DIY Discussion: Smoke ’Em If You Got ’Em

Rob Siegel

One of the more frustrating things with post-OBD-II cars (models built after 1996 that have the standard trapezoidal OBD-II diagnostic port) is when the “check engine” light (CEL) comes on, you plug in a code reader, and get an annoyance code like “small evap leak” or a “secondary air injection.” (See this piece I wrote for Hagerty six years ago for more information about the CEL, what’s serious, what’s not, and how it’s actually useful when you plug in a code reader and find a misfire code.)

Here in Massachusetts, this is often an issue at inspection time, as for decades, the check engine light being on was an automatic inspection fail. Although this appears to be changing—due to the sometimes-limited parts availability for 15-year-old cars, inspection stations now appear to have some leeway on this issue—it’s still downright annoying having the light blazing for what’s likely a very minor issue. And no, it’s almost never due to the gas cap being loose. In my opinion, that’s perilously close to an urban myth. I’ve certainly never had it be the cause of the CEL being lit.

There’s a distinction between a “small evap leak” code (part of the gas tank vapor capture system), a “secondary air injection” code (that system helps the catalytic converter to oxidize partially unburned fuel), and a lean-running code that may be due to a vacuum leak that’s actually reaching the intake manifold. The first two are relatively minor emissions-related codes. The third is more serious because if your engine is running too lean, it can cause intake valves to burn. But there’s a tool that I find invaluable in all three of these situations that’s well within the budget of most DIY mechanics.

It’s a smoke-tester, or as commonly called, a “smoker.” It’s a container into which you pour a smoke-generating fluid, usually mineral oil. The container has a heating element that usually needs to be clipped to your car battery like a timing light. The cheapest consumer-grade ones need to be connected to a source of pressurized air, requiring you to have an external air compressor to run them, but you can find ones with a built-in air pump for only slightly more.

The output of the smoker is a small flexible rubber hose with a little nozzle like you’d use to blow up a raft. You block off the car’s tailpipe and the throttle body by wrapping a rubber glove over them to force the smoke to come out the leak. Then you disconnect one of the car’s vacuum lines, plug up the port where the line was attached, connect the smoker’s nozzle to the hose, turn the smoker on, and let it heat up. A stream of smoke from the heated mineral oil will soon flow into the engine’s tangled weave of little hoses and out the leak, and you’ll likely be able to find the source by simply following the smoke.

Sometimes a certain amount of trial and error moving the connection to different vacuum lines is needed, as solenoids enable and disable some of them while the engine is running, so the path to the leaky section may not always be open. But most of the time I’ve used the smoker, the problem has been found quickly, and turns out to simply be a rotted section of cloth-and-rubber vacuum hose that cost me next to nothing to replace.

Case in point: This past fall, my daily-driver 2003 BMW E39 530i lit its CEL, and when I plugged in the code reader, I found that it had thrown two secondary air injection codes. The secondary air injection pump in these cars does sometimes go bad. When I do this kind of troubleshooting, I first want to know if the hardware itself has died, so I temporarily powered the pump directly off the battery. It spun happily, so either it wasn’t being turned on when it was supposed to, or there was a leak.

I then realized that this was déjà vu—the car had this exact issue when I bought it eight years ago. And that was when I’d bought the smoke tester. Mine is an $80 eBay special that literally looks like Billy Bob had a side hustle turning paint cans into smokers. But it works perfectly.

classic car vacuum leak smoke detector test
Not kidding about the paint can.Rob Siegel

Although the paint-can-style unit is still available on Amazon or eBay for about $60, nowadays you can buy one for $70 that looks like it’s actually built for the application instead of like it used to hold Coventry Gray latex. And for as little as $10 more, you can find one with its own air pump so you don’t need to hook it up to an external compressor.

amazon smoke tester
One of several $70 smoke testers available on Amazon. No, I don’t get a commission, just FYI.Amazon

I connected my paint can (I mean smoker) to my E39’s plumbing, and a few minutes later, sure enough, smoke began pouring out from the back of the engine—exactly the same spot where it did eight years ago.

classic car vacuum leak smoke detector test
Well, I’d say that’s pretty definitive.Rob Siegel

And as was the case back then, I found that a section of rubber/cloth hose had rotted away, leaving a little one-way check valve hanging in the breeze.

classic car vacuum leak smoke detector test
Gotcha.Rob Siegel

I replaced it with a new section of hose, smoked it again just to be certain, and was surprised to find that smoke continued to pour out a nearby location that took me longer to understand and pinpoint.

