TBI with High Idle in PARK

[JJK10]

Hi folks I'm working on a TBI to Vortec (L-31) conversion into an 1983 Chevy K10 and it seems that the tuner I'm working with is having a hard time get the idle down when the truck is in park. I'm hoping someone has some suggestion as I have tried everything and the only way I can get it to idle slower in park is to jumper the A&B pin and force the IAC closed then unplug it and adjust the throttle blades open and she idles nicely at 700 rpms. I have followed the base idle setup procedure but as soon as I plug the IAC back in it opens up and the Idle goes to 850... The tuner tells me the target idle is set at 600rpms in the chip but it just doesn't get there. Now a bit of history on the truck! it started it life as a 6.2 diesel and when I purchased the truck the guy told me he purchased a wrecked 1987 with a TBI and moved all the components over to the 1983... in all fairness it looked as though he did a great job, everything was nicely done..... but even with previous motor and a different tuner the truck idled high in park. With the new GE crate L-31 I replaced about everything except wiring! could it be a wiring issue with P/N switch? Or a tuning issue? The BLM's are running about 117 in Park...... I'll try to attach the Tunerpro log but currently its not working. Thanks Much

[PlayingWithTBI]

Check your grounds to D1 and A12. On one of the electrical service manuals (can't remember which one) for a GMT400 truck it said high idle can result due to a bad ground there.

http://www.gearhead-efi.com/gearhead-efi/wiring/747ecm1.jpg

[brian617]

What is your IAC value when this occurs?

[JJK10]

44 in closed loop... engine temp about 200. Thanks, Jason

[sturgillbd]

Do you have any error codes stored? 7747 idle logic isn't as good as the later PCM's. On the 7427, if you don't have a VSS connected, it will do the same thing. You have to disable the error code for the VSS and the problem goes away. You may try that and see if it helps the high idle.

[Daveo91]

700 seems high for base idle to me. I aim for around 500-550 when I set that, Should be lower than target idle, not higher. If it’s higher, ECM will be trying to lower rpm with the IAC, but it can’t. And I think you can actually damage the IAC that way. But I might be misreading your post on this....

Another thing to look at in the tune... there’s a setting for IAC differential between neutral/park and drive. Perhaps that is set wrong, i.e. it doesn’t drop it enough when you go to park.

[brian617]

What is your TPS voltage at idle?

[JJK10]

Check your grounds to D1 and A12. On one of the electrical service manuals (can't remember which one) for a GMT400 truck it said high idle can result due to a bad ground there.

http://www.gearhead-efi.com/gearhead-efi/wiring/747ecm1.jpg

Thank you... I will double check the grounds

[JJK10]

700 seems high for base idle to me. I aim for around 500-550 when I set that, Should be lower than target idle, not higher. If it’s higher, ECM will be trying to lower rpm with the IAC, but it can’t. And I think you can actually damage the IAC that way. But I might be misreading your post on this....
With the ODB1 jumped and the IAC forced closed I set the idle on the throttle blades at about 525..

Another thing to look at in the tune... there’s a setting for IAC differential between neutral/park and drive. Perhaps that is set wrong, i.e. it doesn’t drop it enough when you go to park.
You could be on to something! but I have no ability to check that... Thanks much for the input!

[JJK10]

What is your TPS voltage at idle?

about .9 with the RPM at 825 and a BLM of 115 when I first put it in park after a long drive.... IAC count 44

[dave w]

Maybe the information in this link will be helpful: http://www.gmcmidwestclassics.org/Web%20pages/Tuning%20the%20TBI.pdf

dave w

[JJK10]

Maybe the information in this link will be helpful: http://www.gmcmidwestclassics.org/Web%20pages/Tuning%20the%20TBI.pdf

dave w
Yes Sir! that's the direction I followed. it just idles up once you plug the IAC back in. Thanks

[brian617]

about .9 with the RPM at 825 and a BLM of 115 when I first put it in park after a long drive.... IAC count 44

Try adjusting the voltage down to somewhere between .50 and .65

[stew86MCSS396]

In post #1 you said tuner set idle target to 600 rpms. As already mentioned twice, You need to set minimum idle 125 rpms below 600 rpms.

[JJK10]

Try adjusting the voltage down to somewhere between .50 and .65
I'm sorry I missed up the Absolute pressure volts is .9 the TPS is .55... Thanks

[brian617]

In post #1 you said tuner set idle target to 600 rpms. As already mentioned twice, You need to set minimum idle 125 rpms below 600 rpms.


I think everyone is getting confused. If he had the minimum idle set too high the IAC would zero out. You have to read the original post a few times to understand what he did.

