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Discussion Starter #1
Hello Everyone,

I have a complex problem that I would love help narrowing down. Thank you in advance to any contributors.

I don't drive my e46 often and so this problem has been dragging on for some time. I love this car and want to keep enjoying it.
Last January I brought the car in to the dealer with an intermittent Service Engine Soon condition. They diagnosed a torn intake hose and I approved the work. After picking up the car I noticed immediately that at mid-range (2500-3000 RPM) the car would surge under acceleration. I parked the car and inspected a few days later. I found that they hadn't finished reassembly. My intake hose was properly clamped but the entire intake box was not secured and appeared to be lifting when the DISA would open (around 2800 RPM, I've read). I informed my service writer but elected to just buy the bolts and reattach rather than return to the dealer for what seemed like a trivial problem. The surging improved but was not eliminated and has persisted.
This behavior DOES NOT present when the outside temperatures are below 32F or if the vehicle has not reached operating temperature. High humidity/ rain also seem to minimize or eliminate the problem.

Today the dealer re-inspected the vehicle and found no issues with the original repair. They smoke tested and confirmed everything has been reattached properly.

Condition: Surging in gears 3-6 under moderate to heavy throttle (highway passing or on-ramp acceleration). RPMs will surge 200-500 RPM with clutch engaged, then RPMS drop and vehicle accelerates as normal. Climate and operating temp impact impact behavior.

Intermittent code: P0174 - lean

Potential problems:
1. Oxygen sensors. I have read that surging can be related to oxygen sensor degradation even without codes that reference oxygen sensors. I question why this would only happen after the faulty vehicle service.
2. Idle Control Valve. I am wondering if the intake air bypassing the air filter could have fouled the ICV. Does anyone have any experience with this?
3. Crankcase Ventilator. I replaced this part roughly 50,000 miles ago. The vehicle is rarely driven less than 1 hour at a time. It is my understanding that vehicles driven mostly long distances have a lower failure CCV rate.
4. Clutch. Despite having many manual vehicles I have had a clutch fail, so I am admittedly ignorant to what clutch failure feels like. I always assumed that clutch failure would be more prevalent in lower gears and my problem only presents in upper gears. Furthermore, would humidity and temperature impact the behavior of a failing clutch?

Ruled Out:
1. DISA. The dealer mentioned at the Jan 2019 service that I may want to consider replacing the DISA. They detected a leak during the smoke test. Upon my own inspection I discovered the dealer has also neglected to re-install the O-Ring for the DISA. I inspected the DISA later and followed the testing procedures I found here. It is operating properly. I have replaced the O-Ring and nothing has changed.
2. Transmission. I have considered and logically ruled out transmission failure as the problem is presenting in multiple gears.
3. Vacuum Leak. The vehicle reportedly has passed a dealer performed smoke test.
4. Mass Air Flow Sensor - I have removed and cleaned the sensor. Nothing has changed.
5. Fuel Filter. Fuel filter was changed as good measure (and with crossed fingers) in December 2019. Condition did not change.
 

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When the transmission is in gear and the cultch is engaged (foot off pedal) the engine should be directly connected to the drivetrain. Engine rpm should increase as vehicle speed increases and there should be no surging of rpm. The engine should be directly connected to the drive wheels by the clutch through the transmission. It sounds like your clutch is starting to slip. Manual transmissions can't slip like an automatic, it's just gears. The engine works harder to accelerate the car in higher gears due to the gear ratios so it's more likely to notice a worn clutch slipping. How many miles are on the clutch?

Sent from my S61 using Tapatalk
 

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I vote slipping clutch, too.

DO NOT go back to that dealer- if they can't change a boot,
they'll make a mess of your clutch, too!
And leaving THOSE bolts out will be harder to fix.

t
transmission currently on floor.
 

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Discussion Starter #4
When the transmission is in gear and the cultch is engaged (foot off pedal) the engine should be directly connected to the drivetrain. Engine rpm should increase as vehicle speed increases and there should be no surging of rpm. The engine should be directly connected to the drive wheels by the clutch through the transmission. It sounds like your clutch is starting to slip. Manual transmissions can't slip like an automatic, it's just gears. The engine works harder to accelerate the car in higher gears due to the gear ratios so it's more likely to notice a worn clutch slipping. How many miles are on the clutch?

Sent from my S61 using Tapatalk
I was thinking that at one point too and actually have purchased a full clutch and flywheel kit for replacement but I would hate to spend all that money and have the problem persist. Does it make sense that low temperatures and even high humidity would virtually eliminate the slippage?
 

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Discussion Starter #5
I vote slipping clutch, too.

DO NOT go back to that dealer- if they can't change a boot,
they'll make a mess of your clutch, too!
And leaving THOSE bolts out will be harder to fix.

t
transmission currently on floor.
To be fair, the service manager was very responsive and seemingly took swift action when I notified him of the problem. Unfortunately, I have also been assigned to a new service writer who I am less than impressed with. It's never good when you speak to the actual technician and he tells you how impressed he is with the condition of your car and then the service writer patronizes you about repair costs v. value. I have the last year of the e46 and this guy just doesn't get it. I'm keeping the car because I love it and I fully acknowledge that is likely not the smartest financial move. I also highly doubt my next BMW, which I plan on parking beside my e46, will prove as reliable as this car has been.
 
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