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Thread: Basic EVAP system for LS applications

  1. #1

    Default Basic EVAP system for LS applications

    Well, I was gonna put a huge detailed thread about how I built a sweet EVAP system for my Jeep CJ off-road wheeler rig, but after I got done typing out a huge detail and data filled post the site auto logged me out and I lost it. So that ain't happening again. PM me if you want to know what I did and how I did it.

  2. #2


    I've gotten into the habit of copying the text from a lengthy post like that into the clipboard BEFORE hitting the submit button, if the post is rejected then you can simply paste it back in and try again, or if there's a problem with the website, paste it into notepad and save for later.

    I'm actually interested in how you did your EVAP system, maybe just post up some pictures and answer any questions raised from there?

    I started to design my own system a while ago, but never finished it. I wanted to include rollover protection so I had a series of one-way check valves on the tank vents, attached to a custom vapor/liquid separator, then to a charcoal canister with a solenoid controlled by the ECU for evacuation.


  3. #3


    OK, I'm back. You're right Bigfella237, I should have copied/saved all the text before I hit post quick reply. That would have solved it.

    This system is for LS MPFI using the Holley Dominator ECU. One thing to keep in mind is get your tune correct first and then shut off learning to use this EVAP system, because the computer will sense the small vacuum leak it creates and it will affect your tune after awhile of driving around. Don't let it mess up your tune.

    What I did for EVAP was as simple as I could. This is an offroad LS powered Jeep CJ, so I tried to make a system that sort of, kind of, worked like the original. Only using modern technology to control it, but I kept that as basic as I could. My knowledge is limited, as you are about to find out, so that's why I kept it basic. The way I did it would work for any car though. My table needs more refining, but the purge valve does pull a vacuum, as verified by my thumb when the engine is revved up.

    Anyhow details about it. My engine is a 2003 6.0 liter LQ9 out of an Escalade. Used an Escalade charcoal canister, but any canister from a GM truck will work. Used the original OEM GM purge solenoid that sits atop the intake manifold. Lost the original nylon hose with quick connect fitting. Had to go to JY and get one. Pretty much any GM car from the early-mid 2000's has all the quick connect fittings you need, or Amazon has them. I used nylon fuel line for the vacuum portion of the system, so it won't collapse. Used rubber fuel line for every thing else.

    Here is a diagram of my system. Click the pic to make bigger. Ignore the tank pressure sensor and fuel level sensor. Those are needed for the OEM GM system. I am using the OEM vent valve, only because it has a nice filter built into it and keeps the system clean. It is normally open/powered closed (for leak checking the OEM system), so it just sits there open all the time.

    Click image for larger version. 

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    Pics of the OEM GM purge solenoid (sits on top of intake, right behind throttle body). Most people take it off and install block off plate. Myself included, till now. Also in the pic is the rollover check valve I used. It's from a Dolorean (like from Back to the Future haha), but looks just like OEM Jeep CJ.

    Click image for larger version. 

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    Pic of the liquid check valve I used. This device prevents liquid fuel from going into your charcoal canister during refuel.

    Click image for larger version. 

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    This next pic is the rollover valve and OEM Jeep CJ liquid check valve (but I'm using the aftermarket LCV pictured above)

    Click image for larger version. 

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    There's 3 ports on the charcoal canister. The vent port is the large 5/8th one. The smallest 1/4" port is the fuel line from the rollover check valve. The 5/16th port goes to the purge solenoid atop the engine.

    Click image for larger version. 

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    Click image for larger version. 

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    Well that covers the parts to use and how to hook it up. I'm sure I forgot stuff. Ask questions if you have them. Next up is the part that is the mystery. How to get the Holley EFI to control the purge valve.

    The purge valve is pulse width modulated solenoid (PWM) and is ground triggered. I used my fan #2 output and configured it to be PWM - There are vids online to explain how to do that. I'm not going into that portion. The OEM purge solenoid has a frequency range of 5 to 30 hertz. I saw other people choose 16 hz as a starting point so that's what I did. I chose a MAP vs TPS strategy to control the valve. You can see how I set up the table below. Sorry about the pic size. I couldn't get the screenshot to be any bigger, but essentially I'm opening the valve more and more the heavier the load and I also open it quite a bit during throttle closed decel. Vary the duty cycle as needed to get the valve to open when you want it to. Pretty basic. Ask questions if you have them.

    Click image for larger version. 

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    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by JeepCJ-6; 07-22-2019 at 12:20 PM. Reason: Fix pic size, add info

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