Ok, here's another update. This time I jacked the front end up and forced the uppermost heater core hose down a couple of inches so that the radiator cap was the highest point in the cooling system. I drew enough coolant out of the radiator to make the level at least an inch below the bottom of the filler neck of the cap. I didn't want it absolutely full because I didn't want a big mess if things expanded.

Some notes on my engine, etc:

I have a '94 pickup 4x4 with the 22re. It has 150,000 on the bottom end, but head is a brand new EB/Topline with O/S valves. Cam is EB's 268. Intake is K&N cone, AFM is stock (untouched). Exhaust is LC headers (4-into-1), 2 inch pipe from cat to muffler, 2.25 out to exit. Cat is almost brand new (cheap aftermarket from Checker) and flows fine.

I have a vacuum gauge in the cab, and it's T'ed in to the PCV hose. I also have an O2 gauge (A/F ratio) in the cab. O2 sensor is a Denso from Sparkplugs.com. It's almost brand new and hasn't given me any trouble. O2 sensor location is right in front of the cat, and it's the 4-wire (heated). The TPS was previously tested and is set correctly.

I started the engine.

Now, I took a lot of notes in order to document my observations. I'm sure I missed testing some things, but I did cover quite a bit. Here it goes:

-After starting engine, the idle rose to 1900 and 20 pounds of vacuum. After 45 seconds or so, I depressed the throttle lightly, and the O2 sensor started to oscillate voltage. Oscillations showed a lean mixture.

-After 2.5 minutes, the O2 was still oscillating lean, and the rpm's were at 1750. (Note, although I can't remember the exact details, I did notice that early on before reaching operating temp the O2 sensor would swing on it's own without appyling throttle. At every point after warm-up, it would not oscillate unless the throttle was applied. A very slight depression of the throttle would bring the rpm up to 2500 and the O2 sensor would oscillate.)

-After about 5 minutes the thermostat opened, spewing some coolant out the filler neck. At that point the engine started to lope.

-It settled into a consistent lope between 1600 & 2000 rpm. The cycle time between the high and low rpm took approx. 2.5 seconds. The vacuum cycled between 19 & 20 pounds. This is with the idle screw all the way in. When I experimented by turning it out to raise the rpm, it caused the loping to increase in frequency. I then screwed it all the way back down.

-When applying throttle all the way up to 4000+ rpm there was a very obvious miss in one or more of the cylinders. However, I don't think I could detect that miss at the "normal" 1600-2000 loping.

-After engine was fully warm, and throttle was applied to get the O2 sensor to oscillate, it seemed to oscillate at stoic. However, it would jump around a little. I couldn't detect any particular pattern. And, I think it did this to some extent before I did the teardown and swapped heads.

-I topped the coolant off right up to the top of the filler neck. There were bubbles being released every time the lope caused the idle to drop. This went on for several minutes, and then the bubbles ceased. Even after the bubbles ceased, the loping persisted without any change. I put the radiator cap back on.

-I disconnected the TPS, causing the engine to cease loping and go into a high-rpm idle. I plugged it back in, and the loping continued.

-I then unplugged the O2 sensor. This didn't stop the loping, but it did cause the "missing" to cease when depressing the throttle. I plugged it back in.

-I then jumped the two terminals in the diagnostics box that lower the rpm for setting timing and checking codes. This caused the idle to smooth out at 1600. Codes 21 & 41 were flashing.

-I shut off the engine, pulled the EFI fuse for 2-3 minutes, and put it back in. Started engine back up with jumper still in. No codes flashed this time. One difference is that the idle dropped to a steady 1500 (instead of the previous 1600).

-I released the jumper, and the idle kicked up to 2000. But this time there was no loping. Barely touching the throttle would cause the lope to come back, but if I let off the throttle, the idle would smooth back out to 2000. There was not perceptible miss when idling at 2000. When revving it up, the miss was noticeable.

-I also grabbed my propane bottle and cranked it way open. I checked for vacuum leaks, and none were detected. Applying it to the air filter caused the engine to stumble and almost die.

-Unplugging the vacuum line (the big one on the front side of the valve cover; not the pcv line) increased the rpm and eliminated the loping. Plugging it back in brought the loping back.

-I unplugged the vacuum line to the EGR valve, and there was no difference in engine performance. Applying vacuum to the valve would almost stall it.

After over an hour of doing all this, I shut her down. If someone can make sense out of all this mumbo jumbo, even though most of it is probably meaningless, I would be thrilled.

There seems to possibly be a connection between the LC headers that I have and the way they react to the new head. I thought the O2 sensor readings/fluctuations, etc. were interesting, but I don't know what to make of it.