Engine missing at high idle & above 35 mph

Pontiac 6000, 2.8 V6. Enigne wasn't running when purchased due to faulty crankshaft sensor (posted previously). Now that the car is driveable I find it misses (like bad ign wires) when driving; also very noticeable at high idle speeds, I'm guessing in range of 1200 to 1500 rpm. Tried different set of wires and examined front plugs. Wires didn't change anything and the plugs look decent so I'm ruling these out as the cause at this time. I experienced a similar problem on a Lumina 3.1 but that unit ran rough at low idle and driving. That problem was traced to two bad injectors. This car runs smooth at low idle but rough at higher speeds. Shorting out plug wires individually doesn't point to any one cylinder as a likely culprit.

(for the Lumina I built a simple gadget which goes inline with the ignition wires and allows me to short to ground the spark to any plug of choice. I find this home made tool quite handy and would post a picture if I knew how. lol)

Anyone experience similar problems? Would defective injector(s) work decently at low idle yet cause problems at higher speeds unlike the Lumina which was at all rpm's?

Thanks

check the coil pack, could be breaking down under load

check the coil pack, could be breaking down under load

Before replacing the coil pack I thought I would check the resistance of the injectors. I thought they should be accessable from the ECM rather than remove the plenum.

Well not so fast. No 1 and No 6 injectors are accessable at the top of the engine, no problem. But I notice from the drawing:
http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?pageId=0996b43f8039380f (http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?pageId=0996b43f8039380f)
that they group the injectors front and rear.

Silly question. It would seem to me that all three injectors would receive signal simultaneously. Shouldn't only one cylinder receive a pulse? Why inject fuel into the other two cylinders since they won't be at the correct position to fire?

Are you getting consistant misfires on same cylinders, or is the misfires random?

I have seen a plugged catalytic converter on one bank, causing random misfires on the same bank.

Are you getting consistant misfires on same cylinders, or is the misfires random?

I have seen a plugged catalytic converter on one bank, causing random misfires on the same bank.


They are random misfires. I wasn't able to detect any weak cylinders (using my device). The engine tone and rpm changes approx equal for each cylinder when the plug wire is shorted.

"catalytic converter on one bank". ???
The converter is further back on the exhaust after the two banks have been joined.

" random misfire " can also be cured by flushing injectors.

The best way to clean Injector's is out of the car, and them to a diesel shop, were they can micro bath, blowout, and the flow rate tested.

Don't forget the new fuel filter!

Thanks for the tip about taking the injectors to a diesel shop. I wondered about that possibility.

Update:

Installed a good set of plugs and wires salvaged from wrecker. ($10. OK so I'm cheap. lol). That improved it somewhat but still not good enough. So decided to pull the plenum and checked the resistance of each injector individually. It is possible to pull one injector wires off on each bank and then read the resistance of the two remaining in that bank. Being that the injectors are connected in parrallel the reading should be about 6 ohms. The resistance was a little low I thought. After I got it apart I found no. 5 injector reading 9.7 ohms so I replaced it with another used one that tested just over 12 ohms. The car runs beautifully now. Lots of power and smooth.

If you are noticing small vibration in gas pedal when accelerating between 1200-1300 rpm try different alternator believe it or not.

or just remove serpentine belt for a minute or two and test drive around the block to eliminate mechanical vibrations