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911 3.2 Engine Misfiring
Welcome to another thread on a misfiring 3.2 Motronic engine.
I've had this engine in the track car since 1991 and at the last track event, it started acting up once again. Driving under heavy throttle, the car would misfire at higher RPMs. It ran well most of the first day, but toward the end of the day the misfiring started to present itself. It felt like it did when the AFM had worn tracks, so I changed it out for one of the spares, but the problem persisted and may or may not have changed RPM's where the problem would present itself: Full or part throttle up near 5500 - 6000 RPM in any gear. Not all the time, but enough to be annoying. It wasn't the rev limiter as it didn't sound the same as when the rev limiter is reached. It kind of felt like it was ignition breaking up. Plug wires were only a couple seasons old, and plugs looked ok, so I thought it might be the coil. I swapped several spare coils in and the RPMs where the misfire occurred seemed to change with the various coils. (Where did all the coils go? Can't seem to find any in stock.) Anyway, the problem persisted. Looking in the distributor cap didn't show any surprises. Couldn't see any traces of carbon tracking or cracks.... Until, I pulled off the rotor and looked at the underside of the rotor. I've never seen a rotor arc through to the distributor shaft, but that is what it looked like to me. Anyone else ever see a rotor that seems to have developed an arc through the rotor to the distributor shaft? It looks like burn marks on the rotor shaft and burn marks on the underside of the rotor. I haven't yet tested the car. I'll have to brave driving it hard up and down the street a few times to check, but I'm guessing this is the problem. I'll post back if changing the rotor seems to solve the issue.  
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John Flesburg 2016 981 Boxster S.................| 1983 911 Turbo - (White) 1974 911 3.2 - Red Car........... | 1974 914-6 3.2 - (Silver) 1974 914-6 3.2, GT -(Red).......| 1974 914 - 2.7 GT Clone (TBD - Saphire?) 1971 914 (TBD)..................... | |
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09-13-2023, 12:27 PM
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I’ve not seen that either, but if there’s a crack in the rotor I could see how it might happen. Look at it really close.
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1973.5 911T with RoW 1980 SC CIS stroked to 3.2, 10:1 Mahle Sport p/c's, TBC exhaust ports, M1 cams, SSI's. RSR bushings & adj spring plates, Koni Sports, 21/26mm T-bars, stock swaybars, 16x7 Fuchs w Michelin Pilot Sport A/S 3+, 205/55-16 at all 4 corners. Cars are for driving. If you want art, get something you can hang on the wall! |
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09-13-2023, 12:42 PM
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To follow up (I hate posts that don't come to conclusion).
The marks on the rotor weren't the root cause of the misfire. I ran the car afterwards with the same exact results. The car would start and run and idle fine, rev fine while parked. Under load, it would misfire or break up before reaching redline on the tach and prior to hitting the rev limiter (6850). Put the car back on the lift and started checking everything once again and found nothing wrong. Fuel Pressure and delivery were fine (pumped over a liter in 30 seconds, pressure 30 psi with vacuum and about 40 psi with no vacuum) Ohm readings on both flywheel sensors Oscilloscope readings on both flywheel sensors Even voltage rise as opening the AFM flapper door Temp Sensor was around 3300 0hms Idle switch works Full throttle switch works Cylinder Head Temp Sensor ohmed out ok both cold and hot Cylinder Head Temp Sensor changed to RICH when unplugged Looked at O2 sensor, not certain how to test this one New plugs, wires test ok Ignition coil checks out to .4ohm between poles, and about 5500 ohms from 1 to primary I can hear the idle stabilizer operate with key on. After all that, I still had the same exact issues. Out of frustration, I took out the speed sensor (DG) and set the gap between the sensor ring and the sensor. (I have an old sensor with a .8mm washer glued to the end that I set them with). I noticed that there were some small metallic debris stuck to the end that I cleaned off, but saw nothing out of the ordinary. I changed the DG speed sensor thinking that it couldn't hurt and while I was there... I tried the car after changing the speed sensor and no more problem pulling all the way to redline and the rev limiter in the lower 4 gears. I think my problem is solved. In any event, I did oscilloscope test the speed sensor that was changed, and while cranking, I didn't see anything abnormal and it ohmed out right around 1020 ohms, so I'm not certain that the test procedure for the speed sensors is as good as it could be. Wanted to share my experiences in case the information could help others. Happy 915 day!
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John Flesburg 2016 981 Boxster S.................| 1983 911 Turbo - (White) 1974 911 3.2 - Red Car........... | 1974 914-6 3.2 - (Silver) 1974 914-6 3.2, GT -(Red).......| 1974 914 - 2.7 GT Clone (TBD - Saphire?) 1971 914 (TBD)..................... | Last edited by johnman001; 09-15-2023 at 04:07 PM.. |
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09-15-2023, 04:02 PM
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 Flywheel sensors for reference
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John Flesburg 2016 981 Boxster S.................| 1983 911 Turbo - (White) 1974 911 3.2 - Red Car........... | 1974 914-6 3.2 - (Silver) 1974 914-6 3.2, GT -(Red).......| 1974 914 - 2.7 GT Clone (TBD - Saphire?) 1971 914 (TBD)..................... | |
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09-15-2023, 04:08 PM
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Join Date: Jul 2013
Location: Brisbane, Australia.
Posts: 2,648
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awesome result. Thanks for posting.
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Always learning. www.aircooledporsches.com.au See me bumble my way through my first EFI and TURBO conversion! https://youtu.be/bpPWLH1hhgo?si=GufVhpk_80N4K4RP |
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09-16-2023, 01:03 AM
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Location: Lomita, CA
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Quote:
and rev a hot engine numerous times. If it dies, the speed sensor is weak.
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Dave |
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09-16-2023, 05:04 AM
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