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TBI not working properly

Tiny
EKECKSR
  • MEMBER
  • 1991 GMC C3500
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 110,000 MILES
Truck listed above Sierra SLE C3500 5.7 TBI. The engine will shut off when both injectors are plugged in. It will start and rev up with one injector plugged in, does not matter which side is plugged in. It will not start unless the gas pedal is held to floor. Both injectors have excellent spray patterns when running or turning the ignition key on. The engine will shut off within 5 to 8 seconds as if it ran out of fuel when the second injector is plugged in.
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Monday, October 11th, 2021 AT 4:29 PM

11 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

Have you checked the fuel pressure to make sure the regulator is working? Too much pressure can cause this. Here is a link that shows in general how to test pressure and the regulator:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Next, have you checked for diagnostic trouble codes? If not, that is the next thing I would recommend. This vehicle has an OBD 1 system which doesn't require a scan tool to retrieve codes. All you will need is a short jumper wire or even a paper clip. Here is a link that explains how it's done.

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Let me know what you find or if you have other questions.

Take care,

Joe
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Monday, October 11th, 2021 AT 8:51 PM
Tiny
EKECKSR
  • MEMBER
  • 6 POSTS
Hi Joe,

Around 4 to 5 weeks ago I replaced the fuel pump which is located in the tank and the fuel filter in addition to flushing tank and lines. Also replaced ignition control module located in distributor. Fuel pressure was fine then, and truck has been running great, use it to trim trees, etc, has a 30' boom. When this issue started several days ago it started immediately and idled great until I started pressing on the throttle. When giving it fuel it began running slower and slower then died. The only way I can now start it is by unplugging one injector, doesn't matter which one, hold the accelerator to the floor and it will start and run great, it will also rev up with awesome power, plug the other injector in and it will stall out within seconds. I have not checked for codes but will in the next several days. Thank you for such a quick response.
Ernie
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Tuesday, October 12th, 2021 AT 5:35 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Ernie,

The idea that you need to hold the throttle to the floor likely indicates the engine is flooding. By doing this, it shuts fuel down. With one injector, you are not getting too much fuel. When you plug the other one in, it increases.

By chance, have you noticed if there is any gray smoke from the exhaust when it finally starts? Does the exhaust smell rich? Also, by chance have you checked fuel pressure to confirm the regulator didn't fail?

Let me know.

Joe
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Tuesday, October 12th, 2021 AT 5:57 PM
Tiny
EKECKSR
  • MEMBER
  • 6 POSTS
I have not checked the fuel pressure since installing the new fuel pump. I really haven't noticed any smoke coming from the exhaust. Recently when driving it before it failed, I did notice an abnormally strong fuel smell at times when pulling a steep incline, loading the engine to its max. I'll re-check it, maybe it is the fuel regulator. I may go ahead and pick one up tomorrow for GP since they aren't that expensive. Thanks again for all your input, this one has me stumped, lol.
Ernie
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Tuesday, October 12th, 2021 AT 6:09 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Ernie,

If possible, let me know what you find. Can't guarantee it's the regulator, but based on the description, it sounds like it's getting too much fuel.

Let me know.

Take care,

Joe
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Tuesday, October 12th, 2021 AT 8:29 PM
Tiny
EKECKSR
  • MEMBER
  • 6 POSTS
Hello Joe,

Overhauled the TBI today which included the fuel regulator. When I started the truck, it ran good for 5 minutes and I shut it off. When I went to start it again it presented all the same symptoms as it had before. I don't think an idle control valve would solve anything. Possible computer?
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Sunday, October 17th, 2021 AT 2:54 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Ernie,

Have you checked to see if there are diagnostic trouble codes stored?

Joe
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Sunday, October 17th, 2021 AT 9:22 PM
Tiny
EKECKSR
  • MEMBER
  • 6 POSTS
I did this afternoon. Code 15 is showing which is a Temp Sensor. I did try and start it and it ran good until I bumped the pedal to slow it down and of course it shut off. I'll get a sensor and see what happens.
Thanks
Ernie
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Monday, October 18th, 2021 AT 12:18 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

I need you to check something. Disconnect the temperature sensor. Use your multimeter to see how much resistance there is. Then based on the ambient temperature, see if the ohms align with the scale below.

I have a feeling, based on the ECT sensor's signal, the computer may be overcompensating for fuel. By disconnecting the one injector, you are cutting the supply in half.

For example, if the ECT is indicating it is -40F below zero and it's 80 degrees outside, the computer would add fuel for the engine to run. It would be too much and cause flooding. If the ECT is indicating it is 210 degrees and the ambient temperature is 50, it will want to lean things out. It's only a theory at this point.

Let me know what you find.

Joe

See pic below.
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Monday, October 18th, 2021 AT 6:02 PM
Tiny
EKECKSR
  • MEMBER
  • 6 POSTS
Thanks for all your input. I picked up an ECT sensor for $17.00 today and installed it. Truck has never run better, it is a dually bucket truck with a utility bed and actually spun the duals, briefly of course. For 30 years old I've only had to do minor repairs other than a transmission this year. Let's hope it runs awhile. Thanks again, until next time!
Ernie
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Tuesday, October 19th, 2021 AT 2:16 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi, Ernie,

So, it was the sensor. That's good to hear. Interesting how a simple part like that can cause so many problems.

Regardless, I'm glad to hear it's running well for you again. Also, please feel free to come back anytime in the future if you have questions or need help. You're always welcome here.

Take good care of yourself.

Joe
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Tuesday, October 19th, 2021 AT 6:28 PM

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