Kia Optima evap codes

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My son has a 2007 Kia Optima 4 cylinder with a couple hundred thousand miles. It's due for the CA smog inspection, but has the CEL on. Codes are P0455 (large evap peak) and P0451 (evap pressure sensor range). Our first reaction was to replace the gas cap with a new Stant. No luck, code came back on. So we took it to a local mechanic with a Yelp good reputation. They said they did a system pressure test and found no leaks. So they replaced the gas cap again, this time with a new OEM unit. A few days later the codes are back. We called the mechanic and they basically said they will just throw parts at it until it gets fixed. Not very cost-effective for us. I do all of my own routine maintenance and even the occasional timing bet and clutch, etc. but have never attempted to solve an evap problem. In an attempt to avoid going back to the mechanic, do you have any suggestions on how I repair this myself? My research tells me that if it's not the gas cap, it's either the purge valve solenoid, charcoal canister or pressure sensor. Only the purge valve solenoid is easily accessible on this car. How can I more accurately pinpoint the problem?
 
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In my experience, evap codes can be a difficult diagnosis. My only advice would be to get a 2nd opinion on the pressure check. In the meantime, here's a Kia diagram that I found a while back and have hung onto ever since. This might help you should you decide to start replacing parts.

kia evap system diagram.png
 
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I just went thru this on my 01 Tacoma. (And we have a 2016 Optima-hence the interest)..I tested my vacuum solenoid and it was ok. I didn't have the equipment to check the pressure sensor and I kept seeing it was unavailable or it was $400, so I bought one from Ebay with the same part #. It was off a 96 camry for $25, and it worked perfectly. BTW, if that CEL is off for around 100 miles get it to inspection station quickly. The computer needs to see a certain number of cycles or miles to show ready.
 
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They did a "pressure" test? Or a smoke test? Some cars can only accurately be smoke tested when running. Hyundais and Kias over a few model generations have a fuel tank air filter (only available from the dealer) that is supposed to be swapped periodically, but never is. It can cause evap issues. I doubt it would evidence in a gross evap leak. Hyundais and Kias also have fairly common issues with the pigtail to the canister vent valve in the rear. It usually results in a voltage code relating to the valve itself, though. Finally, the monitors programmed for these evap systems sometimes can register a leak in the system due to a valve not opening/closing correctly just a few times in a larger number of operations, such as 3 failures in 100 operations. I have seen many Hyundais and Kias reject aftermarket gas caps, but that seems to have already been covered. A competent shop that will carefully smoke test the vehicle would be the way to go. Throwing parts at these systems is almost never the way to go. On your fourth part, $500 in, you may find that a vacuum line off the intake is simply split and leaking when the engine is torqued during driving, which can't be found at idle. There's generally no "slam-dunk, common, easy" fix for these systems. Someone just has to spend time with it and be knowledgeable about what to check and how to check it.
 
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I think solving the pressure sensor problem first might cure the gross leak. I could be wrong, of course. But it would be worth looking at the live data.
 
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Check the charcoal canister for small hair sized cracks. Check where the canister is bolted into its bracket(s), the bolts go through the canister body and can crack over time. From my experience a P0451 is many times just a bad evap pressure sensor. You will need a vacuum guage and and scan tool with live data to confirm its operation.
 

bamorris2

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Originally Posted by KalapanaBlack
They did a "pressure" test? Or a smoke test? Some cars can only accurately be smoke tested when running.
I spoke to the shop, said it was a smoke test. Thank you everyone for the suggestions.
 

bamorris2

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We took the car back to the same mechanic, as they agreed to do additional diagnosis for no additional charge. They called and said that they found a very small leak and want to replace the charcoal/vapor cannister and a sensor, both of which require removing the fuel tank. They said they cannot guarantee it will fix it though. What's the best way to proceed? Should I get another opinion from a 2nd mechanic? I'd just hate to pay almost $500 and it still not be fixed.
 

bamorris2

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Originally Posted by JTK
Can you smell gasoline anywhere around the fuel filler, tank or evap canister?
I can't smell any gas. Certainly not around the filler neck. The canister is up by the fuel tank, so hard to get my nose near it. But no, no noticeable odor of fuel.
 
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JTK

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It sure seems to point to the pressure sensor. The shop said you went to didn't troubleshoot that at all?
 
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Sounds like you need to find another shop to look at your car. If this shop says that they cant guarantee their work, it generally means they are not 100% on their diagnosis. I would try to find another shop capable of doing real diagnostic work, not just firing a parts cannon and praying. Most likely it is the canister and the sensor , but there is also the possibility it is not. This vehicle needs a true diagnosis , that means a thorough smoke test .The evap sensor needs to be tested with a bi-directional scanner and vacuum guage to verify operation. Also a thorough visual inspection of the whole evap system is in order. Most likely isnt the cause but I have seen spiders in the filler vent , spiders clogging and cracking the fuel vent filter as well. Tell the shop to basically test dont guess.
 
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It's probably a long shot, but check the wiring to the pressure sensor. You would assume the shop did this, but maybe not. I had a P0452 evap pressure code on my Silverado that wasn't fixed by a gas cap. I also didn't smell fuel, but checked the filler clamps and the evap hoses as well as I could. The sensor is in the fuel pump, on top of the tank. I unplugged and replugged the connector to be sure I could access the sensor by feel from underneath, and then ordered a new pressure sensor. When it came, I unplugged the connector again and about 5 inches of the harness came off with it--some varmint had chewed through the wires. A new pigtail on the old sensor fixed my code.
 
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I would like to see if the pressure sensor reacts accordingly to vacuum and or pressure in the system. This will tell you if you're chasing a sensor issue or something else. You'd need a scan tool with live data and bidirectional controls and a smoke machine though.
 
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Originally Posted by mattd
I would like to see if the pressure sensor reacts accordingly to vacuum and or pressure in the system. This will tell you if you're chasing a sensor issue or something else. You'd need a scan tool with live data and bidirectional controls and a smoke machine though.
To be completely fair, how many repair shops are either capable of doing that or have the inclination to do that? If I were to take wild guess, 90% of shops are nothing but the part replacers, especially when it comes to EVAP diagnostics.
 
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This is very true unfortunately. If you have a competent tech that is supposed to be what your diagnostic fee pays for. There are not many people that can accurately diagnose evap systems for some reason. I know If i suspect an issue with a pressure sensor that's the first thing I do on a customers vehicle.
 
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