safulop

The temp 2 is new; the wires are indeed suspect, with a damaged boot, although the signal coming from it was checked. I believe I will never figure this out unless the connection of the high idle to the engine operating temp can be understood.

Mrmerlin

OK so replace the connector to the temp 2 cut the wires back till you find fresh corrosion free wire

safulop

Alright but, is there a theory of how the Temp II circuit could cause this issue? The car is running great otherwise. This afternoon it idled correctly once again, after I had disconnected the battery and was running AC. Generally it tends to idle correctly when the AC is on, and tends more often into the high idle when the AC is off.

safulop

I checked the Temp II wiring, it is pretty good insulation under the boot, there is no exposed wire or evident corrosion. The connection seems solid because no amount of playing with the wire has any effect on the running engine. Normally if you've got intermittent wiring it is pretty easy to get it to go in and out like a pair of dying earbuds.

Is there a particular relay that could cause this if it's on its last legs?

safulop

^^^^^^^^^^
E.g., I see that the LH relay and fuel pump/O2 relays are both original. Since the strange idle symptoms interact with the power supply (battery reset helps, running power accessories helps etc.) and only set in at normal operating temperature, at this point I'm going to guess the LH relay is causing strange behavior of the LH circuit controlling the idle.

John Speake

I haven't read all this thread, but needing to clear the adaptation parameters by a disconnect the LH suggests a faulty O2 sensor. Try a disconnect, then unplug the O2, then reconnect and run the car.

OR the slight air leak is messing up the idle adaptation which is separate to the O2 adaptation.

safulop

^^^^^^^^^^^
Don't worry John, this thread is getting like TL;DR, as the kids say.

Fact is, I highly doubt a simple O2 sensor fault here, partly because this one has only about 25K miles on it since 2011, but also because the system and idle work flawlessly for long stretches between symptoms cropping up. Moreover, we passed a smog check recently--unlikely with a faulty O2 sensor I think.

I'm also quite certain there is no air leak.

I'm replacing the ancient relays on both the LH and O2 sensor circuit, we'll see if that settles things down.

safulop

Alrighty now,

possibly the last word on this thread...

I realized while digging in the relay panel that had mis-counted the positions because XXVI is empty; so in fact I had already replaced the critical relays, and today I only replaced the ancient reverse light relay.

Still acting up a bit, wants to race the idle by 200 rpm sometimes, but generally when it happens, a little rev-up will settle her down. The only thing I can figure would behave that way would be the ISV, because the precise amount she wants to race is variable. On a bad day, she'll idle at 1100 in neutral and will not settle down. On an average day she'll race up to 1000 in neutral, and will settle down after one or two rev-ups. On a better day, she'll idle up around 900 in neutral, and will settle down immediately after one rev-up.

So, a 928 quirk par excellence.

safulop

Just a brief update to put this strange business back on the radar.

The symptoms described above are steadily improving. In the past month, the idle has not been racing up as much, and I have always been able to settle down the slight racing with a single rev-up to about 3000.

She does idle rather high at cold start in colder weather. Like, a 32 degree overnight and she idles up to 1800 rpm when first started. Takes about 3 minutes to settle down to a reasonable idle, then runs perfect after we start rolling.

I'm guessing this all has to do with some combination of my Porken S300 chip and new (aftermarket) ISV under the intake. My thought from the beginning was that the ISV is somehow faulty and is getting "worn in" somehow as we use the car.

Another interesting thing is that the racing idle is sensitive to the electrical loading of the system. It never happens when the headlights, wipers and rear defog are all going, or when the A/C is running. It tends to happen on brisk mornings when very little electrical appliances are in use.