88 TBI injectors pulsing after motor dies

Wowza! Lots of info all at once! I really do appreciate the help. I'm a very visual learner/teacher, so the video is my way of telling y'all what exactly the problem is without trying to explain just how the flux capacitor is fluxing lol. I also want to learn and do it myself instead of taking it somewhere and telling some random dude to just "fix it".

Anyway.....

This truck is bone stock, the radio gave out 20 years ago and we didn't even replace that. My grandpa bought it in '92 and put 160,000 miles on it, he died in '14 and it's only had about 7,000 miles since then. He was a mechanic for 40 years and took VERY good care of the drivetrain.... the body is another story, but not important...... now I own it and use it for odd jobs here and there.

To answer your question:

Q: Can this happen at any time, hot or cold? Or does this failure only happen once everything is fully up to temperature?

A: this can happen whenever. I started and drove it (cold) about a half mile yesterday and it quit on me. After monkeying with cleaning everything, I drove it around the block (about 3 miles) and brought it back to idle in the driveway. After 5 or so minutes of idle, it quit again.

Q: is there anything aftermarket?
A: No, besides the cat being removed last summer. It was wasted along with the old Y-pipe, so I replaced the Y and put a straight 3 inch where the cat was. Seen about 2,000 miles since then.

I tried unplugging the brown/black EST wire and start it up, no such luck without it dying instantly.... I will say though that today is the first time the issue is present at startup, it will usually happen after leaving the driveway.

Moving on to checking connections and grounds.

Thanks again for all the info thus far!!!!

Just for fun, I wanted to see how long it took again to act up. At first startup it wanted to idle down and die, but I got it to stay alive. Idled about 10 minutes before it died. I pulled the TPS plug as it was starting to die and it made no change. This time it seemed like on the the passenger injector was firing after stall. That was the only one shooting the excess fuel into the throttle body anyway.

P.S. it was idling indoors with the hood up, thermostat never opened.

14reamdalton said:

This time it seemed like on the the passenger injector was firing after stall. That was the only one shooting the excess fuel into the throttle body anyway.

Click to expand...

I'd be checking the wire going from the ECM to the injector. It won't spray unless it's grounded. The injectors are fed 12VDC all the time, the ECM grounds them when it wants them to fire.

14reamdalton said:

Just for fun, I wanted to see how long it took again to act up. At first startup it wanted to idle down and die, but I got it to stay alive. Idled about 10 minutes before it died. I pulled the TPS plug as it was starting to die and it made no change. This time it seemed like on the the passenger injector was firing after stall. That was the only one shooting the excess fuel into the throttle body anyway.

P.S. it was idling indoors with the hood up, thermostat never opened.

Click to expand...

Sometimes when 'power' electronics goes bad it will break into oscillation instead
of just suddenly quitting. I have a couple of questions:

Q1: Once the engine stalls and the injector(s) are still chattering, if you turn the
key from ON to OFF and back to ON again, will the chattering continue, or will
it cease?

Q2: Assuming that the chattering in Q1 continues across a power cycle, I would
be interested if you could do the following: Malfunction occurs, engine stops,
injectors still chattering, turn key to OFF, pull the plug to the ICM, turn the
key back ON, and then what happens? Our 2 choices would be:

* Chattering stops. Assumption that ICM was the one that broke into oscillation
and was sending spurious signals up to ECM.

* Chattering continues. Since there's no stimulus from the ICM, this would tell
me that the ECM is the component that is breaking into oscillation?

Disclaimer: The GMT400 in my driveway is a Vortec, which is different than a TBI setup. So
I'm having to rely purely upon the TBI section in the older FSMs for troubleshooting approaches.

But the more we learn about your failure before you replace a part the more info that
others can learn from. (In English, I am living vicariously through your real-world
troubleshooting. :0) Seriously, Schurkey's comment about another member having
a similar issue and it was the Ignition Control Module makes sense to me.

Anyway, I would be interested if cycling the power changes the behavior, or does the
suspect circuit just break into oscillation and stays out of control for some period of
time before recovering? BTW, I appreciate the fact that you are trying to take the time
to understand this instead of just shoveling parts at it.

