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A Mechanic's Life - Tales From Under the Hood



  • thecardoc3thecardoc3 Posts: 1,521
    Assuming an understanding of the system (which you noted as step 1), I'd say that the next step is to replicate conditions that should cause that DTC to set, in order to verify that the OBD system is working correctly.

    In a round about way that is correct. The code sets because the computer seals the system by closing the cannister vent valve and then opening the purge valve in order to pull the system into a vacuum. (about 8"-12" in water) Then the purge valve is then turned off and the computer watches the vacuum bleed up rate. By knowing the fuel level, and therefore how much space is air/vapor in the tank it can calculate if the bleed up rate is excessive (leak detected) or not (system sealed).

    So the first step is to command the vent valve closed with the scan tools bi-directional 'controls. The vent valve will make an audible "click" when it opens and closes.
    The Cobalts cannister vent valve could not be heard to be closing/opening.
    That means the system cannot test itself. Two questions need to be thought of and then proven/disproven at this time.
    If the problem is electrical, why isn't there a code for the vent valve's circuit?
    The customer (allegedly) already replaced the vent valve so does that mean it's a mechanical issue with the "new" valve?

    With these two questions in mind it's time to do pinpoint testing on the vent valve and it's circuit.

    The purge valve is tested in a similar fashion, first command it to operate with the engine off and simply listen for it. If you cannot hear it, then specific testing for it, and/or it's circuit is required. If you can hear it then you need to start the engine and see if you can control the vacuum to the system. The Cobalt's purge valve passed these checks.

    1. How long did it take to diagnose, and then fix, the issue, and 2. Appx. much did the customer end up paying?

    The time to locate the exact failure was about half an hour, the time to actually repair the wiring harness damage that I found was about an hour, including doing the post repair testing that was required to make sure that the car would then pass it's monitor the next time that it ran. Our diagnostic/electrical labor rate is $115/hr. People often gasp at that but in reality a shop hanging brakes or doing suspension work or exhaust is realizing more profit per hour than we are with that diagnostic rate and they don't have to spend a dime to do that kind of work they way we do with scan tools, software and schools.

    1.5hrs at $115 with the sales tax and he was out the door with the car working correctly for about $185. Consider that he had already spent over $400 while failing to fix this over the last year.

    Now one of the things we don't do is go on a treasure hunt on the cars. I don't go over the whole car trying to find a grocery list of services to sell. We go straight in at the problem that the car came in for, and straight back out. I wouldn't be able to keep a job at a chain store working like that, I wouldn't be selling enough to keep the management happy.
  • Stever@EdmundsStever@Edmunds YooperlandPosts: 38,910
    The moment someone touches the cap once the light has come on, the only thing that they really accomplish is make it impossible to prove if the cap's being loose caused the light to be on. To a technician it ends up being an intermittent failure condition and you can only assume the cap must have been loose, you don't actually know for certain if it was or was not.

    Heaven forbid if I need to get to your shop and I'm two hours away and my fuel light just came on too. :shades:

    The Escape recall was interesting to me because the only other reflash fixes I remember had to do with driveability issues, like shift points. Being able to reduce the risk of an engine fire with software is a pretty good trick and illustrates how the new cars aren't your father's Oldsmobile.
  • qbrozenqbrozen Posts: 16,886
    Our diagnostic/electrical labor rate is $115/hr. People often gasp at that

    Hell, that's cheap compared to a BMW dealer.

    I didn't get to answer your question originally, but I did just go through this on my volvo. Its a '98 and I've already had extensive experience with another '98 some years ago. Code was the massive evap leak. First thing I did was go to the dealer and buy a gas cap because, in my experience with this car, it is what was stated above: the most common issue.

    Cleared the code and drove for about 5 days before it came back. There are 2 more common things on this car. One being the purge valve and the other being clogged vent. Before testing the valve, I decided to get underneath and inspect the vent. No clog. But then I inspected the canister and after removing some brackets in the way, I found a split air line. Removed, trimmed, reinstalled, and cleared the code again. So far so good, but I'll want to see a couple of weeks without a light before I feel I've solved it.

