by Sam Dillinger (comments: 0)

2001 Jeep Grand Cherokee P0455

2001 evap emissions system

This is the first in what will be a series of articles detailing what I see/have seen for specific Pcodes, as well as questions I have actually received through the contact page on our site

So let’s dive right in with a early 2000’s model Jeep Grand Cherokee with a 4.0 liter engine, automatic transmission, and a P0455. This Pcode deals with the evaporative emissions system of the vehicle, and is indicating a large leak in the system. The definition of the P0455 is “evaporative emissions system large leak detected”. The wording of the Pcodes isn’t hugely important. I say this because the name can vary from one scanner to the next. The most important thing is recording the Pcode correctly because that will lead you to the correct diagnostic information, regardless of the name the scanner being used assigned to the Pcode. In this instance, this was the very first submission to the website. The customer who wrote to me had already replaced the gas cap, as well as some other preliminary checks.

Let’s start with the basics. The evaporative emissions system on your vehicle is used to capture unburned fuel vapors after your engine turns off, as well as vapors from the fuel in the tank itself. The system is sealed off from the atmosphere, at least it is when it’s working properly, and will recycle these vapors back into the intake system to be burned when the engine is running. This system is required by the government to contain hydrocarbons that come from fuel and are harmful to the atmosphere. It’s composed of several different items like: the fuel tank, vacuum lines, a leak detection device, a charcoal canister, a purge solenoid, and the intake manifold itself. These are mostly the base components, and there can be some slight variation between different vehicle/engine/manufacturer combinations.

When it comes to setting a Pcode for this system, it can be pretty misleading and/or confusing. The vehicle has to undergo a very specific set of circumstances before it can test the system with the leak detection device, and depending on the Pcode set, and your driving habits (by driving habits I mean how far and how often you drive the vehicle) it could take a week or more for these tests to occur and actually trigger a Pcode, if there is a failure. This all means when you make a repair and reset the check engine light, you may be under the false impression that the repair was effective because the conditions haven’t been met for the test to take place again. Just something to be aware of.

So as the name implies, this Pcode is indicating a large leak in the vacuum system for the evaporative emissions. Don’t mistake this as meaning the engine has a vacuum leak. A specific type of vacuum leak at the intake manifold can be the cause, but it doesn’t have to be. The most common cause on any vehicle that gets this Pcode is the gas cap. There is a seal on the outside edge of the cap that can fail, as well as a seal inside of the cap itself that can fail. NOTE: the seal inside the cap is usually the one that actually fails when your gas cap “goes bad”. It gets corroded from the fuel fumes and stops being able to seal. As I noted at the beginning, this customer had already replaced the gas cap and had the same issue.

For this vehicle specifically, most of the main components are located in the rear of the vehicle. The fuel tank is at the very rear of the vehicle and located inside the left rear inner fender well is the charcoal canister, the leak detection pump, and some of the vacuum lines that go between these 2 components, go to the fuel tank, and travel up to the purge solenoid as well as the intake manifold. The vast majority of the problems you're going to find will be in the inner fender well where the canister and the leak detection pump are located. There are vacuum hoses that split after time and will leak, as well as some elbows used to connect the vacuum lines to the components. These elbows corrode too and will get a really loose fit, causing a leak. Begin by taking this system apart and inspecting thoroughly. TIP: when looking at rubber vacuum hoses for a leak in the evaporative emissions system, look for a white-ish film that will accumulate on the hose itself. This is from the fuel vapors and will lead you right to the source of the leak.

If you don’t find anything here causing the problem, look at the fuel tank. By this I’m being literal: look at the tank itself. Look as far toward the top as you're able to. If the fuel pump seal (which is pretty much at the top dead center of the fuel tank and can’t be seen without lowering the tank down) is leaking, you’ll be able to see a trail of dried fuel coming straight down the body of the tank. Depending on how recently the tank was filled, it may even still be moist or wet.

The next inspection will be the vacuum lines that go from the leak detection equipment and the fuel tank, up to the engine compartment. The majority of these lines will be a hard plastic, and they don’t crack very often, unless they’ve been impacted. Usually a cursory inspection of the plastic is all that’s needed here. Once you find more of the rubber vacuum hoses, inspect as you have the other ones.

The final stop is the engine compartment, specifically the intake manifold. These vacuum lines will have rubber connections that go to the manifold as well as to the purge solenoid (the purge solenoid is mounted very close to the intake, making it pretty easy to spot). Some of these pieces of rubber are pretty small, so be sure to inspect them carefully because they could be cracked on the bottom, out of sight.

If all of the vacuum components don’t turn up the source of the leak, you’ll need to visit a shop for further diagnosis. The equipment needed in the next step will be highly specialized and very expensive. A smoke machine (it actually pumps smoke through the entire system making an external leak much easier to spot) will be needed. There will also need to be a scanner that’s capable of forcing the leak detection system to run it’s tests, a manual vacuum pump, and possibly further tools. If there are no obvious external leaks there could be an internal leak in the pump, canister, or purge solenoid. I’ve actually experienced all of these instances, and while they’re not the most common cause, I wouldn't go so far as to say they're uncommon either. For the customer we’re talking about in this case, they found the leak in the rubber hoses near the leak detection pump located in the left rear inner fender well. This is by far the most common place the failure will end up being, so I would always recommend a thorough inspection here to start on this Pcode.

As always for my members, if you'd like more specific information for this Pcode, the diagnostics associated with it, or other questions, come drop me a line at!

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Meet Sam Dillinger

My name is Sam Dillinger. I've been a professional, dealership technician for 18 and a half years. My first introduction to mechanical repair was when my own vehicle broke down in the fall of 1995. I was 18 and couldn't afford to pay to have it fixed. So I borrowed tools and asked a ton of questions and, eventually, was able to replace the clutch on my truck by myself. During the course of that project, I found that I really enjoyed having a wrench in my hand and figuring out the puzzle of disassembly and reassembly of a motor vehicle.

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