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My 98 Mazda 626 2L ATX has a fully electronic EGR valve controlled by the ECU. I've been trying to figure out a rough idle problem for a long time, which gets worse when the engine is under load, such as if the brake is held down in drive, if the A/C is turned on, or even if the radiator fan comes on, all of which cause more of a drop in RPMs than I would expect.

On a hunch I decided to check the EGR, and the ForScan software is showing that two of the four EGR control motors are "ON" basically all the time, even right after starting up the engine before it's even warmed up. I don't know if the ECU is reporting what it's commanding, or what it's detecting.

I'd checked the resistances on the pins with the engine off a long time ago and they were all in spec at about 22 Ohms, so I had ruled the EGR out based on that. However, now I'm having second thoughts.

So, is it normal for the EGR to be active at idle? If not is it possible that the ECU is turning it on due to some other problem, or that the ECU is simply showing it on because it's stuck open? If it's stuck open wouldn't that cause some kind of DTC?

EDIT 17/7/2017

I pulled the EGR off and cleaned it. It seems to be seating fine. It's the type which is a single plunger held closed by a spring. The only thing the ECU can do is open it. There's a picture of it here. Cleaning it didn't have any effect on my idle quality. I also tried unplugging it, which also had no effect.

Robert S. Barnes
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2 Answers2

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At idle, your EGR valve should be closed (read: not activated), allowing no exhaust gas to get into the intake air. At idle, any exhaust blended in with the fresh air might indeed cause your engine to have a rough RPM.

A rough idle combined with a fault code for excessive exhaust flow of your EGR system is a good indicator that it is well worth trying to clean your EGR valve.

Actually, the test for finding out if your EGR valve is still able to open goes like this: Run your car at idle, apply 12 V to the EGR valve's solenoid (making sure you don't fry the ECU!), and if the engine starts to run very rough or even dies, bingo, your EGR valve is able to open. Your problem, of course is the other way round. I've had the same issue, and cleaning the valve did fix both the check engine light (because of an EGR excessive flow error) and the rough idle I got with a warm engine.

Note: From what I know the EGR valve is supposed to open only when this condition is met:

 (Engine neither at idle nor at full power) AND Engine not cold

Which translates to:

Engine is warm and at medium power
zebonaut
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I think that the reason I'm seeing two of the EGR related PIDs reading on even during idle is because the EGR is a stepper motor and not a solenoid. Basically the PIDs are EGRMC1 through EGRMC4. The MC stands for motor control and I found a Mustang related site with WSM entries explicitly referring to it as a stepper motor, and listing the testing procedures for it.

That also would explain why disconnecting it even when two of the control signals are "ON" would have no effect. When those two are ON and the other two are OFF, it's in the closed position.

Robert S. Barnes
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