P0446-Evap Emmis Control System

[Mikevan10]

OK, here'e the deal:

 

1997 2.2 Legacy

 

CEL on. Fault code 446("Evaporative Emmission Contrl System Vent Malfunction"). This same code had come up about 2 years ago and I replaced the vent solenoid valve at that time. I do NOT know if that actually fixed this fault at that time because my CEL light stayed on but I probably had other "faults" which would have kept it lit. I did not have access to a scan tool and did not take it to Autozone to get the codes scanned at that time.

 

So this week I got hold of a scan tool. Read the faults. There were four lsited and P0446 was one of them. Cleared codes. CEL off for first time in years!! Let idle for a few minutes and CEL stayed off. Shut down and restarted. CEL came on in a few seconds. Scan codes. Only one - P0446.

 

Disconnected solenoid vent valve and measred resistance across its terminals: 26.4 ohms. Haynes manual says this should be anywher from 10 to 100 ohms so it appears (from that test) that the sol. valve is ok.

 

Measured voltage at power side of solenoid valve connector: 13 volts when key is turned on.

 

Did a point by point (tedious) check of the wiring between the "downstream" side of the solenoiod vent valve connector (White with blue stripe) and the ECU. Everything checked out fine. Total resistance between valve connector plug and pin in computer connector plug is 0.7 ohms. That sounds ok, right?

 

Removed solenoid valve from vehicle and connected it to a 12 volt battery. Solenoid "pulled-in" just fine. Nice positive click. Tested by mouth and valve is closed when de-energized and open when energized. Exercised valve by connecting and disconnecting power about 20 times. Seemed fine.

 

Reinstalled solenoid vent valve. Made sure all connections were secure. Cooencted scan tool and cleared codes. Started engine and CEL did not come on. Let it run for a while and fault code P0446 popped up in about 5 minutes!!

 

Hair is getting pulled out. What could possibly be happening? Is the vent valve truly an ON-OFF deal or does the computer proportionally open the valve up?

 

Sorry for the long post but I wanted to be thorough (like I was in my check-out procedure).

 

Someone please!!!

 

Thanks,

Mike V

[nipper]

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hrmm

 

can you smell fumes coming out of the hoses? Could be clogged filter in the evap can (if they still have them). A cloged evap line?

a gremlin?

Loose connection?

 

 

nipper

[Mikevan10]

Thanks for reading my post Nipper.

 

No, I do not smell any gas fumes at all.

 

Actually, I wonder if the computer actually is monitoring the anything other than the electrical circuit. Do you think it is somehow monitoring the gas vapor side?

 

Also, does anyone know under what conditions the computer tells the valve to open?

 

I beleive my testing ruled out a loose connection except at the ECU connector plug itself but the pins there looked clean.

 

Please, any suggetsions? I need to fix this in order to pass inspection!

 

Thanks again!

 

Mike V

[nipper]

have you checked the purge valve? Haynes doesnt even mention this valve, unless its the two way valave they are discussing..

 

nipper

[porcupine73]

Hi Mikevan10,

Actually, I wonder if the computer actually is monitoring the anything other than the electrical circuit. Do you think it is somehow monitoring the gas vapor side?

Maybe...I see reference to a 'Evaporative Vapor Pressure Sensor', at least in alldata for my '96 Legacy, but I can't find any more info for it so I don't know if it really exists.

[ferret]

Mike, Detach the following attachment. My attachments are filling up so I have to remove some this week. It's right out of the FSM for the P0446. I'll bet you have a high resistance from the drain valve return back to the ECU. You already stated you have 12v at the solenoid.

This fault states the ECU does NOT see that 12 volts before it 'energizes' ( provides a ground via a semiconductor ) so it sets the CEL.

 

P.S. this is for a 98 Forester, but most ECU connections for the evap system are the same across Subaru's

[Mikevan10]

Thanks much Ferret!

 

I quickly read through the procedure and (unfortuneately) I believe I have already accomplished most of what they describe.

 

You said "I'll bet you have a high resistance from the drain valve return back to the ECU" but I did measure the resistance from the drain valve plug connector to the ECU plug connector and it was only 0.7 ohms. I believe step 10AJ7 in the procedure says this needs to be below 1.0 ohms so (again unfortuneately) that looks ok.

 

I am not sure I fully understand your comment "This fault states the ECU does NOT see that 12 volts before it 'energizes' ( provides a ground via a semiconductor ) so it sets the CEL.". Are you saying that if the resistance in the circuit between the drain valve and the ECU were too high there would be too much voltatge drop there and the ECU would then see too low a voltage?

