Thats some interesting information regarding the PCM running on an open loop at start up.

I'm having an issue with my 97 S-10 4.3 that sound like what you just described.

It starts and idles like brand new but had a bad hesitation after a few minutes of run time.

I noticed a while ago, when I first start it,,, it run at around 750 rpm for about 3 or 4 seconds then it drop to what sounds like 500-550 rpm.

I'm thinking the cycling you described is what is happening with mine but when it goes back to the closed loop is when I'm having problems with the hesitation.

ALL o2 sensors were changed over the past year but that doesn't mean they're not bad.

I'm not getting a SES light or codes,,, nothing, just bad hesitation.

 

def check the o2 sensors, also, i can't remember if the 98 4.3's had the same problem with the distributor cap crossfire, once she warms up and adjusts it could move the spark far enough over to cause that, since it's 1 & 3, would have to re-time with a scan tool that can read your camshaft retard position. if the distributor cap is turned too far one way or the other it can cause the crossfire. Just and idea, but the 02 sensors are a good one also, they don't kick in until the pcm decides it's ready to talk to them, which 3-5 mins sounds about right for warm up time.

but main reason i say timing is in order to do intake gaskets you have to pull the distributor, and those things are WAY pickey about making sure the camshaft retard timing is 1-2 degrees on.

 

well i believe i found the intruder. Captain Hook is onto something with the O2 sensor reaction time being slow. i pulled #1,3&5 plugs to check compression. all good 155-160 for all three however replaced the carbon black plugs then i swapped the front bank O2 sensors then hit the road. and to my total surprise the 25% long fuel trim at idle moved to Bank 2 now. so tomorrow morning I'm ordering a new O2 sensor for bank 2 and test drive the heck out of it.
Side Question: what kind of gas mileage does everyone's blazer get in town on an average? mine has pretty much always gotten about 34-36 miles on a 1/4 tank.

 

Yup, lazy O2 sensor. Most O2 sensors will cycle from rich to lean in about 50 to 100 milliseconds, and switch from lean to rich in 75 to 150 milliseconds. If the O2 sensor takes too long to reverse readings, it's an indication that it's getting "lazy" and may need to be replaced. Each time it reverses and crosses the desired .45 volt target, it's called a "cross count". P0133 is a slow driver side sensor and P0153 is slow on the passenger side. By the time either one of those set, and turn the SES light on, the problem is very noticeable. Bosch sensors are known to be slower than AC Delco sensors. You can test them on a graph, which is the most accurate, or compare the cross counts of each sensor to each other, (less accurate).

I don't even check my 98 4X4 for city mileage... it gets what it gets Highway mileage with A/C on and cruise set no more than 70mph, it will consistently get 21mpg. If I drop to 62mph, mileage goes up to 24mpg, (couple ounces of acetone in the tank and it goes up even higher, but that's a whole different story ). 210K miles on the original engine, trans & rear axle, fuel injector upgrade, fuel pressure tweaked to 63psi, camshaft retard set at zero degrees, all AC Delco ignition parts, & fuel pump.

 

Put a new O2 sensor in and truck runs so smooth now. thanks for all the help and info guys.

 

I just went through the same thing. I replaced upper plenum gaskets today thinking I finally had in figured out. I replaced about anything you could think of over the last year to fix this problem. Full tune up,removed and cleaned distributor,new exhaust,fuel filer, modified my air intake,sea-foamed twice in 6 months,cleaned throttle body and MAS and then it hit me. The F'ing gas cap! $10.00 and problem is now fixed. Sometimes you think to hard and over look small things. I did.

 

I'm still having an issue. Go to pass somebody on the highway give it some gas to shift the trans. down then it will start running rough. Let it coast for a little bit seems to clear up and go away. Weird? Haven't figured this out yet. Any hold gear heads it feels like when cars used to float a value. Any help out there?

 

MrMatt1977,
Does the SES light work? Does it remain on, or flash, when the engine is running?
Are there any DTC's in memory or pending?
Check fuel pressure and leakdown: Key ON, engine OFF, fuel pump running: pressure must be 60psi to 66psi and must remain above 55psi for at least 10 minutes after the pump shuts off.
Check camshaft retard, (must be checked/adjusted whenever the distributor is removed).
AC Delco 41-993 plugs .060" gap.
AC Delco cap & rotor, (yes it does make a difference).
AC Delco or Autolite Professional series plug wires, (not some cheezy chain store brand).

Post your results.