I am having the exact trouble. Mechanic said the miss was in cylinders 5 or 6 according to the code. Tune up time, so he replaced plugs and wires. Said the code keep reading a miss in 5 or 6, jumping between them. Then replaced intake manifold gasket, thinking it was a vacuum leak. Still misses badly. Now they say injector Spyder (have no idea what that is!) and I must take to a dealer. $800 so far and still missing badly. HELP! Have seen other posts about the spyder (?) but no real answers/ Thanks 2000 Blazer s-10

 

Most chain type auto parts stores will retrieve the Diagnostic Trouble Codes, (DTC's) for free. Post them here in the order they are retrieved. While you're there, rent a fuel pressure tester and we can explain how to use it and take the readings.

 

Was the distributor cap and rotor replaced? I would pull the cap and look for signs of arching and corrosion.

 

X2 cap and rotor. Makes a huge difference!

 

I had same kind of problem! No codes, but bad misfires. I changed allmost everything: Maf, map, sparkplugs, wires, ignition module, coil, distributor cap and rotor. Problem was distributor, dont know how it was broken, but changin it fixed problem!

 

Check the basics first. By that, I mean firing order, loose wires or cap/rotor, and spark gap. It's even possible you have a cracked plug.

The "mechanic" did at least change plugs and wires, but from your post, overlooked the cap and rotor.

Be careful though. The distributor base is prone to CRACKING when changing the cap and rotor. After having mine crack, I'll tell you to make sure to take the threadlocker off and do not overtighten. Or, use the old screws and don't overtighten. Some people have said to use a very thin wrap of teflon tape around the screw to help keep it in, but again, use at your discretion.

Keep in mind, if you happen to break the base, you'll have to:
A: Ziptie the cap to the distributor (like I've done for now until I get my new metal dizzy in) OR
B: Install a new distributor (I got an all metal version so I'll never have it happen again) and then you'll have to use a scan tool capable of reading cam retard in order to set the distributor to minimize ignition crossfire.

Here is a good read on what happens when the bushing in the distributor goes bad:
Chevrolet Blazer 4.3 Misfire Under Light Load 2000 - 2500 RPM

 

The link brings up a very real possibility for the OP's cylinder 5 & 6 misfires however, some of the information regarding camshaft retard, what it does, and how it directly affects ignition timing, is not accurate... take it with a grain of salt Camshaft retard should always be checked during diagnostics if the engine has a misfire. If camshaft retard is out of spec, it will cause a misfire, and it will get progressively worse the farther off it is. The author spotted the problem but didn't recognize its significance, (cam retard was off by 14 degrees!). This also means base ignition timing was off by 14 degrees! Worn distributor bushings will also cause a misfire adding to the problem. Scan tools can not accurately diagnose worn distributor bushings. It must be checked with an oscilloscope to accurately and positively identify it. The article does not mention the ability to adjust camshaft retard on the 4.3L engine. From the factory, there's ~15 degrees of total adjustment that can be done. If it can not be adjusted within factory spec, the distributor hold down bracket can be modified, (cheap & easy) which makes it fully adjustable. Most higher end scan tools have the capability to access the camshaft retard data stream. The el cheapo ones that the auto parts stores have can not access it. Desired spec is zero degrees with a tolerance of plus or minus 2 degrees, after snapping RPM above 2000 and back to idle.

Both of these possibilities are worth looking into concerning your misfire.

 

I have had the Cap, wires and plugs replaced. The Injector spyder and intake gaskets also. THis corrected the misfire and eliminated any trouble codes. It still is very hard to start in the mornings, after sitting all night. Once started, after a lot of grinding and almost catching, then finally starting, it is fine the whole day, but refuses to fire right off the next morning. No trouble codes. ,

 

Have you checked the fuel pressure and leakdown?

 

^^^X2. In the first response, (post #2) it was suggested that you check fuel pressure, along with retrieving the DTC's and post them. Not sure what your "mechanics" credentials are, but, each time he makes an incorrect guess, he makes more money, at YOUR expense Eventually he'll guess right and solve the problem. The question is.... can you afford all of the wrong guesses before he gets it right? Let us help you.