Author Topic: Obd1 code 21 on 1987 tbi 350  (Read 610 times)

Offline lehman368

  • Newbie
  • Posts: 20
  • Newbie
Obd1 code 21 on 1987 tbi 350
« on: May 08, 2019, 11:20:51 PM »
My truck ran fine for two weeks after getting out of storage then I got a code 21 fought. So then I replaced the tps sensor. It ran fine for about 3 weeks then back to the way it was when I got the 21 code. It has a longer cranking start up. then fires and starts up also fine isles fine for a while then some times idle will raise and lower. Not all the time. Drive ok until you get to 35-45 -50 mph continuous at that speed then you can feel either engine up and down or slight miss or transmission has a slightly chugging. I do not have a tac in the truck. It Feels almost like you have a tire out of balance only at those speeds then increasing speed runs fine.But Then you come to a stopped slow down and the engine just stumbles like it wants to stall. Few times it did but will start or fire right back up quick start up. When running ruff You feel the need to drop it in into neutral and press the gas to keep it running while stopped. Then you can put back into drive and it takes off but with a slight hesitation. Runs decent as long as your pressing the gas peddle. Then again if you get to that consent 35-45-50mph it just feels like engine has that slight miss. It seems to Accelerate decently straight through past the slight hesitation to 5- 65-70 mph. If anyone has any input on trouble shooting is dilemma Iím having I can use some help solving this issue. I do have GM service manuals in hand havenít come across any thing that is helping. I replaced Throttle position sensor twice. It looks to me like there is no adjustment in the hole position on this years type sensor.
Thanks for any help. Love the truck and want to drive it. I had it to a local non GM garage to help in troubleshooting but no help to me. Truck has 86k on it and ran great the last 3 years. Just started this when I got it out of storage this spring.  I Replaced fuel filter new distributor cap button wires and ignition control module and temperature sensor last year. I put fuel treatment in the tanks before I stored it for winter and have ran more then three tank of fuel out in each tank. Thanks for any help Tom


Sent from my iPhone using Tapatalk

Offline Irish_Alley

  • Tim
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 13269
  • Family is not an important thing. It's everything.
Re: Obd1 code 21 on 1987 tbi 350
« Reply #1 on: May 09, 2019, 12:51:22 AM »
the symptoms you describe are consistent with a bad tps or a bad signal from the tps.
take a read here on how to test the tps
http://forum.73-87chevytrucks.com/smforum/index.php?topic=29996.0

if that doesnt turn out you also might want to try to find someone with a code scanner that can read live data on a OBD1. i know snap on made one years ago and its most commonly called "the brick". great tool to have and you can read your tps signal as the ecm sees it.

other things to try is checking your ground wires/cables. since the problem cleared up with a new tps for 3 weeks i would believe that would again be your issue so i would start there (ive seen new parts be bad right out of the box and some only last weeks)
If you canít tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes

Offline lehman368

  • Newbie
  • Posts: 20
  • Newbie
Re: Obd1 code 21 on 1987 tbi 350
« Reply #2 on: May 09, 2019, 01:06:24 AM »
the symptoms you describe are consistent with a bad tps or a bad signal from the tps.
take a read here on how to test the tps
http://forum.73-87chevytrucks.com/smforum/index.php?topic=29996.0

if that doesnt turn out you also might want to try to find someone with a code scanner that can read live data on a OBD1. i know snap on made one years ago and its most commonly called "the brick". great tool to have and you can read your tps signal as the ecm sees it.

other things to try is checking your ground wires/cables. since the problem cleared up with a new tps for 3 weeks i would believe that would again be your issue so i would start there (ive seen new parts be bad right out of the box and some only last weeks)




Sent from my iPhone using Tapatalk

Offline VileZambonie

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 17706
Re: Obd1 code 21 on 1987 tbi 350
« Reply #3 on: May 09, 2019, 06:11:05 PM »
Follow the published diagnostic system test
,                           ___ 
                         /  _ _ _\_
              ⌠ĮĮĮĮĮ'   [☼===☼]
              `()_);-;()_)--o--)_)

Offline lehman368

  • Newbie
  • Posts: 20
  • Newbie
Re: Obd1 code 21 on 1987 tbi 350
« Reply #4 on: May 12, 2019, 09:57:52 PM »
Yes we probed the TPS sensor it looked like we may have had a bad wire in the connector plug replaced the plug ran like a champ for two  days then started acting up again. I looked at the grounds some what but didnít remove or take apart. Which ground should I look at the one one the fire wall or the cowl support or both


Sent from my iPhone using Tapatalk