Stelvio Stumbling at idle.

Hello all! I got a Stelvio in April and have been working on it since to get it into good order. I have fixed a tremendous amount of things, but am now experiencing a new issue that I just cannot figure out.

Some buddies of mine encouraged me to post here. If anybody could throw some ideas my way, I would be so so grateful. The below is not meant to be curt or rude, just as clear as possible for ease of reading. Thank you in advance for any help!!

Issue:
Stumbles at idle. There is a kind of suction pop you can hear from the airbox/airfilter that comes with each stumble. It's rough around idle and below 2500 and if you goose it around 3k there is a delay in response. Video here:

[You must be registered and logged in to see this link.]


Bike:
2014 Stelvio NTX
16,xxx miles


Recent Work:

Throttle Bodies:
Removed and cleaned with throttle body-specific cleaner.
Noticed light oil residue, likely from overfilled sump, now cleaned.
Reinstalled throttle bodies.
Rest TPS multiple times, both along with the throttle body synced
Sprayed carb cleaner all around intake piping after reassembly; no RPM jump.

Airbox:
Installed a new airbox complete with new drain hoses, plugs, and clamps.

Gas Tank and Throttle Body Sync:
Reinstalled the gas tank.
Synced throttle bodies according to a forum-provided instruction PDF,
TPS reset as a part of that process.
I have completed this procedure successfully before, if that helps.

Valves:
Checked and adjusted the valves to .10/.15 mm tonight (along with Beetle map)

Fuel Injectors:
Cleaned by RC Injectors a week ago.

Spark Plugs
New spark plugs and spark plug boots about 3000 miles ago

Fuel pump:
Replaced with a 2013 pump due to a crack in the housing.

Exhaust:
Found and solved some exhaust leaks. Re-torqued multiple times over multiple heat cycles.


Misc:
Checked Engine Oil, Transmission oil, and CARC final drive oil.

Confirmed fuel lines were not kinked.

Sprayed carb cleaner in one of the stepper motor tubes, reassembled the bike and ran it.

ECU Map:
Issue occurs on both the correct Beetle map with .10/.15 valve lash, stock filter and airbox (minus snorkle)
ALSO happens on correct Guzzitech map with .15/.20 valve lash (per Guzzitech), open airbox and K&N filter.

This sounds like the classic 'Chuffing' or spitting back through the TB's that will occur when either the TB's are worn, (Unlikely on such a low mileage bike but if it has been run with a K&N air filter, an airbox with a hole in or missing or un plugged drain hoses then all bets are off.)

If you are sure it's tuned correctly my next step would be to gradually increase the CO trim, (Assuming it is currently at, or close to zero.) With wear issues with TB's it is sometimes necessary to go up a fair way on the CO, like +20 or higher to subdue the spitting back which is caused by an overly lean mixture due to excess air finding its way around the throttle plates.

Obviously running such high CO figures is less than ideal both in terms of fuel economy and wear but it will be a useful diagnostic experiment to se if you can eliminate the 'Chuffing'.

Oh, I assume it's wearing the stock pipe? You did clear the trims as part of the mapping process?

Hey Pete!

Thanks for the reply. The bike had the K&N when I got it at 12,8xx and I found a bunch of air leaks around 14k, which I fixed up (the air box split at the seam and the two drain hoses were gone!!). I did try praying carb cleaner all around the throttle bodies to check for any other leaks and came up with nothing, so I’m hoping I avoided any major damage for now.

It has a Mistral pipe with no dB killer and I reset the trim as a part of the map install. Though, the map change to Beetle’s and trim reset didn’t change the stumbling at all.

I also had a bit of oil in the airbox, which mixed with the dirty air, was a bad combo. I’m thinking the stepper motor might be in a sad state. Do you think a gunked up stepper motor could do this?

It may be a contributor but I'm afraid that the damage to the TB's may already of been done. As I said, try going up on the CO trim until it mitigates the issue. If it doesn't we'll have to look further but too much oil, shitty air filtration, (Both filter type and unsealed airbox, possibly from new.) and missing hoses coupled with an overly rich map, (If it was running one of the GT generic 'Throw a shit-tonne of fuel at it' maps) are a recipe for TB destruction, even at low mileages.

Sorry.

Thanks for the help!

I adjusted the trim in increments of 5 up to 35 and it seemed to fix it. Unfortunately, that means it likely is the throttle bodies.

Am I safe to ride the bike with the +35 trim while I save up money for new-to-me throttle bodies?

Is there any repair/restore work I can do to the throttle bodies? Get the re-machined perhaps?

If you find someone who can repair them please let us know. I've been looking for over a decade with no success.

Oh, as to running it with the CO trim at 30+? Your fuel economy will be dreadful and you'll greatly reduce the life of your rings as well as probably end up with a fair bit of fuel in the oil. It's not a good thing.

