Miss at idle on a 2001 Blazer 4.3

I have been trying to fix a miss at idle on my Blazer so I can get it through emissions inspected. I bought it knowing that it had a gas leak and some other issues. It had an intake coolant leak so I replaced the intake, gaskets, distributor gear and spider injection while I had it apart. I replaced the cat and all three O2 sensors. I also replaced the cap and rotor with accel pieces. The plugs and wires were replaced right before I bought it. I replaced the fuel pump, fuel lines and fuel filter. I replaced the MAF with one from NAPA. The cam retard is at 0 and steady. This thing runs good with no miss fires above idle. I have checked for vacuum leaks at the sealing surfaces and replace all of the vacuum hoses. I looked at idle desired air flow and it is 40 with the tune wanting 39. I scanned the LTFT to MAF and it is only off a couple percent. Hopefully some one has an idea because I'm about out of them.

Ken

What kind of plugs are they?

A good brand like AC delco or cheap shelf parts?

Is the gap right?

Check engine grounds?

They are the AC Delco iridium's. The cylinders miss firing are #1, #2 and #3. I even tried swapping plugs and wires with cylinders that were not miss firing to see if the miss firing would move but it stay with the same cylinders. I did not think about the grounds but I'm out of ideas so I'll look at them tomorrow thanks

Ken

Also the gap on the plugs is at .060 which is what is required.

Ken

Is it an actual miss that can be felt or just one being seen?

Its hard to tell but I do think I can feel it. The DTC does set (0300) unless you leave it idle for awhile. Another thing is that the miss fire does not happen until the motor gets up to operating temp. No miss fires show up as the motor is warming up. It also stops above 800 to 850 rpm. I've thought about trying to adjust the idle mixture to see if its a mixture issue or not. Does anyone have a suggest for the best way to adjust just the idle mixture?

I noticed some else that my LTFT's are quite different between the left and right banks. My right bank is in the -6 to -7 and the left is +2 to +3 at idle. I tried leaning out the idle and the miss fire is better but still there. the #2 miss fire is about twice as many as the 1 & 3 miss fires. I'm thinking that I might have a defective injector setup even though I just put it in. Anybody else have any thoughts

Couple of test left to do - Take the fan belt loose - fire it up and see if the misfire is still present - sounds like it will be in this case...

Next - you don't have mixture screws - your most likely just adjusting the idle screw as it's fuel injected... With the fact that it's only misfiring hot - this usually means you have carbon built up on the intake valves or intake valves that are not fully seating whether it's from worn seats, valves or valve springs... Best thing you can do for this even though it won't be a quick test of any sort is go purchase and install 2 bottles of the sea foam cleaner to put into the tank and drive it around aggressively... If it's carbon - it should get better - if worn parts - it'll actually possibly get worse since some of the carbon will be cleaned out that was helping to seal the valves...

If you installed a reputable spider injection system or changed it over to an actual port style injector system? It shouldn't have a fuel issue, especially with the problem only being present hot... You can have bad O2's causing this - will cause one bank to overly richen or lean out causing misfires...

Anyway - more info and things to check...

Thanks GHuggins. I do have a thought about the carbon buildup. Wouldn't it miss at higher rpm's also and not just at idle. The other thing that I've notice now is that it stops miss firing when its in gear idling with the brake on. I will try removing the belt tomorrow. The O2's are delphi's with about 5000 miles on them. They are from my Blazer I just sold and were working fine. I modified the Y-pipe to flow better so I removed it with the O2's before I sold it and put it with the O2's on this Blazer. The mistake I think I made was the injection is a BWD from Advance Auto. Thanks again

Adiabatic efficiency is much higher with higher rpms - minor valve sealing issue won't show itself at higher rpms like it will at lower...

If the miss goes away with belt removal - your done :)

Update on the misfire. It is almost gone. I replaced the MAP sensor since the miss would go away in gear but that only helped a little. I started looking at the ignition wires since they were a cheap brand (international autoparts). I got a set of NAPA Belden wires. When I removed the wires #2 and #1 wires were about 2000ohms and would change ohms when you would twist the wire around. The Belden wire were all around 900ohms. This made a big difference. The misfire went from a few hundred per cycle to 10 to 15 on #2 and almost none on #1 & #3. I did look at the secondary air injection but everything checked ok except the right bank was leaking so I replaced it. I did notice something that if you remove the hose from the pump side of the check valve and start it up, you can feel quite a vacuum on it. So if you had a leak in the system if could cause a misfire issue. Not the case with mine since there was no leak in mine. So for now I'm going to deal with the misfire since it is so small. Thanks everyone for the help.