TBI issues

Tiny
JEEPSTER1975
  • MEMBER
  • 1987 GMC C2500
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 155,000 MILES
GMC V2500, 5.7 TBI. Starts, idles and runs okay at low RPMs, but will hesitate and cut out under load or sustained off idle RPMs. If someone holds it at a sustained fast idle, roughly 2,000 RPMs for me, I can see both injectors momentarily stop spraying fuel, thus causing the stumble. New fuel pump, filter and can of seafoam. Any ideas?
Thursday, February 10th, 2022 AT 12:14 PM

8 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Okay. That is good info. So at least we know the injector cutting out is the issue so now why is it cutting out?

We can assume it is not fuel pressure since you replaced the pump and filter, so we need to move over to the driver circuit.

We need to test the injector to see if we have a wiring or ECM issue and it is cutting out because it is not being grounded.

Here is a guide that will help with this:

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

Basically, we need to use a test light or injector tester to find out if we are losing ground through the ECM when the injector cuts out.

Please see the wiring diagram below and let me know if the light is flickering when this is cutting out.

Thanks
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Friday, February 11th, 2022 AT 2:56 PM
Tiny
CARADIODOC
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Hi Kenny. This might save some time. Had a friend with the same issue. Once he saw the injectors stop spraying intermittently, he found the two terminals in each cap/connector had to be picked to tighten them for a better connection. Funny thing is he had a very misleading clue. He searched for this for over a year and a half as it only acted up in very humid weather when trying to accelerate. Always idled fine.
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Friday, February 11th, 2022 AT 3:18 PM
Tiny
JEEPSTER1975
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Thanks. Just to clarify, both of its injectors at the same time losing spray. So, my assumption would be that it is whatever wire or connector that triggers both. Correct?
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Saturday, February 12th, 2022 AT 5:36 AM
Tiny
CARADIODOC
  • MECHANIC
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You're right; I'm probably wrong. This occurred on a friend's Astro Van with a 4.3L engine, but as I recall, only one injector quit at a time. When Keeny K continues, he's going to look for what both injectors have in common when they both quit at the same time. If that is fuel pump / supply-related, the fuel spray will drop off relatively gradually as fuel pressure bleeds off. This could also be sensor-related, in which case the spray would quit instantly as though a switch had been flipped.

Next, if you look at the very top of the diagram he posted, both injectors are fed from the same 10-amp fuse. A poor connection there, up to the splice, would affect both injectors, so that's a good suspect too. After that, each injector has its own driver circuit in the computer. It's not likely both of those would quit at the same time. Here's where what the two driver circuits have in common would-be better suspects. That could be a timing sensor input or some of the circuitry inside the computer. In most cases, a failing timing sensor, (crankshaft position sensor or camshaft position sensor), will also cause loss of spark and injector pulses at the same time. All you can see visually is the loss of injector pulses. You might consider connecting an inductive pickup timing light to a spark plug wire to see if spark is being lost at the same time. On newer models, loss of both spark and injector pulses cause around 95 percent of crank / no-starts or intermittent stalling. Loss of just spark or just fuel accounts for only the other five percent.
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Saturday, February 12th, 2022 AT 6:12 PM
Tiny
JEEPSTER1975
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Wow. Great feedback guys. Really appreciate it.
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Sunday, February 13th, 2022 AT 6:34 AM
Tiny
KASEKENNY
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Yeah. Thanks Randy. I think you are on the right path with this.

To address the thing that they have in common, that is the ECM. So, if the injectors are both operating the same, and you prove with a test light that the light is flickering the same as the injectors are then we know the ECM is most likely the issue.

The only issue with this is the ECM has separate driver circuits for each injector. As Randy said, they are fed by the same power source, but they are grounded separately through the ECM. It still can be the ECM, but I would not replace it based just on this info. I would test this with a test light and get more proof on it.

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

Let us know what you find because if this is not just an injector issue then as Randy said, we have other issues.
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Sunday, February 13th, 2022 AT 6:50 AM
Tiny
JEEPSTER1975
  • MEMBER
  • 4 POSTS
Found it! Loose wire in the connector for the air pump control valve. Before I broke out the meter or a test light, I just wiggled and checked all wires, connectors and fuses while it was idling. Moved the wire for the air pump valve and the truck died. Had my son hold it in place when I restarted the truck and revved it up, and shazam, problem solved. I'm assuming it was just loose enough to stay connected at idle, but vibrations from movement and rpms wiggled it loose on and off. Still have a few other "tune up" things to chase to smooth it out, but it no longer cuts out and stops spraying. Thank you!
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Sunday, February 13th, 2022 AT 3:40 PM
Tiny
CARADIODOC
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Dandy. I'd like to know more about this so I can add it to my memory banks.
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Sunday, February 13th, 2022 AT 4:05 PM

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