Howdy, long time reader first post and join...
So I just came back from a 600 mile hunting trip in my 91 240 DL with 210k mi and started having problems with it. Ran fine during the trip but not now... The first day back one time the engine seemed to stall as I was accelerating. The next day on my home from work (30 miles later) it refused to idle, barely ran, and I had to drive home with one foot on the brake and the other on the gas to keep it running (5 mi). I just did a full tune-up before I left. I have already tested the Mass airflow,it tested at 3 ohms cold(points 2,3) and 175ohms(1,4) hot, also the car barely runs with it disconnected at start up. I have checked the EGC thing, and its holding. Side note the plugs were pretty thick on carbon buildup for 1000 miles... Also I checked the fuel pump and the fuel by removing the fuel line from the injector input and starting the car. Fuel sprayed past my thumb like a good garden hose. The fuel sat up for 2 hours and it looked clear and perfect with no separation. Now what to do...
EDIT: for pictures of the spark plugs see https://volvoforums.com/forum/album....pictureid=3024

 

Sounds like you've checked many of the usual suspects. Those plugs look gnarly dude! Air filter fresh? Checked the fuel pressure regulator? Idle should change when the vac hose is removed. If the diaphragm has ruptured, it can dump fuel into the crankcase. So...smell the vac hose for gas fumes. A stale smell is ok...strong fresh raw gas smell is no bueno.While you're at it, take a whiff of your dip stick...if the oil smells of gasoline you have a problem. Generally, a well functioning O2 sensor will tell your ecu to lean the mixture but obviously that isn't happening. Either the O2 is dead or something has failed beyond the ecu's capacity to compensate. A failed engine coolant sensor can make the car run rich to a point but usually straightens out once the car is warm. So...check the above stuff and report back! J/K...hopefully it's an easy fix. My money is on a dead O2 sensor...it's amazing the stuff that can cause!

 

You might also check for codes, see the LH2.4 section here:
http://www.brickboard.com/FAQ/700-90...neOBDCodes.htm

 

What is fuel related and parallel to the drive shaft opposite the 1st muffler? Because that just started leaking gas. The oil did not smell like gas, but the vacuum hose off the fuel pressure regulator (i think that's what I pulled off there is a picture https://volvoforums.com/forum/pictur...pictureid=3025 ) did, When it was removed the idle increased, and the fell back when it was replaced. Also if its the o2 how do I test it other than getting a new one. Also on Sept 1 my car passed emissions testing crazy good. Yes I replaced the air filter in sept, also I did a thermostat in July.

 

Yes that is the fuel pressure regulator in the picture.

The main fuel pump is just before the rear driver's tire.

A working O2 sensor will have a voltage that quickly swings back and forth between .2 and .8VDC with everything connected, during a warm idle. You measure the voltage by peeling back the rubber boot on the connector on the thick green wire, on the firewall. A bad sensor will be "lazy" or have no voltage changes.

As already stated this could be the o2 sensor, coolant temp sensor, AMM, or fuel pressure regulator. The fuel trouble codes (or lack of) will at least help point you in the right direction.

 

So I found that the fuel filter had rusted out and was leaking replaced it. Tested the fuel pressure regulator, which was now leaking. After all that it ran great, for 2 hours, the the fuel pump went out. A paycheck later I replaced it with a bosh after dropping the fuel tank and replacing the worn out sock which was throwing particulates... Now it's running great for a 100 mi trip