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All, I just wanted to share what I found through trial and error and lots and lots of searching this forum, other forums, and online in general. I've had EVAP (Evaporative Emission System) leak error codes (P0455, P0456, P0457 or the GAS CAP message) multiple times over the past couple of years that finally annoyed me enough that I decided to launch a quest to fix it. That quest started with a trip to the service department but ended in me taking charge of finding the solution myself. Below is what I learned in hopes it may help someone else.

Bottom line, it's obvious Jeep's EVAP system isn't rock solid - problems are all too common. Personally, given I believe EVAP systems to be a bunch of hog wash to begin with, I wish there was a way just to pull the system out and be done with it. You could certainly do that, but I don't know of a good way to fool the computer into not throwing a big light up on your dash and codes in the system. If someone knows how, post it - I'm sure there are folks that might prefer to give it a try versus fixing it, particularly if they're beating their Jeeps on the trail, as the EVAP system is a delicate beast.

Codes: Below are the codes your computer will usually show if you have EVAP leaks.
- P0455 - EVAP system error (gas cap or other)
- P0456 - Small leak detected
- P0457 - Large leak detected

OEM MOPAR Part Numbers for items listed in the diagnostic section:
  • Gas Cap: 52100552AG (MSRP: $22)
  • EVAP Canister: 4891781AB (MSRP: $63)
  • EVAP Leak Detection Pump: 4861962AA (MSRP: $35)
  • EVAP Leak Detection Pump Gasket: 52129436AA (MSRP: $5)
  • Purge Valve: 4891731AA (MSRP $20)
Clearing Errors:
You can clear your gas cap message but clicking the little odometer reset button in the dash. Of course, if the issue still exists, it'll come right back next time you start the Jeep. You can also get multiple codes from the above list, and when you do, your odometer will be twice the brightness along with a nice little orange engine light in your dash. You'll need to connect an ODBII reader to read the codes and reset if desired. (You can also clear it by unplugging your battery, but again, if the problem still exists, it'll come right back within 50 miles or less, usually less)

Diagnostic/Fix Recommendations IN ORDER:

Step 1. First and foremost, change your gas cap. Yes, it sounds like bologna, but many, many issues are in fact a bad seal on the gas cap. When you do this, DO NOT use an aftermarket gas cap. I'm not saying it won't work, but you're in troubleshooting mode right now, and the OEM Jeep gas cap is cheap, and you can depend on it sealing properly. Once your issues are taken care of, that's the time to get that fancy billet aluminum gas cap if you want it. If this doesn't fix it, move to step 2.

Step 2. Check your EVAP canister. The EVAP canister is a black box on the underside of your carriage, just left of the gas tank. It has two plastic hoses coming out of it. It is totally unprotected, and if you like to offload, it's super easy to bust it/crack it. It's a hunk of plastic with charcoal in it. Some people choose to relocate it, and many aftermarket folks make skid plates to protect it. If it's broken or cracked or the plastic hoses look messed up, replace it. It costs around $30-$40. If you do replace it, MAKE SURE YOU HAVE A GOOD GASKET. There is a little green gasket between the EVAP canister and the little white box (EVAP Leak Detection Pump) attached to it. This gasket is critical - otherwise, it will leak. I personally put a little bit of silicone grease on the gasket to ensure it has a good seal. If you loose it or the gasket isn't in good condition, replace it. No dice? Move to Step 3.

Step 3. Check your hose near your battery. There is a black plastic hose that runs to your Purge Valve (a part of the EVAP system) that runs directly next to the right side of your battery. In fact, it touches it, and many folks have issues with the back side of this plastic hose corroding/rubbing till it causes a small pinhole. Feel the hose, particularly the part of the hose touching the battery. If it's rough and not smooth like the rest of the hose, you can temporarily and tightly tape it with electrical tape or replace that section. You can replace it with good quality fuel hose obtainable at any parts store, or you can even get short replacement EVAP lines with rubber ends from places like AutoZone for just this type of thing. I found a nice generic one from AutoZone that worked perfectly for mine. Still no dice? Move to Step 4.

Step 4. Replace your Leak Detection Pump. This little pump is what basically helps the computer determine if there is a leak. The pump is not exactly bullet proof in design of operation or longevity, and it's a VERY COMMON issue that it doesn't work properly causing a false alarm on EVAP leaks. The pump is cheap enough that if the above hasn't worked, I would replace it before moving to steps 5 and 6. Still no go? Try Step 5.

Step 5. Inspect all of your EVAP system parts and lines/hoses. Do this with your fingers feeling for loose connections and any holes or roughness in the lines/hoses. Also inspect very closely with your eyes and a really good, strong flashlight. See any bad hoses? If so, replace them. You can splice in a good quality 3/8" fuel line or buy the OEM replacement hose. If you use fuel line, just make sure you have a good firm connection and use a good setup of hose clamps for a quality seal. Still no dice? Move on to Step 6.

