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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.

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    According to Wikipedia: en.wikipedia.org/wiki/Exhaust_gas_recirculation ...EGR is omitted at idle. But consider this: EGR control motor being "ON" can mean the ECU is trying to actuate the motor to the "OFF" position, but the motor isn't working. So, unless you know what the scan software results actually mean, don't try to narrow down the cause too much. Rough idle can be absolutely caused by EGR, though.
    – juhist
    Commented Jul 16, 2017 at 10:39
  • Oh, and at full load EGR is also omitted, but at partial load it is in use. So, if the same problem happens at partial load as well, it indicates the cause would be something else than EGR (or else you have two separate problems resulting in rough running).
    – juhist
    Commented Jul 16, 2017 at 10:43
  • Also consider that the egr motors could be on trying to close the egr if it is seized : a common problem on some cars. Have you checked if it is moving?
    – Solar Mike
    Commented Jul 16, 2017 at 14:33
  • @SolarMike Hmm, it does say in the WSM to check for an initialization click sound from the EGR when starting or stopping the engine: EGR valve initialization is closing action of the valve observed before and after the engine is started, and when the engine is stopped. I'll need another person for that though. But the main question is if it's showing "ON" during idle is that normal?
    – Robert S. Barnes
    Commented Jul 16, 2017 at 15:14
  • I did ask if you had checked if it is moving....
    – Solar Mike
    Commented Jul 16, 2017 at 15:18

2 Answers 2

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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
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  • so what could make it open at idle when the engine is cold?
    – Robert S. Barnes
    Commented Oct 5, 2017 at 9:28
  • Well, I needed two attempts to clean the valve in my car. Maybe if it doesn't close all the way and maybe if it doesn't really, completely seal off the path for the exhaust, you might still get some rough idle. Also, the throttle valve, the air intake filter and maybe the MAP/MAF sensor might cause the ECR to give you weird symptoms. Think of it this way: The better the path for the fresh air (good, clean filter; good throttle valve), the less it matters if there is still a small amount of exhaust mixed in. The mixture is made by the ratio of the pressures of the clean air and the exhaust.
    – zebonaut
    Commented Oct 5, 2017 at 10:25
  • What is the definition of warm?
    – Robert S. Barnes
    Commented Oct 27, 2017 at 9:10
  • @RobertS.Barnes Actually, your question is better than I first thought when I read it. The dumb answer would be: When your coolant temperature meter shows approx. 90 °C - but the real question is: When does the ECU (engine control unit) decide it's warm enough to activate the EGR valve. However, I don't know the value in degrees...
    – zebonaut
    Commented Oct 27, 2017 at 13:22
  • I logged some data, and the first time the EGR is activated the engine was only at 54*C, load 39%, rpms 1673.
    – Robert S. Barnes
    Commented Oct 30, 2017 at 22:14
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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.

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  • This is the first time I've heard there's a version of EGR valves being moved by a stepper motor. Until today, I only knew about those being activated by vacuum or a solenoid. Troubleshooting a stepper is a bit more on the tricky side. As a start: How does the controller know the valve is at one of its end positions? And how do we activate the stepper to check if it is moving? The questions become more... Sorry...
    – zebonaut
    Commented Oct 27, 2017 at 13:24

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