TECHNICAL LIBRARY Understanding the PCV and AOS Systems · On the 9X7.2 and 9X1 Por

TECHNICAL LIBRARY Understanding the PCV and AOS Systems 7 On the 9X7.2 and 9X1 Porsches, the engine crankcase vacuum reading is normally in LOW VACUUM READING: the region of -14.0” to -16.0” H2O (Inches of water). When an oil separator fails, the 8 -Missing engine oil cap and or oil cap O-ring. crankcase vacuum can rise to -20.0” to -25.0” H2O, or even much worse (-30 to -40.0” -Cracked or broken spark plug tubes, if applicable. H2O) depending on the extent of AOS failure. 9 -Disconnected or broken dipstick tube, if applicable. Normally vacuum is measured in inches of Hg (Inches of Mercury) but in this case we -Make sure the oil dipstick is fully seated, if applicable. measure it in H2O (Inches of water). Water is much lighter that Mercury. When the air oil separator starts to fail on a Boxster (986-987), you will generally start to -Anything that can leak air can leak oil and vice versa. see a substantial amount of tailpipe smoke on cold engine start-up. When the oil -Torn AOS boot, broken or cracked oil filler tube, Dipstick and/or crankcase crossover separator finally fails, the exhaust tailpipe smoke can reach epic proportions; almost like accordion pipe. it is manufactured from an anti-mosquito smoke machine. On the 996’s & 997’s, when the oil separator fails, it usually doesn’t create much exhaust tailpipe smoke, if any. It’s Please note: These readings are at Sea-Level and at operating temperature. With almost like a silent killer or costly surprise. It is a good idea to check your vacuum reading elevation these readings should be lower in value due to thinner air. at each engine service to monitor the health of your AOS. SPORTS-CAR NOMINAL VACUUM READING: However, there are instances other than tailpipe smoke that can indicate your AOS is going bad. Two such symptoms caused by an early failing non-symptomatic AOS are M96 & M97 Nominal crankcase vacuum is -5.0” (-4.0” to -6.0”) of H2O (not HG). ignition misfire and fuel trim faults. On 2005 and older models the DME fuel trim monitor MA1 Nominal crankcase vacuum is -15.0” (-14.0” to -16.0”) of H2O (not HG). does not operate immediately from cold start-up but the misfire monitor typically does. If the AOS is in its early stages of failure, it can produce some very high vacuum values i.e. HIGH VACUUM READING: -40.0“ H2O, but only for the first 3 minutes of cold engine operation, usually without any trace of tailpipe smoke. Hence the AOS can cause a lean running condition that can The only thing that can cause an excessively high manometer vacuum reading is a affect the fuel trim values but without any other symptoms. You can get misfire faults faulty AOS and or sludge. and the check engine light (CEL) will be illuminated but there are virtually no other The result of a faulty AOS (with high vacuum readings) will sometimes cause the symptoms. This is where having tools such as a Durametric along with the CR Tools engine to make a honking like sound; this is caused by vacuum/air being forced AOS Analysis Tool can help you diagnose and catch an AOS failure before serious backwards past the crankshaft seal(s). Note: this does not have to be accompanied engine damage can occur. by tailpipe smoke. More often than not it is non-symptomatic. A high vacuum reading is an internal vacuum leak and this causes a lean running condition usually leading to Normally when an AOS fails, it can cause a lean running condition that will most likely an illuminated check engine light (CEL). illuminate the CEL. The only way to really know what’s happening is to measure the crankcase vacuum and then monitor the fuel trim numbers in the Actual Values section of A failed AOS (with high vacuum readings) can cause the engine oil pressure warning the Digital Motor Electronics (DME) with a high quality scan tool/tester to see exactly light to illuminate. This is usually from too low of engine idle, a cause and effect what’s going on, otherwise you’re just guessing. So what do the readings actually mean? condition created by a faulty AOS. If the AOS has a catastrophic failure and causes a hydraulic lock condition do not panic, remove all the spark plugs, turn engine over slowly by hand with wrench, get engine oil out of the cylinders slowly and gently. Perform engine leak down and compression tests on all cylinders to confirm engine condition and then replace spark plugs with new. Remove exhaust muffler(s) and drain engine oil from exhaust system. It is always recommended to remove central resonance tube from intake manifold to remove engine oil with a vacuum device (e. g. Mighty-Vac), complete assembly, replace faulty AOS and start engine. Run automobile at idle in a safe area until smoke subsides. Be forewarned, it will smoke a lot. LNEngineering LLC | 125 Gladiolus St | PO Box 401 | Momence IL 60954 Web: http://www.LNengineering.com | Email: support@LNengineering.com | Phone (815) 472-2939 | Fax (413) 280-9041