing wrong with your alternator/regulation system...the engine is just running too slowly to get adequate charge from the system. It sounds like you only have a "low idle speed" problem at startup. I suspect that a temperature sensor is not working properly...the engine 'thinks' that it is at normal operating temps when it is really cold. That would not cause the computer to send an error code, because the sensor signal is not seen as "improper" by the computer; it is merely reading a 'valid' temperature, just not the correct temperature. When the engine is cold, the computer should set a higher than normal engine speed until it warms up.
Find the offending temp sensor and install a good one. That should fix your problem. {Others on this Forum may be able to guide you in how to diagnose which sensor is defective.}
Find the offending temp sensor and install a good one. That should fix your problem. {Others on this Forum may be able to guide you in how to diagnose which sensor is defective.}
I'm going to try and find one of these sensors locally today and provide an update after I install the new sensor. Hopefully this does the trick so I can get back to enjoying the ride
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
Ok. I did some serious research in my 1982 shop manual and it looks like the coolant temperature sensor located on the front of the intake manifold is the only sensor that provides information to the ECM. The other sensor on the left head only sends info to the instrument cluster and the electic cooling fan next to the radiator.
I'm going to try and find one of these sensors locally today and provide an update after I install the new sensor. Hopefully this does the trick so I can get back to enjoying the ride
I'm going to try and find one of these sensors locally today and provide an update after I install the new sensor. Hopefully this does the trick so I can get back to enjoying the ride
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Pro
Joined: Oct 2007
Posts: 525
Likes: 0
From: Charlotte N.C.
Let's try this: With the vette running, pull the hose off the Power Brake Booster Vacuum Filter. If the idle pics up, the Vacuum Filter could be bad. Also, check your EGR valve with your fingers pushing up from the bottom, if it bogs down its fine. Check the top plate directly under the tb's for leaks, also check fuel pressure and if its low it could be the Fuel Pump. The TP sensor should be at 5.25 volts. These are some of the things you can check first, keep us posted and good-luck.
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
Update
Let's try this: With the vette running, pull the hose off the Power Brake Booster Vacuum Filter. If the idle pics up, the Vacuum Filter could be bad. Also, check your EGR valve with your fingers pushing up from the bottom, if it bogs down its fine. Check the top plate directly under the tb's for leaks, also check fuel pressure and if its low it could be the Fuel Pump. The TP sensor should be at 5.25 volts. These are some of the things you can check first, keep us posted and good-luck.
While I'm at Bloomington this morning, I'm going to try and find a new filter for the brake vacuum to see if that works. I'll send a new post after I install the new vacuum filter.
Thanks for the suggestions Pepsiman.
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
Have you checked the idle air control motor? That is the primary method used by the computer to control idle speed.
You already know that every time you create a vacuum leak that the idle speed rises so it
seems likely that the IAC is stuck and unable to bypass air past the throttle plates as needed.
You already know that every time you create a vacuum leak that the idle speed rises so it
seems likely that the IAC is stuck and unable to bypass air past the throttle plates as needed.
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
Have you checked the idle air control motor? That is the primary method used by the computer to control idle speed.
You already know that every time you create a vacuum leak that the idle speed rises so it
seems likely that the IAC is stuck and unable to bypass air past the throttle plates as needed.
You already know that every time you create a vacuum leak that the idle speed rises so it
seems likely that the IAC is stuck and unable to bypass air past the throttle plates as needed.
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
If you arent aware, the IAC motor actually controls a cone shaped tip that moves in and out to regulate how much air bypasses the throttle blades. If it lets more air bypass then the idle speed rises. If it closes then the idle goes down. They are prone to getting gummed up and sticking. Since your car wasnt driven for a while it is easy to see how it could be stuck. If you unscrew the IAC motor from the side of the throttle body you can clean it up and try to move the pintle manually. A little movement can free it up pretty often and it will go back to working properly.
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
If you arent aware, the IAC motor actually controls a cone shaped tip that moves in and out to regulate how much air bypasses the throttle blades. If it lets more air bypass then the idle speed rises. If it closes then the idle goes down. They are prone to getting gummed up and sticking. Since your car wasnt driven for a while it is easy to see how it could be stuck. If you unscrew the IAC motor from the side of the throttle body you can clean it up and try to move the pintle manually. A little movement can free it up pretty often and it will go back to working properly.
TR
Racer
Joined: May 2007
Posts: 285
Likes: 0
From: Ga.
If you haved changed the CTS start looking at the IAC Motors. 1st check the postion of each. If they are closed you will have a very low idle. Take them out and set them to 1 1/8" and reinstall them. If they go back closed, start looking at your ECM connectors. Most likely the connectors that go on the ECM are corroded, or have lost their tension. Take a small flathead and bend them back out some. There was a GM memo concerning this problem. I only found out after chasing my tail for 6 months. My main problem was the ground connector that controlled all of the engine sensors.
Don't do like me and buy parts that you didn't need. Believe me I have a collection of all of the sensors. I even changed out the ECM 5 times looking for the problem.
