I checked all the vacuum lines and they are in great shape.

Replaced the MAF with a spare (actually the original, been running with the spare since I found it), no change. The MAF connector pins look tight. Cycled it a few times.

The 1990 FSM has very comprehensive tests for the harnesses. Checked the ones to the MAF, ISC, TPS, engine coolant temp sensor, inhibitor switch, power steering oil pressure switch (tested the switch operation also). Results normal.

Turned on the air conditioner while idling, and the idle bumped up 100 rpm or so, so that seems to be working okay.

Figured it would be worthwhile to check the components on the vacuum line, so I tested the purge control solenoid valve with a hand vacuum pump w/vacuum gauge and that's working normally. The PCV valve tested okay. Connecting the vacuum gauge to the nipple on the throttle body that the purge control solenoid vacuum line connects to drew a steady 19psi vacuum.

Running out of logical ideas, I tested the crank angle/TDC sensor harness and its fine.

About the only thing I haven't messed with is the fixed SAS, but since I swapped the throttle body with no change, I don't see how that could be the problem.

I'm wondering if maybe the fuel pressure regulator is acting up; it's got a vacuum line connected to it?

I have noticed something unusual.
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Idle seemed to operate more normally, with an exception. 1800 at first startup with the engine cold, then after a minute the idle bounced around for awhile, down to 1300, up to 1700, repeat, repeat, repeat, until it warmed up more, then dropping down to a steady 1000. Flaky fast idle air valve?
This reappeared. I thought putting in the original ISC corrected that, but it happened again today when warming up from a cold start. The bouncing rpm's also occasionally happens when I'm driving. It's most pronounced when I'm driving down a slope, foot off the accelerator, in Drive, coasting at 40mph. The engine is at 1700rpm, then drops to 1200 quickly, then back up to 1700 and so on. When this happens, my Autometer vacuum gauge shows about 20psi when the engine's at 1700, and drops to 17psi at 1200. Strange.

This thing's got me stumped. <img src="/forums/images/graemlins/confused.gif" alt="" /> I gotta get this figured out; while I can live with the high idle in 2H, when I drive in 4LO with these Aussie gears, it bucks like a bronco when putting it in gear. Hard to hold it back with the brakes in 4LO also.

1990 Monty LS 3.0 A/T LWB. KYB front shocks, OME rears and coils. 33x10.5 BFG Muds. 5.29, 2.85, RD110, RD46. 2 bouncys. Aisins. Monte Disco swaybar disconnects. GenII t-arms, idler arm, UCA's, V45W front brakes. 2in BL.