Two things I suspect either the IAC motor is bad and giving a false step reading (the IAC position is 44), or there is another parameter that is causing the high idle. This is the point when a data log and a .bin file is a must.

[PlayingWithTBI]

I think everyone is getting confused. If he had the minimum idle set too high the IAC would zero out. You have to read the original post a few times to understand what he did.

Two things I suspect either the IAC motor is bad and giving a false step reading (the IAC position is 44), or there is another parameter that is causing the high idle. This is the point when a data log and a .bin file is a must.

The confusion is "minimum idle" - when you set minimum idle you set it by adjusting idle with the throttle blade stop screw (with IAC bypassed) to 125RPm below target idle. If you don't you'll burn up the stepper motor in the IAC since the ECM will continuously try to close the pintle more to reach "target idle" speed. The IAC can not zero out the idle if the blade shaft stop is holding it open.

[brian617]

ECM will continuously try to close the pintle more to reach "target idle" speed. The IAC can not zero out the idle if the blade shaft stop is holding it open.

Still confused. If the minimum idle were too high the ECM would drive the IAC pintle to 0 steps and be unable to lower the idle further. However his IAC count is 44, way higher than I shoot for when setting.

I dont bother with minimum idle spec. I want my TPS voltage between .50 and .65 and I want my IAC steps around 15 counts on a hot engine no load.

[brian617]

I can get it to idle slower in park is to jumper the A&B pin and force the IAC closed then unplug it and adjust the throttle blades open and she idles nicely at 700 rpms.


I have followed the base idle setup procedure but as soon as I plug the IAC back in it opens up and the Idle goes to 850

Read these two sentences separate. Not as one.

[PlayingWithTBI]

Still confused. If the minimum idle were too high the ECM would drive the IAC pintle to 0 steps and be unable to lower the idle further. However his IAC count is 44, way higher than I shoot for when setting.

If the minimum idle wasn't set 1st the IAC may be already bad.


I dont bother with minimum idle spec.

You may get bit if you don't...

[brian617]

You may get bit if you don't...

How?

[PlayingWithTBI]

Read these two sentences separate. Not as one.

OK, my bad - I missed that.

[JJK10]

Thanks everyone! I'm working with Old School Efi and I believe he is going to try to burn a Manual chip to see if that might bring the in PARK idle down. Any other ideas would be welcome! long story short the wiring and the tuning approach (until now, David is trying a manual bin) are the only two things that haven't changed and at the end of the day how can you have AIC counts in the 50's and at 850 rpms in park? Also worth noted that this engine was transplants from a donor truck by previous owner. Any help would be great. Thank you!

[dave w]

Manual Trans .bin attached.

dave w

[brian617]

how can you have AIC counts in the 50's and at 850 rpms in park?

Something is driving the IAC to 50 counts, the result is an idle of 850 rpm.

Your IAC at hot idle, no load should be around 15 counts.

With no .bin file or data log the rest of us can't help much.

I'm sure Dave will get it sorted out.

[JJK10]

Something is driving the IAC to 50 counts, the result is an idle of 850 rpm.

Your IAC at hot idle, no load should be around 15 counts.

With no .bin file or data log the rest of us can't help much.

I'm sure Dave will get it sorted out.
Dave is working on it! but I cant help but think something in the wiring is causing this high idle.... had the same issue with the L05 engine I removed. Maybe if Dave gets time he can post my last log... it might make for some fun discussion. Thanks Brian617

[dave w]

Log file attached.

dave w

[JJK10]

Log file attached.

dave w
Thanks Dave!

[JJK10]

HELP!
After nearly a year of working on and off on this HIGH idle in park.... its still there! I've tried different computers, different throttle bodies, different chips, different tuners and still no luck. My next try is to start looking into the wiring. Remember from my previous posts that this issue has been there every since I got the truck. The PO was the guy that installed the harness and dropped in the TBI engine from a donor truck. At the end of the day how can the engine idle high and still have IAC counts in the 50's? It seems that the IAC wiring is bad or something is tell the engine to idle at 900, the computer. Please if anyone has any ideas I would be so grateful. Thank you, Jason

[tayto]

is this vehicle still tbi? if so don't worry about setting base idle by jumpering the iac closed. set the the base idle screw so IAC counts are 5-15 with engine hot. is the throttle body the same before new engine? has the throttle shaft been bushed? I have not come across a TBI unit in the last 10 years that DOESN'T need to be bushed. a worn throttle shaft can make idle untamable.

