Hesitation, bog and backfire

93 Astro 2WD 4.3 w/TBI. 120,000mi. Symptoms: When cold ( OAT around 55 ), Engine starts immediately. Idle settles down nicely. Little if any roughness noticed. If driven right away, vehicle will sluggishly accelerate and sustain speed up to tested 60MPH. As engine warms up, engine will sputter and can be coaxed to backfire if throttle is opened quickly to 50%. If you accelerate gently, you can reach and maintain steady speed. If you try to accelerate from speed, you get stumble and hesitation. While sitting, vehicle in park, you can slowly raise engine speed to 3 or 4000RPM which it will smoothly maintain. If you mash the throttle from idle, it will backfire and stumble. I've been chasing this problem for over a week, moving from one probable cause to another. 10,000 miles ago: New cap, rotor, wires, plugs, fuel filter. Problem just started. Items addressed: EGR inspected, cleaned and reinstalled. PCV valve replaced. Couple of suspect vacuum lines replaced.New air filter. TBI unit disassembled, cleaned, put back together with all new o-rings and gaskets. Diaphragm OK. Note that with a cold engine (fuel enrichment mode) it is OK.The problem starts just as the engine begins to warm up. What next?

you should test your fuel pressure-tbi should have 11-15 psi pressure-if it is low-you will lack power under accel. also check exhaust backpressure-your cat. converter could be starting to deteriorate and plugging off.

I guess the next step is to check fuel pressure. My reason fo far for not figuring the cat or fuel pressure is the culprit is this: When cold, the engine wil accelerate OK. (This thing has had a flat spot in acceleration since we got it 40,000 mi ago) Because it will sustain high RPM with or without load (driving at 60 or sitting still with RPM raised ) I ruled out fuel supply problems (fuel pressure, fuel filter). If the cat was plugged the engine would gradually lose power as you tried to sustain higher speeds. Not the case. Only after engine begins to warm up does it start these symptoms.

So once it goes into closed loop operation it begins to fail?

If I assume that closed loop refers to engine operation when it reaches correct operating temperature, then yes, however, the symptoms start before it is up to full operating temp. NEW ISSUE: Coolant level may have been very low. I just noticed a trail of coolant running down the back of the motor which I found to originate at the coolant fitting at the rear of the intake manifold. Vehicle is parked on stone parking pad and never noticed a puddle. Upon inspection I opened up another Pandora's box of PITA. The special fitting which screws into the manifold was totally rotted and when I wiggled it, it broke. Tried easy-out to remove broken part still in manifold, no dice.Tomorrow's new engineering will be to tap what is there and install 3/8 pipe nipple and connect to return coolant line after I cut off special fitting end with a piece of heater hose. So, once the leak is fixed and coolant level is topped off, I can then see if the low coolant level was fooling the engine temp sensor causint the computer to lean out the mixture too much too soon. It would be great if this would be the end of the story. Stay tuned!

Situation update. Intake manifold was tapped with 3/8 NPT and short galvanized nipple screwed in. Metal coolant line was stripped of compression fitting parts and connected to nipple with piece of 5/8 coolant hose and clamps. About 1 1/2 gallons of coolant were needed to top it off. Engine started fine. Because its dark out, I was able to see some arcing from a couple of the plug wires. Hmmmm. These things are less than 2 years old. Test drive showed improvement in drivability, however there was still bogging and hesitation. Only now it seemed to be more of a regular misfire. With vehicle in park, stabbing the throttle from an idle, I could still make it backfire. Next step I'm going to have to replace the plugs, wires, cap and rotor. If this isn't the cure, then I will have to test the fuel pressure.
How do you check the fuel pressure? Do you hook up the pressure guage to the supply line to the TBI? Because the fuel pump is electric, do you just turn on ignition to activate the pump and read pressure?

I too have an Astro with a bad idle that I am trying to correct.

For fuel pressure, from the Haynes manual (not from experience):

"On CMFI and CSFI engines, install a pressure gauge equipped with a bleeder hose onto the Schrader valve on the fuel supply line [ picture looks like a valve top rear of engine]...turn the ignition key on (engine off) several times to pressurize the system (this activates the fuel pump for 2 seconds each time the key is turned to the ON position."

I think you have the CMFI; Haynes says you should have 58-64 psi with engine off, key on. With engine idleing pressure should decrease 3-10 psi.

It also notes that the pressure should hold. If it bleeds, there is a leak.

good luck
Pete

I you read above he states he has TBI and thus he should only have a fuel pressure of 9-13psi.

Problem solved: Since the only time I've had to work on this thing has been weekday evenings, the early darkness was my friend. The one time I started the engine, I noticed two spark plug wires arc against the valve cover. I had originally assumed that the wires would not be suspect since they were less than 2 years old.So, before I delve into fuel pressure tests, I'd change the plugs, wires, cap and rotor. Voila! No more bog or backfire. Just proves that you shouldn't assume anything. Always work from the most logical and easiest solution in your process of elimination of causes of malfunction.
Thanks to everyone for your input and help.

Glad to hear you got it and that it was an easy fix too!