Hello friends,

We have a 97 mercury villager van, and my wife has been complaining of a rough idle when the van first starts. Irratic RPM's, it would come close to stall, then rev itself past 1000 RPM, then back down. When I get in the van, it seems to run fine. However the check engine light has been on in it for a while.

So I took it to advance auto to have them put the scanner on it.

3 errors were found

EGR valve
Knock Sensor
02 Sensor

Now I was told that the EGR valve regulates gas and air flow in the engine. And I searched on the internet and found that the knock valve has something to do with engine timing. And that the EGR valve may be giving me the 02 sensor error.

The EGR valve is about $80, the knock sensor is $130. I'm at a delima as to which it may be. Both seem to be likley culprits of a rough idle at start up.

Your thoughts?

The car has about 200,000 miles on it. And once you drive it, or when ever I drive it, it seems to run fine. It's only on a cold start.

I would clear the codes and drive it as normal until the light came back on and then read what code or codes are coming back.
The EGR could cause those symptoms.
But there are many other things as well.
If the EGR is setting a code, by all means, replace it and have the passages inspected.

A vehicle that old may well benefit from a new O2 sensor.

In the mean time, consider having the fuel induction system, including the throttle body, professionally cleaned.
Have you had a tune up done in the past? Worn plugs require far more energy to fire when the plugs and the engine are cold, and when they have cold, raw fuel dumped on them.
I hope this helps...