Tech To Tech
One of the most difficult problems to diagnose can be caused by vacuum leaks. Vacuum leaks can cause problems on some or all cylinders, and leaks in vacuum lines can cause critical actuators to fail to respond.
Vacuum leaks generally tend to fall at the end of the process of elimination in the diagnostic procedure. This means the vacuum leak is considered after eliminating the ignition, compression, fuel delivery and sensor problems. A time-consuming procedure at best.
Spray carburetor cleaner is probably the most common technique used for finding vacuum leaks. I have used this method with varying success. The first problem with this method is the possibility of liquid "carb" cleaner igniting and causing a fire. This has happened to me three times in my career. Twice from fumes getting too close to a distributor and once from a hot exhaust manifold. Well-serviced carbon dioxide (CO2) fire extinguishers close at hand saved the day in all three incidents. Secondly, breathing the fumes from vaporized carb cleaner is not only unpleasant, but unwise.
Carb cleaner will not give a good indication of the size of a vacuum leak. Say you disconnect a tiny vacuum hose and spray carb cleaner into the line. At idle this will usually stall an engine, however, a leak this size may not cause any driveability problems. This is a major problem with this technique. Spray carb cleaner on most EGR shafts with the engine running and the liquid will run down the shaft and show up on a tailpipe reading the same is true for throttle shafts. How do you know what is too much and what is OK?
Carb cleaner will be of no use in determining a leak under an intake manifold on a "vee" engine, unless you disconnect the crankcase fresh air inlet and the PCV valve and have the "guts" to spray into the crankcase. While the possibility of a crankcase explosion is quite low, the result is too dangerous for this procedure.
Vacuum leaks into the crankcase can be tested by blocking the PCV and fresh inlet, and then connecting a vacuum gauge to the dipstick tube and checking vacuum. If there is a vacuum leak into the crankcase and there are no gasket or seal leaks in the crankcase, then the leak will show up as high crankcase vacuum.
Carb cleaner is not very effective at finding leaks on the underside of manifolds and in vacuum lines that run through wiring looms. Short of soaking everything under the hood, you just aren't going to find some vacuum problems with carb cleaner.
Oil is another alternative to carb cleaner. The idea is the oil is thick enough to seal the vacuum leak for a short time. This is a good technique if you have found a leak at the top of a manifold gasket by using carb cleaner and it seems a significant leak that you can test for size by using oil. However, oil is difficult to apply in many places and has limited uses.
Propane is probably the best alternative to the above techniques. A fire is usually short-lived if one occurs and is much less likely. Using one of the available propane tools gives better access to tight places. Propane--like carb cleaner--tends to "clump" together in the air and sometimes when testing it gets pulled into the air horn giving false indications. Always test with the air filter connected to the carb or throttle body to help avoid this problem.
Ultrasonic leak detectors can be used with limited success. I bought one when they first came out and was disappointed by the performance. I'm sure they are better now, but background noise still seems to be a problem. In discussing this with other shop owners, these devices don't seem to have caught on very well.
Small vacuum leaks affecting engine driveability can be tested by using high- and low- speed cylinder-balance testing and recording the four-gas values per cylinder. A small- or medium-vacuum leak will be less significant at higher air flow rates. The problem here is that many combustion problems will resolve at higher speeds, so this test must be backed up with positive confirmation of a vacuum leak.
Enough bad news? OK, now for the good news. First, if I sound enthusiastic about this product it is because I am. Second, if I sound like a salesman for this product that is also true, I am working for Emi-Tech on sales. So hear what I have to say with some skepticism and don't come to any conclusion until you have tried VacuTec for yourself and talked to other VacuTec owners.
The VacuTec machine produces smoke. With the engine off, a hose is connected to any vacuum connection into the intake manifold. With the machine on, it will fill a manifold with smoke in about 30 seconds. Any leak will become immediately obvious by the appearance of smoke leaking out. You get a direct indication of the size of the leak by the quantity of smoke. The smoke is in the air, so this will give a positive indication of EGR shaft leaks where liquid fails. This is the first test we run now, eliminating vacuum leak possibilities in less than five minutes. We tested a 1.9 liter Ford with no driveability problems. Smoke came out of a wiring loom in no time. The small nylon vacuum hose that supplied the EGR solenoids was broken 12 inches down inside the wiring loom. While the customer had no driveability problem because this leak was so tiny, his EGR valve was not functioning and could have caused pinging problems under load.
We test every car that comes into the shop for intake manifold and crankcase leaks. We were surprised by the number of crankcase leaks that are only vacuum leaks. What I mean here is that with medium-sized crankcase leaks and a properly functioning PCV system, no oil leaks out. With the engine running, the PCV vacuum in the crankcase overcomes any tendency for oil to leak out. We all know that the PCV is a controlled vacuum leak that is calibrated to the engine. So on a speed density system with a MAP sensor this usually doesn't present a problem. A MAF or vane airflow is another story altogether! These systems are designed to measure all the air consumed by the engine. On these systems, the crankcase leak becomes false air that bypasses the sensor and can cause driveability problems, especially at idle and low engine speeds where the air leak represents a larger percentage of total air flow.
The VacuTec machine is ideal for finding crankcase oil leaks. With the PCV removed and the smoke hose inserted in this hole, and with the fresh air inlet to the crankcase plugged, the crankcase effectively becomes a "sealed chamber." You can also insert the hose into the dipstick tube, but this will prevent your finding a leak here which is quite common. In absolute terms, the crankcase never really becomes a sealed chamber because air will travel past the pistons, so don't be surprised by smoke in the intake during this test.
The smoke will show a crankcase leak anywhere that it doesn't have to travel through liquid. So you wouldn't see smoke coming out of a leaking drain plug. We have had no trouble finding and isolating leaks anywhere else in a crankcase, including front- and rear-main-seals. The smoke pump is regulated to bypass at one PSI to prevent blowing seals or dislodging a vacuum line when testing a sealed system such as the HVAC hoses under the dash. Oil fouling of an area is no obstacle to finding a leak. No matter how dirty an area is, we have had no problem finding the exact location of a leak. We were surprised to find many crankcase vacuum leaks that leaked no oil because of an efficient PCV system. Leaking valve covers, oil filler caps and dipsticks (in that order) are the most common problems we've seen.
The VacuTec machine is ideal for finding exhaust leaks, body leaks, evaporation system leaks (the "I smell gas" complaint), and leaks from any sealed chamber. The possibilities for this tool are only limited by your imagination. Finally, a simple solution for a complex problem.
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AutoInc. Magazine ®, Vol. XLIV No. 5, May 1996