Boxter with various codes/rough idle/misfire

[ziggyrama]

Greetings, my wife recently purchased a 1998 Boxter with 30k miles from the original owner. A month after we got it, it began having some issues including idling rough, showing multiple codes including a vacuum leak and an occasional misfire. After some research, I replaced the gas cap, replaced the fuel tank vent valve, and cleaned the MAF sensor, but it is still not running properly. I've read more that perhaps the issue is due to a bad resonance flap actuator or vapor canister purge solenoid? I don't have a lot of experience with this car, so I feel like I am just chasing the issue - any advice would be appreciated. Thanks

[JFP in PA]

Welcome to RennTech 

P0306 is a misfire on cylinder #6

P0447 indicates an open circuit (disconnected electrically) on the EVAP cannister shut off valve.  Possible wiring issue or valve itself.

P1318 also indicates a misfire on cylinder #6

 

[ziggyrama]

Thank you JFP! This is the valve located near the throttle body under the intake? Does this part generally fail and need to be replaced, or do the connections sometimes just need to be fixed? I noticed it is $350 on Pelican so wondering if I should plan on purchasing it. 

[JFP in PA]

14 hours ago, ziggyrama said:

Thank you JFP! This is the valve located near the throttle body under the intake? Does this part generally fail and need to be replaced, or do the connections sometimes just need to be fixed? I noticed it is $350 on Pelican so wondering if I should plan on purchasing it. 

No, it is located at the EVAP system cannister in the wheel well (item #6):

 

And don't buy from pelican, use board sponsor Sunset Porsche

[ziggyrama]

will do, thank you. I will disassemble the wheel well again and take a look.

[JFP in PA]

11 minutes ago, ziggyrama said:

will do, thank you. I will disassemble the wheel well again and take a look.

When you do, check that the valve has electrical power before doing anything.  If there is something wring with the wiring, changing the valve will get you nothing.

[ziggyrama]

yes, makes sense. Is there is a constant 12v to the valve once the car is on? I also read that there may be a clogged line to check for as well.

[JFP in PA]

9 minutes ago, ziggyrama said:

yes, makes sense. Is there is a constant 12v to the valve once the car is on? I also read that there may be a clogged line to check for as well.

That is possible, but would throw a different code if it was  Your code is an electrical issue.

[ziggyrama]

6 minutes ago, JFP in PA said:

That is possible, but would throw a different code if it was  Your code is an electrical issue.

Ok, didn’t realize it was specific to an electrical issue. Are there any fuses or other issues I should check first or is it part of another system? We did have new stereo equipment installed, now I’m wondering if something happened. 

[JFP in PA]

The code read out is as stated: "Open circuit, check valve voltage supply, check valve for function, check trigger wire for continuity"

[ziggyrama]

After some testing, there is 12v between the ground wire that clamps to the fuel line and the blue/red wire on the valve.  The yellow wire doesn’t appear to have anything present on it. Does that sound correct?

 

Update: removed the canister and valve and can confirm valve opens manually when I apply 12v directly 

Edited March 1, 2023 by ziggyrama

[JFP in PA]

According to the wiring diagram in the OBD II manual for your car, the valve should have two wires, #2 that receives a power signal from the DME relay terminal 87; and the #1 wire that runs to the DME terminal #7 (the DME supplies the valve's ground when it needs to run).  I do not have the color codes for those two wires.

 

Have you checked the valve to see if it functions?  Unplug the harness connector at the valve, there should be a 22-26 ohm between pins 1 and 2 on the valve, and you should see battery voltage on the wiring harness connector pin #1.

[ziggyrama]

The valve tests at about 24 ohms and has constant 12v on pin 1 when the ignition is on. Could you point me in the right direction to find the relay so I can test it? 

[JFP in PA]

Slot #1 in the trunk, but if you have power at the valve harness connector, it should be working.

[ziggyrama]

Thanks. I was able to remove the relay and test that it is working properly so now I’m not certain now what the issue is. 

[JFP in PA]

You haven't mentioned two items: removing and testing the valve itself and testing the triggering wire back to the DME pin for continuity.

[ziggyrama]

I tested the valve and it operates manually when I connect it to 12v. I will test the continuity between the relay and the trigger wire. Thank you. 

[JFP in PA]

The trigger wire runs from the valve to the DME, the wire from the relay simply supplies power.

[ziggyrama]

Thanks for the clarification. Do you have any information on the DME and what pin that wire is on?

[JFP in PA]

The second, or trigger wire, runs from pin #1 on the valve to terminal pin #7 at the DME.

[ziggyrama]

On 3/2/2023 at 1:58 PM, JFP in PA said:

The second, or trigger wire, runs from pin #1 on the valve to terminal pin #7 at the DME.

Thanks for the advice.
 

Now that the weather has cleared, I was able to resume testing and the voltages test fine. I’m at a loss now as to what to check for next. 

[JFP in PA]

1 hour ago, ziggyrama said:

Thanks for the advice.
 

Now that the weather has cleared, I was able to resume testing and the voltages test fine. I’m at a loss now as to what to check for next. 

But did you test the wires between the sensor and the DME for continuity or shorts to ground?  Your code is for an open circuit, indication of a wiring discontinuity. 

[ziggyrama]

10 minutes ago, JFP in PA said:

But did you test the wires between the sensor and the DME for continuity or shorts to ground?  Your code is for an open circuit, indication of a wiring discontinuity. 

I will re-test continuity between the valve wire 1 and the DME; if there is voltage at the valve, wouldn’t that indicate it should be working ok?

[JFP in PA]

Not if there is also a short  on the same wire.  Test for continuity and presence of ground on each wire while the harness is disconnected from the DME and the sensor.

[ziggyrama]

After some testing, there doesn’t appear to be any continuity between pin 1 on the valve and pin 7 at the DME. Actually there is a very low resistance of about 70 ohms between pin 1 and ground.