Fixing what someone else 'fixed'

For me, anyways, the hardest diagnostics are for the ones that another person created when fixing an original concern. I would be willing to bet that no one intentionally does something in a repair that will cause a different problem down the road, or even knowingly for that matter. However it does reinforce the reason to gather information from a customer so we know what previous repairs have been done to a vehicle and keep an open mind. Even if the customer’s concern appears completely unrelated to the previous work performed, it just might be correlative.

Now as for customers, it amazes me what lengths people will go to try to sneak by an emissions test instead of repairing a vehicle correctly. The vehicle in particular is a 2005 Mazda MPV that was due for its yearly smog test. According to the owner, for the past two years they have been driving around with the Malfunction Indicator Light (MIL) illuminated for most of the year and then having the code cleared a few days before getting the emission test done and driving about 30 miles to set all but the Evaporative Emission (EVAP) readiness monitor. To pass an emission test in our state requires that there be no more than one monitor unset on 2001 and newer vehicles under 8,500 GVRW. It does not matter which monitor is unset, unless the vehicle previously failed for a catalyst monitor, then the catalyst monitor must not be the one unset. Another point is that pending codes will not cause a failure. So if the code hasn’t triggered the MIL on, it will still pass.

So as the customer explained it, for the past couple of years there was always a P0455 Large EVAP Leak Detected Code that could not be repaired. When they would take the vehicle to an auto parts store for a free scan, they would end up being sold a gas cap. Another repair facility, which performed most of the customer’s repairs, had looked at the vehicle but said they could not find any leak in the system and recommended an OEM gas cap. The customer stated that there were no repairs other than normal maintenance done recently but had an oil leak repaired a couple weeks before the MIL illuminated for the first time, more than two years ago.

However, the shop that performed the repairs convinced the customer that the MIL for the EVAP leak was not related to an oil leak repair. I can understand this from the shop’s point of view, because the two are completely separate systems and we have all had the “ever since you” type of customers. There were no records of diagnostics other repair facilities had performed on the vehicle, but the customer assured us that a Google search had shown that a gas cap was the cause for the code even though the first three gas caps proved them wrong. One thing that stood out in the customer interview was that the MIL now would return much sooner when the code was cleared and driving a short distance, whereas previously, it took nearly two weeks.

Because there now was no chance to set enough monitors to pass an emission test before the MIL reilluminated, the customer knew they would have to actually get the vehicle fixed this time. When a MIL turns back on fairly quickly, it is very unlikely that the cause is an evaporative emissions system fault, especially in an area where summer temperatures rarely are below 100 degrees Fahrenheit.

Starting at the Beginning Looking at the vehicle code list, I found the P0455 (Large EVAP Leak), but also found a code P0037 (HO2S Heater (Rear, RH) Circuit Low). The O₂ sensor heater test also is not a one-trip failure, but the monitor does tend to run to completion much more frequently, so I started with a quick visual inspection. When I put the vehicle on the lift and inspected the rear oxygen sensor, I found the harness side of the oxygen sensor baking on the exhaust manifold. It appears that the wiring harness retainer had not been correctly reinstalled and allowed the O₂ sensor wiring to drop down. Easy enough, there was the short to ground, so a new O₂ sensor was put on the repair order.

Now time to diagnose the EVAP system code. I have seen a lot of technicians start to diagnose an evaporative system leak by immediately putting smoke in a system and looking for a leak. Being that this is a large leak, that might not be a terribly wrong thing to do, but I like to verify that I actually have a leak before smoking the system. In all cases though, the first step I take is to visually inspect the system. Sometimes you find a cracked hose or even one that is disconnected, but this vehicle appeared to have everything attached.

As always, it is a good idea to check for Technical Service Bulletins (TSBs) or possible reflashes for the code. Several hours and countless parts have been installed on vehicles when everything was functioning as designed, but an unpredicted variation in a parameter or calibration caused a test to fail.

This vehicle uses a Leak Detection Pump (LDP), which, on most vehicles that use it, has a high failure rate. In fact, some techs refer to the LDP as an acronym for “Likely Defective Part.” Some manufacturers allow an EVAP system seal through use of a scan tool, which is what I try to use when available to verify both the integrity of the LDP and the command circuitry of the Powertrain Control Module (PCM). On this Mazda Leak Detection Pump, the changeover valve (which is part of the LDP itself) is what seals the vent to the atmosphere.

Because most of the newer EVAP systems are non-intrusive, meaning they do not have a test port, I like to test the system at the hose connection to the purge valve. While I am there, I also attach a MityVac vacuum pump to one side of the purge valve and confirm that it holds vacuum and does not leak down. I then use the bidirectional controls on the scan tool to cycle the purge valve to verify it works and is not restricted. This one passed the test. What I did find when I leak checked the system was that the flow meter ball dropped to the bottom of the gauge, indicating that the system was sealing.

