'91 305 TBI stalled out, got towed, never started again

[TheAutomaTom]

1991 Chevy C1500 Scottsdale, 305 5.0L TBI, standard tranny, 309k miles, super fun to drive.
Backyard find. Major issue was distributor, but replaced a bunch of parts because everything looked crummy and I wanted to be confident it could go.
A few months of driving her around with no issues, it stalls out as I pull into a parking spot. It turns over but does not start. Ended up getting towed back to the house.
Educated guess made me feel like the timing chain may have had a hiccup, so I picked up an OEM replacement. The old chain was a little sloppy, but this did not solve my issue.

One quirky thing I found, it started at 30' ATDC with the timing advance unplugged (like when you want to set the timing). I could even tune it down to the 0' spec and it idled well. However, after the motor was turned off, it would not start again with the timing set there, or anywhere under 30' ATDC. With the timing advance plugged in, it would bounce around 50'.

No progress has been made, and as of today it no longer starts in any circumstance. The following is my present diagnostics report and history firing the parts cannon...

November to December 2023
Distributor w/Ignition Module
Spark plugs
Plug Wires
Coil
Idle Air Control
MAP Sensor
EGR Valve
PCV Valve
Fuel Filter
Oil Pressure Sensor

May to July 2024
Timing Chain Set
Throttle Position Sensor
Knock Sensor
Fuel Regulator guts replaces (was super gross)
Replaced the Distributor (it was under warranty)
Tried a new Electronic Spark Control
Tried a new ECU, although it did not come with the 2 chips

Recent Diagnostics
Compression test looks good
Each plug has spark
Literally have pulled the timing cover off 3 times to make sure the marks line up. Nothing indicates the mechanical timing is an issue
With an electrician buddy, a lot of the wiring harness has been checked for crossed/ pinched lines from the ECU.
Map sensor checks out per some online voltage specs
Noid light blinks on the injectors
Fuel pressure checked before and after the filter hits 15psi, and bleeds off in about 30 seconds.

So, what's left to do? I could get an Ebay ECU for $100 that comes with some chips to try out? I could convert this thing to a carburetor (worst case scenario on the wallet)? Any advice?

 

[docstoy]

Do you have fire/spark when tested/cranked using spark tester for hei ignition ? Do you have fuel coming from injectors during crank ? Have you downloaded fuel and emissions service manual from sticky thread above it is titled 88-98 service manuals. Look in symptoms that will help steer you the way you need to go , as it looks by the amount of parts you've thrown at this truck. Not trying to be SA but SWAG parts replacement is expensive. Look thru the f&e manual for your truck it will help you and if any questions someone here will help answer them. Best of luck

 

[TheAutomaTom]

"Do you have fire/spark when tested/cranked using spark tester for hei ignition?"
Yes

"Do you have fuel coming from injectors during crank?"
Yes

I will check out the service manual, thank you for pointing that out to me.

Swag'n & grin'n,
Tom

 

[WICruiser-97]

It sounds like an ignition timing issue as you say you have compression, fuel and spark which only leaves the timing of the spark.

Given your previous timing strange issues I am guess that the distributor gear is not rotating the distributor consistently. I would pull the distributor and look at the condition of the gear and the gears attachment to the shaft.

 

[docstoy]

Ano5her thought when you had timing marks aligned on chain /gears did you look at distributor cap/ rotor to see if were also in correct position could be tooth off by what your saying on timing ck. I'm talking about distributor since you have it at 0 degrees and would only run at 30 degrees. Thinking distributor gear has 12 teeth (12 ×30 = 360)

 

[TheAutomaTom]

I can add pics later, but yes:
Timing marks went on just like the old gears (stamped GM) aligned at 12 o'clock with cylinder 1 peaked on compression stroke. I used where that rotor was pointing to start with the plugs firing order.

That new distributor has since been replaced, and the symptom has not improved. I know there are gears, but the inner part of that rod is like a flat head screwdriver, so it only goes in 2 ways. Spinning it 180' makes no difference (provided the plugs are also rearranged).

Again... This truck ran great for 6 months, then just stalled out and quit, with no weird noises or anything.

Thank you for taking a swing!

 

[TheAutomaTom]

"...only leaves the timing of the spark."

I would love to hear from someone who has experienced symptom where "motor only starts with timing advance unplugged." That was the main, unique thing, for a while.

Unplugging that wire is part of the process of setting base timing. Plugging back up means that the ECU takes over timing after 400 rpm. I can't find an issue with the physical wiring. A replacement ECU didn't impact the issue, and everyone says the 2 chips basically can't go bad... But wtf, right?

 

[docstoy]

The inner part of distributor shaft which is flat, slips into the oil pump drive. If having problems installing rotor in position that you would like simply use a flat tipped screwdriver (long in length) to rotate shaft for easier distributor installation

 

[TheAutomaTom]

IDC which way the rotor wants to face, because I stuck the #1 spark plug where it points at TDC.

 

[Schurkey]

1991 Chevy C1500 Scottsdale, 305 5.0L TBI, standard tranny,

A few months of driving her around with no issues, it stalls out as I pull into a parking spot. It turns over but does not start... ...feel like the timing chain may have had a hiccup, so I picked up an OEM replacement. The old chain was a little sloppy, but this did not solve my issue.

Did you verify True TDC and compare to the timing mark on the damper?

What is the cranking compression of all eight?

it started at 30' ATDC

ATDC not BTDC?

With the timing advance plugged in, it would bounce around 50'.

50 degrees ATDC or 50 degrees BTDC?

No progress has been made, and as of today it no longer starts in any circumstance.

Has it run poorly enough, and sprayed enough fuel without starting, to foul the spark plugs?

Compression test looks good

WHAT ARE THE NUMBERS?

Each plug has spark

Spark TO the plug, or spark across the plug gap?

So, what's left to do?

All the above, plus connect a scan tool. Look for "codes", but more important, look at the data stream.

Timing marks went on just like the old gears (stamped GM) aligned at 12 o'clock with cylinder 1 peaked on compression stroke. I used where that rotor was pointing to start with the plugs firing order.

With both gears having the marks at 12:00, the pickup coil teeth should be exactly aligned, and the rotor points to #1 terminal on the cap.

"...only leaves the timing of the spark."

I would love to hear from someone who has experienced symptom where "motor only starts with timing advance unplugged." That was the main, unique thing, for a while.

Can be an ignition module problem. Could be an ECM problem, or a wire-harness issue.

A replacement ECU didn't impact the issue, and everyone says the 2 chips basically can't go bad.

ANYTHING can "go bad".

But it is unlikely for those two ICs to have problems...unless you touched the pins in the process of switching them from one ECM to the other.

IDC which way the rotor wants to face, because I stuck the #1 spark plug where it points at TDC.

Playing Pin-The-Tail-On-The Distributor by shifting plug wires from where they should be to where they align with the rotor is poor practice, but it can work.

"I" would put it together the way it's supposed to be.