classic car vacuum leak smoke detector test
Okay, maybe I don’t gotcha.Rob Siegel

I eventually figured out that it was coming from the idle vacuum solenoid located deep under the intake manifold. I replaced the vacuum lines leading to and from it, but the casing to the valve itself turned out to be cracked. Replacement of the valve solved the problem.

classic car vacuum leak smoke detector test
Okay, really gotcha.Rob Siegel

But the tricky idle solenoid issue notwithstanding, trust me: These minor leaks are usually from rotted-out hoses, and this sort of smoke testing is totally within the DIY realm. If you can change spark plugs, you can do this, and the $70-ish spent for the smoker will more than pay for itself the first time you don’t have to pay someone for all the hours they spend figuring out how to make the CEL go dark.

The cool thing about the smoker is that it’s useful for more than just check-engine light problems on OBD-II cars. I’ve used it multiple times on my vintage cars. Unplug the vacuum line from the distributor, plug the smoker into it, block off the exhaust and carburetor inlet with rubber gloves, and you can quickly find intake manifold gasket leaks and bad o-rings in warm-up regulators.

Smoke ‘em if you got ‘em, right?

***

Rob’s latest book, The Best Of The Hack Mechanic™: 35 years of hacks, kluges, and assorted automotive mayhem, is available on Amazon here. His other seven books are available here on Amazon, or you can order personally inscribed copies from Rob’s website, www.robsiegel.com.

Read next Up next: $53M Mercedes-Benz W196 R Is the Second-Most Expensive Car Ever Sold at Auction

Comments

    I love the irony that the red smoker from amazon is still a paint can (for a spray gun) 😛

    This is one of those tools I keep telling myself to buy but never get around to actually do it.

    We have had one of the paint can testers for about 8 years now. Worth every penny to quickly find leaks that would otherwise be difficult to locate.

    Extremely useful article, Rob.

    I actually once had the gas cap being loose causing the CEL to come on.
    Happiest fix ever.

    Happened to my sister in law’s Mazda many times because she kept forgetting to tighten the gas cap till it clicked several times–despite my numerous “suggestions”.

    I have also had it happen; reinstalling the cap fixed the issue. I think it was on either my 1998 or 2002 Grand Prix.

    I have solved more CELs with my trusty smoker than I can shake a stick at. One common BMW offender is the CCV valve built into the valve cover of later BMWs.

    In a lot of cases, I will ignore the codes and grab the smoker first. People think codes are causes… no they are symptoms, and if you replace the sensor associated with the code there’s a good chance you are throwing money away

    Usually, smoke emanating from anywhere on my car is frowned upon, but this is a valid tool for diagnosing certain problems – one that I’ll wager a majority of newer DIY’rs weren’t aware of. Thanks for bringing it front and center, Hack!

    Thank you for an extremely well written and helpful article. I bought one off Amazon and have yet to use it but will after reading this article. As my 87 MB 560sl which has about 8,000 ft of vacuum lines that all seem to leak at some time especially after almost 40 years. While I am at it I might as well “smoke” the 84 Mustang, 90 Mazda B2200. I better buy some more baby oil.

    If the car is old enough to have a crankcase downtube, wouldn’t you need to block that and the oil filler cap?

    It all depends on how much flow your smoker produces. If the flow is good enough, you don’t need a (mostly) leak tight system for it to work.

    I’ll apologize up front, but I think Italian & British cars from the ’70s had that as standard equipment, often located behind the dash or under the hood…

    Yes, well, I vividly remember the shifter-through-the-dash on my Alfa Spider, and shifting it into 3rd which pinched a wire against ground, and smoke beginning to pour out…

    Every one calls it the “check engine light” but as far as OBD II readers are concerned the CEL is refereed to as the MIL (Malfunction Indicator Lamp), for instance “MIL Status” is a check on the OBD reader to verify that the light it self is working properly such as not burnt out or that it hasn’t been removed.

    Hi Rob, curious as to how much the replacement valve cost? It looks like what Porsche calls a crossover valve. My understanding is there are several of them in the 911. My 2017 911 just had its water pump replaced under extended warranty. Also the crossover valve was replaced under the same service. The valves have a reputation for being a problem. They look like your valve.

    I once had a vacuum advance canister leak so be careful pulling the hose at the distributor and looking for smoke everywhere else.

    It has been my experience that BMW’s should come with a smoker as part of the factory tool kit due to the frequency of use to find the latest issue.

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