Off idle tip in stumble

czimpel

Boost...gotta get some!
Trying to fix an off idle tip in stumble. Where do I start? Iac is at 60-70 at idle, tried closing throttle plates more, seem to make it worse.
 

earlbrown

runs with scissors
Open the throttle plates as far as you can get away with. Make sure the IAC reading is at least '1' in park.


That lowers the abruptness during the transition from the small IAC passage to the large TB.
 

czimpel

Boost...gotta get some!
I'll try that tomorrow. Thanks. I was actually thinking backwards, I thought I read somewhere that trying to get the plate closed as far as possible was good. Thanks for clearing this up.
 

czimpel

Boost...gotta get some!
Thanks Eric, I will try that as well.

Do I start with one pound at a time in either direction?
 

750H.P.V6

Brutal 6 Racing
Is your cam sensor set right? If you have it set 25 degrees after TDC on the exhaust stroke not the compression stroke the fuel injector sequence will be wrong and result in an off idle stumble.

Neal
 

czimpel

Boost...gotta get some!
750H.P.V6 said:
Is your cam sensor set right? If you have it set 25 degrees after TDC on the exhaust stroke not the compression stroke the fuel injector sequence will be wrong and result in an off idle stumble.

Neal

I've never had it out, but I bought a caspers cam sensor tool. I'll check that next. Thanks.
 

czimpel

Boost...gotta get some!
I tried adjusting the idle screw, but I can't get the iac numbers below 60. I pulled the iac and it looks clean, could it be bad?
 

earlbrown

runs with scissors
that's not an idle screw. It's a minimum air bleed screw.

Screwing it in will cause the throttle blade to open and the IAC to go down numerically.
 

czimpel

Boost...gotta get some!
Throttle body.jpg
When I screw it in it only raises the idle RPM, the iac numbers go up to 120 when it is screwed in all the way. Am I doing something wrong?

This is the screw I'm messing with.
 

earlbrown

runs with scissors
When the engine is idling, it get's air from two places. Through the throttlebody and through the IAC circuit. When you open the throttle blade up the IAC closes to maintain idle (and the IAC numbers go down). When you screw it in, it closes the butterfly and the IAC has to open up to keep the engine breathing. (like you've been seeing)

Keep in mind that when you're moving that screw you're messing with the TPS settings too.

With a warm engine, put that screw where about 2 threads are showing next to the throttle stop. Turn the key on and set the TPS to .42V if it needs it. Then start the car. Blip the throttle a few times and let the engine learn where it wants to set the IAC and note the number in both park and drive.

The more you run the screw in, the lower the IAC numbers will be. Every time you make an adjustment turn the car off and restart. Make sure the TPS stays in spec everytime you adjust something. It's a back and forth balancing act to get them set. It's just the nature of the beast.

Personally, I like my IAC numbers as low as I can get them without hitting "0" to minimize tip-in stumble. With my car the lowest I can ever get it to read is at an offramp on the interstate. My car will read 02 IAC in OD in that instance. (as long as it reads something higher than 0 the ECM has control over idle)
 

mgmshar

Active Member
Your IAC and TPS are probably so far out-of-whack now that it might be a good idea to start from scratch.

First, do the minimum idle air adjustment. This will also cause the ECU to reset the IAC:
  1. Warm the engine to full operating temps
  2. Turn engine off and then turn the key back to ON
  3. With the IAC connected, jumper pins A and B at the ALDL.
  4. Wait 30 seconds and unplug the IAC
  5. Remove the jumper at the ALDL
  6. Start the engine
  7. Adjust engine RPM to 500-550 in Drive using the throttle stop screw on the throttle body.
  8. Shut the engine off and plug the IAC back in.
  9. Restart the engine
  10. Set TPS to .40-.44 volt or whatever your chip manufacturer specifies
The above procedure should reset the IAC and get you close to final values. Then, to fine tune it, follow the procedure below:

(note that everything below should be done with the car in PARK, not DRIVE)
1. Start the car and get it up to operating temperature.
2. With the engine-off, key-on, adjust the TPS per the GNTTYPE procedure so that you get the following on the Scanmaster:
0.40 to 0.42 volts with your foot off the gas pedal.
Over 4.0 volts when you push the gas pedal to the floor.
(note: many people say you need to have at least 4.6 or more volts when you floor the gas pedal. This is not true. Just make sure it's comfortably over 4.0 volts when you floor it. 4.25 volts would be fine. You should not have to ream out the TPS slots as is sometimes suggested. Try sliding the TPS all the way forward and then rotating it to achieve the 0.42 volts - this usually helps to get the highest WOT value possible.)
3. After securing the screws that hold the TPS, start the car. Make sure that at idle, your TPS volts are BELOW 0.46 volts (I make sure mine are in the 0.42 to 0.44 range for a margin of safety). On most cars, the TPS volts jump by about 0.02 when you start the engine - i.e. if you set the TPS to 0.40 volts with the engine off, you will probably see it jump to 0.42 volts after you start the engine.
4. Once the car is warm and in closed-loop, look at your IAC reading at idle. Make sure they are in the 15 to 40 range, with 20 to 25 being"perfect".
If the IAC reading is too LOW, then turn the throttle body screw a little counter-clockwise (i.e. CLOSE the throttle plate a little more).
If the IAC reading is too HIGH, then turn the throttle body screw a little clockwise (i.e. OPEN the throttle plate a little more).
(Warning: if the TPS volts go over 0.45 volts while turning the throttle body screw clockwise, you are no longer in idle mode. Shut the car off and go back to Step 2.)
5. Repeat Steps 2 through 4 above until you get the TPS and IAC readings to be in good values:
TPS at 0.40 to 0.42 volts with the engine-off, key-on.
TPS reading over 4.0 volts when you floor the gas pedal with the engine-off, key-on.
IAC reading between 15 and 40 with the car running at idle, engine warm, in closed loop.

It's an iterative process and takes a little time, but once you get there, you should have a good idle and minimize the chance of tip-in stumble. As others have mentioned, biasing the IAC to the low-end of the acceptable range might help improve your tip-in problem.

One other thing to keep in mind - the exact IAC steps required to idle the car changes every day. If the incoming air is hot, it will open more to maintain idle speed (higher counts). If the incoming air is cold, it will open less (lower counts). So, if you set your TPS and IAC on a warm day, don't be surprised if the IAC steps are lower on a cold day, and vice versa. This is OK as long as they are generally in the ranges shown above.

The good thing about the above is that it costs no money to do as long as you have a ScanMaster or some other scan tool.

Good Luck
 

czimpel

Boost...gotta get some!
mgmshar said:
Your IAC and TPS are probably so far out-of-whack now that it might be a good idea to start from scratch.

First, do the minimum idle air adjustment. This will also cause the ECU to reset the IAC:

[*]Warm the engine to full operating temps
[*]Turn engine off and then turn the key back to ON
[*]With the IAC connected, jumper pins A and B at the ALDL.
[*]Wait 30 seconds and unplug the IAC
[*]Remove the jumper at the ALDL
[*]Start the engine
[*]Adjust engine RPM to 500-550 in Drive using the throttle stop screw on the throttle body.
[*]Shut the engine off and plug the IAC back in.
[*]Restart the engine
[*]Set TPS to .40-.44 volt or whatever your chip manufacturer specifies

The above procedure should reset the IAC and get you close to final values. Then, to fine tune it, follow the procedure below:

(note that everything below should be done with the car in PARK, not DRIVE)
1. Start the car and get it up to operating temperature.
2. With the engine-off, key-on, adjust the TPS per the GNTTYPE procedure so that you get the following on the Scanmaster:
0.40 to 0.42 volts with your foot off the gas pedal.
Over 4.0 volts when you push the gas pedal to the floor.
(note: many people say you need to have at least 4.6 or more volts when you floor the gas pedal. This is not true. Just make sure it's comfortably over 4.0 volts when you floor it. 4.25 volts would be fine. You should not have to ream out the TPS slots as is sometimes suggested. Try sliding the TPS all the way forward and then rotating it to achieve the 0.42 volts - this usually helps to get the highest WOT value possible.)
3. After securing the screws that hold the TPS, start the car. Make sure that at idle, your TPS volts are BELOW 0.46 volts (I make sure mine are in the 0.42 to 0.44 range for a margin of safety). On most cars, the TPS volts jump by about 0.02 when you start the engine - i.e. if you set the TPS to 0.40 volts with the engine off, you will probably see it jump to 0.42 volts after you start the engine.
4. Once the car is warm and in closed-loop, look at your IAC reading at idle. Make sure they are in the 15 to 40 range, with 20 to 25 being"perfect".
If the IAC reading is too LOW, then turn the throttle body screw a little counter-clockwise (i.e. CLOSE the throttle plate a little more).
If the IAC reading is too HIGH, then turn the throttle body screw a little clockwise (i.e. OPEN the throttle plate a little more).
(Warning: if the TPS volts go over 0.45 volts while turning the throttle body screw clockwise, you are no longer in idle mode. Shut the car off and go back to Step 2.)
5. Repeat Steps 2 through 4 above until you get the TPS and IAC readings to be in good values:
TPS at 0.40 to 0.42 volts with the engine-off, key-on.
TPS reading over 4.0 volts when you floor the gas pedal with the engine-off, key-on.
IAC reading between 15 and 40 with the car running at idle, engine warm, in closed loop.

It's an iterative process and takes a little time, but once you get there, you should have a good idle and minimize the chance of tip-in stumble. As others have mentioned, biasing the IAC to the low-end of the acceptable range might help improve your tip-in problem.

One other thing to keep in mind - the exact IAC steps required to idle the car changes every day. If the incoming air is hot, it will open more to maintain idle speed (higher counts). If the incoming air is cold, it will open less (lower counts). So, if you set your TPS and IAC on a warm day, don't be surprised if the IAC steps are lower on a cold day, and vice versa. This is OK as long as they are generally in the ranges shown above.

The good thing about the above is that it costs no money to do as long as you have a ScanMaster or some other scan tool.

Good Luck

Amazing info, thanks for the help. I love that you actually have a laid out procedure for this.
 

mgmshar

Active Member
Thanks for the compliment, but I did cheat. The minimum idle air adjustment procedure is right off the gnttype.org web site and was previously posted by another board member. The IAC/TPS setting procedure is my version of the one that is on the gnttype.org web site. I had previously posted it in another thread.

Give it a try and post back on whether or not it fixes the tip-in problem.
 

czimpel

Boost...gotta get some!
Finally got around to setting the iac and tps again. Everything worked like a charm and the stumble seems to have went away. Haven't had much time to drive it, but I hope it works.

Thanks for the help guys!
 

czimpel

Boost...gotta get some!
Well, took the car out, got it warmed up good again, and the stumble came back, but it was much better than before.

Dropped the FP about 3 psi and it seems good now. Will the 3 psi cause problems under boost now? Running 120s w/E85.
 

czimpel

Boost...gotta get some!
One other thing that I notice is if I blip the throttle, it seems very lazy coming back down to idle speeds. If I rev it up, it takes about 5 seconds for it to settle back down. Is this normal?
 

czimpel

Boost...gotta get some!
Had some time to get out and drive today. The tip in stumble is still there once in a while, some more fp tweaking will probably help.

My idle in drive is between 900-1000 rpm which seems high, I also think it may be causing some 2nd gear starts. Iac numbers are between 10-20 now.

I've checked for vac leaks, nothing found yet.

Any thoughts?
 

Eric Stage I

TurboTweak Guy
The IAC is programmed to come down slowly. Also the idle is programmed at 900rpm. All that is normal for big injectors like that. It shouldn't cause a 2nd gear start though.
Regards,
Eric
 

czimpel

Boost...gotta get some!
OK, I'll double check the Tv adjustment for the 2nd gear starts.

How far can I safely turn the FP down?
 
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