Last post on Dec 02, 2013 at 3:15 PM
You are in the Maintenance & Repair
What is this discussion about?
Auto Repair, Hatchback, Truck, Sedan
This topic is primarily for professional mechanics, current or retired, or ardent amateurs who would like to share the suprises, victories, tricks and challenges of working on the modern automobile. All Forums members are invited, of course, to ask technicians about their work, or comment on your own experiences dealing with mechanics.
If you have a maintenance or repair question about your vehicle, please use search to find one of our Maintenance and Repair discussions, or ask a question in Edmunds Answers.
#647 of 4850 Re: Chevrolet Cobalt, large evaporative system leak [xwesx]
Dec 11, 2012 (2:23 am)
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.
#648 of 4850 Re: It isn't wrong to do the job right! [thecardoc3]
by Stever@Edmunds HOST
Dec 11, 2012 (6:49 am)
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.
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.
#649 of 4850 Re: Chevrolet Cobalt, large evaporative system leak [thecardoc3]
Dec 11, 2012 (7:23 am)
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.
#650 of 4850 Re: Chevrolet Cobalt, large evaporative system leak [thecardoc3]
Dec 11, 2012 (10:05 am)
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).
#651 of 4850 Re: Chevrolet Cobalt, large evaporative system leak [qbrozen]
by MrShift@Edmunds HOST
Dec 11, 2012 (10:10 am)
Where I live common labor rates are at $140/hr now. I'm fine with that, if they are efficient AND correct.
#652 of 4850 Re: Chevrolet Cobalt, large evaporative system leak [thecardoc3]
Dec 11, 2012 (10:50 am)
...to 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?
#653 of 4850 Re: Chevrolet Cobalt, large evaporative system leak [qbrozen]
Dec 11, 2012 (2:39 pm)
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. .
#654 of 4850 Re: Chevrolet Cobalt, large evaporative system leak [thecardoc3]
Dec 11, 2012 (3:02 pm)
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.
#655 of 4850 Re: Chevrolet Cobalt, large evaporative system leak [thecardoc3]
Dec 11, 2012 (3:13 pm)
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.
#656 of 4850 Re: Chevrolet Cobalt, large evaporative system leak [qbrozen]
Dec 11, 2012 (4:18 pm)
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.