Good luck!
Don't do like me and buy parts that you didn't need. Believe me I have a collection of all of the sensors. I even changed out the ECM 5 times looking for the problem.
Good luck!
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
If you haved changed the CTS start looking at the IAC Motors. 1st check the postion of each. If they are closed you will have a very low idle. Take them out and set them to 1 1/8" and reinstall them. If they go back closed, start looking at your ECM connectors. Most likely the connectors that go on the ECM are corroded, or have lost their tension. Take a small flathead and bend them back out some. There was a GM memo concerning this problem. I only found out after chasing my tail for 6 months. My main problem was the ground connector that controlled all of the engine sensors.
Don't do like me and buy parts that you didn't need. Believe me I have a collection of all of the sensors. I even changed out the ECM 5 times looking for the problem.
Good luck!
Don't do like me and buy parts that you didn't need. Believe me I have a collection of all of the sensors. I even changed out the ECM 5 times looking for the problem.
Good luck!
TR
Last edited by toddrob; Jun 28, 2009 at 09:41 AM. Reason: remove unintentional quote from Driveshaft
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
This morning I took the air cleaner assembly off and inspected the IAC on both TBI's. The rear IAC is fully retracted, the front IAC is extended only slightly, about 1/8", into the inspection cavity. As I understand it, this condition would not allow more air to be diverted around the TBI plate and may in fact be causing the low idle condition on cold start. Unfortunatley, I need to go buy an 1-1/4" box wrench before I can take the IAC's out to clean them.
I also inspected the ECM, which appears to be in outstanding condition. No corrosion what-so-ever was found on any connector including the ground connector to the frame.
As long as I have had the car it has not seen rain and it is stored inside by both myself and the previous owner so I am not surprised.
I'll put up another post after I have cleaned the IAC's and given it another try.
I also inspected the ECM, which appears to be in outstanding condition. No corrosion what-so-ever was found on any connector including the ground connector to the frame.
As long as I have had the car it has not seen rain and it is stored inside by both myself and the previous owner so I am not surprised.
I'll put up another post after I have cleaned the IAC's and given it another try.
Racer
Joined: May 2007
Posts: 285
Likes: 0
From: Ga.
You can take the IAC motors out with a pair of channel locks. They are not on to tight with the fine threads. Please go back to the ecm connectors and bend the prongs out for more tension. That is what my car kept doing. The only way you would find it for sure would be to do a pin to pin check with an VOM. Once you fix that problem, you will hear the difference when it cranks.
Thread Starter
Advanced
Joined: Aug 2005
Posts: 88
Likes: 0
From: Naperville Illinois
Long stinking day. I should be in St Charles enjoying the nice weather with the other enthusiasts at Bloomington Gold.
So I started out by checking the engine code, much to my surprise it was code 14 CTS short circuit. I just friking replaced the CTS. There's $38 down the drain. I decided to put the old CTS in, reset the trouble code memory, and now the engine light no longer comes on. Go figure.
Next I took out the IAC's and cleaned them. Put them back in, started the car and waited to see what happended. The rear IAC started in the retracted position and then after a short time it moved to the closed position. The front IAC started in the closed position and never changed. I took it back out to see if I could persuade it to move and the $%@pintle broke off. $128 at NAPA. I checked RockAuto and found an original AC/Delco part for only $68 online. I bought it and should have it in a couple of days.
Then I checked the forum... and see Dirtbuster1's new post. Wish I would have checked it in between taking out the IAC a second time.
Might have saved me from buying a new IAC. So it goes. When the new IAC comes in, I'll also put a little tension on the prongs of the ECM as suggested. My luck I'll be buying a new ECM next...
Toddrob.
So I started out by checking the engine code, much to my surprise it was code 14 CTS short circuit. I just friking replaced the CTS. There's $38 down the drain. I decided to put the old CTS in, reset the trouble code memory, and now the engine light no longer comes on. Go figure.
Next I took out the IAC's and cleaned them. Put them back in, started the car and waited to see what happended. The rear IAC started in the retracted position and then after a short time it moved to the closed position. The front IAC started in the closed position and never changed. I took it back out to see if I could persuade it to move and the $%@pintle broke off. $128 at NAPA. I checked RockAuto and found an original AC/Delco part for only $68 online. I bought it and should have it in a couple of days.
Then I checked the forum... and see Dirtbuster1's new post. Wish I would have checked it in between taking out the IAC a second time.
Might have saved me from buying a new IAC. So it goes. When the new IAC comes in, I'll also put a little tension on the prongs of the ECM as suggested. My luck I'll be buying a new ECM next...
Toddrob.
Racer
Joined: May 2007
Posts: 285
Likes: 0
From: Ga.
Did the pintle break, or just come unscrewed? If it just came unscrewed, just hold the spring back & screw it back in. I have a collection of the IAC motors if interested. 2 are AC Delco put in to confirm what was going on with my ECM. I will make you a deal. They came from RockAuto.