 

I will consider the procedure more carefully and perform any checks that I have not already done. But I am seriously afraid I will get to the very last step on page 193: "CONTACT WITH SOA"!!! Don't think I could afford that....

 

Again, thanks very much for your comments and consideration!

 

Mike V

[ferret]

In the past I have been fooled by a voltmeter measuring ohms. When you use the meter, you are usually sending 1.5v at a few ma thru the test circuit. But when you put a 'load' on it such as the drain valve coil, you might find that at the ECU you do not have 12v, but much less. ( STEP 1 ) And if you do have 12v at the ECU input connector, you may be looking at a replacement ECU as the internal semiconductor that's supposed to apply ground when the ECU tells it to, may be defective. In which case you either replace the ECU, or live with the CEL.

 

PS..say hello to the 'valley' for me.....used to visit relatives there regularly as I grew up in W-B....and before 'Wild Water Kingdom' we used to call it Dorky Park.

[Mikevan10]

OK. I will interpret the Step 1 instructions as saying to disconnect the ECU, turn the ignition on and measure the voltage between the Purge valve pin on the ECU connector and ground. Correct? This is not what I would call the "OUTPUT SIGNAL FROM THE ECM" in the instructions. Not sure why they would call it that.

 

As far as "living with the CEL", I could not give a darn about it. This is ALL about passing state inspection and if the CEL light is on for ANY reason, I do not get a sticker and risk a $100.00 fine every time the car leaves the driveway.

 

Thanks again,

Mike V

[Mikevan10]

UPDATE:

 

I did the Step 1 check on the P0446 troubleshooting procedure from the factory manual (thanks to Ferret) and I get battery voltage at the appropriate pin (ECU connected, measuring probe inserted from rear of connector) when I turn the ignition on. HOWEVER, when I turn ignition OFF, battery voltage REMAINS at this pin for several minutes and then drops out. I was able to repeat this test several times. I believe that the power should drop out IMMEDIATELY upon turning ignition off, right??? So now I am suspecting a problem with the "Main Relay" that controls the power to this circuit. Trouble is, my Haynes manual does not tell me where this relay is. Can anyone help? Ferret? Nipper? Anyone?

 

Very frustrated but at least I have found what seems to be an anomaly.

 

Thanks,

 

Mike V.

[nipper]

Haynes manula page 12-4 upper right hand of page.

 

nipper

[Cougar]

Mike,

 

What Ferret is saying about this problem is right on. The ECU completes the circuit for the valve by making the connection to ground. This means when you measure voltage between the ECU pin and ground that there will be near full voltage at that point when the valve is not turned on. When the valve is on the voltage will be near zero volts.

 

The circuit is monitored by the system by looking for a current in the circuit. It is hard to say if the voltage dropping off slowly is really a good thing or bad thing. You are seeing a capacitor discharge most likely.

 

Since you are getting voltage to the valve on the supply side, that eliminates the Main relay as a possible problem. If the voltage never goes to zero on the ECU pin then the problem would appear to be with the ECU or something else that tells the ECU to open the valve.

 

Good job on your troubleshooting proceedures. You are doing the correct things.

 

If the problem is with the ECU it can be fixed probably. In other model ECUs, I have replaced IC's in them that make the connection to ground, thus fixing the problem.

[OB99W]

Mike,

 

Ferret and Cougar have both given you good advice. However, I'd like to add one more thing. I believe the ECU (ECM) drain valve control pin (B134 pin 10) should remain high (not pull to ground) when the ignition switch is only placed in the "ON" position (engine not started); you should see close to battery voltage under that condition. However, once the engine is started the ECU can pull that pin to ground, and the voltage can then go low (typically under 1 volt).

 

I'd suggest hooking up the voltmeter to B134 pin 10 (back probe, as before) and ground, so that you can see the meter from the driver's seat. Begin with the ignition switch off -- the voltage should be near zero (relay contacts open). Then turn the key to "ON" -- the voltage should come up to near battery voltage (ECU powers relay coil via B135 pin 19, thereby closing relay contacts). Next, start the engine -- see if the voltage drops (monitor it for a while), and to what voltage.

 

If you get back with the results, I might have further insight.

[Mikevan10]

Thanks for chiming in '99! I will run the test you suggest tonight and I'll report back. I have been wondering under what conditions the good ol' computer tells the purge valve to open. Sounds like you understand it to be open pretty much any time the engine is actually running?

