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Troubleshooting Vacuum Leaks
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Pelican Technical Article:

Troubleshooting Vacuum Leaks


2 hrs






SmokePro Machine

Applicable Models:

Porsche 986 Boxster (1997-04)
Porsche 986 Boxster S (2000-04)
Porsche 987 Boxster (2005-08)
Porsche 987 Boxster S (2005-08)

Parts Required:


Hot Tip:

See if you can borrow the tool from a friend

Performance Gain:

Smoother running car, better performance

Complementary Modification:

Replace vacuum hoses and intake boots
101 Performance Projects for Your Porsche Boxster

This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your Porsche Boxster. The book contains 312 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 950+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Boxster owner's collection. The book is currently available and in stock now. See The Official Book Website for more details.

Today's modern cars have a tremendous amount of vacuum hoses and boots contained with the engine compartment. To the uneducated eye, the engine compartment can easily look like the insides of an oil refinery with all the hoses running in and out. As these hoses and boots age and are constantly exposed to hot and cold temperatures, they tend to break down and develop cracks which can then cause vacuum leaks. Unfortunately, when the fuel injection system develops a vacuum leak it will tend to confuse the fuel injection computer's sensors, and the car will cease to run properly. You may get decreased gas mileage, rough idling, misfires, and sometimes a check engine light (CEL) on the dashboard.

Whenever someone contacts me and indicates that their car is running rough, I almost always tell them to check the entire system carefully for vacuum leaks. Without the proper tools, this can sometimes be very difficult. Old rubber boots have a tendency to crack and leak in spots that are not visible to the naked eye. Sometimes squeezing them will show a crack that you can't normally see when the boot is in its initial resting position.

One poor man's way to check for vacuum leaks is to artificially create a leak and see how the engine reacts. There are two primary vacuum systems on the Boxster engine: crankcase vacuum and intake manifold vacuum. You can test for proper crankcase vacuum by removing the oil cap while the car is idling. The engine should change idle and begin to run a bit rougher with the oil cap off. If there is no change in the running of the engine, then you might have a crankcase vacuum leak somewhere (sometimes caused by a failing air-oil separator: see Pelican Technical Article: Air / Oil Separator). To induce a vacuum leak into the intake manifold, you can disconnect one or more of the hoses that connect to the intake. One example would be the hose that connects to the air-oil separator. If you crack one of these hoses open just a bit, and the engine RPM doesn't change, then you might have an intake vacuum leak somewhere.

By far, the best way to test for vacuum leaks is with a smoke machine. Although these are somewhat expensive at about $650, you can rent and/or borrow them from some shops. The machine generates smoke and then blows it through your engine's intake and crankcase. All you need to do is sit back and watch for little puffs of smoke where there is a leak in the system. This can save many, many hours of random troubleshooting and guesswork. For the project car for this book, I wanted to make sure all of my custom-made hoses that were required for the installation of the 3.4 engine into the Boxster were leak-free. I ran the car through the smoke machine and confirmed that everything was airtight and there were no troublesome leaks.

The smoke machine runs on standard household baby oil, and generates smoke by heating the oil. The smoke generated is very similar to the type created in model railroad steam engines. The machine also needs to be connected to a shop air compressor. The compressed air is mixed with the smoke and then funneled through the intake system of the engine. In the case of the Boxster, I removed the intake air filter and used a cone adapter that comes with the machine to seal the smoke hose to the intake pipe. The car should be completely cold when you are testing it for leaks, as the rubber hoses and boots are most likely to leak when they are cold and contracted. You should also remove the mass airflow sensor from the system so that the smoke doesn't build up on the sensor and affect it's operation. Plug the hold for the sensor with some masking tape.

After you power up the smoke machine, it will take a minute or two to fully smoke out the car. If your car is completely air tight (a good thing), then you might not see any smoke. You can check to see if the smoke machine is operating correctly by removing the oil cap in the rear trunk. You should see a steady plume of smoke exiting out of the oil filler (this is normal). On the Boxster, you may also see some very tiny plumes of smoke exiting out of the resonance flapper bearings: this is also normal and doesn't affect the operation of the car or indicate a major vacuum leak. If you do see steady plumes of smoke exiting out of the engine compartment, then investigate further. On the 3.4 conversion engine, there weren't any major leaks, but there were a few minor ones that were found (one of the breather hoses that connected to the radiator tank needed to be tightened).

Although you might think a smoke machine is a limited use tool, it's highly versatile for solving other problems as well. Basically any system that contains air can be tested. You can use the system to check climate control systems, leaky headlamp housings, exhaust systems, A/C lines and compressors (although most A/C leaks are very small and difficult to detect with just a smoke machine). You can also bench test components like radiators prior to installation to make sure that they are factory perfect.

You can even use the smoke machine to detect leaks from door and window seals. First, rollup all windows and seal the car. Then turn on the fresh air fan motor to the maximum setting (do not set the system to recirculate). Using the smoke machine with a diffuser (a wider nozzle that will slow down the flow of smoke), move around the outside of the car and blow the smoke onto the area you think might have a leak. The fresh air fans inside the car will create a positive pressure environment that will push air out through any leaks. By slowly blowing smoke on these suspected areas, you can see the smoke pattern become disturbed by the leaking air. This test of course requires that you do it inside your garage in an environment where there is very still air.

Shown here is the Redline Smoke Pro machine.
Figure 1

Shown here is the Redline Smoke Pro machine. This extremely useful tool is invaluable for finding vacuum leaks within your fuel injection system. The machine's air supply is provided by an air compressor (upper left) that is plugged into a pressure regulator (green arrow). The heater is powered by your car battery (upper right and blue arrow). The compressed air is combined with smoke and then pushed out through the nozzle (yellow arrow).

