Smoke Machines And Vacuum Leaks

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  • greasybob
    Senior Member
    • May 2008
    • 1590

    Smoke Machines And Vacuum Leaks

    This has been a repeating theme for me this winter. Customer calls with a complaint of an intermittent rough idle, maybe check engine light or stalls when cold. They bring the vehicle to the shop and everything is now running fine but the codes are all there, P0171, P0174 maybe slow 02 codes or misfires. By the time they get the vehicle over here it has warmed up and all the data is looking good (maybe still a little lean but not setting any codes currently). Smoke test the intake for for leaks and nothing shows. So I've gone to another technique for checking for vacuum leaks. Before you jump all over me, I understand the risks of spraying a flamable substance around a hot running engine. What I've been doing is set the scanner to watch the upstream 02s and spray a little carb cleaner at areas I would suspect there to be a vacuum leak. If the 02s go rich, then theres your leak. If you consider the difference in pressures between the inside of an intake manifold of a running engine and atmosphere to the little bit of pressure your smoke machine makes I think you'll understand why the smoke mahine has it's limitations when when checking for vacuum leaks. Not to mention that the pressure is now going the other way and the engine is not in motion when smoking it. The following pictures are the most recent example from a 98 2.5 Contour that when smoked only showed a very little smoke from the EGR (normal) and the intake manifold tuner shafts ( sort of normal). The 02s were flat at about 20mV at idle but when the carb cleaner was sprayed at the lower intake manifold base gaskets the 02s quickly responded. This has played out several times this winter with engine that use this type of plastic gasket with the inserted rubber Orings. Upon inspectection you'll find the rubber is compressed beyond it's ability to seal and is always worse when cold.The first look at scan data with the inability to of the smoke machine to find any sizeable leak would tempt you to go after the upstream 02s first. The second graph is the same engine after replacing the intake gaskets and resetting fuel trims.
    Attached Files
    Last edited by greasybob; 03-31-2013, 03:05 PM.
  • Crusty
    Senior Member
    • Dec 2007
    • 2450

    #2
    Here's another method that can get right into tight areas.

    Take a propane torch kit, take just the burn nozzle off the end, leave the 6" tube on and stuff a piece of rubber vacuum hose of whatever length you want on it. next take a piece of 1/4" steel tubing, again of whatever length you like and now you can poke the end of the steel tube into amall areas such as underneath intake plenums.

    The valve can be turned on, open just a little or a lot.

    No more mess and no mess of wet dribbling down onto the hot exhaust manifolds.

    I find Combustion Chamber cleaner in a spray can better than Carb cleaner if you want to use a spray can.
    Attached Files

    Comment

    • greasybob
      Senior Member
      • May 2008
      • 1590

      #3
      So maybe we should ask not only have you "smoked" it for vacuum leaks but have you "fogged" it as well ? You probably had that propane setup back in the carburetor days ? How many times have you seen check for vacuum leaks in a diagnostic chart but no mention of technique ? What do the manufacturers recommend ? What are they teaching in tech schools these days ? Just throwing this out there and wondering.

      Comment

      • Crusty
        Senior Member
        • Dec 2007
        • 2450

        #4
        Funny you should say that. as it happens, that small 2" piece of vacuum hose taped to the longer hose was to hold the propane feed onto the carburetor air cleaner stud-!! LOL

        When the engine pulls in the Combustion chamber conditioner, man does she ever smoke-!! and it can creat quite a fog in the shop if you don't have your exhaust hose on right-! LOL

        From my reading, they all seem to rely on the smoke machines and forget completely about the pressure differential, or that seals and gaskets can move in one direction and seal up and move the other way and allow unmetered air in.

        Far too many times the basic principles are not being taught. Here's where going back to old school principles and first things first will fix what the over relied on technology won't. A old school compound guage, and a newer vacuum only guage and a "technology" reading....ROFLMAO.

        Funny thing about the compound gauge.....the guy doing repairs of the smoke machines calibrate their compouind guages using my "old school" compound gauge that someone once called "crude".....but it WORKS-!
        Attached Files

        Comment

        • Joe Rappa
          Snap-on DSD
          • Aug 2007
          • 2050

          #5
          Here is another Ford with the same problem, diagnosed the same way. Also visible is the Short Term Fuel Trim.

          "You don't build a reputation on what you're going to do"
          Henry Ford

          Comment

          • sandt38
            Senior Member
            • May 2012
            • 187

            #6
            Originally posted by Joe Rappa
            Here is another Ford with the same problem, diagnosed the same way. Also visible is the Short Term Fuel Trim.

            Yep. I always use STFT for vacuum leaks. But a lot of the time intake leaks will be cold leaks (look at your freeze frame data!) and will not show up on a warmed up engine.

            Comment

            • brad_m
              Member
              • Dec 2012
              • 42

              #7
              I have been using carb cleaner and propane for vacuum leaks since forever.

              I've had one fire in 12 years and that was because I got the carb cleaner to close to a distributor. After that about 5 years ago, I far more careful around the ignition system.

              It's a very effective way to find leaks.

              Comment

              • brad_m
                Member
                • Dec 2012
                • 42

                #8
                Originally posted by Crusty
                Funny you should say that. as it happens, that small 2" piece of vacuum hose taped to the longer hose was to hold the propane feed onto the carburetor air cleaner stud-!! LOL

                When the engine pulls in the Combustion chamber conditioner, man does she ever smoke-!! and it can creat quite a fog in the shop if you don't have your exhaust hose on right-! LOL

                From my reading, they all seem to rely on the smoke machines and forget completely about the pressure differential, or that seals and gaskets can move in one direction and seal up and move the other way and allow unmetered air in.

                Far too many times the basic principles are not being taught. Here's where going back to old school principles and first things first will fix what the over relied on technology won't. A old school compound guage, and a newer vacuum only guage and a "technology" reading....ROFLMAO.

                Funny thing about the compound gauge.....the guy doing repairs of the smoke machines calibrate their compouind guages using my "old school" compound gauge that someone once called "crude".....but it WORKS-!
                Any one that calls a U-tube manometer "crude" Needs a serious ridiculing.

                They are one of the most accurate pressure gauges going. The scale only needs to be set-up to suit the purpose.

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