[JJK10]

Thank you Tayto
Yes the truck is still setup with TBI...The TB is not the issue, I have tried a few different setups. This is a real head scratcher I've tried everything except wiring harness stuff. Remember if I jumper the ALDL and drive the IAC closed, then unplug it and start it up it will idle at 500-525... plug it back in and start the truck.... and back to 900 it goes. Something is telling the IAC to open and all I can think is wiring resistance, short or ground. My next step is to build a new pigtail and run the four wires straight from the IAC to the computer and see if that helps

[dave w]

The attached pics show the wiring differences between the IAC for a SBC and the IAC for a BBC. Circuits 441 and 442 wired differently, SBC vs. BBC

Gearhead-efi link to the 1227747 wiring diagrams: http://www.gearhead-efi.com/Fuel-Injection/showthread.php?304-1227747-ECM-Information-42

Seems unlikely the IAC wiring would be incorrect, but maybe?

OTC might still offer an IAC tester: https://www.otctools.com/sites/default/files/103378.pdf

dave w

[tayto]

ohm out the wires and confirm pin locations. before running new/temp wires. my other thought was A/C request, if it is on it will bump up the idle.

[JJK10]

I double checked all this, the wiring is correct for a SBC. Thanks, Jason

[JJK10]

The wires are located correctly and seem to all Ohm correctly ….. I'm stumped

[tayto]

throttle cable holding throttle blades open?

[JJK10]

throttle cable holding throttle blades open?

Thanks Tayto... but that was confirmed long ago! this is much deeper than something that simple.

[stew86MCSS396]

Are you using the manual bin that dave w had posted and the problem still persist? By any chance are you running with a 180 degree T-stat? IAC logic doesn't play nice with colder than stock T-stat unless you change the parameters in the bin.

[JJK10]

Are you using the manual bin that dave w had posted and the problem still persist? By any chance are you running with a 180 degree T-stat? IAC logic doesn't play nice with colder than stock T-stat unless you change the parameters in the bin.

Yes still running the manual bin and unfortunately the issue is still there. I'm not sure what Tsat in there but remember this issue has ran through several different computers, different engine and different programmers.....but same harness...:mad1:

[stew86MCSS396]

Idk which came first...the manual bin or the xdl. The xdl still reports the tune in park, then drive which the idle rpms seem normal but flares back up when back in park. Idk what ill effects i.e. cold starting, this might have since you're running an automatic but have you considered de-pinning the P/N switch? I just find it odd that its reacting to the P/N switch when you're using a manual bin???

[Dehwishwayne]

I have seen exhaust leaks do this before on TBI setups. I have held off posting because honestly the IAC value is throwing me off. but something to look for. usually near the O2 sensor.

Btw this is my first post on this forum. i was an ASE master tech once apon a time. but joined the army in 2008 so ive been out of the pro game for a while. But i retired young so now i piddle a little. Current project is a 72 C50 Tbi swap.

[brian617]

To clear up some confusion and wild guesses what you are seeing in the data pids is Commanded IAC steps, not Actual IAC steps. So think about that for a minute and what youre seeing is what the PCM wants the IAC to be. That means something in the bin. or a related sensor feed back is telling the PCM to park the IAC at the value you see.

[JJK10]

Thanks everyone! I have no ability or skills to mess with the computer/tuning but aside from that I can handle the other stuff. In all fairness I have had three tuners attempt fixing this.... but to me it really seems like the computer is telling the truck to idle at 900 in park or some kind of wiring issue. I'm thinking about installing a new wiring harness...I don't know what else to try. Again thanks for everyone's help, we will get there! Jason

[tayto]

have you verified your p/n switch? unplug it and then look at wiring diagrams and emulate it being in park and neutral by adding or removing a jumper.

[stew86MCSS396]

Are you using the manual bin that dave w had posted and the problem still persist? By any chance are you running with a 180 degree T-stat? IAC logic doesn't play nice with colder than stock T-stat unless you change the parameters in the bin.

I'm gonna recant the T-stat comment as I've looked at some old logs of another '7747 project that I've worked on and temps stayed in the same range as your xdl if not cooler! Don't want you chasing ghost on my account.

[JJK10]

So today I installed a new IAC pigtail from the engine to the computer... Went through the TBI setup and with the IAC forced close and then unplugged the truck idles at 525. Plug it back in and after a few minutes back to 1000 it goes, with counts in the 50's. is there a sensor you think could be causing this? I data logged today but I'm not sure how to post it on this thread. Thanks for everyone's help, Jason

[JJK10]

Not sure if I did this correctly..... but I tried!

[stew86MCSS396]

I'm seeing 2 engine codes 15 Coolant Sensor Low and 34 MAP Low. Don't just change parts. Double check the cleanliness of the grounds to those sensors. You can use a volt meter to verify key on engine off and running volts. http://www.gearhead-efi.com/Fuel-Injection/attachment.php?attachmentid=1940&d=1331599730

[dave w]

Couple Screen Shots from the New Iac Wire.xdl

I agree Codes 15 & 34 are in the log.

