More On The Idle Problems
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St. Jude Donor '07
More On The Idle Problems
After testing every sensor I am able to, and replacing the IAC & MAF, it is time to take it to a pro.
The question is:
What should I set the TPS voltage return at to keep the idle @ ~ 1300rpm while I drive it to the shop 30 miles away? Then it should stay running when I'm using the clutch and brake.
I'm narrowing it down to the burnoff module or a hidden vaccum leak.
Anyone know how to test the Burn-Off Module in an '85?
Thanks!
The question is:
What should I set the TPS voltage return at to keep the idle @ ~ 1300rpm while I drive it to the shop 30 miles away? Then it should stay running when I'm using the clutch and brake.
I'm narrowing it down to the burnoff module or a hidden vaccum leak.
Anyone know how to test the Burn-Off Module in an '85?
Thanks!
Melting Slicks
Joined: Aug 2003
Posts: 3,415
Likes: 404
From: Lakeville MI
To raise the idle via increasing tps voltage (which will raise the IAC counts):
With the car idling. Move the tps sensor manually until the rpm is where you want it. This will most likely be ~50% of the TPS range. Disconnect the IAC.
The IAC should stay in the present position and permit enough airflow for the desired idle speed.
I saw your previous post that mentions the throttle blades are completely closed. With the throttle at the stop you should have enough throttle opening for at least 4 gm/sec of airflow or at least 400-500 rpm. You can open the stop to increase the min air as required. Try something like 6 gm/sec or 600 rpm and readjust the tps.
With the car idling. Move the tps sensor manually until the rpm is where you want it. This will most likely be ~50% of the TPS range. Disconnect the IAC.
The IAC should stay in the present position and permit enough airflow for the desired idle speed.
I saw your previous post that mentions the throttle blades are completely closed. With the throttle at the stop you should have enough throttle opening for at least 4 gm/sec of airflow or at least 400-500 rpm. You can open the stop to increase the min air as required. Try something like 6 gm/sec or 600 rpm and readjust the tps.
Last edited by tequilaboy; Apr 11, 2007 at 09:09 PM.
Thread Starter
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St. Jude Donor '07
I would have never thought to DC the IAC. I imagine it would keep trying to shut the idle down and damage itself, right?
Your procedure effectively has the IAC close its port so all idle control is maintained by the TPS setting. Did I follow that right?
Your procedure effectively has the IAC close its port so all idle control is maintained by the TPS setting. Did I follow that right?
Melting Slicks
Joined: Aug 2003
Posts: 3,415
Likes: 404
From: Lakeville MI
Once disconnected, the IAC should stay in the last commanded position (preferably open to the desired number of counts).
The ecm will stll try to control the IAC, but will have no effect since its unplugged. There is no harm in running with the IAC disconnected.
The ecm will stll try to control the IAC, but will have no effect since its unplugged. There is no harm in running with the IAC disconnected.
Thread Starter
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St. Jude Donor '07
I saw your previous post that mentions the throttle blades are completely closed. With the throttle at the stop you should have enough throttle opening for at least 4 gm/sec of airflow or at least 400-500 rpm. You can open the stop to increase the min air as required. Try something like 6 gm/sec or 600 rpm and readjust the tps.
How can I tell by looking at it that I'm getting 4-6 gm/sec? Is there a tool I may need?
Melting Slicks
Joined: Aug 2003
Posts: 3,415
Likes: 404
From: Lakeville MI
If you could scan it you could see the MAF flow. In the absense of MAF flow data rpm is a reasonable indicator of idle airflow roughly 1 gm/sec/100 rpm.
Simply put: With the IAC fully closed you should set the throttle opening to provide enough air flow for 400-600 rpm.
4+3 cars should idle around 700 rpm warm, so I would recommend setting the min air a little on the high side to 600 rpm or 6 gm/sec as a starting point.
The IAC can cover the additional airflow required to reach 700 rpm with around 18-20 counts.
The IAC flows around 0.05-0.06 gm/sec/count.
