1991 Chevy Suburban with an odd miss/stumble at low throttle

To keep this tractor related, this is the vehicle I use to pull my 1964 Ford 4000 4 cylinder tractor.

This vehicle is my daily driver, a 1991 Chevy Suburban with a 5.7 (350) throttle body injection with 4L60E transmission. It has the stock distributor, coil seperate, not on top of distributor cap. The TBI unit (NOS) was replaced with the rebuild several years ago. It got a new Chinese distributor with the rebuild, but with a GM module.

It has a bone stock engine, a Jasper rebuild, and with about 100,000 miles on the rebuild has developed a cold start stumble, and a very strange ongoing stumble/shudder/miss when cruising at low load. Under acceleration the motor runs perfectly. Every once in a while the motor will stumble like you cut the key off for half a second. No pattern to that.

I replaced the ignition module inside the distributor a while back with a genuine GM module. No change. I replaced the O2 sensor. No change. I removed, checked, and replaced the EGR valve. No change.

I am leaning toward thinking it is some sort of weird vacuum leak that is jacking with the computer.

Ant ideas? What do I check next?
 
Those TBI's were known for sucking in the base gasket and causing a vacuum leak. But usually it would cause fast idle, and not be intermittent. There would be a whistling sound, louder than normal air intake.

Might try unplugging the EGR hose, go for a test drive. Not sure how it gets the vacuum signal, could be in the signal or the valve getting weak.

I hate parts guessing, but has the coil been replaced? Sometimes they short internally, give intermittent weak spark. Get it in the dark, use insulated wire puller pliers, slowly pull the coil wire out of the distributor, look and listen for the coil to start arcing and the engine faltering.
 
Blue Tractor Man,

Honestly, as a "test", block the egr port(s) completely, leaving the electrical connection to the egr intact. I am not a gambling man, however, a small wrench and a plate devised to seal the egr port on the manifold may result in a smooth running engine.

D.
 
Try to twist the TBI body by hand,if it moves, replace the gasket. No small pieces just the gasket.
 
Had very similar problem on 99 suburban with Vortec 350. What it was was arcing in the distributor cap. It took me a while to realize it was not a missfire but a rather a crossfire problem. A good cap from MSD solved the problem.

The difference between yours and mine is that the Vortec uses a "flat cap" where the plug wire outlets come out on the associated side. However, the TBI engines do have a smaller diameter distributor and cap when compared to the early HEI so crossfire may still be a possibility.
 
Can you get a code from it? I believe that vintage is a OBD 1 system. Usually turn the key so many times, press odometer reset a number of times or something and check engine light will flash a sequence.
 
(quoted from post at 09:02:48 06/13/19) Can you get a code from it? I believe that vintage is a OBD 1 system. Usually turn the key so many times, press odometer reset a number of times or something and check engine light will flash a sequence.

That would work if he was driving a Dodge.
 

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