I have a '96 Blazer with the 4.3 and I'm getting a hesitation at part-throttle from 1800-2500 RPMs. My tach needle fluctuates 1-300 RPM during normal driving at nearly any speed. That should sound familiar to anyone reading through the other long posts on this problem. What I haven't done is spend hundreds of dollars replacing parts unnecessarily as I'd really like to understand the functions of certain sensors before I go too far.

I read all the pertinent threads in this forum and other forums and I have done the following: Cleaned the MAF and replaced a rotten vacuum boot going to my PCV valve. Why haven't I done more? Because when I open the loop by disconnecting my MAF sensor all of my problems go away, other than it gets harder to start. Once started it idles, drives and accelerates without hesitation.

By doing this, I think I'm ruling out any mechanical problems with the engine and exhast systems. So, I am wondering about the sensors. My question is, during open loop are there any sensors still functioning, or do they all go offline? What sensors are part of this loop? MAF, IAT, TPS, O2 (3), and Cam and Crank?

Let's start there and see where this goes. Any help understanding how these pieces interact with each other would be greatly appreciated.

 

It doesn't cost anything to pull the distributor and check the wear on the distributor gear. Do a search on this site for code P0300 and you will see many that solved their problem with a new distributor. I had code P0300, hesitation at shifts and at highway speeds, sometimes bad enough to cause flashing Service Engine Light, no torque converter lock-up, but wide open would not be affected. After about 10 miles, engine would be hot and not misfire as much.

I'm not totally ruling out coil, wires, plugs, or sensors, but after throwing money at it with new parts, the distributor fixed my misfire problem. I thought, "How can that be? It only acts up on cold or rainy days... it must be a sensor."

From what I've seen, crank sensors usually just work or don't work, though. I'm sure there are exceptions, but they are pretty simple devices. The service manual states the cam sensor (on the distributor) does not effect timing...only helps detect a misfire to determine which position has it.

I had a big thrill when I replaced the coil. Despite the original one testing out okay with an impedance test, I saw a big difference in misfires. Instead of happening every day, it just misfired on cooler days. Later, it went back to misfiring every time the engine was cold.

Finally, after dealing with the misfire issue for nearly eight months, I gave in and bought a new distributor. Pulling out the old one revealed a pretty well worn distributor gear. The new one from E-bay only cost about $75 delivered. Foreign made, of course, but it is all aluminum. I was surprised by the good quality of components such as brass terminals and reinforced rotor contact. I have not had a single misfire since. Sorry I didn't answer your specific questions, but it did work for me. I did try running in open loop, and it did reduce the amount of misfires, but I would still get them occasionally.

 

Many thanks to Mike for giving me the insight I needed to forego unnecessary part changes and go right to the source of the issue. My new distributor is in and I'm not having the issue anymore (knock on wood).

The technical article for setting the timing after replacing the distributor by SWARTLKK was unbelievably helpful as well. My sincerest thanks.