I've been chasing an issue since putting on my 82mm TB (I know, I know, issues with that have been beaten to death) but it's not the 'normal hiccup' problem as the issue happens immediately at startup.

When it happens, I'll start the car and it will run very rough, feels like it will stall, wideband showing it lean and it's trying to add fuel, then while driving the fuel trims will hit the ceiling of +25% at times. I know this is similar to the hiccup mode people have experienced, but cycling a key changes nothing.

If the ECM was cleared beforehand, the code P2007 will be stored immediately after startup indicating it was set right at startup. No other codes will appear even after days of driving like this.

It has a newer OEM MAP sensor installed (part number 0051537228), and this morning when it started rough I decided to look at the self guided tests and noticed something that never caught my eye before: At idle, it is reading about .06 psi of boost when STAR specifies it should be less than or equal to 0. I believe this is what is triggering this mode. I should also note I've tried running the TB with factory idle rpm settings, as well as raising 50-60rpm like some suggest, it has made no difference.

I've got larger injectors, a 77mm fixed pulley, shorty headers, and a RaceIQ tune. I've been trying to talk to Tony and see if this can be cleared with a tune tweak, but he's swamped and my guess is that a code that sets a different performance map might be hard to do anything about.

Is it possible I have a bad OEM MAP sensor, or with the fixed pulley could I actually be making a slight amount of boost at idle? I've got no vacuum leaks (maybe they would help haha) so I'm at a crossroads. I could undo all the work I did to put in the TB and go back to stock, but it runs SO WELL when it doesn't trip this fault and I know everything is happy mechanically. Any input appreciated!

Showing small amount of boost at idle after warming up for about 2 min.

Code, no freeze frame or other info available.

Pulled from WIS, showing criteria for setting code.

Scary fuel trims!

 

Very possible. Remove the supercharger belt and see if the problem goes away.

 

Just curious, are you running a brand new or used 82mm throttle body?

 

OK, did this and there was no change at all, still showing above 0 and Tony did get back to me confirming this shouldn't be the case. Here's the other kicker - should the SC be spinning with the belt off? See vid below

 

of coarse engine vacuum is high.

 

Well good, I figured that was the case but I don't know how much bleed-by the bypass valve allows at idle and I've never ran with the belt off before.

So, should I warranty the MAP sensor as a next step? The only things that could cause boost reading at idle is an incorrect reading (sensor) or maybe the bypass not opening enough but I've run the tests and watch it move and seems fine... I wish there was an easy access for reading physical boost to compare besides drilling and tapping the manifold.

 

Used genuine Mercedes unit. I think the combo of fixing some potential vacuum leaks and putting this on at the same time amplified a problem that was there before.

 

I would check and make sure your purge line is seated correctly in the supercharger snout. How long has it been since you moved up to the 82mm?

 

Yep, it's solid (I even replaced the rubber grommet when I had the SC out). Smoke tested too and no leaks. I'd imagine a leak would not yield positive boost though so that is what I'm stuck on.

This code appearing started after the 82mm (maybe a month ago?) but because it doesn't always code enough to set a CEL I also wasn't going into STAR looking for it before.

 

I know it isn't what you want to hear but having just gone through 3 used 82mm Merceds TBs I finally had to bite the bullet and go OEM brand new. It was an immediate improvement over the other 3 and removed one more layer of uncertainty in diagnosing the P0106/hiccup. That being said, it sounds like your issue may be unrelated to the hiccup issue. Have you gone through the whole throttlebody reset procedure?

 

I have, and tried just about any combination of ecu reset then tb reset, tb reset then ecu etc. I wish there was a confirmation of some sort but I followed the key on/unplug procedure I've seen posted and can hear noises when it's working so I have to assume it was reset, doesn't seem to make a difference.

I'm just focusing on the boost at idle issue, and if the tb were to blame it would be open at a larger angle, more air in and rpms would be higher which they aren't. I've got my old MAP somewhere, thinking I'll bust my knuckles and see if that changes things...

 

Turns out I tossed the old MAP so that route of testing is out. I did however find a way to get an actual vacuum reading on the boost side by drilling a hole through a rubber plug, then pressing it into the spot where the IAT sensor is (leaving it dangling and plugged in). Vacuum on the inlet side (nipple on top of supercharger) reads -20-21 so right in range.

So I'm seeing physical vacuum on the charge side, and a generic OBD reads vacuum as well, but in STAR it's showing positive pressure, so now I'm even more lost. It's the same signal through the ECM, so why is STAR reading a positive value? Maybe that isn't representative of actual vacuum or boost but a preset 'zero marker' for the computer to expect under idle conditions and my mods just put me over that a tad?

