Cold start: (0-10 °C) Idles at 2400 rev/minute
Warm start: Idles at 3200 rev/minute, no doubt due to the reduced engine oil viscosity.

After touching the gas pedal once, idle speed cycles between 1500 and 2000 rev/min, once every second or two. It sounds like a fuel cut upon reaching 2000 rev/min.

This anomaly began suddenly and has been consistent. The day before, the idle speed was about 800 rev/min, with slight hunting. (maybe ±150 rev/min)

Engine runs smoothly and delivers the usual amount of power. I have not driven it enough to comment on fuel consumption. I suspect it's burning a lot of fuel at red lights. I need to stand on the brakes pretty hard; it idles at 30 mi/hr if I don't.

Disconnected the battery overnight. No change.

Suspecting a stuck idle-air control valve, I disconnected the electrical connector from the IAC. Idle speed dropped to a steady 300 rev/min. (warm engine) That suggests there are no stray vacuum leaks.

Pin 1 on the IAC connector has 12 volts.
Pin 3 has ~160Ω to ground with the ignition switch on; several hundred kΩ to ground with the switch off.
(there is no pin 2)

Pin 1 to pin 3 on the IAC itself: 4Ω.
Pin 1 to the body of the IAC: ∞

The throttle-position switch clicks when the throttle is moved slightly off idle. Have not confirmed its function with an ohmmeter.

Suggestions? I'm hoping somebody out there understands this system and its software better than I do.

Automagic transmission, no turbocharger. 232,000 miles.
Driven every other day or so, entirely city driving, less than 200 miles per week.

 

Go to this link:

https://www.volvoclub.org.uk/faq/Eng...PressureSensor

and look under "Regina Manifold Air Pressure (MAP) Sensor"

That was the culprit on my'91 740. Good news is it's available as a GM part, meaning cheap. There's also a temperature sensor in the airbox that can be faulty.

 

The saga continues:

Not the MaP sensor. Output: 4.8 volts DC when open to the atmosphere; falls toward 1 volt DC as the pressure is reduced.

Checked the tube to the MaP sensor. Despite its age, the nylon tubing looks good as new. The rubber connector is a little stiff, but doesn't appear to be cracked or leaking. Covered/sealed it with a piece of heat-shrink tubing anyway.

Unplugged the MaP sensor. No new fault code, but the engine refused to start. Reconnected it.

City fuel consumption has increased to 12 mi/gal. I haven't done enough highway driving to say what the highway mileage is.

Fault codes
2-3-3 (Adaptive idling control out of limits)
2-2-1 (Lambda sond sensor absent or faulty) and
1-2-2 (Air temperature sensor signal absent or faulty)
Replaced the O₂ sensor, which was completely covered with soot.

#4 spark plug had hard, grey, ashy/sandy deposits.

Disconnected the battery overnight. All fault codes cleared.
Ran the engine for 5-10 minutes. No change in idle behavior, no fault codes.

Unplugged the intake-air temperature sensor. Code 1-2-2, as expected. Reconnected it.
Sensor resistance: 2.6 kΩ at 15°C; 12 kΩ at -20°C.

Latest trick: it puked up 3 liters of coolant, but the temperature gauge suggested it wasn't overheating, and it didn't leak when I applied pressure with a radiator pressure tester, either cold or warm.

 

the other three plugs looked OK (light grey coating, not oily or charcoal ?)

if one cylinder is that different than the other three, I'd be doing a compression test on all 4 cylinders before I go much farther.

 

The other three plugs were sooty, but not crusty.

Compression: 175 - 175 - 171 - 173 lbf/in²

I figgered as much, since it runs smoothly and develops a normal amount of power when you put the pedal to the metal.
It's nice to know the long block's in good shape, even if I'm temporarily flummoxed by today's injection/ignition/emission problem.

 

have you 'smoke tested' for intake air leaks ? various ways you can do this, but the idea is to spray something thats NOT fresh air flooding the various possible sources of vacuum leaks, like around the injectors, around the throttle body and idle air valve, etc.

 

I don't think it's a vacuum leak.
- It started doing this suddenly. One minute, it idled at 900 rev/min. The next minute, 2400 rev/min.
- When I unplug the electrical connector from the idle air control valve, it idles at 300 rev/min. If a vacuum leak were responsible for the fast idle, unplugging the IACV wouldn't have any effect.

It's not the throttle-position switch, either. It switches normally (less than 1Ω) at each end of the throttle's travel. Pin #2 is solidly grounded, (again, less than 1Ω) and pins #1 & #3 are not.

 

ok, then its quite possible your IAC Valve is sticking open... time to pull it and clean it (throttle body spray cleaner works reasonably well on these). if the rubber hoses to it are at all sketchy, I'd plan on replacing them with fresh rubber as they are much harder to install when they are rock hard than fresh pliable rubber.

 

When I disconnect the electrical connector from the IACV, idle speed drops to a steady 300 rev/min. (warm engine) Were a stuck-open IACV responsible for the fast idle, unplugging the IACV wouldn't have any effect.

 

Did you ever figure this out??

 

Yes. It turned out to be a chafed & grounded control wire that was holding the IACV open. But only when everything was assembled. When I removed the connector from the IACV to put an ohmmeter on it, I also un-grounded the chafed wire. After I repaired the insulation, it began idling at 700 rev/min and fuel consumption dropped to the window-sticker estimate.

Never did figure out why it suddenly puked up 3 liters of coolant one day. It did that only once, then resumed its usual consumption/leakage -- about 0.05 l/month.

 

So look at the wires that connect to the IAC. My 93 940 is having rpm problems, where the rpms bounce from 700 to 1,000 rpms. I cannot figure out what is wrong with this damn car.