Code scanner PO0400

Got a 98 Subaru Forester with 130,000mi. Good car so far. Just got a good deal on a scanner ($50) and it reports 1 code - PO0400. I see it mean EGR flow error. Could anyone help me with a most probable cause or a troubleshoot process to narrow it down? My repair manuals on the car are kind of lacking (got 'em on ebay for $10) and I can’t even find the blasted EGR valve! I thought I found it but I think it’s the knock sensor. Damn emissions crap - meanwhile a city bus goes by me and I can’t breath for 10 seconds! We have emissions inspection here and that ‘check engine’ light is the big $cha-ching$ for mechanics. Thanks guys!

This is what it looks like:

Unbolt it and clean it with some carb or brake cleaner.

Thanks RemcoW! So it I did find the EGR valve not the knock sensor.

No problem. Should cleaning the EGR not fix the problem, resist the urge to immediately run out and get a new one. First have a close look at the vacuum hose going to it. Sometimes they get clogged or are cracked.

The key to EGR diagnosis is not finding the valve but finding the whole system. The valve itself will be fed by a small pipe coming off of the exhaust manifold somewhere. That pipe can get plugged up. The pipe often feeds both the valve and a pressure sensor of some kind. The sensor feeds can also be a problem, as can the sensor itself. The sensor data along w/ other info from the engine is what the PCM uses to tell a solenoid to actually apply vacuum to the EGR valve to open it up. This requires a bunch of wiring, an intact & properly functioning solenoid & a good vacuum connection both from the solenoid to the manifold and the solenoid to the EGR valve. You can find these other parts by following things connected to the valve & feeder pipe.

So you have a lot of checking to do - and RemcoW is absolutely right about resisting the urge to just replace the valve. I could probably retire within a year if I could get my hands on all of the perfectly good EGR valves that have gone to the dump. A little eBay work & I’d be good to go.

The P0400 is a flow malfunction, so the first things to suspect are a clogged valve and/or clogged passages into the manifold where the valve connects. So when you pull it out make sure to inspect & clean the intake passages carefully. Make sure you have a new gasket handy for it. The old one probably won’t be worth a hoot. It should be cheap.

But there is also a really quick way to check for flow without pulling the valve. Just disconnect the vacuum line on the valve and rig another small piece of vacuum line onto the valve’s port. Idle the engine while you put some vacuum to the valve. I have a hand held/operated vacuum pump for these sorts of things. If you don’t, you can actually use your mouth but obviously be careful and don’t suck in the fumes - its exhaust! If the valve opens & flow is good the engine should immediately stumble. You should actually be able to stall it if you apply enough vacuum.

If you find the flow restricted I’d assume that is the issue, but otherwise the next thing I’d do is check all of the vacuum lines that look in any way related to the system. If the valve isn’t getting the correct amount of vacuum from the solenoid it won’t open enough. But all the PCM will see is a “flow malfunction.”

If you know anyone with a vacuum gauge you can actually check the vacuum in real time by tee-ing in the vacuum gauge to the vacuum line that actuates the valve.

Thanks cigroller! Lots of info and things to look at. I may have a vacuum pump, but if not, I’ll breath a little exhaust to save a buck. I’m sure I’ve sucked worse things into my lungs.

Okay - tmi, millhunk. :wink:
Hope it works out.

Cigroller - thanks for the EGR valve test! The engine stumbled just like it should and I have a vacuum pump so didn’t have to take a toke of exhaust (Remco - tmi, right? lol!). I tested for vacuum from the intake to the EGR valve and it stops at the device just before the EGR valve. My docs don’t list it, so I don’t know what it is, (it is located directly under the large vacuum hose connector that connects to the brakes) it has a small electrical connector - maybe its a solenoid of some kind? It has 2 small vacuum hoses - 1 goes to the EGR valve and the other goes to small black plastic flying saucer lookin thing higher up in a holder and the saucer has 2 small hoses that go to the intake. Watcha think guys? I also replaced all the vacuum tubing with new.

My EGR system knowledge is general (except for a few cars I’ve specifically messed with) so I can’t tell you for certain where or what things are on the Subie. The bit with the wiring attached is almost certainly the solenoid. The saucer - I don’t know. Maybe some kind of check valve device or something.

The solenoid will not apply any vacuum to the EGR valve except under certain conditions. The engine should be up to operating temp, and probably has to hit at least 1500 rpms before the valve will be activated to apply the vacuum. If your vacuum pump doubles as a vacuum gauge you can tee it into the line and play around with engine speeds to see what kind of vacuum gets applied, though it will apply varying amounts of vacuum depending on other conditions.

