Intermittent TBI Idle Issue

Among some other challenges I have with the tune for my vortec 383 TBI is an intermittent issue where it won’t register as an idle when speed is zero. Causes it to stay in closed loop even though I have the timer set at 12 seconds to go into open loop idle. Also causes a high idle of 750 or 800 even though commanded is 650. Also exacerbates my decel issues that is my other big tune problem.

Happened today on next start after a 30 mile drive. (Two hour delay after 30 mile drive and next start). I was lucky to have my laptop with me so I plugged it in and saw that idle status was not registering even though vehicle was not moving and PCM showed 0 mph vehicle speed. What would cause the idle status to not get set even though speed is zero? VSS? Loose connection somewhere? Ground? Note that shutting engine off and restarting resolves the issue - at least it has the few times this has happened in the past. I’d say it happens less than 1 in 10 times I start engine.

Is tps voltage above .6v? Is it moving around at idle?

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Originally Posted by In-Tech

Is tps voltage above .6v? Is it moving around at idle?

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Thanks, good idea. Kicking myself since I only monitored for a new minutes before I shut it off and restarted and forgot to check that. And I didn't log while it was doing it. When it's running normally TPS voltage is pretty close to 0.5 and not jumping but maybe when this is going on it's behaving erratically. I'll just keep an eye on it.

Any other ideas?

What ecm? There are many things that affect closed loop idle and not just talking about o2 closed loop. There are other idle control strategies mainly "unforseen" especially in the 7747 if that is what you are working with especially if you have changed the desired/commanded idle speed in the cal.

I'm sure we'll all help if we can, just need a bit more info.

-Carl

Quote:

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Originally Posted by In-Tech

What ecm? There are many things that affect closed loop idle and not just talking about o2 closed loop. There are other idle control strategies mainly "unforseen" especially in the 7747 if that is what you are working with especially if you have changed the desired/commanded idle speed in the cal.

I'm sure we'll all help if we can, just need a bit more info.

-Carl

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Sorry, it's a '7060 PCM. And I think I'm pretty much stuck with that stock unit since it's an early (91-93) 4L80e. Here's my other active thread right now - and that thread has a link to my intro thread.

Yeah, I kind of gave up on closed loop idle - it would idle fine for a while and then it would start hunting. I have the timer set at 12 seconds to go into OL idle - rock steady once it's there. Works great, except when the PCM thinks the vehicle is still moving and won't go into idle status.

Thank you for the added information. Pardon, I might have read the details in another post but I think you said you are using a 454 tbi, can you reply with info on what year 454 injectors you are using and also your fuel pressure?

Thanks,
-Carl

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Originally Posted by In-Tech

Thank you for the added information. Pardon, I might have read the details in another post but I think you said you are using a 454 tbi, can you reply with info on what year 454 injectors you are using and also your fuel pressure?

Thanks,
-Carl

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Yep I'm using 454 injectors - don’t know the p/n offhand but they’re 70 lbs/hr. And my fuel press is right at 13 psi. I modified stock regulator to make it adjustable and have a gauge installed at the reg. Thanks

I need a few days to dig into some old notes. From memory I think what we need to do is start with a '93 454 file because of the injector size. It's been quite a while since I have done any TBI stuff but all of this reminds me of something I think was called "quasi sync" mode for the injectors due to their size. Better idle control as well as the transition from decel fuel cutoff and decel enleanment. The 7060 stuff is fairly unique and I think they started using that in '93 and is in use in 94-95 in the 0E mask(7427 puter). If you get to it before me, look into this.

edit and p.s. The high idle stuff was something we had to fix back in the day when I did some work for Whipple. It was much more drastic with the supercharger. Again, many years ago so finding those notes will probably take a bit of time.

Oh, and another thing. I'm not trying to control this thread so, please, any other tuners feel free to chime in. This is good refresher stuff for my failing memory. :mad1:

Quote:

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Originally Posted by In-Tech

I need a few days to dig into some old notes. From memory I think what we need to do is start with a '93 454 file because of the injector size. It's been quite a while since I have done any TBI stuff but all of this reminds me of something I think was called "quasi sync" mode for the injectors due to their size. Better idle control as well as the transition from decel fuel cutoff and decel enleanment. The 7060 stuff is fairly unique and I think they started using that in '93 and is in use in 94-95 in the 0E mask(7427 puter). If you get to it before me, look into this.

edit and p.s. The high idle stuff was something we had to fix back in the day when I did some work for Whipple. It was much more drastic with the supercharger. Again, many years ago so finding those notes will probably take a bit of time.

Oh, and another thing. I'm not trying to control this thread so, please, any other tuners feel free to chime in. This is good refresher stuff for my failing memory. :mad1:

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Thanks so much for the info and willingness to dig up your notes. I'll be excited to see what you find. I've seen the decel enleanment scalar value in my bin and have wondered if that's something that needs adjustment, but I haven't done anything with it. '7060 computer as far as I know is stock to my '91 3/4 ton suburban. I'm pretty sure it was used on all (350 and 454) trucks '91-'93 with 4L80e. There are multiple threads on here that talk about the differences with the early 4L80e - it used software to do the force motor cleaning and AFAIK, the '7060 PCM is the only one that will do that. in '94 they changed the design of the force motor and the PCM no longer was needed to do the cleaning. (I might not have all this exactly right, I haven't done a lot of research into it, but pretty sure I need the '7060 unless I make some mechanical changes to my tranny)

My problem described in the OP repeated today and I was able to log it. Once again, second drive of the day, engine not cooled down from first drive, and PCM refuses to go into idle status, even though vehicle speed reads zero. And, since I'm using OL idle to deal with some erratic idle issues, this problem eventually affects idle causing hunting and a high idle of about 100-150 rpm above commanded. Also exacerbates my weird decel issues. And finally, shutting off engine and restarting resolves the issues: idle status registers when appropriate, will go into OL at idle after 12 seconds (per my .bin), and idle is stable and at commanded speed. And I did check the TPS volts during this time -- I saw two different numbers at different times, 0.47 and 0.49V but it was not fluctuating at idle.

I've attached my .adx and the log file from today. A bit after engine run time of 1900 sec (log file is much less than that, though), I shut off engine and restarted a few sec later. You'll see that before that time idle status never registered. After that, idle status does register and it will go into OL.

Any ideas as to what could cause my PCM to not register as idle status when the vehicle is not moving?

I forgot to mention in the last post: the ADX file I’m using also has a “vehicle moving” status indicator field. For some reason, that always seems to show as “Yes” even when everything else is working properly and the idle status indicator shows “yes”. I noticed that a while ago and just ignored it because, but maybe that is part of my problem.