    Now, I know you aren't happy with my process. Should I have jacked up the car and gone through the whole troubleshooting first? Maybe some people should. But if $24 for the gas cap saved me the 30 mins I spent on fix #2, it would have been worth it to me. And, certainly, $24 total spend is a helluva lot less than a professional shop diagnostic fee. And, no, I'm not returning the cap because I know it to be such a common failure point that I might as well keep this one and know I'm good for about 4-5 years.

    '13 Stang GT; '86 Benz 300E; '98 Volvo S70; '12 Leaf; '08 Town&Country

  • xwesxxwesx Fairbanks, AlaskaPosts: 8,267
    he was out the door with the car working correctly for about $185.

    That's a great deal for him. Plus, he's a happy customer that will be back (but only when another problem comes up that stumps him). If I could have found a local mechanic that could solve my '96 Outback's issues a decade ago, I would have stumbled over myself trying to get to that place! As it was, I spent around that same $400 for no resolution at all.

    Also, thanks for sharing more information on the process - it's very interesting stuff and clearly illustrates the investment required (time, tools, and education/training).
  • Where I live common labor rates are at $140/hr now. I'm fine with that, if they are efficient AND correct.
  • srs_49srs_49 Posts: 1,394 actually repair the wiring harness damage that I found ...

    So, what caused the damage to the wiring harness?

    And didn't you tell us about another problem (SUV liftgate?), that people were throwing BCMs at and that turned out to be a wiring harness issue?
  • thecardoc3thecardoc3 Posts: 1,521
    Cleared the code and drove for about 5 days before it came back


    Now, I know you aren't happy with my process. Should I have jacked up the car and gone through the whole troubleshooting first? Maybe some people should

    If there is something that I'm not happy about it's the fact that there are those who keep claiming that all someone has to do is tighten or replace a fuel cap for an evbap issue when what you have here is the reality, the caps rarely fix the evaporative system problems. .
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,267

    As we talk about the complexity of modern cars and all these systems they have, I still can't help but appreciate how "clean" cars operate today. Last night, I used my plow truck ('76 F250) for about an hour clearing mine and my neighbors' driveways along with our cul-de-sac. I had my coat, boots, gloves, and hat donned at the time.

    This morning, when I put on my coat to go to work, it still smelled like "old truck." You know, that mixed fume smell that only an old vehicle can create. ;)
  • qbrozenqbrozen Posts: 16,886
    the caps rarely fix the evaporative system problems

    I can't speak for all makes models. I only know what I know, which is a cap solved exactly the same code on my last '98 S70, as well as the same code for dozens of other owners.

    I would never claim that to be the fix for any other make/model unless I knew it to be a common problem for that car.

    '13 Stang GT; '86 Benz 300E; '98 Volvo S70; '12 Leaf; '08 Town&Country

  • imidazol97imidazol97 Crossroads of America: I70 & I75Posts: 17,682
    The cap has been the problem on my leSabre 3 times over 8 or more years. Each time cleaning the o-ring and the mating surface, lubing same, and reinstalling then waiting for the car to have the requisite conditions to run the check for leaks showed the problem had been solved.

    This message has been approved.

  • Stever@EdmundsStever@Edmunds YooperlandPosts: 38,910
    Even if the cap is the problem in only 1% of the cases, with 300 million cars on the road, that's still pushing two million cars (even after throwing out ~100 million older ones that may not be OBDII cars - like Xwes's lol).
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,267
    Hey now, don't make fun of my old beast(s)! :cry:

  • Stever@EdmundsStever@Edmunds YooperlandPosts: 38,910
    OBDII came out in 1996. My Subie made the cut with a few months to spare. :D
  • thecardoc3thecardoc3 Posts: 1,521
    Replacing the gas cap first failed to fix 100% of the cars that have had an evaporative emissions issue that then had to be taken to a shop to be properly diagnosed and repaired.
  • thecardoc3thecardoc3 Posts: 1,521
    OBDII officially came out in 96. Honda drug their feet and got in trouble with some of the vehicle systems. Some manufacturers like GM and Ford actually had the systems on the road as early as 94, they just didn't call it OBDII.

    Evaporative systems didn't need to be fully enhanced fleet wide until 99. The non enhanced systems couldn't test for leaks, and only had to prove that they could/would purge.
  • srs_49srs_49 Posts: 1,394
    Replacing the gas cap first failed to fix 100% of the cars that have had an evaporative emissions issue that then had to be taken to a shop to be properly diagnosed and repaired.

