Randon misfire: adding auxilliary vacuum pump to fix misfire at idle on 2000 Blazer
 

I have a 2000 Blazer with a weak cylinder (#6) that idles roughly and occasionally misfires on that particular cylinder. cylinder 6 has 120# and the others have 170# of compression at the moment. The thing runs smooth and strong at all other times...not surging, misfires, etc. while driving once the throttle is tipped. But when i let it sit and idle for more than about 30 seconds the engine starts to misfire on cylinder 6...popping out of the tail pipe and throwing up a code for misfire on cylinder 6. I believe i may have an intake leak but have no apparent signs of coolant seepage at this point. All of my vacuum operated systems seem to be working fine...occasionally the heat switches from the floor to the dash when driving up hills for long distances...but otherwise no other inconveniences. I have inspected for vacuum leaks and ran a vac test which shows it does drop below 16 psi at idle...to about 12psi. or so (I didn't let it run long as I fear fouling the plug eventually). I have been checking other posts regarding misfires and from what i believe i may be experiencing a lean misfire condition at idle due to the drop in vacuum and how that drop in vacuum effects sensors...leading to the fuel being trimmed back and causing a lean condition at idle. As cylinder #6 is weak it misfires right away. Does this make sense to anyone?

Anyways...the reason i'm posting is i would like advice on installing an aftermarket vacuum booster to help increase my engine's vacuum at idle to see if that helps. I know that hot rodders like to use these to increase vacuum on high compression engines to help ensure enough vacuum at idle for braking. The ones i have seen online are installed inline between the intake and the brake booster and include a selenoid that kicks the pump on only when vacuum drops.

Again...does this make sense to anyone out there? I really am just trying to avoid replacing intake gaskets on this thing as the last time i had it done the shrapnel from the removal and cleaning of the gasket surfaces ended up down in my front main bearing, taking my crank and mains out and ruining my engine, necessitating replacement with the one that's in it now. I know these 4.3's are notorious for leaky intakes...have owned 2 S-10's over the span of 16 years with each getting 2 engines apiece due to intake leaks and the subsequent bearing failure following intake gasket replacement.

Have you raplaced the plugs, wires, cap and rotor? What does the number 6 plug look like? Have you checked for vac leaks along the plenum and throttle body using brake fluid to listen for an idle change? Its also possible that it may be an injector issue. Having said that I have no experience with aftermarket vac boosters, but would check into the other possibilities first. I have never heard of a 4.3 needing a booster.

Yes I did get a chance to replace the plugs, wires, and cap/rotor with ac delco's about a month ago. the #6 plug has always looked perfect...all the plugs have burned perfectly...never any soot or glaze, etc. just nice and light gray/brown and clean. the motor hardly uses any oil...leaks a bit...but not an oil burner. i have sprayed carb cleaner around the intake and throttle bore with no effect thus far...no decrease in idle. i may try some propane and see if the idle goes up to see if it will be easier to detect a leak. i worried about injectors but have no starting issues and it runs strong at all speeds once i tip the throttle. gas mileage is great right now...about 22 average highway which made me think it was running lean. i did do a throrough intake cleaning a month ago...went through 3 cans of Chemtool. no obvious leaking or puddling inside the intake...no bare spots anyways. oh well...

Not sure where you hail from, but I saw a recall that was put out for vehicles with California emissions due to carbon build up in the fuel injector poppets. It would cause a misfire and show as a misfire with a diagnostic tool along with the service engine light coming on. I have a similar issue you are having and I am in Pennsylvania (except mine is the #4 cylinder). Me thinks that possibly this may have effected more than just California emission vehicles. I am replacing my clutch right now, but plan to dig/research into that poppet issue a little deeper in the next few weeks.