 

Since P0446 seems to ONLY be related to the electrical circuit (no one has opined otherwise nor has anything I have read specifically indicated otherwise), I think we have, at this point, boiled it down to one of two things:

 

1. Bad connection between the ECU and the B134 connector.

 

or

 

2. Problem internal to the ECU.

 

Cougar has been man enough to go in and do internal repairs on the ECU. Now that's persistance!

[OB99W]

[...]I have been wondering under what conditions the good ol' computer tells the purge valve to open. Sounds like you understand it to be open pretty much any time the engine is actually running?

No, and I'm sorry if I left you with that impression. What I was trying to say is that it wouldn't be open with the ignition switch just "ON", if the engine wasn't running. Even with the engine running, the ECU still decides when to open the valve. Do let us know what happens when you get the chance to check things as I suggested.

 

 

Since P0446 seems to ONLY be related to the electrical circuit (no one has opined otherwise nor has anything I have read specifically indicated otherwise), I think we have, at this point, boiled it down to one of two things:

 

1. Bad connection between the ECU and the B134 connector.

 

or

 

2. Problem internal to the ECU.

 

Not necessarily. Ferret pointed out the pitfalls of using an ohmmeter, and even a voltmeter can lead to false conclusions under certain circumstances. The main thing one has to appreciate is that a voltmeter is purposely designed to have a high internal resistance (and therefore draw minimal current) in order to limit its influence on circuit operation. The problem with that is when you're looking for a resistive connection (or corroded wire, etc.) by voltage drop, unless the circuit itself is drawing current, you may not see it. (Ohms Law says that a voltage drop is equal to the current flowing through a resistance times that resistance. Even very high resistance won't cause a significant drop if the current drawn isn't sufficient.)

 

The other factor is that corrosion can lead to intermittent contact that often changes just by moving things. Oxidation can be broken down by movement (or sufficient current), but reform in a while. You might measure low resistance, but still have a problem a bit later.

 

 

Cougar has been man enough to go in and do internal repairs on the ECU. Now that's persistance!

I'm glad that he does. I also do component-level repairs when it's merited. Often, the cost of a replacement electronic module, or sometimes availabilty issues, are good motivators . It's also a good idea from an ecological standpoint.

[Cougar]

That's exactly right what you say about the economic motivators OB99W. The first time I woked on a ECU was when it failed to turn on the fuel pump in my 88 GL-10 wagon. Subaru wanted around 600 dollars for a new one and a rebuilt cost around 300 dollars. The problem ended up being with a TO-92 style driver transistor that cost me about $1.60. I also repaired another one for a shop that had the very same problem. That one was a quick fix after working on the first one.

 

Thanks also for the backup on the lighting problem.

[Mikevan10]

Today's update:

 

Ran the tests that OB99W suggested and here's the results:

 

Voltmeter connected between ECU connector B134 pin 10 (back probe, connector still hooked up to ECU) and ground.

 

Ignition off: 0 volts

Ignition on: 12.5 volts

Engine running: 13.7 volts

 

I let the engine idle for several minutes and reading did not change. Revving engine had no effect (voltage may have increased a little due to higher alternator output at increased engine speeds). Engine was up to operating temperature.

 

Apparantly, the ECU was not gorunding/completing the circuit.

 

Any ideas what to try next?

 

Also, where can I procure a Subaru Factory Service Manual?

 

Thanks again,

 

Mike V.

[Cougar]

You are correct Mike. The ECU is not turning on the valve. The voltage will go low when it does.

 

I can't tell you what makes the ECU turn on the valve. Maybe it is always supposed to be open when the engine is running or there may be some other external control to the ECU for that. You may be able to get another ECU from a salvage yard pretty cheap to see if that will solve this problem. I would be glad to work on yours if you want but if you can't be without the car for a while that won't work out.

 

You should be able to find a manual or factory set of manuals (total 8 of them I think for your year) on Ebay. A full set may run about $160 dollars but that is a bargin really over the factory new prices. Once you have them you won't let them go.

 

Here is a sale for a CD that is pretty good. I have one myself.

 

http://cgi.ebay.com/ebaymotors/1996-98-Subaru-Outback-Legacy-service-repair-manual_W0QQcmdZViewItemQQcategoryZ119140QQihZ006QQitemZ160027017331QQrdZ1QQtcZphoto

[Cougar]

Mike,

 

One other thing you could try in order to pass the IM test is to place a ground jumper on the ECU purge valve control pin. This will open the valve and hopefully clear the code though, depending on the circuit design it may not. In that case you are sunk. If it works then you can get a replacement ECU later on. This procedure may also cause a different code to appear. The system may see the ground on lead at startup and say there is problem with the load. Then you are still sunk.

