By Brad Bergholdt
November 13, 2012
There are many reasons for an engine to idle rough or hesitate on acceleration, but most of them would occur at all times. If the engine runs fine for a short time before acting up, this would lead me to focus on the EGR system. This is an emission control strategy that allows a small quantity of Exhaust Gasses to be Recirculated through the engine a second time. Adding a small, carefully regulated quantity of exhaust to the incoming air-fuel charge reduces combustion temperature, and the formation of NOx, a nasty emission compound.
I wrote about this a while back regarding a GM vehicle, but have since heard of quite a few other makes and models of vehicles encountering similar issues. Many but not all vehicles have EGR. Some, like a 2001 Escape with a V-6 engine, use a vacuum-operated EGR valve and others use a purely electric EGR, which isn't prone to the problem I'll mention.
EGR is not supposed to occur when the engine is cold, or when idling. If so, engine roughness may occur. If an Escape runs well when the engine is cold, it would appear the computer controlled electric solenoid (valve) controlling the EGR valve is functioning normally. What may be happening is the control solenoid's vent filter has become restricted, which causes the EGR valve to linger open when the engine returns to idle. This is a serviceable and inexpensive part, located beneath a removable black plastic cap which snaps on to the top of the EGR control solenoid. Unsnapping the cap reveals a removable foam thimble, which can be either cleaned or replaced.
On other vehicles with a less obvious vent filter location, or perhaps a non-serviceable integrated filter, one can temporarily disconnect and plug the vacuum hose leading to the EGR valve. Its location, along with that of the control solenoid, should be indicated on the under-hood emission control information label — if EGR isn't mentioned, the vehicle likely does not have one. Try driving the vehicle for a few minutes to check for a difference in performance. If it markedly improves, an EGR fault exists. If the check engine light illuminates, don't worry; it'll go out within a day or two and the stored trouble code will self-erase in about two weeks. Long-term disconnection of the EGR is not advised as it is illegal, increases emissions and may lead to engine-damaging detonation, or spark knock.
I should add that an engine that idles rough cold or hot may be the result of an EGR valve that does not fully close. The cause is often a build-up of carbon on the valve seat. Rough running while under way can result from a faulty EGR position or exhaust pressure sensor.