Will start randomly and also doesnt start randomly

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eddie1712

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Hello everyone hope everyones doing good, so recently ive started to develop an issue where my 94 c1500 5.7 will randomly not start in the mornings and then will randomly not start after normal trips around town. Ive read around other threads and hoped it was just a bad coil and or bad icm causing this but after replacing both things she continues to do the same thing, when cranking the injectors also do not fire or pulse but I have a brand new fuel pump and filter which have 4ish months on them, I also do get spark but my injectors wont pulsate but then again I thought that was due to a bad icm. Any thoughts or suggestions would be amazing
 

Road Trip

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Hello everyone hope everyones doing good, so recently ive started to develop an issue where my 94 c1500 5.7 will randomly not start in the mornings and then will randomly not start after normal trips around town. Ive read around other threads and hoped it was just a bad coil and or bad icm causing this but after replacing both things she continues to do the same thing, when cranking the injectors also do not fire or pulse but I have a brand new fuel pump and filter which have 4ish months on them, I also do get spark but my injectors wont pulsate but then again I thought that was due to a bad icm. Any thoughts or suggestions would be amazing

Greetings eddie1712,

First of all, welcome to the GMT400 forum!

That's an interesting problem description, and it sounds like you are barking up the right tree.
In order to get your TBI fuel injectors to dispense fuel, we need the following things, working
from the injectors back:

1) The injectors can be cycling, but if there's no fuel pressure ahead of the injectors, nada, no joy.
Turn the key to the ON position, and now listen carefully for the timed 2-second fuel pump operation.
Can you hear it? Assuming the pump ran, continue on to the START position and does the engine fire up?

If there are any issues with step #1, we will need to observe the fuel pressure during the failure. (And hopefully
also during good operation so that the differences in behavior are easy to tell apart.)

2) Assuming you have the specified fuel pressure at all times during step #1, (either start or no start) then
we are now troubleshooting from the perspective of "good fuel pressure, but no injector pulsing."

The electrical control path will be: Injector coils < PCM Injector Control circuitry < Ignition Control Module < Pickup Coil in distributor.
This control signal is implemented by the PCM grounding injector #1 via pin A16 or injector #2 via pin A9. No grounding = no current
flow in injector coil = no plunger/ball valve movement. (See attached)

And on the other side, the (hopefully) constant power source for this circuit is via the 10 A INJ fuse, which in turn is fed by the Ignition Switch.

That's the entire circuit. To verify my description, check out p. 3A-22 of the '94 Driveability & Emissions Electrical Diagnosis Manual:

You must be registered for see images attach


So again, from the symptoms you described I would concur that the Ignition Control Module and the coil are
in the middle of the suspect circuit. Assuming that the new parts are good, here are the possible scenarios
to choose from:

* Ignition Control Module is a victim of a missing input signal from a defective pickup coil. @Schurkey has a
great picture of how they physically go bad -- possibly he could add his picture to this thread?
Result: ICM does not generate/send the all-important 'Reference' pulses to the PCM = no injector pulses.

* Pickup Coil and ICM are good. (aka: Good distributor.) But wiring between dizzy and PCM is marginal, sometimes
just good enough, other times not. Especially if truck starting is influenced by a change in weather. Visually inspect
every inch of the wiring & connectors on both ends of the wires identified above.

* All preceding good, but coil drive (in ICM) and/or coil itself are flaky. Cross-check is that IF no start was only after
hot soak, but cold starts are always OK? But you are experiencing both hot & cold no starts, plus we have new part
substitution with no change in behavior.

* Once you have carefully inspected the wiring between these parts, assuming all is good, then now we need to drive
to the ends of each side of this circuit. Both hot & cold no start (plus otherwise good operation once started) makes
me not want to check the injectors at this time. On the other hand, how new is your ignition switch? Or possibly a
better question is how many total cycles has your ignition switch accumulated since it was placed into service?

****

There you have it. As a troubleshooter it's always a good first step to identify all components in the success chain
of an intermittently failing function. But the real magic is to take the entire list of components involved and relist
them in order from highest probability to lowest probability.


Now we have to stop here and add in one more detail. According to the FSM, the ICM is responsible for generating/driving
the coil with strictly base timing from 0 to ~400-450 rpm. Once the rpm goes above this threshold (ie: the engine starts)
the ECM takes over, adding value via carefully computed ignition timing. So the fact is that if you are checking for
sparks at cranking speeds and getting them this doesn't totally absolve the PCM.

But pulling this all together, let's see if we can't build a big picture, troubleshooting-wise.

Assuming good sparks 100% of the time spent cranking, we need to verify fuel delivery.

1) IF the PCM is driving the fuel pump for 2 seconds when the key is turned to ON, good. Otherwise,
stop here and figure out why the fuel pump is intermittently not being driven for the priming cycle?

2) Now for the fuel pressure check during a successful 2-second fuel system prime. Is it correct?
If not, stop here and fix any/all fuel pressure issues.

3) OK. We have fuel pressure, but we now need to drive the injectors with timed millisecond current pulses.
We need solid 12v from the Ignition Switch through the 10A INJ fuse, and of course good wiring all the way
to the injectors. And on the other side of the injectors we need the PCM to be grounding each injector for
the proper amount of time in order to sustain combustion. You may need a noid light (or a scope) in order
to see the injectors being driven properly?

But as the old salts used to tell me, "Check the obvious before going for the exotic." Fuel pump running
for 2 seconds? Fuel pressure good? Ignition switch still capable of supplying a solid 12v every time it's
cycled by the human? Only after all these have been proved good do we start to suspect the $$$ PCM.

****

The above is just food for troubleshooting thought. Which reminds me, if you haven't already done so,
go where I did and get yourself a copy of the '94 Factory Service Manuals. One of the strengths of this
forum is that thanks to the efforts of others we have links to download these manuals. Used for all
they are worth, these FSMs remove 98+% of the guesswork out of keeping these old war horses on
the road & earning their keep. Check over here: ('88+ GMT400 FSMs)

Apologies for the length of the above, but the more intermittent the problem, the deeper the dive required
into the theory rabbit hole. The diagram above is your friend.

Happy hunting, and when you discover the root cause please come back and close the loop with your fix,
for this will help others researching similar intermittent no-start failures by giving them one more proven
solution to consider.

Cheers --

EDIT: Attached TBI injector cutaway. credit: Hemmings TBI article: TBI design history & theory
 

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  • TBI Injector cutaway (arrows) - Chevrolet TBI article - The Online Automotive Marketplace  Hem...jpg
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docstoy

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Another thought, before problem started happening, did you or a service center do any maintenance or parts replacement ? Thought being if work was done in tight or confined area then wiring/connectors may have been moved around or pulled that could have caused intermittent connections or connectors not being latched/connected as they were or should be.
 
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