CO trim is only active to 3000 RPM, so it will run pretty rich up to 3000.

We could try tweaking the idle fuel table, which is active to ~2600 RPM, and only on a closed throttle. You might then be able to reduce the CO value to try to reduce the overall amount of fuel being used.


I’ve not tried this before, you’ll be first.

bettle,

I'd be down to experiment with it if you are! I can even plan on some back-to-back MPG data if we want. I have been fascinated by fueling since getting this Stelvio and would be thrilled to be a test case, so to speak.

beetle wrote:

CO trim is only active to 3000 RPM, so it will run pretty rich up to 3000.

We could try tweaking the idle fuel table, which is active to ~2600 RPM, and only on a closed throttle. You might then be able to reduce the CO value to try to reduce the overall amount of fuel being used.


I’ve not tried this before, you’ll be first.

Ah, I got it wrong. I thought CO trim added fuel across the entire map. In which case over-fuelling would be less of an issue, at least on longer trips. Around town it still would, it would for me anyway as I spend a lot of time chugging about between 2,300 and 3,200 in urban areas simply because the fueling is good enough to allow me to do it. With a stock map, especially the early ones, it was a bloody nightmare in comparison.

I'd wondered if it might be possible to adjust the map in that area but hadn't asked you simply because every set of TB's would be different and therefore every one would need a custom map. I assumed you'd go nuts if I suggested it!

I used to think it affected the entire map, until Meinolf did some deep-diving into it. The RPM limit it affects can be modified as well. That's an interesting thing.

Quote :

I'd wondered if it might be possible to adjust the map in that area but hadn't asked you simply because every set of TB's would be different and therefore every one would need a custom map. I assumed you'd go nuts if I suggested it!

I sure would.

Well that puts my mind at rest a bit and probably also explains why the couple of bikes we service that we have had to go stupid on the CO don't drink fuel like a drunken sailor on shore leave necks rum!

Hey all!

Quick update: I was wanting to run down any possible mechanical things before relying on map tweaks, so I got in and cleaned up the stepper motor with some Gumout. Seemed to clear up the stumble pretty well. Idle is now pretty damn solid with CO trim at 0.

Gunna ride it for a bit to make sure, but that may have been a big part of the problem. Wanted to chime back in to contribute another small data point to the forum!

Had to flip the battery around and use a metal straw to make it happen, so I’ve attached some pictures.

[You must be registered and logged in to see this image.]

Well that's a happy result! Don't forget to reattach and plug the airbox drains, (In my opinion you can pretty safely just cap the rear spigot on the airbox off. I've never seen any oil or water in that hose.) The one at the bottom front of the airbox though does need to be there and be plugged. And don't even think of using the K & N air filter. It won't assist performance and Will damage your TB's and motor.

I’ll double check the airbox lines. One of the first steps in this 6-month diagnostic journey was discovering the previous airbox had split, so I replaced it entirely along with new drain lines and plugs.

No more air filter swaps for me. As much as I enjoy working on my bike, I'm very much hoping I can put the tools away until the next service

Thank you again for your help, Pete and Mark! Very much appreciated.

Nice work Zack.

I really appreciate your approach to getting all the mechanical items in order before tweaking things.
Too often I've seen tweaks and band aids upon tweaks and band aids until failure and confusion.

Thanks for posting the outcome!
I hate it when someone leaves a thread loose and dangling.
The CARC World Collective gained a bit more knowledge and experience when concluded for better or worse

Hey all!

Another update: While the initial cleaning of the stepper motor cleared up the stumble, it started causing some rough running. It seemed to be stuck open causing the lean surging anybody with stock maps is familiar with. This would go away when I cleaned it, but would return the next morning.

So, I capped off the ports on the TBs and plugged the lines coming off the stepper and the low end throttle response improved massively, but it’s still running rough and is idling very rich. Re-synced TBs and reset TPS, same issue. Do we know for sure if the ECU adjusts fuel to accommodate the stepper valve opening and closing?

If so, how have people deleted their steppers without fueling issues? Still working that one out and searching around the forums. Wanted to update anyway.

The ECU doesn't change the injector pulses with respect to the stepper, because it doesn't know what position the stepper is at. There's no feedback. That's why there's no service warning when the stepper fails, or doesn't move.

The ECU only uses the input from the TPS, temperature & pressure sensors, and the phase sensor.

beetle wrote:

The ECU doesn't change the injector pulses with respect to the stepper, because it doesn't know what position the stepper is at. There's no feedback. That's why there's no service warning when the stepper fails, or doesn't move.

The ECU only uses the input from the TPS, temperature & pressure sensors, and the phase sensor.

Thank you for the info! I’ve been so curious!