Volvo 850 sometimes runs rough and gets low gas mileage sometimes runs fine

SOURCE: idle low then hi, and gas smell, and bad gas mileage

This should show up easily on the ECU diagnostics computer at a mercedes dealership. Basically your fuel mixture is way too rich causing your car to be heavy on gas, have a raw gasoline smell, and not run right. Something like this can be caused by a defective oxygen sensor (but that usually produces a check engine light) or it might even be a defective ECU. There is really not way of knowing for sure until you do a proper diagnostics where they hook up your exhaust at the same time and do an emmission test. In most cases it's some sensor that needs replacing.

SOURCE: 2005 chevy colorado runs rough and gets bad gas mileage.

Sounds like it needs spark plugs and wires, and a fuel system treatment.

SOURCE: 1993 volvo 850 heater blower works sometime tell me why?

These models had problems with the blower motor power stage, which is what controls the fan speed, and the blower motors wear out. There a good chance it needs both!

SOURCE: volvo 850 won't start occasionally

Hey Kolby!

Sounds like you've got an issue with your alarm module, causing the immobilizer to activate (also known as a "start inhibitor"). I'm not sure what type of alarm you've got, but this will probably help you find out if there is something wrong with it:

There is a panel with relays on it behind the dash. At the bottom of the steering column, by the drivers feet, there is a panel. Remove it to get access to the relay panel (just a couple of torx screws, no breakable clips as far as I can remember). Remove the "double size" relay (probably marked 210-211). Put a wire between the terminals 50E and 50F. This will hotwire the immobilizer and make sure that it always activates when turning the key.

Drive around with the wire in place instead of the relay for a while to see if this helps. If it does you'll know that the problem is related to the alarm system. Keep in mind that you will be without alarm and therefore probably also without remote central locking while doing this.
Make sure that the wire is connected firmly enough so it doesn't come loose. You can also put a fuse on it if it makes you sleep better at night ;)

Good luck!

SOURCE: The car starts and immediately stalls unless I

Hey Jruthesq

The 850 is equipped with an Idle Air Control Valve (IACV) which supplies the engine with air when idleing (a necessity since the main throttle body valve is completely closed when no throttle is applied, thus the engine would lack air without the IACV when idleing). The IACV is suspended in a rubber mount on the front of the throttle body itself, and is easily removed and inspected/cleaned. If it's dead a good cleaning usually gets it going again. Also make sure that the air from the IACV is unhindered on its way from the IACV to the throttle body (it's actually common for the holes in the throttle body to be gunked up so bad by hardened carbon deposits that one sometimes needs a nail or even a drill to open it up again).

Just thought I'd mention this since you said that the gas needs to be feathered for the engine to start. When you press down on the gas the main valve in the throttle body opens up, closing the IACV and supplying the engine with enough air.

Oh and if your fuel pump relay is a green one marked 103 you should replace it with the new and improved red one. The relay is located under the fuse box cover under the hood. Remove the 4 torx srews holding the cover in place and you'll see the relays under there.

Good luck mate!