Hello Owen,

My knowledge of this is limited, but I'm also curious. I had an experience with my 91 240 a few years back that left me stranded a few times. The car started normally but the gauge cluster was completely wonky. I put in a spare 87 (LF 2.2) box as a test and all lights and gauges functioned normally, but because the VSS is wired differently in that year I got a 311 and a non-op cruise control. Non abs car.

I drove the car for a year like that before repairing the original box and never noticed the high idle you describe. There may have been other factors involved or maybe I missed it, but on my everyday driving I encounter red lights on the highway and think I would notice a 1100 rpm idle, so it's probably the latter.

To test my understanding of Art's explanation I will make some assumptions which may be wrong. I would be happy to be corrected.

It seems that the ECU has a high idle option. Perhaps this is the same high idle that comes on for a few moments at start up as the one in your post. In a car with a functioning VSS, the ECU defaults to this high idle at a certain vehicle speed, and reverts back to the lower idle when below this speed. Perhaps this function is reset with key off, so that it restarts normally. To a tackless person this may not be noticeable. Without a VSS the ECU defaults to the high idle.

The above is probably not right because without a VSS, how would the ECU know that the car is rolling and hence move it into high idle ? Maybe the TPS?

As to why, a couple of things come to mind but I'm definitely shooting in the dark. Volvo warns in it's manuals to not tow an automatic over 20 mph or over 20 miles. This only applies to automatics so the issue must be the transmission. If you're running downhill at 6o mph with throttle off, does this somehow simulate towing, and the higher idle alleviate it somewhat? It's a stretch, I know.

You say "an idle is an idle", but not necessarily. The downhill trip I mention above with throttle off has the rear wheels driving the motor to some extent, and may be beyond the ECU's ability to understand what's going on. In any case it's a different matter than when the car is idling without moving.

Maybe emissions are less at the higher idle under these conditions?

Sorry if you're more confused than before you read this. I know I am.

Regards,
Peter