94 5.7L Idle Miss/Stall-for the experts

I've seen a ton of posts on similar idle miss and stall problems, but none that seem to nail a common solution:

Problem on a 94 5.7L TBI - Idle Stumble once warm idle is reached (approx 700rpm). Irregular idle stumble is annoying in Park/N, but in gear, when idle is down at 625rpm, the stumble usually cuases a stall. By 'stumble', I mean a 100rpm or so drop, then recovery.
The IAC does seem to react to the 100rpm stumble but not fast enough to keep it from stalling.
Vacuum guage shows the vac drops 1-2in at every stumble, but is a steady 16in at all other times (at 6500' elevation). No pushrod clacks or other noises that I'd associate w/a sticking valve.
I can't seem to correlate the stumble to any other telemetry I see on the scanner, except that the TBI pulse width opens up a bit from (.9 to 1.1ms) with the stumble, seems to be a reaction *to* the stumble rather than the cause. MAP voltage also climbs about .15v at the stumble.

Have verified:
Stumble occurs in both CLOSED or OPEN loop - no difference
EGR is closed at idle (cleaned and monitored) - no change
Other vacuum leaks (PCV hose, EGR vac) were found and cured - no change
IAC cleaned, then replaced - no change
MAP replaced - no change
New fuel filter
Injector flow appears fine, no abrupt change in spray pattern during stumble
TPS telemetry is steady thru the stumble and stall - shows 0% throttle at idle, linear (good) everywhere else
Plugs, cap, rotor, coil all replaced - no change, no plugs showed coolant or other pbms
Coolant temp telemetry is steady thru the stumbles
O2 sensor shows quick response, full range (.2-.9mV range from lean-rich)
No rpm, temp, injector pulse, MAP, TPS, O2 sensor spikes when it stumbles

Oh, when the A/C is requested/on (causes the ECM to bump up the idle a bit), the idle is PERFECT

Still need to verify timing and fuel pressure. My scan tool is good but not a graph-type, so I COULD be missing spikes that occur between the 0.5 sec samples - Any other ideas? Gracias

You don't mention what your fuel trims are at when the miss is happening. I own a 90 GMC K1500 that had a rough idle for quite a while. In my case, BLM (long term fuel trim) would get down to 108. This means that the ECM thinks the engine is running rich so it is removing fuel. This would happen the longer the engine ran.

I found that the PCV valve was not sealing well at the valve cover so I replaced both gromets and the idle quality improved greatly. The engine is tired so the idle is not perfect but a lot better than it was before I replaced the PCV gromet. It felts like it wanted to stall but never did.

I also found that the vacuum hose to the MAP did not hold a vacuum which I replaced at the same time that I did the PCV gromet.

Post your BLM and INT values when the idle problem happens and maybe we can steer you in the right direction.

//2000CAYukon

Thanks 2000CAYukon for responding

My cheapie scan tool (auto xray) does not indicate fuel trim in it's GM program - I believe I only have the pulse width telemetry for the injectors, but I'll check again. Is there another name for the trim, like 'Block Learn'?

PCV grommet is good, MAP hose is good (that's where I confirmed the 16" vacuum, after first seeing it on one of the TBI's front vacuum ports). I don't like the MAP voltage though - Both old and new MAPs show idle MAP voltage at .8v, bumping up to 1.1v during the stumble - my other GM shows 1.5v at idle (4.3l w/CPI, but at the same altitude).....which is why I changed the MAP to begin with - trying to recover the voltage to Chilton spec (1.4v diff betwene key on-engine off vac, and idle vac) - I have abt 2.2v with either MAP - may not be issue at all, though.

Thanks 2000CAYukon for responding

My cheapie scan tool (auto xray) does not indicate fuel trim in it's GM program - I believe I only have the pulse width telemetry for the injectors, but I'll check again. Is there another name for the trim, like 'Block Learn'?

PCV grommet is good, MAP hose is good (that's where I confirmed the 16" vacuum, after first seeing it on one of the TBI's front vacuum ports). I don't like the MAP voltage though - Both old and new MAPs show idle MAP voltage at .8v, bumping up to 1.1v during the stumble - my other GM shows 1.5v at idle (4.3l w/CPI, but at the same altitude).....which is why I changed the MAP to begin with - trying to recover the voltage to Chilton spec (1.4v diff betwene key on-engine off vac, and idle vac) - I have abt 2.2v with either MAP - may not be issue at all, though.

BLM is Block Learn which will be the long term fuel trim
INT (or Integrator) will be the short term fuel trim.

128 is neutral for both. The MAP voltage changing is due to the drop in vacuum that you are seeing.

Does the EGR solenoid hold vacuum (on the TBI side)?

//2000CAYukon

Thanks for the insight - I do have both of those streams on my scanner (always wondered how to use that data), so I'll check later this week when the truck returns. I did not check the EGR solenoid, just the EGR valve itself - that's a good idea.

Would you expect both the long and short term trims to be stable, over time, at 128?

Thanks for the insight - I do have both of those streams on my scanner (always wondered how to use that data), so I'll check later this week when the truck returns. I did not check the EGR solenoid, just the EGR valve itself - that's a good idea.

Would you expect both the long and short term trims to be stable, over time, at 128?

You should take a look at http://www.customefis.com/GMEFI.html which explains BLM and Closed Loop.

What I found with my truck was that at idle BLM was going down over time and as the BLM got lower, the idle would get worse.

My theory was that the leak at the PCV gromet caused the crankcase to fill with unburned gases and the ECM thought the engine was running rich so it leaned it out to the point that the idle would suffer.

I am sure having 127K miles on the engine does not help either and that there is some unburned fuel getting past the rings and/or exhaust valves. In my case the idle was much better after the PCV gromet but still not perfect. I also have some blue smoke (valve seals) at start up (which could also cause the ECM to think it is running rich).

//2000CAYukon

One thing I see you didn't mention is the pickup coil in the distributor. Used to cause some misfire and stumble problems but mostly when wet outside. The cause was the dist. shaft allowing oil to go up and contaminate the coil. They came out with a grooved shaft but I forget what year. Still happened with the grooved shaft but not as much. Look for greenish oxidation on the pickup coil connecter. If so I would replace it.