P0442 engin ligh

mdoziermdozier Member Posts: 7
edited May 2018 in Lincoln
Town Car 2004 I get this code, I can turn it off then it comes back on again about 600 miles later. I’ve tried changing the gas cap with no luck. Any idea what else to try! P0442 evaporative emissions small leak detected


  • eliaselias Member Posts: 2,209
    rubber hose from fuel tank/filler-neck to the charcoal canister likely has dried out and has a crack. inspect/replace it and the clamps used to attach it.
  • mdoziermdozier Member Posts: 7
    Thanks, I will give it a try
  • PF_FlyerPF_Flyer Pennsylvania Furnace, PAMember Posts: 9,372
    Start simple...Common Problems That Trigger the P0442 Code. Defective or damaged fuel cap. Distorted or damaged Fuel Tank Filler Neck. Small tear or puncture in the Evaporative system hose(s) and/or Carbon Canister. Defective Fuel Tank Sending Unit gasket or seal.

    Given the age of the vehicle, hose problems are possible Things dry out.
  • thecardoc3thecardoc3 Member Posts: 5,592
    What anyone trying to diagnose that code really needs to know is how exactly how that test is run, that way they can test the car the same way that it tests itself and go straight at the problem instead of just trying to hunt for it.

    When the PCM runs the evaporative emissions monitor (test) it starts by first purging vapors from the canister and tank, and then it commands the canister vent valve closed. The PCM monitors the fuel tank pressure sensor and looks for a drop in pressure in the fuel tank, if it exceeds eight inches in water vacuum the large leak test passes and the system then moves onto the small leak test. To run the small leak test the PCM commands the canister purge valve closed (turns it off) while it leaves the vent valve on (closed) and monitors the fuel tank pressure sensor to see how fast the vacuum that was pulled decays. If the vacuum bleeds off too fast, the test fails.

    The easiest way to locate the leak is to simply make the test run manually while blocking off pieces of the system to see if the rate of decay can be altered. If you have a fully functional scan tool the purge and vent valves can be commanded bi-directionally and the fuel tank pressure sensor signal monitored and it is then very easy to block off different sections of the system to locate the problem. If you don't have a scan tool that will make those commands it can still be done but you need to use jumper wires to command the vent valve closed and pulse the purge open as required to pull a vacuum on the system while the fuel tank pressure sensor signal is monitored with a volt meter. To block off different sections all you need is a couple of large needle nose pliers. When you clamp off the right hose(s), you slow the vacuum decay and you have found where the leak is.
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