TBI miss at idle, what am I missing?

Truck 94' GMC Yukon, It has had a miss at idle for years, runs perfectly at any rpm above Idle. Every year or so, I attempt to fix it.

I launched a "Special Vehicle Operation" against this time. Replaced every possible problem. I've cleaned and resealed the TBI, new distributor, new 8mm plug wires with ceramic boots, new coil, new properly gapped plugs, new O2 sensor, new vacuum lines, new ecm ground wire, new PCV valve and grommet.

I have also set timing to 0* with the advance wire unplugged, reset the base Idle(IAC) to 600rpms following the proper procedure. Literally all sensors have been replaced in the past year. EGR is 2 years. Fuel pressure is 11-12 psi steady

Idled smooth as silk for about 2 days, now it randomly misses yet again. Its never stalled, but its infuriating.

Anything else I can try?

edit: TPS, IAC, coolant sensor are around 3 yrs old now. At the time these were replaced, they had no affect.

Following as I have the same dang problem

1. Have you verified that the indicated TDC is ACTUAL TDC? The damper ring hasn't slipped, the timing pointer isn't bent?

2. Connect a scan tool, look at the data stream. Replacing all the sensors is crazy. I'd be looking at sensor signals to see if they make sense. A sensor can work perfectly, and the computer can still get bad signal if the wire harness is damaged. What are the short- and long-term fuel trims at idle and off-idle? Spark advance works? O2 cross-counts?

3. I have heard that fueling problems can happen when the intake passages and intake valves get covered in carbon. I'd be trying some Techron Complete Fuel System Cleaner in the gas tank, and about a gallon of water drizzled as a "urine-stream" alternately between the two throttle plates of the TBI unit; with the engine at 2000+ rpm. In fact, I dump a 20 oz bottle of Techron in the gas tank at every oil change. Buying Top Tier fuel is also strongly recommended. Top Tier fuel has added detergent to keep the fuel system clean.

4. When MY '88 K1500 starts idling rough, it's time to replace the intake gaskets. I'm on the third set with this long-block. But this is likely due to a poor-fitting intake manifold which isn't a common problem. I have photos but not on this computer. I'll have to post 'em later.

I dont have a scan tool, I have a couple of mechanic buddies that are looking for one, nobody seems to have OBD1 scanners anymore.

I dumped some fuel cleaner in my last tank, sprayed water into the intake last year, didnt seem to have much effect. Ive done that on carbed vehicles in the past, and usually get a huge puff of smoke.

Also, my mileage is down, 12.8mpg on last fill. Normally run 15-17mpg.

As I have no scanner, I cannot verify very much, but the timing tab does look normal. When I reset the timing with the advance unplugged, it stayed very steady, hopefully indicating a healthy timing chain.

Check out the actron cp9690, obd1 and 2.

I know you said you replaced everything but, with what brand/quality did you use? I had a miss at idle when it ran great otherwise. I sprayed water on the coil, while idling, and the engine almost died. The coil was a Pertronix Flame Thrower and with only ~2000 miles on it. Swapped it out and it's been running fine ever since.

New parts aren't always good parts.

If good with laptop you can download Tunerpro for free/donation. Good for data logging if you invest the time to learn it.

I agree with above that data logging is a must to sort out what is going on with these old TBIs. Too many variables to hunt and peck

Looks like I need a scanner, as far as the coil, I replaced that after I noticed some corrosion on my ACCEL coil I had on it, think the new one is "standard motor products". Made no difference, although I did clean up the coil mounting bracket and put some di-electric grease on it, so good on that. Not a single miss now for 3 days, then its started again as I pulled into my driveway.

On the hunt and peck side of things, I had a flakey knock sensor throwing bursts of knock counts causing problems once. Found it by the data stream though. On mine, the port in block was also gunked up so knock sensor wasn’t in the water jacket.

Use an ignition scope or do a cylinder drop test to determine which cylinder is misfiring. If it’s just one, focus on that. With the money you’ve spent in random parts you could have paid for an hour or more of diagnostics.