Not to threadjack, but I think we have a similar situation here, and may be able to help each other out even though the vehicles are slightly different..............

I too have put on lots of parts and labored over this motor: Delco wires, Delco platinum plugs, Accel coil and module, new pickup coil, "free flowing" cat, Dynomax catback, fuel filter, all new vacuum lines, intake gaskets when I pulled the intake to clean the EGR passage in the head/intake/solenoid, a couple oil changes with Mobil 1, a couple nice doses of Seafoam, fuel pressure checked out good in all conditions, etc. It runs well until it warms up, but then misses at very light throttle pressures, especially before shifts.

I just got done pulling the plugs and the #4 plug was mostly fouled. I rechecked all my wire routing, added some insulator and whatnot to the plug wires and all dist/coil wiring, and it still does it. Going to try an Accel cap and rotor next I suppose.

 

I'm at my wits' end with this blazer and it's engine!
Feels as if I have tried and changed everything, but no luck!
compression is good, after new fuel pump and spider fuel pressure is fine, ignition is fine. So I don't know what to try next. But as you wrote, it gets worse when it gets warm/hot.

Have a look at my other thread, might me something to this:

https://blazerforum.com/forum/engine-transmission-35/problem-95%25-gone-two-days-%25-%A4-45039/

 

I have the same problem. is 45 mph at a shift point for the motor or something?

 

I saw that other thread and have been thinking about that; I can't put much logic as to why that would make a difference, but was thinking of putting a breather type filter on that valve cover anyhow. I'll let ya know.

I put a "high perf" Accel cap and rotor on last night and covered every inch of plug and coil wiring in fresh insulator. It helped in all areas except where the miss occurs of course.

Another thing I have been thinking of is about how my #4 cyl is the one where the most issue is according to the plugs.....this is also the cyl with the EGR port from the head. This could possibly mean the EGR solenoid is acting irratically, but that would normally throw a code even with OBD 1.

I'm going to start checking grounds, ground/power wires, and connections at the ECU today.

Also, my ABS light came on recently and I have no codes which leads me to believe I have a grounding issue somewhere.

Best of luck to ya; keep throwing ideas out there and hopefully someone can track this issue down.

 

I don't know what engine you have, but if its the cpi unit it could be a leak at your FPR or spider just above the #4 cyl. Slow dripping fuel down the cyl that makes a missfire.

 

Did some more troubleshooting today. I used a stetoskop and put it to the fuel tank. Even now with a new fuel pump I can hear the pitch go up and down togheter with the eratic idle. But what I don't know is if the idle affects the pump or the pump the idle.

I also put the stetoskop to the fuel filter and there I heard a sound best described as a fast "TIKKATIKKATIKKATIKKATIKKATIKKA", this sound gets faster with higher idle. I can't imagine what woul cause this sound to come from the fuel filter. I know its the fuel filter and nothing else.
I tracked this sound all the way along the fuel line to the test port on the back of the engine. The sound was not as loud at the engine, but its clear that this sound follows the fuel lines from the filter.

 

Checked the spark plugs again today, they where like new!
That takes a fuel leak off the table, right?

 

What you are likely hearing at the fuel filter is the injectors opening and closing. It makes sense (to me) that it would be louder at the filter given it's larger area for the stethoscope to hit against. The sound is actually caused by a pressure wave that shoots back down the fuel line as each injector closes off. This phenomenon is called "water hammer".

As far as plugs ruling out a fuel leak, they may, but the definitive proof would be a fuel pressure test...

 

I have a fuel leak some where, that I'm almost sure of. But with the plugs looking brand new, I don't think it's at the CPI. After shut off the psi still jumps up to 60, 10 minutes later the psi has dropped to 10. There is noe external leaks, so that leaves the tank and the spider. The fuel pump setup is new, togheter with the tank, the CPI is also new. One of these has to be leaking right? Hope I don't have recived bad parts one more time. This would make that the third time this year.

The most recent symptoms are small "PUFFS" comming from the exhaust at idle. Soot around the exhaust tip and still some surging at take off and acceleration. The jerking is also still there, but not so much as before.

 

Isolate the pump by installing the gauge at the filter. If the pressure still drops, the check valve in the pump is bad. If it holds steady, then you have a leak inside the plenum.

From your descriptions of the exhaust, you have a leak inside the engine still.

When you replaced the spider, did you also replace the 'nut kit' (internal lines)?