Actual vacuum from charge side at idle, my connections might not be 100% tight but this should be close.

Vacuum reading from Torque App.

 

Well I got off my lazy butt and swapped my stock TB back on. So far I've had no rough starts or pending codes and it seems to drive well. Another member had a thought about backpressure and exhaust restrictions on these engines, even w/ shorty headers I'm still dealing with a small opening to the factory cats and maybe the combo of the larger TB is creating too much pressure inside where the MAP takes its readings.

I'll report back after another few days of driving and see if that has eliminated it. The annoying thing is that it actually drove better w/ the 82mm despite the map switching, I'm back to having that 'cough' feeling when pulling away from a steady speed. I know that's common w/ the fixed pulleys but the 82mm TB seemed to overcome it better.

If the problem doesn't come back, maybe I'll find an 80mm and hope that it's the goldilocks porridge of the 3...

 

I just saw your other thread of going to an 80mm TB. Did you still have the issue of positive boost at idle with the stock TB?

 

I can say at this point the 82mm TB was definitely the cause of the lean running map and rough idle. It's been a week of multiple cold starts, driving the car easy and hard, and I've had zero issues with the stock TB back on.

As far as the boost at idle, the real head scratcher is that STAR still shows the same values as before. What has changed since swapping TBs is that the physical vacuum, BEFORE the MAP (i.e. vacuum gauge plugged into one of the nipples on top of the SC at the back) increased when I put the 74 back on. So it went from 17-18 in of vacuum to 20-21, which is what Tony at RaceIQ said his car runs at too. The reading through OBD2 from the MAP remained roughly the same (19-20) and that positive reading in STAR didn't change.

My theory is that we should all ignore the STAR readout, even though it clearly shows a value that is out of parameters. I think it was a wild goose chase. That value is the result of an internal ECU calculation since it doesn't match the MAP readings or physical vacuum even with atmospheric values subtracted, so it must just be a value used in whatever map the car is running. For closure's sake, I'd love to have this STAR menus readings from a bone stock car, as well as another car that runs a 77mm fixed pulley. That would confirm to debunk my theory, but at this point it wasn't the main cause.

I know I had no vacuum leaks since it's now running flawlessly (and I did smoke testing prior). My belief is that the 82 TBs simply allow too much air in their closed state to pass by the throttle plate. I would imagine this is marginally worse with used ones as things have broken in already, and why buying a new one may have improved but not fixed this issue since the tolerances are likely a bit tighter, but will get worse as it wears. The 85 and 90mm are based on different platforms, and are performance oriented, so I bet the tolerances are tighter on those as well. These 82s are from other factory cars that are not forced induction, so having some extra pass-through air is no big deal and maybe preferred for smooth running. The theory posted tons of times on this forum is that some cars are lucky and some aren't. I think it's the other way around, that some throttle bodies are lucky and some aren't.

Whatever it is, the MAP sensor did not like the airflow it saw and based on my wideband readings, it's very dangerous to drive your car in this state. It also rarely set a CEL on the dash (which surprises me with how bad it ran at times, pegging fuel trims etc), so anyone with an 82 TB that doesn't have wideband should really consider getting one, or at least frequently check with STAR or an MBII scanner for pending codes in the ECM as well as watch your short term fuel trims. These won't show up in a generic OBD2 scanner.

 

This is an interesting theory and I'd be curious what a 90mm and 82mm look like side by side closed. When you mention watching the STFT, what values are you looking out for?

 

Just take a look at the last picture in my first post on this thread. When it was in a different map after pending that fault code, fuel trims would max at +25% and the car would still be running lean. When it didn't trigger a code, you shouldn't see more than about 10% during regular driving and the mean line would be closer to the middle, not shooting up and staying there. I'd be moderately on the throttle, not going crazy, fuel trims at +25% and my AFRs would climb to 18. Not good!

 

Ah yes, I see that now, I was used to looking at it in the Torque Pro app. I would see that same behavior as well with the 82mm but usually after cruising at 80-90mph for a while with light throttle.

 

Yeah that was me at 35 mph driving normally. The car was not happy with me haha.

 

Regarding your thoughts on the 90mm I found this post: https://mbworld.org/forums/market/798353 that is a close up of the 90mm BBK. It looks like it still has a slight gap between the blade and the housing. This being a 90 vs 82 I'd imagine that the amount of air leaking by would be greater than the 82mm and still cause some issues.