Got a REAL stumper for the experts

[FbodTrek]

NO the ground issue would affect it in gear while driving. What is your spark KV? = Coil output
I have seen all kinds of issues on these old trucks over the years, distributor pick up coil is another issue. Find the Tan wire w/white stripe (I think) on the firewall single wire with an inline connector, unplug the wire (that will put the system into open loop base timing) how does it respond.
Don't forget to check the throttle body vacuum fittings for being clogged with carbon, do you have good vacuum to the MAP sensor?
Does propane enrichment stablize the idle?
Check the throttle shaft where it goes into the body, they wear and can cause vacuum leaks large enought o lean out. I used to have a kit to install bushings in them.

With the Brown tan timing wire unhooked, it goes to 0*, power doesn't return as I'd imagine it wouldn't with no advance. Advance works normally with it connected.

Propane enrichment DOES stabilize the idle, same as the effect when unplugging the 02sensor.

Shaft is in good shape, really the whole TB unit is, it was the first place I looked as I was convinced it was plugged passages or a vac leak when I first got it.

 

[ne ga pappy]

If it stabilizes with propane, then it can't be a timing or ignition issues... gotta be fuel. hmmmm....

 

[FbodTrek]

Oh, and for those asking about Codes, the Only code it gives off is code 12, which IIRC is "it's all good" code

 

[FbodTrek]

If it stabilizes with propane, then it can't be a timing or ignition issues... gotta be fuel. hmmmm....

Yessir, everything points to lack of fuel at all states. Regulator seems to be functioning fine as the pressure doesn't drop going to the unit under any load

 

[Constitution Hour]

With the Brown tan timing wire unhooked, it goes to 0*, power doesn't return as I'd imagine it wouldn't with no advance. Advance works normally with it connected.

Propane enrichment DOES stabilize the idle, same as the effect when unplugging the 02sensor.

Shaft is in good shape, really the whole TB unit is, it was the first place I looked as I was convinced it was plugged passages or a vac leak when I first got it.

I would not expect increase in power with the est in bypass, how does it idle? Remember incomplete combustion can mimick a lean condition, if the o2 sensor is seeing a rich condition due to poor combustion/timing.
Check the timing at idle and 2000 rpm with the est wire disconnected. The timing should NOT advance, also check ac voltage at the pick up coil it should be .300 or higher most good ones will clock in around .650-.750 anything below .300v ac is bad.
I am willing to bet you have a bad pickup coil in the distributor. Open loop is enough fuel enrichment to mask the underlying issue,once it is in closed loop the ecm is leaning out the fuel mixture casuing the symptoms, the problem I suspect is poor/incomplete combustion.

 

[FbodTrek]

I would not expect increase in power with the est in bypass, how does it idle? Remember incomplete combustion can mimick a lean condition, if the o2 sensor is seeing a rich condition due to poor combustion/timing.
Check the timing at idle and 2000 rpm with the est wire disconnected. The timing should NOT advance, also check ac voltage at the pick up coil it should be .300 or higher most good ones will clock in around .650-.750 anything below .300v ac is bad.

Will check the timing now at 2k rev DECON

Would the plugs show lean under an incomplete combustion (dry lean?)

I'll check the PU coil for sure, hadn't thought of that yet as i've been stuck ina brain loop thinking it's fuel related.

BBshortly

 

[devin25gun]

Fuel pressure sounds about right before the vortec motors started which was around 60 Sounds like a o2 sensor which will cause a lean conditon at idle and surging and stalling. Could also be a worn distributer shaft causing erratic timing. Also check to make sure the base idle is set with the iac and tps. Theres a base setting procedure under chevy tbi idle reset. If it isn't set correctly it can't read the butterfly opening in the tbi and the computer adjusts the idle up and down. Good luck

 

[ShawnZR-1]

Shaft is in good shape, really the whole TB unit is, it was the first place I looked as I was convinced it was plugged passages or a vac leak when I first got it.

Wait, is this a new TB? Did you swap/replace the TPS?

 

[jaybb]

I had a similar problem on a Chevy Tahoe. Took it to the dealer and had Throttle Body Cleaned.$100 and it was fixed.. My symptoms were: truck battery voltage would drop when I applied the brakes.. Truck ran great on the gas but idled as if it wanted to cut off.. Happened a couple of times thru the years I had it.. Each time I had the throttle body cleaned and it fixed it. Not sure, just giving you some options

 

[Constitution Hour]

Will check the timing now at 2k rev DECON

Would the plugs show lean under an incomplete combustion (dry lean?)

I'll check the PU coil for sure, hadn't thought of that yet as i've been stuck ina brain loop thinking it's fuel related.

BBshortly

Yes the plugs could look white as snow with incomplete combustion, Ecm will shorten injector dwell time to lean it out causing an actual lean condition.
When in open loop,no fuel control full rich=smooth/er idle.