Electrical engineers always talk about equivalent circuits, and what follows is an air flow system represented as one. Here, Voltage is Pressure (relative to atmospheric), Capacitance is Volume, Diodes are Checkvalves, and resistance is resistance. Ground is atmospheric pressure.
Once the operation of the system is explained, a section on troubleshooting the system is presented. For advanced testing you will need a vacuum gauge that can read up to 20 inHg (a MityVac works great) and an assortment of vinyl and fuel hose.
The Engine block has three components: the cylinders generating pressure (D), the cylinder leakage (E) that lets combustion gases to leak into the crankcase, and the crankcase volume that stores this gas (C). The Crankcase Breather Tube allows two exits for this gas: the hole in the barrel canister to the throttle valve mouth and the angled Suction Jet Pump nipple.
SUCTION JET PUMP
This patented contraption (US 6,270,321 - get a copy at www.freepatentsonline.com) provides a means to boost the vacuum provided to the Brake Booster. One end of this unit connects to the engine intake manifold, and so is at some negative pressure. Gases flow continually through the connection to the Crankcase Breather Tube (from either or both of the throttle value connection or the crankcase). This flow is sent through a venturi tube, which creates a partial vacuum relative to the intake manifold pressure, essentially boosting the effective vacuum at P1. The schematic represents this additional vacuum as another battery, since with a more or less constant flow from the angled nipple it's constant.
Lets look at what happens when the engine first starts. P3 is close to atmospheric pressure and P2 goes immediately to around -18PSI. The two checkvalves A and B immediately start passing air from the Brake Booster, so the pressure at P1 falls to just above P2.
However, there is now a flow from P3 to P2, which creates a vacuum in the Venturi tube. This tube is small, and even though it is pressurized now way below the intake manifold (p2), it can only support a small gas flow from the relatively large Brake Booster. Thus, the pressure at P5 starts falling below P2, and gradually the Brake Booster pressure becomes less than the intake manifold pressure.
The Green Checkvalve essentially provides a second flow path from the Brake Booster to the intake manifold. When the brake is used, the Brake Booster pressure becomes more positive (trending towards atmospheric pressure), and thus this valve works in tandem with the Suction Jet Value until the Venturi Effect starts having some affect. In addition, it also provides some redundancy in case the Suction Jet Pump should fail.
BARREL CANISTER OPENING
The Barrel Canister has an internal plate within it that has at least one hole approximately 1/8" wide. Apparently this servers two functions: condensed oil and water can flow down into the throttle entrance, and a source of air so that for really tight engines, the pressure at P3 never gets too low.
The most commonly reported failure is a plugged Venturi. The angled nipple going to the Crankcase Breather Tube completely clogs, and no gas flows through it. However, it appears that the two internal checkvalves continue to function, so the Brake Booster system is normally not affected.
So, lets see if it in fact is clogged. With the engine off, pry off (or unscrew) the clamp holding the small angled tube to expose the angled nipple (the left clamp in this photo):
With the nipple exposed, start the car. Put a finger over the nipple and see if you feel suction. When the valve is functioning, you will both feel suction and hear a small pop when you pull your finger away.
If you have a vacuum gauge or MityVac, you can plug that into the exposed nipple - it should read over -15PSI.
If you feel nothing and hear no pop, well, the venturi is clogged. Replace or clean the Suction Jet Pump (see related Audiworld article on replacing this unit). I had luck using Carburetor and Choke spray cleaner with the unit removed, but ordered a new unit anyway. [In the end, the cleaned old unit was probably just fine - I could tell no difference in pressure or suction after installing the new one.]
MORE COMPLEX TEST
Suppose you feel vacuum, but aren't sure if its sufficient.
The system is stressed at expressway cruising speeds, when the amount of gas leakage is high and engine manifold pressure is low. In this case, D becomes larger than at idle, and pressure starts to build in F. The flow into the Suction Jet Pump cannot increase, so P3 increases. The only way for the engine to lose pressure is through the Suction Jet Pump, the small path to the throttle valve, or by leaking around the various engine seals.
New engines have tight rings and and a corresponding high E value; as they age, E decreases allowing more flow into the crankcase. The system shown here cannot adapt much to increased engine blowby - over time, the pressure is going to build within the crankcase. Audi's recent TSB (C15-04-02 dated August 31st, 2004 and titled Diagnosing Oil Leakage from Gaskets) would lead one to conclude that the "problem" threshold is around 0.43 PSI.
This is why a clogged Venturi within the Suction Jet Pump is so onerous - crankcase pressure increases dramatically at P3 and starts forcing oil around the camshaft seals (as well as others no doubt).
The above mentioned Audi TSB details one method for testing the Suction Jet Pump, but only for cars with automatic transmissions. The test mentioned here can be used for any car.
- home made manometer using 3/8" OD vinyl tubing (from local hardware store)
- 1 foot of 3/8" ID fuel hose
Build the manometer (google manometer, or here is one site: http://www.komar.org/faq/manometer/). Get lots of tubing - say 16 feet or so. Put manometer in passenger's side, snake the tube out the window, under the hood, and into the engine compartment.
Remove the gasket that separates the engine compartment from the battery side (so as to not crush the hose when the hood is shut).
Connect the vinyl tube into the fuel hose, pull out the oil dipstick, and slide the fuel hose over the dipstick tube.
Grab a significant other or friend, and have he/she monitor the manometer as you drive around. WARNING: DO NOT DRIVE ALONE AND WATCH THIS DEVICE, AS IT COULD PROVE FATAL TO YOU OR TO OTHERS. GET A SPOUSE/FRIEND TO DO THE VIEWING!
The liquid level will go down on the engine side when positively pressured. On my 1999 A6 with 150,000 miles on it, I got about 1" of positive pressure at 65MPH on a local interstate.
A pressure of 0.43 PSI would displace one side of the manometer about 5", so my 1" translates into about 0.08PSI of positive pressure
APPENDIX A - EXPERIMENTING WITH THE SUCTION JET PUMP
Disconnect the Brake Booster side of the SJP, and plug the hose with a stopper. Connect a vacuum gauge to the exposed SJP nipple. Remove the hose that connects the SJP to the Barrel Canister (recirculation pipe), and cap the SJP nipple.
Start the engine. Observe that the vacuum is around 18 inHg - this is pure manifold vacuum as the SJP is not doing anything special (yet). Now, uncap the SJP nipple (that normally connects to the Barrel Canister) - watch the vacuum increase to around 24 inHg! The SJP does in fact boost the vacuum by around 6 PSI!