Tootie Pang

iTrader: (2)

I recently did plugs, wires, cap, rotor, pickup coil, knock sensor, and ICM.

I have not done the ESC.

Timing set at 8

Engines misses at low RPM idle in gear but not in N or P. It also misses under load. It literally "goes flat" around 4,000 RPM

This seems to be an under load condition. Idling in N or P is very very little load. Idling in D is some load and then it goes up from there.

Any ideas where I should start?

All of the parts I used were AC Delco.

Current plugs are NGK Iridiums.

Fuel system has been thoroughly checked.

I recently did my valve stem seals and springs. I set the lash at TDC so that there was just starting no play on the pushrod up and down and then one more full turn of the rocker nut. Valvetrain sounds solid.

GeneralDisorder

iTrader: (1)

Those Multecs are prone to that behavior. They short out when hot. I know you put those back in but I wouldn't trust them when hot.

Stethoscope and unplug them when it's misfiring. Also ohm them hot.

GD

Tootie Pang

iTrader: (2)

I had done that before- hot and cold. They were all good. 14 ohms-ish. The engine with the Bosch D3's behaved that way too.

GeneralDisorder

iTrader: (1)

Have you checked out how they sound with a mechanic's stethoscope and tried unplugging each in turn to see how it affects the idle and misfire condition? They can still be clogged even if the coil is good.

GD

ULTM8Z

A long shot, but if you have a bad engine ground the spark energy can actually escape out of the wires and find an alternate lower impedance path to ground. This happened to me many years ago...

Run the engine at night in pitch darkness. If this phenomenon is occurring, you'll see arcing out of the wires to nearby pieces of metal. If you lose spark energy like that, that's energy that isn't getting to the plugs, and you can get misfires like that.

Do you have a scanner? Without some means of diagnostic like that, you can get into the mode of throwing parts at the car unfortunately...

BTW, I just recognized your screen name from the DIY PROM thread about the Bosch-III's. It could be somewhat improbable as well, but malfunctioning Multecs possibly behaving the same as improperly tuned-for Bosch-III's...?

Tootie Pang

iTrader: (2)

Thanks. It was poorly routed spark wires that got baked, cracked, and shorted. New wires on the way and loom!

GeneralDisorder

iTrader: (1)

Multiple cylinder misfires will drive the O2 sensor lean and make the ECM ratchet up the BLM's. You see unburned oxygen from a misfire causes the O2 (it reads oxygen) to assume there wasn't enough fuel to consume all the oxygen in the cylinder and so a misfire results in a perceived lean condition, and the ECM adds fuel.

So the bad plug wires actually could make the Bosch D3's appear to run lean when in reality they were fine.

GD

Tootie Pang

iTrader: (2)

Yes but the two wires I fixed are cylinders 2 and 4- not in the O2 sensor path. From only an injector swap, my BLM's went from pegged 160 to a little low (but in range) almost immediately. Now, patching those two No.2 and 4. wires and route-clearing all the wires, the idle is now super smooth and steady and the engine only misfires above 4,000 but much less. However, the BLMs are consistently a little lower than before. So something changed on the driver side bank as well.

I have three threads, in different forums, so all three will merge to one and will continue here: https://www.thirdgen.org/forums/tech...inside-my.html

ULTM8Z

You can measure the impedance of the wires vs other wires on the engine. If these two repaired ones significantly out of family compared to the others, it could mean that the core got damaged along with the jacket due to contact with the exhaust manifolds... and you're not getting sufficient spark energy to the plugs (sufficient for idle and light throttle, insufficient for high load/rpm).

So, which injectors do you have on there now? The Bosch-III's or the Multecs?

I also wouldn't abandon the Bosch-III's just yet either... confirm with Southbay what you have and then retune for them. I saw a significant improvement over my older Bosch injectors when converting to the Bosch-III's... can imagine you'd see improvement over those old Multecs...

Tootie Pang

iTrader: (2)

I am running the Multecs until I get the engine running 100% factory smooth. And yes, got new AC Delco wires coming.