GOT_CAMARO?

It seems like at the end of summer the car has some new mysterious driveability issue... This year is no different: Hunting idle, lean misfire 1000-2000rpm while driving.

If I drive the car, very light on the gas between 1000-2000rpm, there is no perceivable misfire. As soon as I lean on the throttle, say going up a hill in 5th at 50mph it stutters until I back off the throttle a bit.

Now if I were to continue accelerating through the stuttering, as soon as the rpm reaches above 2000rpm it feels like a someone literally flips a switch and the car pulls cleanly to 5000rpm without much of a hiccup (from time to time it continues to misfire to 3500rpm but it is conditional) If I do a WOT run from first through fourth the pull is smooth, but bit lacking in normal power. Not once has the car backfired during or after its fits of misfiring.

The problem arose the day after I replaced the m/c solenoid. I set the lean stop height as specified in my ’86 service manual and drove the car that night. It drove smoothly in all throttle positions and varying load, even up hills on moderate throttle in 5th at 50mph. The idle seemed to be a tad unstable so I decided to make adjustments in the morning.

The following morning I readjusted my idle mixture screw to 3.5 turns out and tweaked the IAB while monitoring dwell. Took the car for a drive, WOT up the road through 1st and 2nd, ran like a champ shifted to 4th to cruise, then the aforementioned problems arose.

Since the carb was the last thing I tweaked, I put all the settings back to what they were before, no dice. For good measure, I tore the carb apart and cleaned all the passages with compressed air, assembled, made initial adjustments etc. Car still missed and idled rough.

At this point I honestly feel like I have a vacuum leak caused lean misfire and rough/hunting idle. I re-set the IAB to factory and my dwell would peg rich <10* and while monitoring my O2 sensor voltage it would vary from ~0.450v to 0.019v to 0.890v and continue to do so.

So I began my search for the vacuum leak: checked all vacuum lines at the carb with both carb cleaner and pulling 20in of Hg, found only one leak, EVAP control valve, replaced with no improvement. I am not running EGR or AIR and all associated vacuum lines are capped. I also replaced the o-rings on the IAB valve, no improvement. Spraying carb cleaner around the manifold to carb and manifold to did not yield a change in rpm. So I am left with a possible internal vacuum leak or maybe the PCV valve is allowing too much flow, unlikely unless I can be convinced otherwise. Also manifold vacuum is 15in Hg with a slightly varying needle, it varies corresponding to the idle hunting.

The most annoying part is I swear I can hear whistling coming from the passenger side front of the engine, but when I try to hone in on it with a piece of tube jammed in my ear, I can’t locate it, even while spraying carb cleaner… Of course this in the general vicinity of the alternator, so I’m not sure if it’s whining/whistling…

So my question is: can i verify a vacuum leak on the inside of the intake manifold before I take it all apart? AND Does this truly seem like a vacuum leak related problem?

Thanks for any and all responses!

P.S.
q-jetted 355 minus almost all emission equipment (EVAP still in play) more info upon request.

GOT_CAMARO?

I suppose I'll update this for anyone who may come across it. I've replaced the intake manifold gaskets, but I'm still getting the misfiring. Albeit, it is more subdued and my idle has smoothed out a bit. Right now I'm completely at a loss as to what's causing the misfire, I really thought it was a vacuum leak. I've included a link showing what the dwell, O2 voltage and vacuum looks like at idle. The vacuum is a little, off, I think. Does this all seem normal? I adjusted the IAB again to attain the dwell shown.

Video:
It would be great if I could get some suggestions.

t-top havoc

iTrader: (3)

Suspect a vacuum leak? Engine at idle, spray carb cleaner around intake mating surfaces & all vacuum lines-- idle change will show where a leak is.
Cap, rotor, plugs & wires in good condition?

GOT_CAMARO?

I did think I had a vacuum leak which is why I went so far as replacing the intake manifold gaskets. I did the whole spray carb cleaner deal with no success, even pulled a vaccum on all hoses connected to the carb with no success in finding a leak. Haven't checked the cap, rotor, and wires yet since I'm still not convinced there isn't a vac leak. Though the plugs are white, the kind of white indicative of a lean burn. I feel like I'm running in circles...