[Mikevan10]

OK, I went ahead and tried what Cougar suggested: I attached (rear connected) a ground jumper to the purge valve solenoid pin (B134 pin 10) and, after clearing the fault codes, ran the engine. As ususal, after starting the engine the second time, the CEL came back on. Upon reading codes, I had two of them: P0446 (as usual) and now also PO443 (which also somehow realtes to the EVAP system). So I cleared the codes and retsted. Now I am back to just getting P0446 after starting the engine twice.

 

So, my friends, is the consensus (conclusion?) that my ECU is defective???

 

Any other possibilities?

 

I do understand Ohms law and that voltmeters are high impedance devices and ohmeters only flow low currents, etc. But the checks I did make all indicate the harness is fine. I have also visually inspected virtually all of this circuit and everything looks to be in fine condition. The solenoid pulls the purge valve in with a positive "click" when I ground B135 pin 10.

 

Suggestions?

 

Running out of hope,

 

Mike V.

[Cougar]

You have done the correct testing Mike. It looks like you have gone as far as you can go with this problem short of getting into the ECU. You may want to do that. I know I would check it out, and you have enough savy with a meter to check into this. So lets go for it.

 

Some folks have repaired ECU problems by finding simply bad connections to the outside connector. Who knows, maybe that is what this problem is also. I would find the connection to pin 10 inside the ECU and see if is ok. If that is fine then you could trace the circuit back to see where it ties to a component on the board. Chances are that component (most likely a IC) is damaged and needs to be replaced. I have some IC's left over from other jobs and it may match yours if that is the case.

[nipper]

OK, I went ahead and tried what Cougar suggested: I attached (rear connected) a ground jumper to the purge valve solenoid pin (B134 pin 10) and, after clearing the fault codes, ran the engine. As ususal, after starting the engine the second time, the CEL came back on. Upon reading codes, I had two of them: P0446 (as usual) and now also PO443 (which also somehow realtes to the EVAP system). So I cleared the codes and retsted. Now I am back to just getting P0446 after starting the engine twice.

 

So, my friends, is the consensus (conclusion?) that my ECU is defective???

 

Any other possibilities?

 

I do understand Ohms law and that voltmeters are high impedance devices and ohmeters only flow low currents, etc. But the checks I did make all indicate the harness is fine. I have also visually inspected virtually all of this circuit and everything looks to be in fine condition. The solenoid pulls the purge valve in with a positive "click" when I ground B135 pin 10.

 

Suggestions?

 

Running out of hope,

 

Mike V.

 

sometimes you just have to swap them out and find out.

 

nipper

[OB99W]

[...]So, my friends, is the consensus (conclusion?) that my ECU is defective???[...]

Perhaps, but before going there I have yet another question: To retrieve the DTCs, are you using a full-functioned scan tool, or just a code reader? If it's a scan tool, it should allow you to obtain freeze frame data related to set codes. If you can do that, let us know what that data is (since only one code is being set, the frozen data should relate directly to the conditions that particular code was set under). That info may (or may not, unfortunately) help pinpoint what's going on.

[OB99W]

[...]Some folks have repaired ECU problems by finding simply bad connections to the outside connector. Who knows, maybe that is what this problem is also. I would find the connection to pin 10 inside the ECU and see if is ok. If that is fine then you could trace the circuit back to see where it ties to a component on the board.[...]

Could be, Cougar. I'm not privy to the ECU's internal B134 pin 10 circuit, but you've got to figure that it's an I/O situation; it's apparently monitoring the voltage, as well as being the pull-down for the valve's coil. Certainly if the pin connection isn't being made at the ECU, it would fit both conditions Mike has experienced -- the ECU wouldn't see the DC coming through the valve's coil, and it also wouldn't be able to pull the pin low. Just a thought.

[nipper]

Could be, Cougar. I'm not privy to the ECU's internal B134 pin 10 circuit, but you've got to figure that it's an I/O situation; it's apparently monitoring the voltage, as well as being the pull-down for the valve's coil. Certainly if the pin connection isn't being made at the ECU, it would fit both conditions Mike has experienced -- the ECU wouldn't see the DC coming through the valve's coil, and it also wouldn't be able to pull the pin low. Just a thought.

 

closely examin the terminals in the connector. Make sure they are al the same height. It is possible to pop the individual terminals out of the connector, but practice on one from a junker. That is the one true way to clean it, then pop it back in. Make sure the battery is disconeected for this.

 

 

nipper