If you get the full low down on the system & can figure out how the solenoid operates you can test it directly. Some are energized constantly at 12V & then the PCM cuts power to apply vacuum. Others are off & commanded on w/ 12V to apply vacuum. Once you know how yours operates you can either apply/cut the 12V power supply to see if it stumbles/stalls the car at idle. Don’t do that w/out getting specs on the Forester system. I don’t want to think I helped you burn up a perfectly good solenoid.

Do you know how much vacuum (in Hg) it takes on the EGR valve to make the engine stumble. You’d have to find the specs for a Subie system, but on a Ford it only takes a couple of inches. During regular driving it typically only applies as much as 4in., though will end up as high as 8. If you had to pop it pretty hard to get the engine to stumble then it might mean that it just needs to be cleaned up.

Of course, it may just be that one or more of those hoses you just replaced were bad & now you’re done.

I’ll see what I can find out about that black saucer thing. If I had to guess what it is I’d say it’s a pressure regulator cuz it looks like one, and it has 2 vac tubes on the manifold side (vacuum side) and only one going to the EGR/solenoid.

Yeah, I had to put a pretty strong vacuum on the EGR to get it to stumble. I think I’ll pull it and clean it and see what happens - but you’re right - first, I might as well try to reset the code and wait, cuz it could have just been a bad hose. Thanks pal, you’ve given me a better understanding of another system on my Subie.

The black saucer-type thing is the vacuum modulator. It basically determines the proper timing for the EGR to open. Most Asian cars have them, and they are pricey to replace. But, typically not the source of your problems. There are actually three inputs, two vacuum lines from the throttle body and a riser line from the exhaust from underneath. Heated exhaust from the exhaust through the riser activates the modulator. This line is prone to getting blocked with carbon, making the EGR circuit not activate. It is easily replaced with a length of emissions-rated hose. The riser line can be cleaned out with a pipe cleaner or wire.

To check the operation of the vacuum modulator, connect a vacuum gauge between the modulator and the EGR valve with a tee. When the engine is cold, there should be no vacuum to the EGR at 2,500 RPM. With the engine warmed up, the vacuum to the EGR should be 2.5 to 3 in-Hg with the engine at 2,500 RPM. If you get no reaction from vacuum, you have a problem. On my wife’s 1992 Celica, my problem was a dirty throttle body with clogged vacuum ports. There was no vacuum getting to the modulator at all, and a thorough throttle body cleaning fixed the problem.

Thanks knuckles - I’ll check that other line to the modulator. Weather’s been bad, so I’m waiting for it to clear up. This one’s not worth freezin my ass off. Subie runs fine.

I checked the vacuum between the modulator and the EGR valve. I get good vacuum out of the modulator, but at running temp and approx 2500rpm, I get nothing out of the device between the modulator and the EGR valve. This device has an electrical connector so I’m assuming it’s a solenoid.

I put a external vacuum on the EGR valve and got the engine to stumble a bit, so I think I’ve got a good EGR valve. Doesn’t it look like the problem is that electrical device (possible soleniod)? Thanks.

Yes it is most likely the solenoid that controls the vacuum flow & thus operates the valve. What you describe probably means either that the solenoid or its power supply are no good - OR that whatever info the PCM uses to decide when to activate the solenoid is no good. In the latter case, the problem will not be in the solenoid or its wiring - but rather in its control.

You’re going to need to hear from someone who really knows the Subaru EGR system, what inputs the PCM uses for deciding on EGR function, and what inputs it uses to figure out whether the system is doing its job. I can tell you all about a Ford system - but not Subie.

One thing I do know is that these solenoids are either normally open to manifold vacuum or normally closed to it. The PCM will send it power (probably 12V) to either keep it closed when it should be (if normally open), or open it up when it should be (if normally closed). You can do a quick check of the solenoid itself by giving it direct voltage. On a Ford system, e.g. the solenoid is normally closed. If you give it a direct 12V from the battery at idle it will stall the car - one shadetree mode of checking the solenoid. Two cautions about that - one is that these are often duty cycle solenoids & you can’t just keep steady voltage on them or there’s a good chance you’ll burn it up. Second is that on a Subaru it might actually be a lower voltage. So don’t do anything that will damage the solenoid just in case there’s nothing wrong with it to begin with.

Anyway, that all adds up to - you need to get a good, full description of how Subaru’s system is designed. If no one here turns up with it you might look for some dedicated Subaru forums.

cigroller - Thanks. Yeah, I thought about it and that solenoid is the only electrical device in the system. So, that means just what you said - something controls that device (PCM or whatever). I’ll see if the Subaru service center here will give up some info on the system. The docs on this car are VERY scarce - both Haynes and Chilton don’t make any repair manuals for it. I can’t find anything for it except what I bought on ebay and that’s a bit spotty.