Well, we have seen this post a number of times, but here goes. I have been battleing rough idle on my 93 850 for about a year now. No fault codes, and no failures, but a pronounced rough idle. I have folowed this board and have applied my mechanical knowledge as best I can. Machinist for 19 plus years now.
I have tried Cap rotor plugs and wires. Fresh fuel filter. Motorvac cleaning at Indie shop that I have been a customer at for 20 Plus years. New O2 sensor. cleaning Throttle body, no EGR air pump on 93. Checked vacuum lines. Purchased RPM sensor, not installed yet. Removed Cam sensor, visual check cleaned RPM and Cam sensor conection. Replaced Fuel pump relays. Checked fuel pressure at idle and received 40 PSI reading.
Only thing left in my mind is to replace the fuel pump. Rough idle seems more pronounced when tank is down to 6-7 gallons of fuel. I am debating buying the whole pump assembly, or just the replacement cartridge. Any thoughts on this on would be greatly appreciated. Only other thing I could think of would be a faulty injector, but I do not know a good way to inspect those as a DIY project.
Original owner, 84000 miles. 6th volvo owned

hi there,a fuel pressure regulator can cause rough idle and poor gaz mileage.check also for vacuum leak.try to pull your fuel pump out and visually inspect it,sometimes you will have a dirty screen or partially detirioated hose at the top of the pump.good luck.charles.

You can check your injectors by removing the fuel rail. Check them visually but you can also check by putting the injector in a jar (still on fuel line)
disconnect coil so the car wont start or attempt start. Turn the key as to start the car. Watch the spray pattern. Should be conical. If not try to clean it with varsol or something. (The jar is to collect the gas as it sprays) While the rail is off you can inspect the injector seals and the fuel pressure regular.

Spraying water on injectors (cover the spark plugs and rotor)
at idle will make the engine cough if injector seals are bad.
Spraying start gas or propane on vaccum lines will change rpm
if there is a vacuum leak.

[From 700-900 FAQ]

Dmitri

Hmmmh!

Have you checked all the vacuum lines and fittings with a vacuum gauge?
Other possibilities:

Evap valve - mounted on the side of the air filter box.
Inlet manifold gasket - could be leaking.
Idle air control motor - might need cleaning.(this does control the idle).
Fuel pressure regulator.

It does sound more like a vacuum/air problem rather than a fuel one. Could you check fuel pressure and see if there a leakdown?

Matt
1993 850 GLT 130,000 miles

Hmmmh!

Have you checked all the vacuum lines and fittings with a vacuum gauge?
Other possibilities:

Evap valve - mounted on the side of the air filter box.
Inlet manifold gasket - could be leaking.
Idle air control motor - might need cleaning.(this does control the idle).
Fuel pressure regulator.

It does sound more like a vacuum/air problem rather than a fuel one. Could you check fuel pressure and see if there a leakdown?

Matt
1993 850 GLT 130,000 miles

My 93 idles rough too, I gave up runs fine otherwise, good mpg,
good power for a n/a engine, My indy says the 93 all do it

My 97 850 runs smooth a glass