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Smoke Test

On my Buick, the “service engine soon” notice came up. I don’t seem to be having any problems; however, the light would not go out. I took it to a repair company near me that seems to get good recommendations. They called to say they needed by permission to do a “smoke test” to find a possible leak not easily found at a cost of $119. I have not heard back as to what they say. I was hoping I could get some advice before their call. Thank you.

The service engine soon light could be turning on for something as simple as the need for an oil change or the gas cap is loose. Your mechanic should be able to give you the code that comes up from their reader, it will be something like P1234, ask the mechanic for it and post it here. Without knowing the year mileage history of your car no way to figure out what is going on. GM cars from the mid to late 90’s had an intake gasket that would fail and that may be why they are doing the smoke test.

I’m not clear on what advice you want - I’m not quite sure if you’ve already given them permission, or you want us to tell you whether or not to give them permission.

Without knowing what code it threw, we can’t tell you if the smoke test is necessary or not. If it threw P0442 then that indicates a possible leak in the evaporative emissions system, which takes gas vapors and sends them up to the engine to be burned. It’s a sealed system, and leaks make it not work properly.

Often this code is caused because the gas cap isn’t sealing properly, or wasn’t tightened enough, or was left behind the last time you fueled up. But it’s also possible that the code was caused by a hole somewhere in the system that can be difficult to find. The easiest way to find it is to shoot smoke into the system and look to see where the smoke comes out.

The price is roughly inline with what it usually is for a smoke test.

When I get this code my first step is to go take the gas cap off, and then put it back on and make sure it’s tight. Then I clear the code (you need a special tool to do this, but usually an auto parts store can do it for you if you don’t have one) and drive around to see if the code comes back. If it does, then I replace the gas cap with a new one, clear the code, and drive around again. Only if the code comes back a third time do I conclude that there may be an actual leak. The gas cap is the weakest link in the system from a leaks perspective because it’s the only one being manhandled on a weekly basis. It’s a really good possibility when you see this code that the gas cap is to blame, and so there’s no point paying for a smoke test until you’ve ruled out the gas cap as a culprit.

Concur w/advice above, if the light is coming on due to an evap problem, it’s probably just the gas cap seal. If they’ve already tried that and it didn’t fix the problem, a smoke test is a common way to diagnose evap problems. The evap system is supposed to be air tight to prevent gasoline vapors from escaping and causing air pollution. They hook up a machine and force some smoke into the appropriate place, and then look all around the engine compartment and fuel tank area to see if any smoke is coming out; if so they’ve found where the leak is. The price quoted seems reasonable enough.

Speaking of gas caps . . .

A few days ago, I was working on a Blazer, with a rotten gas cap seal. The seal was split, you could see it. No evap codes, no pending codes, no codes in history. The evap monitor ran to completion and passed

Then there was a Jimmy, with an evap major leak code. Closed the vent valve, put smoke in, leaked past the cap. The cap was tight, the seal looked great. the filler neck was not deformed in any way, so the cap was clearly bad

Just the opposite of what you would expect. Of course, I also replaced the cap on that 2003 Blazer, out of principle, because the seal looked like crap. Even if it’s not a problem right now, it may be, in the near future

@db4690 the reason it did not set a code is because the monitors never ran. In the early days of emissions controls the manufactures were less than willing to go along with the program, so they set their cars software up to where some monitors like evap would almost never run. The EPA caught on and tightened the standards. I believe it was the 2005 model year that the evap system had more stringent rules so it would actually set codes.

@BLF $119 for a smoke test is a fair price as said above.


Are you suggesting that perhaps after the latest battery replacement, for example, the evap monitor did run to completion, and everything was fine at that time? But the parameters to run the monitor and fail with the now rotten seal were not met recently?

I’ve seen plenty of evap codes on pre-2005 GM vehicles. Which leads me to believe that perhaps GM was “playing ball” before the EPA shortened their leash

I believe you may be referring to enhanced evap systems . . . ?

My memory may be wrong, but I seem to recall that it was implemented around 2000, give or take a year or so . . . ?

There have been so many changes over the years I may be off in my years. I can remember seeing cars with gas caps completely missing and no codes. Next time I have a chance I’ll try to look up the monitor parameters.

I don’t see a problem with the shop’s request to perform a smoke test and there may not be any noticeable symptoms.

Not related directly to the OP’s comment but a couple of evenings ago my wife and I stopped at a Sonic drive-in for a burger. There was a lengthy line in the drive through and the second car behind the one at the drive-up window was a 2013 BMW X5. I noticed the gas lid was open and the cap inserted into its holder rather than screwed into the filler neck.

I mentioned to my wife that I might ought to get out and say something so the better half thought it would be the right thing to do.
I get out, walk over, and tap on the passenger side window. The lady lowered the window and I offered to put the cap back in and close the lid. She thanked me profusely for catching it and said that she had just plain forgotten about it while gassing up that morning.

When questioned about any CEL she glanced down and said there was not one and there had been no indicators coming on at all.
Anywho, buttoned it up and sent her on her way. So; all day long motoring around and no CEL even with the cap completely removed.

There are few conditions that have to be met for the evap monitor to run

And there have to be a certain number of consequitive failures, for the light to come on

The fuel level has to be between 1/4 and 3/4 for the monitor to run, and the engine coolant temperature has to be below a certain point

It’s possible the light would have come on the next morning