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Old 11-30-2018, 10:18 AM   #1
oltronj
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TBI wont go to closed loop

91 Chevy V3500 454 with a 4L80E and a 205 tcase, trucks seems to run good but noticed the gas mileage has dropped (went from not so good to bad), started poking around and put a 3 wire O2 sensor in it just to see if that would help. I learned of the trick to jumper the 2 terminals on the diagnostic port and start the truck to get the check engine light to flash to tell if it is in closed loop or open loop and sure as heck it was just staying in open loop all the time. So what all will keep the computer from going into closed loop? I tested the coolant temp sensor that is by the tstat housing and it checked out ok at all the temps I checked it at. I am gonna test the O2 sensor and see what it is doing. I have read that the Bosch O2 sensors can give these trucks fits and I dont remember what brand came with the 3 wire conversion kit so if it looks suspect it will get a new AC Delco one. Any other suggestions on what to check or information on this would be appreciated.
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Old 11-30-2018, 11:09 AM   #2
cadillac_al
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Re: TBI wont go to closed loop

Is the OBD generating any codes?
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Old 11-30-2018, 12:12 PM   #3
DieselSJ
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Re: TBI wont go to closed loop

Get an ALDL cable and do some live data scanning. It will tell you if it is in open or closed and you should be able to see what is preventing closed loop. Remember that you will NEVER have closed loop at idle. I have also never heard of having the check engine light flash for open vs closed loop.

Get a cable from here -
http://aldlcable.com/

They also supply a CD with a couple dozen different free programs for live monitoring and data logging on OBD1 systems. I found sensors that were bad that weren't triggering a CEL using this.
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Old 11-30-2018, 12:56 PM   #4
oltronj
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Re: TBI wont go to closed loop

I had never heard of the flashing check engine light for open or closed loop until the other day either, I did find it in a post from ChevyTech on here. I will be getting a scanner to get a good look at everything eventually but until then it is just the digital multi meter. It also is not throwing any codes and the CEL is not on. Why won't it go into closed loop at idle? Does anyone know what the required parameters are for it to be in closed loop?
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Old 11-30-2018, 04:37 PM   #5
DieselSJ
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Re: TBI wont go to closed loop

Quote:
Originally Posted by oltronj View Post
I had never heard of the flashing check engine light for open or closed loop until the other day either, I did find it in a post from ChevyTech on here. I will be getting a scanner to get a good look at everything eventually but until then it is just the digital multi meter. It also is not throwing any codes and the CEL is not on. Why won't it go into closed loop at idle? Does anyone know what the required parameters are for it to be in closed loop?
Can you link to the post about the open/closed loop using the CEL?

No closed loop at idle because it isn't designed to.

Mine didn't throw any codes either, but I had sensors that were definitely bad. Forget the scanner and just get the cable I referenced above and use a laptop. Less money and way more functionality.
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Old 11-30-2018, 05:34 PM   #6
oltronj
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Re: TBI wont go to closed loop

http://67-72chevytrucks.com/vboard/s...d.php?t=206478

post number 9
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Old 11-30-2018, 09:15 PM   #7
oltronj
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Re: TBI wont go to closed loop

I dont have a laptop and would have to get one of those, the "scanner" i was looking at is made by 1320 electronics and you just bluetooth it to your phone and it gives you all the readouts on there. It is 75 or 80 bucks I believe on ebay. Here is a link to their website

http://www.1320electronics.com/
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Old 12-03-2018, 02:41 PM   #8
andyh1956
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Re: TBI wont go to closed loop

I got a set up called Turbo Link for the TBI GMs.
It displays a screen on your laptop.
Kinda a "Poor Man's" HP Tuner...
You can find them ever so often around.
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Old 12-04-2018, 06:31 AM   #9
cadillac_al
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Re: TBI wont go to closed loop

A paper clip is all you need.
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Old 12-04-2018, 10:58 AM   #10
hatzie
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Re: TBI wont go to closed loop

You can get working dell D620, D630, D820, & D830 laptops that have RS232 serial ports on evilbay for around $100, sometimes quite a lot less, running Windows XP.
I've seen a lot of cheap or even free Windows XP generation laptops floating around that will work just fine for this kinda thing.
I have an ancient HP NX7400 to do old software and hardware things. It's in the same class as the Dells but it doesn't have an RS232 port... OTOH I already had it on a shelf in the closet.

Those will run TunerPro and WinALDL among other ALDL software packages. Don't put them on the internet. Do the USB stick shuffle from your safe computer.

If you have an Android phone or tablet you can use one of the 1320 electronics bluetooth ALDL interfaces with the various ALDL apps.
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RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.

Last edited by hatzie; 12-04-2018 at 11:03 AM.
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