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996 Engine Races to Redline at Startup

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Have stepped in here to help revive a 1999 911 Carrera, with 70K miles and a shady service history. Vehicle was purchased used at 50K and ran without fault until about a year ago. At that point the service history gets very muddy and the car sat unused because of an inability to idle. Now, a year later, I been asked to step in and troubleshoot the engine problems.


On initial inspection, no fault codes were present other than P1602, due to the battery having died. Following battery renewal, engine cranked readily but could not achieve a stable idle. During the course of attempting to nurse an idle, the engine suddenly raced to maximum RPM at which point the engine was immediately shutoff.


Continued inspection revealed the presence of a rat's(?) nest in the air filter housing, a missing throttle housing retention hardware, untightened clamps for the air intake boot supplying the throttle housing and a disconnected large diameter breather hose running from the air/oil separator to the intermediate piece to which the throttle body bolts. Upon further disassembly the mount gasket for the throttle body was noted to be fatigued and leaking air. The air filter had been chewed through and numerous small pieces of air filter element with rubber base attached were found in the MAF sensor's housing.


The air filter housing was thoroughly cleaned, the air filter element renewed, and the debris present in the MAF sensor housing was extracted. The MAF sensor was removed from its housing and inspected: no gross faults were noted and the unit was reassembled. The rubber sleeves and the resonance tube supplying the intake distributors were inspected for the presence of oil; the throttle body mount gasket was renewed, the throttle body thoroughly cleaned and then reinstalled with all of its proper mount hardware. Engine oil level dipstick and oil filler cap were renewed, due to fatigued seals. The disconnected hose supplying the throttle body intermediate piece was reconnected.


Following completion of this service the engine was started and observed for performance. The engine started readily and idled, within reason, as long as the MAF's electrical connector was left disconnected. The idle was not perfect however and a vacuum leak was apparent. Everytime the engine was started with the MAF connected, it immediately raced to redline and its speed could not be attenuated by blipping of the accelerator pedal.


With the MAF's wiring harness again disconnected, the engine was allowed to idle long enough to check for stored fault codes. With the engine thoroughly warmed up, both the oil filler cap and the engine oil dipstick could both be easily removed from the engine.The following codes were listed:


P0102 - Hot film MAF sensor (not necessarily indicative of a faulty MAF; present because the MAF connector was disconnected during the fault code check)

P0300 - Misfire detection

P0304 - Misfire cyl 4

P0305 - Misfire cyl 5

P0306 - Misfire cyl 6

P0150 - O2 sensor ahead of cat conv. bank 2

P1316, 1317, 1318, 1319 - misfire, emission relevant for cyls 4-6


Following code retrieval, the idle control valve was removed for inspection; approx. 40% of the valve's mount gasket was missing, permitting a vacuum leak. The valve was seen to be functional in both directions by use of a 9V battery check.


Based on this information, is it possible the MAF is responsible for this grossly elevated idle speed? Or does the fault more likely lie with the idle control valve? The next logical diagnostic step would be to fix the ICV gasket and then attempt to run the engine with the MAF sensor connected and the ICV disconnected. If the engine idle speed is stable and within limits at that point, would it be safe to say the MAF is erroneously signaling the ICV to upwardly adjust the idle speed?? If replacement of the MAF is warranted, is the consensus that the Bosch units are suitable replacements for the factory branded component? I understand the manufacturer is Bosch, just curious as to how they have been holding up over the Porsche boxed units.



Edited by jandersen
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The MAF has little to do with idle speed - as a matter of fact, you could disconnect it and the car will likely idle fine.

If your car is a C2 check your throttle cable - it may have come off or is broken and jammed open.

Barring that occurrence then unplug your MAF and see if the end starts and idles. I have never seen a MAF cause that but it would eliminate the MAF.


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Engine will start and idle with the MAF disconnected. Its is only when the MAF is connected that the RPM immediately race upwards. The throttle cable is intact and connected properly; lumen of throttle body is clean, butterfly flap is functional.


Can a faulty MAF trigger the idle control valve to increase idle speed?

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  • 1 month later...

I can't think of a fault in the MAF that would lead to such behaviour. I'd say the reason for a normal idle rpm with MAF disconnected is that instead of actual air mass provided by the MAF the motronic then uses a stored/model based value for air mass at idle. I hazard a guess that the engine is actually running very lean as the actual air mass into the engine is greater than the stored value. I.e. you have more air coming in than is required for idle rpm. Either further vacuum leaks, the icv further open than necessary or the butterfly cracked open ever so slightly.

Another thing to check is the throttle position sensor. Make sure it is mounted such that it signals idle and not off-idle when the throttle is shut.


Edited by hardtailer
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