The MAP and CTS look normal on the Dash Screen.

dave w

[dave w]

I wonder what removing the Park input from the ECM connector would do to the idle. The screen shot in the previous post shows "Park" with the high idle.

Manual Transmission 1227747 TBI systems have VSS input without the "Park" switch wiring.

dave w

[JJK10]

I'm going to look into the two codes and I will try removing the Park input from the ecm. The codes seem odd because no CEL ever came on... Thank again everyone and keep the thoughts coming! Keep thinking about the possibility that something was done wrong when the engine and wiring was transplanted from the donor vehicle. Jason

[JJK10]

Well removing the park input from the ECM did not change anything with regards to the HIGH idle in park. Check that off the list!!! any other thoughts on this? is it ok to install back into the loom? thanks, Jason

[dave w]

Before installing the Park wire back into the ECM connector, try the following:
Reset the codes by disconnecting the battery for 1 minute.
With the Park wire unplugged from the ECM, codes cleared (verified in TunerPro) ... try the short cut IAC adjustment by adjusting the IAC screw so that TunerPro RT shows less than 30 steps (ideally 10 steps) with the engine fully warmed up. Verify the TPS voltage is 0.55 VDC after adjusting the IAC Screw.

I've done the short cut IAC adjustment several times with good results. Keeping the TPS volts @ 0.55 VDC is a HUGE challenge when using the short cut IAC adjustment. The TPS voltage can fluctuate when tightening the TPS mounting screws. Adjusting the IAC screw will change the TPS voltage:mad1:

dave w

[JJK10]

Dave,
I followed your suggestion and got the IAC counts down around 6-9 and the TPS set very close to .55 but now the idle is 1100. I attached the log. :mad1::mad1::mad1:

[dave w]

I am currently out of any new ideas.

dave w

[brian617]

At 6-9 counts the IAC will be nearly closed. Can you confirm that by looking at the idle air passage?

What happens if you put you thumb over the IAC passage?

[JJK10]

Yes, its nearly closed. but the throttle blades are waaaaaay open. If I jumper the ALDL and turn the key on, I can pull the IAC plug when the plunger is at about 20 counts, then leave it unplugged and it idles nice. I'm lean toward a new wiring harness! I don't know what else to try.

[dave w]

As a ballpark adjustment for the throttle blade opening, try adjusting the throttle blade opening by pulling a strip of paper about 1/2" wide x 4" long through the throttle blade opening. The paper strip should have a slight drag when pulled through the throttle blade opening.

Note, paper thicknesses will vary depending on the paper. I use a common 20 lb paper, not 16 lb (to thin) or 24 lb (to thick).

dave w

[brian617]

Yes, its nearly closed. but the throttle blades are waaaaaay open. If I jumper the ALDL and turn the key on, I can pull the IAC plug when the plunger is at about 20 counts, then leave it unplugged and it idles nice. I'm lean toward a new wiring harness! I don't know what else to try.

If your throttle blades are "waaaaay open" and your TPS voltage is .55 you have a strange problem. What is the TPS voltage when the throttle blades are closed? Or just a turn open from closed?

[JJK10]

I would open the butterflies, cycle the key and then reset the TPS. I continued that until the counts were near 10, at that point the idle was worse.... ]If your throttle blades are "waaaaay open" and your TPS voltage is .55 you have a strange problem. What is the TPS voltage when the throttle blades are closed? Or just a turn open from closed?[/QUOTE]

[JJK10]

[QUOTE=dave w;83926]As a ballpark adjustment for the throttle blade opening, try adjusting the throttle blade opening by pulling a strip of paper about 1/2" wide x 4" long through the throttle blade opening. The paper strip should have a slight drag when pulled through the throttle blade opening.

Note, paper thicknesses will vary depending on the paper. I use a common 20 lb paper, not 16 lb (to thin) or 24 lb (to thick).

dave w

Thanks Dave, I will give that a try.... but I bet I will be 900+Rpm and IAC count 40+

[dave w]

After adjusting to paper thickness, reset TPS voltage = 0.55 volts

dave w

[JJK10]

Is it possible that this is the issue I'm having? Below is another members post!

So I finally figured out that the park/run IAC offset was too low and added a few steps. Now it idles at 765 regardless of gear selector position. I guess with it too low, the ECM was giving almost as much air in park/neutral as it was in drive, causing the park/neutral idle to be too high. I enjoy learning on my own, but sometimes a little bump in the right direction is helpful...
Please chime in if I'm misunderstanding how that offset was affecting things.

[JJK10]

Tried the paper trick, then reset the TPS with a VM and no change. Idle around 900 and IAC counts near 45.... anyone have any ideas. Thank you