Step 6. Replace your purge valve. Cost of this part isn't great, so you could choose to replace it and give it a try. If you don't want to try that, you can test it by applying 12v to the incoming leads - it's basically a valve. You can blow into it, and apply voltage, then not blow into - if that's the case, it's probably working. It could still have a leak potentially, but that's the basic operational test. If you don't want to try replacing it, or if you feel confident it's working and not got a leak in it, then move to Step 7.

Step 7. You have arrived at the Smoke Test. You can hire your local shop to do this test or do it yourself. Basically, you send smoky air through the EVAP system, grab a strong flashlight, and look for a leak very carefully. Professional smoke test machines are commonly used to diagnose EVAP system leaks in all modern vehicles that have these blasted abomination EVAP systems. Unfortunately, these machines run about $2,000. If you're a stubborn do it yourself kinda guy like most Jeepers are, you can actually make your own for less than $50. Basically, you get a new metal paint can, pipe up some hoses in and out of it, hookup the incoming air line to a propane (low pressure) regulator you can buy at your hardware store, then hook the regulator up to your shop air compressor. Light some incense sticks inside of the paint can, seal the lid, hook the outgoing hose up to the line attached to the purge valve, and search for leaks. (The purge valve is located in front of the battery) There are plenty of youtube videos that show you exactly how to build these and use them. (Search for "DIY smoke tester" on youtube) If you're at this point, you're really out of quick and easy options. You're gonna likely have to do this or get it done to identify the root issue. Good thing is, it's doable, and if you have a leak, with enough time, you'll find it.

What was causing my error codes? My gas cap wasn't sealing every time, and I did have a corroded plastic hose due to rubbing up against the battery. However, I still had error codes after addressing those issues and replacing my EVAP canister. My fix was to replace the leak detection pump. Once I did that, she's been error free since, and I'm crossing my fingers it stays that way.

Good luck!
 

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Great write up! Thanks for the effort put into this post.
 

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thanks for the write up.
I have been messing with this same problem on my 2007 RAM 5.7L, Code P0456. Replaced gas cap, code still showing up.

Now just this past week my Jeep has the gascap error on the odometer and the P0456 code. Gas cap on order.
 

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Just got this error now. Tightened ho the has cap, cleared the code, and now crossing my fingers it doesn't arise again.
 

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Sometimes the gas cap rubber seal gets a bit dry and won't seal properly on the filler neck. Smear some silicone grease or Vaseline on the seal and make sure the filler neck is clean. I have had great results with this over the years.
 

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Good post and thanks. Will be checking into some of your suggestions. Hoping one of them works for me.
 

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Thanks for the write up! I'll be hitting these items tomorrow after I replace the fuel backsplash valve which the dealer refused to do under warranty...
I also asked them to diagnose and repair the trouble code P0456 and they could not figure it out, even with a smoke test. They replaced the gas cap (which was already a brand new after market cap), charged me $100.00 and said "let's see what happens?" "If that does not work we'll replace the gas tank for $1600."
Yeah right! Of course what happened was that the code tripped again and now we are back at square one. Thanks again for the information!
 

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Thanks for the write up. My odometer lit up with the gascap message today. Thank goodness, mine's still under the bumper-to-bumper warranty
 

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Changed out the leak detection pump, gasket and purge solenoid/valve. The cannister looks good, dealer already did the smoke test and inspection so not much else to do... Reset the codes and am crossing my fingers! Thanks for the write up again!
 

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Great first and only post by the OP from Nov '15.
 

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At least it was a good post.

My message turned off this morning without replacing the gas cap yet. It'll be changed tomorrow, though
 

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I had a similar problem with my 2007 Chrysler. It was an ongoing issue for several years. Each time I took it in they would replace something else. Sometimes they would replace the same thing they previously did, stating that the replacement had failed. The fault codes would go away anywhere from 2 weeks to several months but a fault related to the evap system would always pop back up.

After two years and over $1,000 worth of parts and labor they finally found the cause. A dirty filter upstream of the ESIM switch was causing the ESIM switch to fail. After that filter was replaced, along with a third ESIM switch, the problem cleared up and never came back.
 

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thank you for taking the time to post this. my check engine light came on last summer with the small evap leak code. it went away a few days later and hasn't been back since, but now I know what to look for if it does. thanks!
 

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Sometimes the gas cap rubber seal gets a bit dry and won't seal properly on the filler neck. Smear some silicone grease or Vaseline on the seal and make sure the filler neck is clean. I have had great results with this over the years.
Same here. I have three people I turned onto that trick that have reported back it worked. A lot of people replace the gas cap and don't clean the fill neck area that makes contact with the gas cap gasket, and have problems. They waste a lot of time and money looking for a fix, replace a perfectly good gas cap, and all they had to do was wipe the area clean, and smear some silicon grease on the gasket.

OP-Good write-up!
 

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Hey guys, I am having the same check engine light problems.

I am thinking it is the evac leak detection pump, where exactly is it located?
 
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