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The Leak Detection Pump (LDP) - Gaseous

Posted in How To: Engine on June 25, 2014
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If we had a dollar for every time someone told us their check engine light came on and the code was “Large EVAP Leak” caused by the gas cap, we’d be rich. If you live in a state where it is illegal to pump your own gas or you only use the full-serve lanes and can blame it on someone else, when you get an “EVAP Leak” code -- check your gas cap, whether it is “small,” “large,” “gross,” or any other kind. If tightening the cap doesn’t fix it, don’t just run out and buy a new gas cap, new filler hoses, or charcoal canisters. Hoses and hose clamps can come loose or get pinched, and most often the leak is the cause of something coming loose -- not a part going bad.

The codes come up because newer Jeeps pre-pressurize the fuel system to test for leaks. This was a government-required test in an effort to keep gas fumes out of the atmosphere, but it has a side effect as well. It can help keep gas fumes out of your garage. Chasing these codes can be like wrestling a greased eel because many times the check engine light will show up only sporadically, making diagnosis difficult. It can be weather-associated, elevation-associated, or just crazy as the cat lady next door and show up with no discernible pattern at all.

If tightening the gas cap didn’t work, borrow a known good one and try that. Many vehicles on the road today use the same style. If that doesn’t work, sometimes cleaning the gasket around the gas cap or wiping Vaseline on it can make for a better seal. After that, the next step is a visual inspection of the system. We always start at the gas tank and work our way outward. Look for disconnected or dry-rotted hoses, loose hose clamps, and cracks in plastic fuel system parts. Sometimes removing plastic trim and using a flashlight can save a ton of time, as seen on this TJ. Jeep used a lot of hard lines, then switched to rubber lines and back to hard lines again. Pay special attention to those lines, as they seem to crack faster than the longer runs of rubber.

Different years of Jeeps can kick different codes, but most code readers today will also tell you what the numbers mean, and that makes it easier. If you can still do the key trick on your Jeep, then you can search the Internet and learn what those numbers mean. That said, there are many places to check before just dropping your gas tank, spending a couple of hundred bucks for a smoke test at a local shop, or throwing new parts at the Jeep in hopes that something fixes it.

How It Works
The Leak Detection Pump (LDP) is present in all Jeeps capable of popping an EVAP check engine light. The earliest ones were engine-vacuum driven, but later ones had an electric vacuum pump built-into the charcoal canister. What it does is pressurize the fuel system to check for leaks. The whole thing is controlled by the PCM, and it pumps about only about 0.25 psi into the fuel system. If a leak is present, the PCM keeps pumping air into the system until it can figure out how big of a leak there is. After it figures that out, it pops the code that you get to see on your dash. It does this every time you start your Jeep.

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Smoke Test
If none of the visual tests gave you anything, and you can’t scan your system, you might want to try a smoke test before pulling parts. Basically it plugs into your fuel system and pressurizes it with air that has smoke in it to pinpoint where the leak is. When the smoke pours out from somewhere it shouldn’t, then you’ve identified your leak. The smoke is often from mineral oil, but any kind of smoke can work as long as it doesn’t leave deposits. Some parts stores sell smoke test kits for $20-$30. If that fails, there are shops around that can do a smoke test for you. However, if it is the purge valve, something internal in the charcoal canister, or other valves in the system, the smoke can come out of the intake or other odd places. But if it is something as simple as a hose, this is a great way to go.

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