2008 E85 N52 Bog and Surge (and other assorted repairs)

[DenverZ4]

Hello all, just an introduction and a bit of a log of issues on our new to us 2008 3.0i Roadster. My son is 15 and a half and in Colorado he has his learner's permit. His grandfather gifted him his 2008 3.0i Roadster 6spd manual. It is a fun little first car especially to learn to drive manual on, but it has just over 120,000 miles and has some issues we are having fun sorting out and bonding over as father and son. So far we have fixed:

1. Broken radio head unit right knob. It has factory nav, and the right rotary knob did not work. We bought a donor unit for parts on ebay and after replacing the daughter board it is now fully functioning.
2. replaced broken sport button from a donor part sourced from local junkyard.
3. This weekend we are replacing the valve cover gasket and lots of other weeping seals (Valvetronic motor, oil filter housing, ESS sensor, etc.), plan to give it an oil change and new manual transmission fluid.

Major issues we still need to sort and where I am looking for any advice:

"Bog and Surge": The car has a major flat spot under moderate to heavy throttle in the midrange. As mentioned above it is a manual transmission, but it feels like an old automatic transmission car trying to down shift. When you apply moderate to heavy throttle in say 3rd gear at 40MPH, the RPM jumps to 4k, but then smoothly and quickly drops to about 3K before building RPM back up and once above say 4500 RPM the car seems to pull much harder. The car is not throwing any codes. We have chased a few things. We replaced the MAF after some strange readings in INPA - no change to bog and surge. Using INPA we reset adaptations for Valvetronic, throttle, and load control adaptations. Resetting and relearning these did make a big difference on idle smoothness and overall drivability, but flat spot bog and surge is still present. Seems like DISA valve issues others have described, but the 2008 3.0i does NOT have DISA. Current theory is worn VANOS solenoids. I did remove them and clean them up with MAF cleaner. They didn't seem to have much sludge or any buildup. I replaced them in swapped positions (exhaust to Intake and vice versa) the bog and surge is still there but it feels like the RPM range is narrower where it occurs now (I hope not my imagination). So, I am going to put in new solenoids and see if this solves it. Anyone encounter a similar issue?

Tires and Suspension: My father-in-law is old school and I am sure he rotated the tires, but likely had no idea that the M Sports Suspension and the 18 inch wheels on this car are different offset and size front to rear. We found a rear wheel on the front and vice versa. When we swapped them back we noticed that one of the tires was showing steel belt due to wrong camber! And the shocks (and likely struts) are pretty worn out. I haven't even started inspecting bushings, but needless to say we need new tires and at least get the alignment close to spec if possible with worn suspension parts. Any thoughts on almost 20 year suspension parts to check and/or prioritize for replacement - especially if it affects the ability to get in spec for alignment?

 

[enuff_zed]

Assuming the rev counter is correct, this sounds much more like a slipping clutch to me

 

[DenverZ4]

I had considered clutch slippage, but there is no detectable slipping when first starting out from a stop even under heavy throttle, so looked elsewhere. The RPM only surges AFTER it bogs down, a slipping clutch I would think would cause revs to rise faster than the corresponding vehicle speed, not decrease torque and revs when under moderate to heavy throttle.

 

[enuff_zed]

I had considered clutch slippage, but there is no detectable slipping when first starting out from a stop even under heavy throttle, so looked elsewhere. The RPM only surges AFTER it bogs down, a slipping clutch I would think would cause revs to rise faster than the corresponding vehicle speed, not decrease torque and revs when under moderate to heavy throttle.

How do the revs rise then? Can only do that if not connected to the transmission surely?

 

[DenverZ4]

How do the revs rise then? Can only do that if not connected to the transmission surely?

Maybe I miss represented it a bit. The revs rise because I am stepping on the throttle and the car is accelerating. Here is how I reproduce it every time in the same RPM range.
1. At 40 MPH steady throttle in 3rd gear I moderately or aggressively hit the accelerator.
2. Car responds to the accelerator input and revs start to rise normally, at about 4K RPM the engine cuts torque smoothly but distinctly.
3. Car drops to about 3K RPM then smoothly but slowly starts to accelerate back towards 4K
4. After 4K the car quickly accelerates normally all the way to redline.
Note: my foot is still on the accelerator the whole time from step 1 through step 4

 

[enuff_zed]

Maybe I miss represented it a bit. The revs rise because I am stepping on the throttle and the car is accelerating. Here is how I reproduce it every time in the same RPM range.
1. At 40 MPH steady throttle in 3rd gear I moderately or aggressively hit the accelerator.
2. Car responds to the accelerator input and revs start to rise normally, at about 4K RPM the engine cuts torque smoothly but distinctly.
3. Car drops to about 3K RPM then smoothly but slowly starts to accelerate back towards 4K
4. After 4K the car quickly accelerates normally all the way to redline.
Note: my foot is still on the accelerator the whole time from step 1 through step 4

Ah, ok.
Well if it were a carburettor I'd say accelerator pump.
Can't get enough fuel quickly enough to match the air when you open the throttle wide.
So, making a very general assumption that it may be something similar, I am thinking fuel pump or DME map?

However, reading your comment about the Vanos solenoids I think you'd be right to try those first; though I'd expect to see codes for them, cam angles etc.