Now what? The code states that I have a large leak, so that should be easy to verify since it sets every time the EVAP monitor is run. At this point I am trying to rationalize a replacement of the Leak Detection Pump, since maybe it is not sealing completely when the test is run. As for service information and theory of operation of the Mazda LDP, there isn’t a lot. The only tests listed are an air resistance test, which is basically blowing from LDP port A to B and then from B to A to make sure there isn’t anything blocking the component. Then there are electrical resistance tests. I’m thinking if there was an electrical problem with the LDP a different code would have been set.

Failed Test, Sealed System? My service information source listed several possible causes for this code:

· Loose or defective fuel cap

· LDP Malfunction

· Purge Valve Malfunction

· Fuel Tank Malfunction

· Charcoal Canister Malfunction

· Loose or disconnected vacuum hose

· PCM Malfunction

Time for a different diagnostic approach, probably the one I should have used from the beginning. This vehicle has the benefit of the EVAP system monitor test available as a functional test on its scan tool.

Starting the test, the scan tool verifies that certain parameters are in range, including the vehicle’s fuel level (15 to 85 percent full) and a check to make sure the Intake Air Temperature (IAT) and Engine Coolant Temperature (ECT) difference is less than 15° F. The next step was to bring the vehicle up to operating temperature, the scan tool states to raise the engine rpm to 3,500 for three and a half minutes. Finally, the PCM takes over the EVAP system test by controlling the throttle opening to around 2,000 rpm while it performs the self-test.

The results of the EVAP system test verify the illumination of the MIL. The PCM believes there is a large EVAP leak somewhere in the system. However, one thing that I noticed was the Fuel Tank Pressure (FTP) Parameter Identifier (PID) was flat during the entire test. When the purge valve opened, engine vacuum should have been introduced to the evaporative system and I should have seen a negative pressure displayed on the Fuel Tank Pressure PID.

Could this be a defective Fuel Tank Pressure Sensor that is not correctly measuring the pressure change? Well, it’s not the easiest component to get at for testing, so let’s try another approach.

Someone’s Lying I attached a Pico WPS 500X Pressure Transducer to a fuel cap smoke adaptor to measure the true pressure in the fuel tank. Through the Custom Probes feature on the Pico Scope, I set up the scaling to read in inches of water, the same pressure scale used by the PCM.

Running the EVAP system test again, I find a mirror image of what the FTP sensor displayed during the initial test. The test took a total of a minute and a half and showed virtually no change in the evaporative system pressure. The purge valve was opening and closing, so why would vacuum not reach the tank, unless there was no vacuum to begin with.

I tested the source of vacuum at the purge valve and found that vacuum read 0 in/Hg. I traced the vacuum line going to the purge valve and found that it actually came from the air intake boot on the atmospheric side of the throttle body. Well, that can’t be right and certainly explains the lack of manifold vacuum to the purge valve. I used the EVAP Vapor and Vacuum Hose Diagrams in the service info and found that the hose with the vacuum source was supposed to be routed to the rear cylinder head cover. Apparently, when the hoses were disconnected to access and replace the valve cover gasket on the rear cylinder head they were not reconnected to their correct locations. An afterthought of this had me wondering if the direct vacuum to the rear cylinder head cover ventilation port could have caused any engine running concerns. While I did not take notice of fuel trim readings on the scan data for this type of code, I did not notice any drivability issues of the vehicle while bringing it inside, nor had the customer mentioned any.

Looking at the two hoses, I can see where the mistake could have been made. They are the same size and also both line up easily with the position of the purge valve. It is hard to see exactly what I am referring to in the actual picture of the hoses on the vehicle, so I included the correct purge valve and ventilation system hose routing diagrams along with one that shows how the hoses were connected which was causing the failure. I swapped the hoses to their correct locations, cleared the codes and reran the EVAP system test.

This time the system showed a pass on the large leak test. I also had the WPS500X attached during the test and noted the results. Notice that after the test starts, it takes only 20 seconds to reach the target vacuum of -7.5 in/H₂0. I have found that when performing this test on newer vehicles, the scan tool actually monitors target vacuum, so a fault like this one would have been more apparent. The next section is the Bleed Up portion of the test where the size of the leak is determined. This took approximately 30 seconds and the system lost less than 2 in/H₂O. Finally, the Leak Detection Pump released the vacuum and the PCM processed the results. One very nice feature of this test was that it gave results on the before and after repair that we can share with the customer.

While this was not a complicated repair, the diagnostics to find the problem were not found in any of my available information resources. Had I followed the diagnostic flow chart steps in order, I would have inspected the hoses for poor connections to the purge valve, but would have never checked the actual source vacuum at the purge valve. Too often we convince ourselves that a failure must be due to an electrical circuit or a component itself and we overlook items as simple as source vacuum.

Fixing what someone else 'fixed'

Michael Miller | Contributing Editor

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