Simply put: With the IAC fully closed you should set the throttle opening to provide enough air flow for 400-600 rpm.
4+3 cars should idle around 700 rpm warm, so I would recommend setting the min air a little on the high side to 600 rpm or 6 gm/sec as a starting point.
The IAC can cover the additional airflow required to reach 700 rpm with around 18-20 counts.
The IAC flows around 0.05-0.06 gm/sec/count.
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St. Jude Donor '07
That actually made good sense! Thanks!
In the absence of a scan tool, what if I loosened the screw to ~1000 rpm until the engine is warm, then slowly back it off to ~700?
The other wierd thing is it looks like there is some sealant on the TB where the screw head would be. What the heck? Am I missing a cap or is this not supposed to be adjusted?
In the absence of a scan tool, what if I loosened the screw to ~1000 rpm until the engine is warm, then slowly back it off to ~700?
The other wierd thing is it looks like there is some sealant on the TB where the screw head would be. What the heck? Am I missing a cap or is this not supposed to be adjusted?
Melting Slicks
Joined: Aug 2003
Posts: 3,415
Likes: 404
From: Lakeville MI
Additional info:
For good running with the MAF disconnected (in order to test or rule out MAF calibration issues), you would want the min air to be set to 4 gm/sec.
When adjusted in this manner, the default airflow calculation will align reasonably with the actual idle airflow, although it will be slightly rich as the IAC counts increase, since the default IAC airflow factor is calibrated to be greater than the actual IAC flow to keep things on the safe side.
Too much min air will cause the default airflow calculation to underestimate the flow and be too lean. This is at least part of the logic behind the low factory recommended min air setting.
Since you are planning to run with the MAF, a greater min air setting should provide good results, hence the 600 rpm recommendation.
For good running with the MAF disconnected (in order to test or rule out MAF calibration issues), you would want the min air to be set to 4 gm/sec.
When adjusted in this manner, the default airflow calculation will align reasonably with the actual idle airflow, although it will be slightly rich as the IAC counts increase, since the default IAC airflow factor is calibrated to be greater than the actual IAC flow to keep things on the safe side.
Too much min air will cause the default airflow calculation to underestimate the flow and be too lean. This is at least part of the logic behind the low factory recommended min air setting.
Since you are planning to run with the MAF, a greater min air setting should provide good results, hence the 600 rpm recommendation.
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Melting Slicks
Joined: Aug 2003
Posts: 3,415
Likes: 404
From: Lakeville MI
During warm up, the idle speed should drop from 1200 rpm to 700 rpm as a function of coolant temperature, so you should see higher rpm when cold.
It should look like this.
Desired Idle Speed vs Coolant Temp.
Deg C RPM
-40 1200.0
-28 1200.0
-16 1200.0
-4 1200.0
8 1200.0
20 850.0
32 800.0
44 800.0
56 700.0
68 700.0
80 700.0
92 700.0
104 700.0
116 700.0
128 700.0
140 700.0
152 700.0
If you adjust the min air for a warm idle, the IAC will simply add counts to maintain the desired elevated cold idle. A cold engine may need an additional 80-100 counts to maintain 1200 rpm.
Min air adjustment should be performed when the IAC is fully closed (and normally disconnected).
Set it when warm and it should be ok cold since the IAC will provide the extra air (if all is working properly).
From the factory, the throttle stop screw hole was plugged to prevent tampering, but what fun is that.
It should look like this.
Desired Idle Speed vs Coolant Temp.
Deg C RPM
-40 1200.0
-28 1200.0
-16 1200.0
-4 1200.0
8 1200.0
20 850.0
32 800.0
44 800.0
56 700.0
68 700.0
80 700.0
92 700.0
104 700.0
116 700.0
128 700.0
140 700.0
152 700.0
If you adjust the min air for a warm idle, the IAC will simply add counts to maintain the desired elevated cold idle. A cold engine may need an additional 80-100 counts to maintain 1200 rpm.
