Reving engine Turn car on and the engine will rev to 2500. Usually idle at 900. As I was driving today, engine surges and slows down.

autodr

• 260 Answers
• Posted on Dec 01, 2008

SOURCE: At idle the engine revs up and down

engine lite on? would ck to make sure there is plenty of coolant in system. low coolant may cause idle hunting. also could have a vacuum leak or air intake restriction. ck air filter

SOURCE: volvo s4o 1.8 gdi 2003 model, I have just cleaned

sounds like an air leak. check to see if you've left a vaccuum pipe off. you may hear a hissing noise whilst the engine is running. failing that spray a small amount of brake/carb cleaner and listen to a change in revs.
if your sure there's no leak detected then it sounds like a faulty throttle body.

SOURCE: Engine stalls on tickover. Drives

I would suggest looking into this suggestion first then I'd inspect your air filter, spark plugs, and fuel pressure. This is a culprit on many vehicles and overlooked by many people because they don't know what it is or what it does. An IAC (idle air control) motor is designed to adjust the engine idle RPM speed by opening and closing an air bypass passage inside the throttle body. The cars computer or ECM (electronic control module) receives information from various sensors and will output signals to adjust the IAC motor in or out to adjust engine idle speed by controlling engine idle air. An IAC motor can fail one of two ways, either the motor short circuits and stops working or the motor will develop high resistance and cause the IAC control motor to react slowly, either failure can cause the engine to stall at idle. When a trouble code scan is performed it sometimes won't always detect a failed or weak IAC motor. To check the IAC motor remove the unit, with the wires connected turn the key to the "on" position without starting the engine, the IAC should move in or out. If the IAC motor does nothing it has probably failed, replace it with a new unit and recheck system. Note: while the IAC motor is removed clean (use aerosol carburetor cleaner) the passages the IAC uses to control idle air speed, also inspect the IAC for a build-up on the seating (pointed) end and clean as necessary.

Idle Air Control Valve (appearance will vary)
Common Problems
An IAC motor is highly susceptible to carbon and coking build up; if an IAC goes too long without cleaning it can cause stalling and poor idle quality. Some cars are designed with a large vacuum transfer hose that connects the intake manifold to the IAC (idle air control) motor. If a broken or dilapidated these vacuum lines can cause the engine to lose vacuum which will allow the engine to run rough and die. Inspect all engine and accessory vacuum lines to look for missing, torn or dilapidated lines and replace as needed. Any car that is designed with a magnetic non-motor operated IAC like Toyota/Lexus is subject to carbon and should be cleaned about every 40,000 miles to avoid stalling.
Basic Maintenance
To check the IAC motor remove the unit, with the wires connected turn the key to the "on" position without starting the engine, the IAC should move in or out. If the IAC motor does nothing it has probably failed, replace it with a new unit and recheck system. Note: while the IAC motor is removed clean (use aerosol carburetor cleaner) the passages the IAC uses to control idle air speed, also inspect the IAC for a build-up on the seating (pointed) end and clean as necessary. Hope this helps you and good luck