Wife's Galant came up with the "service engine soon" light a couple of days ago but car was running fine. Before I could get it checked, it started running rough. Pattern seemed to be rough idling when stationary but in "drive" and when accelerating quickly or under a load (like up hill). It runs smooth when idling in "park" or when gently accelerating. And it wasn't rough in every instance of acceleration, but only intermittently.

Code was checked at Autozone and printout reads as follows: P0400, EGR system fault, ECM detected too little flow on the EGR system. There were also random cylinder misfires. Autozone guy said the EGR system fault would/could cause random cylinder misfires and randomness rules out a plug/wire/boot issue which would, if present, show as a repeated misfire in a specific cylinder(s).

So I find the EGR valve...on the backside of the engine kinda tucked into the intake manifold.

Anyone have first hand experience with testing/replacing the EGR valve? Can it be removed/replaced without removing the manifold? Better to just replace or should I try cleaning it first? Is there an easy way to test for valve vs. solenoid fault since one part is $60 and the other is $40. Hate to replace a part I don't need to replace.

After some thought, it seems logical that the valve is sticking at least partially open. That would explain the rough running under load and when idling in gear and when first starting up a cold engine. Reintroduction of exhaust gases would normally be prevented under these running conditions so introduction of exhaust gases in these situations would lead to misfires. But under gentle acceleration, cruising and idle in park (when RPM's are higher than idle in drive) the introduction of exhaust gases doesn't cause misfires. Is my understanding of how the EGR system works sound?

 

EGR fully open at all times will cause lean misfire,if your car has individual egr runners like some nissans it is possible to have all ports clogged except one and EGR flow directed to single cylinder
Forget the EGR for now.Concentrate on misfire,it is really easy to check on 4 cyl...use precautions so not to get zapped by high current,pull each spark plug wire or disconnect coils(if you have coil on plug set up) one at the time.
Disabled Cylinder that is least affecting idling is the one with problem.
Try swapping plugs,wires,coils,see if misfire moves within cylinders

 

Not sure but I don't see individual runners for each cylinder, at least not anywhere close to the EGR valve and solenoid. Valve exhausts into one tube.

And the codes were pretty clear in that it was random cylinder misfires rather than with specific cylinders.

Besides, it doesn't look like a port clogging issue or exhaust gas would be prevented from reaching the combustion chamber...which would mean NO misfires...just a bit hotter engine temps because exhaust gas isn't getting recirculated to help cooling. Presence of misfire would seem to indicate it's an open valve problem rather than a closed valve (or interrupted flow) problem.

Thanks for the input. I'm not trying to be argumentative, I'm just thinking out loud as I learn about the EGR system for the first time.

 

Yes you are correct in how the egr system works. Egr flow is blocked at an idle and wide open throttle, it then flows during most other conditions. It is used to control nox emissions gas. It sounds like the egr could be sticking internally and allowing flow. It is not likely a clogged port that would usually set a egr performance dtc and usually doesnt have an affect on driveability but increases emissions. This is likely setting your misfire dtc as well. You can test it by using a hand held vacuum pump and apply maybe 8 IN.HG to the vacuum port on top of the egr, the car should run rough or stall, then after releasing the vacuum the car should recover. It could be gummed up but i would just replace it rather than try to clean it unless you just did it for test purposes. As a side note never just pull the plug wire off with the engine running to find a weak cylinder, it can create a volage spike and fry the coil, always use a spark tester. If you want to find a weak cylinder unplug the individual coil or fuel injector.

 

Sorry i left some things out of the response, it could be the solenoid as well. You would need a vacuum gauge to check. There should not be any vacuum at the egr at an idle. If vacuum is found at an idle you should look at the control side of the system, i.e. the solenoid or vacccum hoses to the throttle body.