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River Rat
 
Dave 86 930 Fl's Avatar
 
Join Date: Dec 2005
Location: 1000 Islands NY
Posts: 405
Boost Issue

Iím experiencing an issue where the engine hesitates/stumbles/misses under boost.

This all started when I overfilled the oil during an oil change. Prior to the overfill, the car ran 13.5 afr off boost and high 11ís on full boost and the car ran like a scalded dog. Ever since the overfill, the car runs rich around 10.3 afr off boost and when the stumbling begins, it briefly runs lean (14+) before I get off the gas. Iím curious as to why the off boost afr mixtures became rich after the overfill, but I suspect that the overfill may have caused the problem. I became aware of the overfill issue at the track when the engine started stumbling and a bunch of smoke blew out the exhaust, so I drained some oil off to get the correct oil level. I checked the air box and I had sucked some oil up and it obviously entered the intake past the metering plate. I cleaned up the metering plate, but Iím thinking that any oil on the mechanism may be adding weight to the plate and as a result, the engine is running very rich. If this is true, would an overly rich condition cause the car to stumble on boost? If the problem is the oil residue on the metering plate, what is the best way to clean it?

BTW, the car is an 86 930 with stock internals, K27-7200, AI fuel head, Andial intercooler, .9 bar, B&B headers. I recently had the motor out of the car in order to replace the head studs and everything looked very good Ö pistons, bearings, cylinders, etc.

Regards, Dave
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1986 930 - K27-7200, Andial Intercooler, Tial Wastegate, Electronic Boost Controller, B&B headers, Bodart Zork, Leask Adjustable WUR, Full Elephant Racing Suspension w/Adustable Spring Plates and Von Shocks, Wevo Transmission Mounts, Fuel Safe Fuel Cell, Forgelines (9"/11"), Hargett Shifter, Innovate AFR, Cobra CF Seats, DAS Rollbar, Coleman Floating Rotors, Lightweight Interior, Forgeline 17" x 9"/11", 2800 lbs.
Old 10-27-2008, 03:18 AM
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Do you have a spare, known-good O2 sensor you can swap in? Perhaps you fouled the sensor which might exhibit some of those symptoms.
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Old 10-27-2008, 04:09 AM
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Cynical Misanthrope
 
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Assuming the car is a US (lambda control), try unplugging the 02 sensor and run it open-loop to determine if the 02 sensor is boogered. I think you may have just fouled the hell out'a the plugs.
EDIT: did you have the intercooler off? Did you pop the throttlebody o-ring?
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Old 10-27-2008, 08:28 AM
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Crotchety Old Bastard
 
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Given the symptoms, it seems likely that you pinched an intercooler O-ring when cleaning up the oil mess.
I assume you changed the spark plugs.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8

Last edited by RarlyL8; 10-27-2008 at 09:26 AM..
Old 10-27-2008, 09:24 AM
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River Rat
 
Dave 86 930 Fl's Avatar
 
Join Date: Dec 2005
Location: 1000 Islands NY
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Thanks for the replies.

The o2 sensor is long gone, it was disconnected many mods ago. I do have an AFR gauge on the car and it appears to be functioning properly.

The symptoms began immediately after the overfill and the only component that I removed from the engine was the airbox in order to clean it. So, I'm assuming that the pressure side of the intake path is properly sealed up, as I did not disassemble any of the components. I did carefully clean off the air sensor plate. After cleaning the oil up the AFR's still were in the low 10's at idle and the car would hesitate badly under boost.

After having these problems at the track, I brought the car home and changed the spark plugs. During the process, I found a broken head stud (crap). So I purchased some new head studs from our very own Henry S. and pulled the motor. The condition of the motor's internals were perfect (40k miles) and most importantly, the pistons looked great ... no detonation. So, the motor is back together and running exactly as it did before the head stud fix ... low AFR's at idle and hesitating under boost.

I'm going to remove the mixture control unit (air flow sensor and fuel distributor) from the motor and give it a good cleaning to remove any oil residue that may be impacting the function of the air flow sensor. I'm thinking that oil may be on the air flow sensor and causing the plate to be lower than it should be, hence causing a rich condition. I'm also going to remove the WUR in order to inspect the vacuum port for oil contamination.

I keep going back to the origin of this problem, I overfilled the oil and it was sucked up into the intake. The oil entered the air flow sensor and this caused the AFR at idle to go down into the 10ís and the motor was missing under boost. Iím not sure how far the oil traveled up the intake path, but it may have made it to the WUR.

Does anyone have any opinion on the effects of oil on the turbo ... can this cause a problem with the seals in the turbo?

I'm working the problem (track day scheduled in 12 days) and I appreciate the responses.

Regards, Dave
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1986 930 - K27-7200, Andial Intercooler, Tial Wastegate, Electronic Boost Controller, B&B headers, Bodart Zork, Leask Adjustable WUR, Full Elephant Racing Suspension w/Adustable Spring Plates and Von Shocks, Wevo Transmission Mounts, Fuel Safe Fuel Cell, Forgelines (9"/11"), Hargett Shifter, Innovate AFR, Cobra CF Seats, DAS Rollbar, Coleman Floating Rotors, Lightweight Interior, Forgeline 17" x 9"/11", 2800 lbs.
Old 10-28-2008, 04:35 AM
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Crotchety Old Bastard
 
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Wow, long way around the horn just to end up where you started.

The oil flood must have been rather sever to affect all you have seen. The WUR does have a vac line attached to the throttle body so it is likely that if oil reached that far it would also find its way into the WUR. I have no experience with that sever a problem but can imagine the impact.
Everything must be cleaned as you stated. A thorough douching of the air meter plate with carb cleaner is a start. A quick check of the WUR would be to swap it out with another.

Please report on your findings and results.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 10-28-2008, 06:17 AM
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River Rat
 
Dave 86 930 Fl's Avatar
 
Join Date: Dec 2005
Location: 1000 Islands NY
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I wouldn't call it a flood, but there was oil on the metering plate and some oil had pooled under it. What's amazing is that I can disassemble a 930 motor and put it back together without any issues, but I can't change my oil properly ... wtf.

If there are any more opinions out there, I'd like to hear them. I'll report my findings as I progress through this journey.

Regards, Dave
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1986 930 - K27-7200, Andial Intercooler, Tial Wastegate, Electronic Boost Controller, B&B headers, Bodart Zork, Leask Adjustable WUR, Full Elephant Racing Suspension w/Adustable Spring Plates and Von Shocks, Wevo Transmission Mounts, Fuel Safe Fuel Cell, Forgelines (9"/11"), Hargett Shifter, Innovate AFR, Cobra CF Seats, DAS Rollbar, Coleman Floating Rotors, Lightweight Interior, Forgeline 17" x 9"/11", 2800 lbs.
Old 10-28-2008, 08:00 AM
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