Road Trip said:

Q2: Assuming that the chattering in Q1 continues across a power cycle, I would
be interested if you could do the following: Malfunction occurs, engine stops,
injectors still chattering, turn key to OFF, pull the plug to the ICM, turn the
key back ON, and then what happens? Our 2 choices would be:

Click to expand...

I'd be looking really closely at that ICM harness, too. It's somewhat common at this point for them to be brittle.

Richard

Quick question from a novice: Is the spray supposed to "pulse"? My TBI system releases a near constant spray/dribble of fuel and seems to run fine. Gets about 10mpg.
Thanks!

OLD BLUE 88 said:

Quick question from a novice: Is the spray supposed to "pulse"? My TBI system releases a near constant spray/dribble of fuel and seems to run fine. Gets about 10mpg.
Thanks!

Click to expand...

Mine also runs pretty constant (or so fast they naked eye can't see it). I am assuming the frame rate of the camera makes it appear to pulse at a lower rate. I did notice that when I took the video, but forgot to mention the camera caused this effect.

14reamdalton said:

Mine also runs pretty constant (or so fast they naked eye can't see it). I am assuming the frame rate of the camera makes it appear to pulse at a lower rate. I did notice that when I took the video, but forgot to mention the camera caused this effect.

Click to expand...

They pulse synchronously, meaning they take turns firing, whenever they get a Distributor Reference Pulse (DRP) to the Ignition Control Module (ICM), to the Computer. So, every time a spark plug fires, one of the injectors fires too. You just can't see them pulse with the naked eye.

PlayingWithTBI said:

…. every time a spark plug fires, one of the injectors fires too. You just can't see them pulse with the naked eye.

Click to expand...

Thought of The Day: Hook up a timing light and strobe the injectors. It’s revealing. Try reving the engine while doing.

IIRC, at least on the OBD1 systems, the injectors pulse in synchrony with the ignition up to a certain RPM, and above they pulse asynchronously. Don't anybody quote me on this, but just be aware the relationship between ign timing and inject pulsing may vary.

OBDII systems are more sophisticated.

OLD BLUE 88 said:

Quick question from a novice: Is the spray supposed to "pulse"? My TBI system releases a near constant spray/dribble of fuel and seems to run fine. Gets about 10mpg.
Thanks!

Click to expand...

Greetings OLD BLUE 88,

A steady looking cone at idle speeds & higher is a normal condition. Any dribbling in the center is a
loss of fuel control. NOTE: If you want to see the alternating triggering of the TBI injectors it's
easiest to see at cranking speeds -- it's obvious then.

I heartily recommend that you continue to familiarize yourself with what the spray patterns look like
when your vehicle is running well so that you have a solid frame of reference if/when fuel delivery
is suspected of causing a no-start or not running properly. (Dating myself, watching countless
accelerator pump shots on good carburetors helped me when I was troubleshooting carbs that
were hard starting, would hesitate when accelerating from a stop, or would sneeze back through
the carb itself. The visual difference between a strong shot vs a weak dribble was obvious...but
only after I looked.)

14reamdalton said:

Mine also runs pretty constant (or so fast they naked eye can't see it). I am assuming the frame rate of the camera makes it appear to pulse at a lower rate. I did notice that when I took the video, but forgot to mention the camera caused this effect.

Click to expand...

When I was watching your video I noticed the visual effect. Back in the day they would refer to the frame rate affecting
the look of a moving object as the 'wagon wheel' effect. (Wiki wagon wheel effect)

Just like the Quadrajet it replaced, the TBI system has a lot of good engineering behind it, and
the more we are familiar with it's normal operation, the easier it will be to know the difference
when something has gone awry.

Now for someone to train their high speed cameras on the TBI setup so that we can see the
alternating injector fuel delivery at speed. (Until the duty cycle turns it into a near-continuous
delivery into a wet intake manifold.)

Good stuff. The more observation & less mystery the better for the TBI wing of the GMT400
community.