With the smoke machine turned on, you can see the trail of smoke that exits out of the nozzle.
Figure 2

With the smoke machine turned on, you can see the trail of smoke that exits out of the nozzle. You don't want to "smoke out" your mass air-flow sensor (MAF), so be sure that you remove it and tape off or plug the hole prior to pressurizing the system (upper left). The Smoke Pro comes with a whole set of adapters that you can use to plug into the intake system. On the Boxster here, we removed the aftermarket cone filter and plugged the intake with the rubber cone adapter that comes with the Smoke Pro (blue arrow).

With the system pressurized, you can check to see if your engine is
Figure 3

With the system pressurized, you can check to see if your engine is "fully smoked out" by removing the oil filler cap. A steady stream of smoke should exit the filler hole. This means that smoke is going from the intake, through the air-oil separator and into the crankcase. At this point, replace the cap and carefully examine your intake for smoke trails that will indicate vacuum leaks.

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Comments and Suggestions:
Nick Comments: I recently replaced a failed water pump on my 1999 2.5 boxster. A few weeks later, I had a major coolant leak from the passenger side R/H Driveradiator followed two weeks later by a failed driver's side radiator. Both replaced.
However, I then had an engine check light come on and the car idle was really rough, surging and stalling. I can hear a hiss from somewhere on the top of the engine so have been searching for a vacuum leak with no luck so far. Is there any link between the coolant side and air/vacuum side that may have been disturbed with all the coolant system work?
October 4, 2016
  Followup from the Pelican Staff: HHMM. Your crankcase vent valve can have a coolant hose attached, that is the only connection I can think of. Some models have it, some don't. You can check if there is a small hose T-ing off the larger hose at the water pump toward the side of the engine. - Nick at Pelican Parts  
Moss Comments: I have smoke tested my 2001 Boxster 2.7 because I have P0103 - exceeds limit value and the only smoke I have escaping is at the resonant or flap valve. Other than that I have no leaks. Can you advice me regarding the P0103 code and what to try next
August 30, 2016
  Followup from the Pelican Staff: P0103 is for a mass air flow signal. COnfirm the signal to the DME is faulty, if so, replace the MAF sensor. - Nick at Pelican Parts  
Nicky Comments: I have a 1997 Porsche Boxster with a 2.5L 5speed maunual transmission. So I took a trip one day just a 45 minute drive to and from where I was headed and after that trip I noticed that my car would rev really high when I start it up and then the rpms would go really low to where my car almost died. And when I put my car in neutral at a stop the engine rpms would still be high but then would slowly go back down. Now that it's summer time my car will rev just over a grand and then die. Someone told me a vacuum leak could be the cause but I'm just having issues finding where it's at or what it could be from. Any ideas?
June 30, 2015
  Followup from the Pelican Staff: Impossible to guess where it could be. I would start by checking the obvious intake ducts and intake manifold hoses and connections. - Nick at Pelican Parts  
mikestrip Comments: I have a 2003 Boxster 2.7L that starts normally with no "check engine" light or anything.

The problem I am having which has existed even before the ECU was replaced is the idle is "searching."

It ranges between 800-1000 RPM. Just slowly up and down while in neutral.

The OAS is brand new as well. The secondary air injection pump turns on as I can hear it on the right side of the engine bay after a few seconds. Do you think this is a possible O2 sensor issue? Or leak in one of the vacuum hoses? I have new spark plugs and coils in it as well.
March 17, 2015
  Followup from the Pelican Staff: If the idle is hunting with no check engine light ON, you may have a vacuum leak or a problem with the throttle housing.- Nick at Pelican Parts  
Sherif Comments: I have a carrera s 996.1 3.4L witha tiptronic automatic transmission zf5hp19 Hl I had a transmissin complete failure I got a used one and after installing it the engine started normally, but the lamp indicators for the gear position was on the 4 th shift and blinking and when I added the oil the indicator went off completly.and none of the gear indicators lamps are on no D no N nothing is on at all and the car does not want to start at all as if the battery is not there ,but the dach board is on and the battery is in a good condition but when I turn the key to start the engine nothing is happening as if u r trying to start the car when the stack shift is on D
Any idea what that could be
January 8, 2015
  Followup from the Pelican Staff: The range switch may be misadjusted, I would start there. You can check function using a scan tool. - Nick at Pelican Parts  
George Comments: When removing the oil cap on my 06 cayman the engine rpms I tease by 500 - 700 is that normal
December 9, 2014
  Followup from the Pelican Staff: Yes. RPM will fluctuate with the oil cap off. - Nick at Pelican Parts  
cdw Comments: car stalls once at 179 degrees. then i easily restart. however engine rpm goes up 200 rpm and stays at 1000 rpm. engine exhaust tone changes a bit deeper. could this be a vac leak on right side purge valve flapper causing internal leak? if so how to fix and also do you have pary in stocks.

October 3, 2014
  Followup from the Pelican Staff: We do have stuff in stock. Give our parts specialists a call at 1-888-280-7799. They can help you find the right part.

A hot stall is a bit of a weird one. I would want to see some engine data to make a call. What is desired and actual engine RPM when starting and up until the stall. What are the fuel trim values? - Nick at Pelican Parts
Rocky Comments: Bmw 318i 2004 touring has about 3 dips in half hour while idling,almost like something is choking the engine,checked all vacuum hoses, pcv diaphragm ok.
Does this with vvt motor,sensor and computer disconnected.With all reconnected 2 codes are generated,27b8 and 2862.eml light and cel light and yellow triangle with ! sign comes on,help please
September 29, 2013
  Followup from the Pelican Staff: These are differential pressure sensor and valve timing faults. You may have an issue with the VANOS system on your vehicle. I would perform a VANOS test using a BMW scan tool. - Nick at Pelican Parts  

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