MaxC1Get!

Hi guys! When I'm stopped at a light, my RPMs rise intermittently to 900-1000 RPM. The engine will rev like this for approx 10-15 seconds. Sometimes when I start the car, the RPMs rev to 2000 but only for a few seconds. Do you have any suggestions as to what the problem could be.

The car is a '96 automatic GXE.

Thanks
Don

Daniel B. Martin

Any air which "sneaks" into the engine from a source downstream of the Mass Air Flow Sensor is called False Air. False Air will cause RPMs to increase. There are several possible sources of False Air. Because your RPM increase is seemingly random the first place to look is the flexible duct which conveys air from the MAFS to the Throttle Body. The engine moves around a bit on its flexible mounts during normal operation. If there is a crack or split in that air duct, engine movement may cause the crack to open and close. Every time it opens, RPMs go up. Every time it closes, RPMs drop.

Other sources of False Air include a damaged or disconnected vacuum line, a stuck-open Positive Crankcase Ventilation valve, and a leaky power brake vacuum line check valve.

False Air is one of those problems which does not set a Diagnostic Trouble Code and turn on the Malfunction Indicator Lamp.

When the engine reaches normal operating temperature the electronic engine management system goes into Closed Loop mode. The Oxygen Sensors sense the concentration of oxygen in the exhaust stream. The Engine Control Module (the computer) uses the signals it receives from the Oxygen Sensors to adjust the fuel mixture. False air puts extra oxygen in the exhaust gas. The ECM responds by directing the fuel injectors to correct by supplying extra gasoline. More air plus more fuel equals higher RPMs.

We commonly refer to the accelerator as the "gas pedal". With modern fuel injected engines this has become a misnomer. It is more properly called the "air pedal".

MaxC1Get!

Daniel

Thanks for the information regarding the sources of False Air. It will help me understand more when I take it to the dealer.

Cheers,
Don