    Think about what you just said - "...that then had to be taken to a shop to be properly diagnosed and repaired".

    So what you're describing is a limited subset of the total population of cars that have had evaporative emissions issues - that is, those that were brought into a repair shop.

    If the total number of vehicles that experience an evaporative emissions failure is 100, and replacing the gas cap solves the problem in 90 of those cases, but doesn't in 10, those 10 are the ones you or someone else is going to see in your shop. So from your or some other shop's experience 100% of the cars (that you diagnosed and repaired) needed something other than a gas cap replacement. You (or some other shop) never see the other 90 cars.
  • thecardoc3thecardoc3 Posts: 1,521
    Experience has proven that what you linked fails to fix the cars, the majority of the time. It is a lack of knowledge and experince that has you clinging onto your perception that a gas cap being loose or bad is the most common cause for evap issues.

    It's almost funny how you are ignoring what it really says.

    Possible Solutions With a P0442, the most common repair is to:

    •Remove and reinstall the gas cap, clear the codes, and drive for a day and see if the codes come back.
    •Otherwise, replace the gas cap, or
    •Inspect the EVAP system for cuts/holes in tubes/hoses

    By far the "most common" repair is a close tie between the filler neck corroding and leaking, and the vent valve failing to seal for one reason or another so the car simply cannot test itself. Why is it that your link doesn't mention either one of those? That's what you get when someone who really doesn't know anything about how the system works, nor how they fail, and yet they still try to give people advice.

    In many way's it's similar to an instructor, VS a teacher who each are presenting a continuing educational class for auto repair. You can take the greatest teacher in the world, and while he/she may do a great job teaching if they have no idea that the material they are trying to present is flawed, that makes the training, just like your link be worthless. It takes a truly qualifed instructor, who knows how to do the work to recognize when there are flaws in the material and he/she must correct it so that the educational material is worth the techs time to attend.

    Is there a reason why you don't want consumers to understand how the cars really fail, and what it takes to diagnose and repair them, correctly?
  • Stever@EdmundsStever@Edmunds YooperlandPosts: 38,910
    edited December 2012
    I know you don't see many new cars since you don't do warranty work, but I doubt that many 2012 models with the CEL on throwing a P0442 have corroded filler necks.

    Well, up here in the UP, it's a definite possibility. :shades:

    [Edit] This is good timing. Check out this cry for help over in Edmunds Answers. A 2005 Ford Five Hundred throwing P0442. Place your bets. :shades:
  • thecardoc3thecardoc3 Posts: 1,521
    Take note of the enabling conditions for the onboard computer to run a P0442 test. Especially note the line I highlighted in BOLD

    Conditions for Running the DTC


    The following conditions must be met prior to ignition OFF.

    •Before the ECM can report DTC P0442 failed, DTCs P0446, P0455, and P0496 must run and pass.
    •DTCs P0112, P0113, P0116, P0117, P0118, P0120, P0125, P0128, P0443, P0446, P0449, P0452, P0453, P0463, P0503, P1106, P1107, P1111, P1112, P1114, P1115, P1516, P2101, P2108, P2120, P2125, P2129, P2138, P2610 are not set.
    •The diagnostic runs once after a cold start drive cycle.
    •The start-up intake air temperature (IAT) is between 4-30°C (39-86°F).
    •The start-up engine coolant temperature (ECT) is less than 30°C (86°F).
    •The start-up IAT and ECT are within 8°C (15°F).
    •The barometric pressure (BARO) is more than 74 kPa.
    •The ambient air temperature is between 2-32°C (36-90°F).
    •The engine run time minimum is 10 minutes.
    •The vehicle has traveled more than 5 kilometers (3 miles) this trip.
    •The ECT is more than 70°C (158°F).
    •The fuel level is between 15-85 percent.
    •The ignition is OFF.
    •A refueling event is not detected.
    •DTC P0442 runs once per drive cycle when the above conditions are met.
    •One test occurs at ignition OFF after a drive cycle, and may require up to 45 minutes to complete. No more than 2 tests per day are allowed.
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