Last month my 940T started idling a bit rough (100-200 rpm up and down).
It's not constant, and doesn't seem to be affected by engine or ambient temperature.
The car has never stalled.
No funny smells or noises in the engine compartment. (except the up and down idle)

My first thought was that the vacuum line on top of the intake manifold was clogged up, so I cleaned it out. (helped a little)
I then cleaned the MAF sensor and checked the air filter.

Last night I took apart the throttle body and idle air controller and thoroughly cleaned both with carb cleaner.
The car now accelerates and sounds much better than it did before the cleaning, but a slightly rough idle remains.

The external fuel pump (roughly under driver's seat) has been making odd high pitched noises since I got the car back in August.

Any thoughts on what to check next?
I'm probably going to replace the fuel filter anyway, as it is 4 years old, and probably has 80K mi on it.

It's also been suggested that a dirty EGR valve can cause a rough idle, however I can't seem to find one on this car.

 

I called a 25 yr master Volvo mechanic (friend of a friend) who walked me through resetting the idle to the correct range.
It's now idling at 700 rpm, but still rough.

 

It's good you found someone who actually knows LH2.4 Jetronic. Most folks think since the idle is controlled by the ecu that base idle can't be set. I think your rough idle is caused by a vacuum leak. People don't change out their hoses often enough and turbos are harder on 'em as they carry not only vacuum but positive pressure under boost. Many leaks are imperceptible until the hose is removed. I've chased them often enough on my own cars that I'm of a mind to swap them all out, including the larger brake booster and iac hoses. Swapping every single hose is not only a periodic maintenance thing, it rules out vac leaks from hoses. Other possibilities would be the intake gasket and/or injector seals. You can find those leaks using judicious sprays of carb cleaner.
Here's how to set base idle on the LH2.4 engines...

Adjusting the LH2.4:
1. Adjusting Idle Linkage Rod.
Ensure that when the cable drum is pushed off its endstop with a 2.5
mm feeler blade, the gap between throttle lever and adjustment
screw is from 0.1 and 0.45 mm. (This is the adjustment screw that
mounts on the throttle lever, not the adjustment of the larger nut
that attaches the lever to the throttle plate shaft). To achieve the
above spec, adjust the throttle link rod (ball-socketed connection rod
between drum and throttle lever)
2. Checking the Throttle Body (Plate and Throttle Position Switch)
Adjustment
Connect an accurate tachometer then warm up the engine. Let it idle in Park with a/c off. Pinch off the hose between the air intake and the
IAC valve (don't damage the hose with something sharp!) Idle speed
should drop below 500 rpm, or the engine may stop: both are
normal. If idle speed does not drop, adjust as below.
3. Adjusting the Idle Speed
On the front side of your throttle body, there is an adjustment screw.
Loosen the lock nut. Start the engine and turn the adjustment screw
until idle is 480-520 rpm. Switch off the engine and tighten the lock
nut while holding the adjustment screw so it doesn't turn.
4. Checking the TPS Adjustment.
Check the gap between the adjustment screw and the throttle lever
with a feeler gauge. Insert a .45mm feeler gauge. There should be
no click from the Throttle Position Switch on the rear side of the
throttle body when the throttle is closed. Then insert a .15mm feeler
gauge. There should be a click from the TPS when the throttle is
closed. If these are incorrect, then adjust as below.
5. Adjusting the TPS.
Loosen the TPS adjustment screws holding it to the throttle body.
First turn the TPS clockwise (away from the electrical connector) until
it stops. While keeping your finger on the throttle disk so it won't
move, turn the TPS counter-clockwise (toward the connector) until
you hear or feel a click. Continue turning until it stops, then tighten
the screws. Go back to step 3 above to check the adjustment.

 

I replaced all the vacuum hoses except the new looking IAC hoses which I trimmed and put new clamps on.

Good news: the brake pedal feels better and acceleration is smoother.
Bad news: idle is still rough.

I did both injector tests (unplug one at a time and carb spray) and intake manifold test with no positive results.

Any thoughts on what to do next?

 

I forgot some stuff.
About a month and a half ago I got some bad gas (I suspect watery) the car performed horribly until that gas was gone.

A large bottle of Techron has been run through the car.

 

What I thought was the EGR turned out to be the heater valve.
So now I've got better acceleration, braking and heat.

But still a rough idle..

I apparently missed the fuel pressure regulator's vac hose.

 

I got the last two vac hoses, long heater recirculation valve hose and the one to the fuel pressure regulator.

90% improvement, hopefully a new fuel filter will make up the last 10%.

 

Fast forward 3 months.
In total:
New fuel filter
rebuilt matching Bosch injectors
new spark plugs
new vacuum hoses
new intake gasket
new throttle body gasket

Throttle body, MAF and IAC valve cleaned.

The only code I get from the on board diagnostic is 1-1-1. (no fault detected)

It accelerates like it should and the turbo is actually working correctly.
In fact it works well enough to smoke smart *** kids in tricked out rice burners.

Yet this funny idle persists....
I'm at a loss here, could it be electronic?

 

Thanks for the detailed rundown of your troubleshooting. I just got my '93 940T a couple months ago, and it's been idling low (so low it stalled a few times today while the AC was cranking... it's been hovering in the 500-600 range.) My manual says it should be at 750, but it looks like I have a few other things to check out as well.

I haven't been much of a gear-head since I worked at Goodyear in high school, so I'm a bit nervous about touching a throttle body (I broke a bimmer messing with its throttle body a few years back)... being a rank amateur, should I avoid this or is it relatively safe as long as it's put back the way it was?