Min air adjustment should be performed when the IAC is fully closed (and normally disconnected).
Set it when warm and it should be ok cold since the IAC will provide the extra air (if all is working properly).
From the factory, the throttle stop screw hole was plugged to prevent tampering, but what fun is that.
Last edited by tequilaboy; Apr 11, 2007 at 10:11 PM.
Thread Starter
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St. Jude Donor '07
Hey TB!
I tried a bubbification. I taped a spacer on the TB throttle control. The idea was to 'fake' accelerator use. Then I started it up and it ran OK. It took about 5 minutes to level out. It was hunting for idle for a bit. Then it leveled off @ 720 rpm and held relatively steady.
I took the time to use my torch to search for leaks. None to be found.
This might be as simple as a TB adjustment and/or timing. We'll see! Good enough until I can get it on a scanner next week?
What do you think?
I tried a bubbification. I taped a spacer on the TB throttle control. The idea was to 'fake' accelerator use. Then I started it up and it ran OK. It took about 5 minutes to level out. It was hunting for idle for a bit. Then it leveled off @ 720 rpm and held relatively steady.
I took the time to use my torch to search for leaks. None to be found.
This might be as simple as a TB adjustment and/or timing. We'll see! Good enough until I can get it on a scanner next week?
What do you think?
Melting Slicks
Joined: Aug 2003
Posts: 3,415
Likes: 404
From: Lakeville MI
Sounds like progress.
I second the motion to check the timing.
You may see some benefit with additional advance, but start with the recommended 6 degrees initial advance with the EST disconnected.
It should run around 23 degrees advance at 700 rpm when the EST is connected.
Also be sure to correct the tps voltage in the event that you re-adjust the throttle stop. It may be an iterative process to get it right, since one adjustment will affect the other somewhat.
Once your happy with the idle rpm and tps voltage, disconnect the battery to clear any learned memory and then let it idle for awhile on the next restart with both a/c on and off for several minutes.
I second the motion to check the timing.
You may see some benefit with additional advance, but start with the recommended 6 degrees initial advance with the EST disconnected.
It should run around 23 degrees advance at 700 rpm when the EST is connected.
Also be sure to correct the tps voltage in the event that you re-adjust the throttle stop. It may be an iterative process to get it right, since one adjustment will affect the other somewhat.
Once your happy with the idle rpm and tps voltage, disconnect the battery to clear any learned memory and then let it idle for awhile on the next restart with both a/c on and off for several minutes.
Last edited by tequilaboy; Apr 12, 2007 at 07:07 PM.
Thread Starter
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St. Jude Donor '07
I have not checked the timing. But this idle problem came on rather suddenly. It has always kinda hunted in the last 2 years I have had it, so maybe the tweaking I have done has finally expose it?
Melting Slicks
Joined: Aug 2003
Posts: 3,415
Likes: 404
From: Lakeville MI
Ok, my misunderstanding.
Its always a good idea to check the timing if only to rule it out as part of the problem.
Changes that affect the airflow and rpm also have an effect on the calculated load which along with rpm determines the spark advance.
In the idle rpm region, the spark advance table (rpm vs load) should be tuned to provide some torque stability. Small rpm and load changes should not result in big swings in spark advance or big changes in the torque output which would be difficult to control with the IAC alone.
If the timing is skewed, some of the designed in stability may be lost and the changing spark advance as a function of rpm and load can make for an even more unstable idle.
It is best to start within the know good (stable) parameters.
Its always a good idea to check the timing if only to rule it out as part of the problem.
Changes that affect the airflow and rpm also have an effect on the calculated load which along with rpm determines the spark advance.
In the idle rpm region, the spark advance table (rpm vs load) should be tuned to provide some torque stability. Small rpm and load changes should not result in big swings in spark advance or big changes in the torque output which would be difficult to control with the IAC alone.
If the timing is skewed, some of the designed in stability may be lost and the changing spark advance as a function of rpm and load can make for an even more unstable idle.
It is best to start within the know good (stable) parameters.