The Mercury News Weekend

Getting to the bottom of a rough-running engine

- By Brad Bergholdt

I have begun to have a problem withmy 2001 Ford Escape with theV6 engine. It starts fine in the morning and runs OK for a few miles, and then it begins chugging and loping when I come to a stop. It also hesitates to take off but then drives OK again after I get to higher speeds. Any idea of what could be the cause before I make a service appointmen­t? — Karen

There are many possible reasons for an engine to idle rough or hesitate on accelerati­on, but most of them would occur at all times. Your comment that the engine runs fine for a short time before acting up leads 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 GMvehicle, 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 use a vacuum operated EGR valve (addressed here) 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 it’s idling. If it does, engine roughness such as you described may occur. Since your Escape runs well when the engine is cold, it appears the computer controlled electric solenoid (valve) controllin­g the EGR valve is functionin­g normally.

What I believemay be happening is the control solenoid’s vent filter has become restricted, which causes the EGRvalve to linger open when the engine returns to idle. This is a serviceabl­e and inexpensiv­e part, located beneath a removable black plastic cap that snaps onto the top of theEGR control solenoid. Unsnapping the cap reveals a removable foam thimble, which can be either cleaned or replaced. Try removing the filter and drive the Escape for a short time without it to see if this corrects your idle loping/roughness symptom.

Onother vehicles with a less obvious vent filter location, or perhaps a non-serviceabl­e integrated filter, one can temporaril­y disconnect and plug the vacuumhose leading to theEGR valve (its location, along with that of the control solenoid, should be indicated on the under-hood emission control informatio­n label— ifEGRisn’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 (spark knock).

I should add that an engine that idles rough cold or hotmay be the result of anEGRvalve 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 faultyEGR position or exhaust pressure sensor.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, California. Readers may send him email at bradbergho­ldt@gmail.com; he cannot make personal replies.

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