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Fuel cut at 5k-ish rpm. Overboost?


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Went for a spirited drive up some mountains and it was awesome! All was well till I got down.

 

I was overtaking at 3/4th to full throttle when the car seemed to cut out for a second then go back to normal. I didnt think much of it till it happened again a while later.

 

I hooked up my OBD bluetooth dongle and scanned with Torque, no codes.

 

I had just gotten my laptop with Romraider so hooked it up and went back to testing with it displaying some parameters and noticed boost peaking at almost 19psi. Watched the screen while replicating the cutout and was seeing about 17-18psi, then it would cut at about 5k rpm.

 

No ECU/TCU codes, I checked everytime I felt it cut out.

 

Car runs great, the cutoff is the only problem.

 

Stock powertrain except for custom 2.5" Y-pipe and Borla mufflers.

 

Stock tune.

 

I'm thinking what was happening is fuel cut due to overboost?

 

Would love to hear ideas on troubleshooting this.

 

EDIT: FIXED!! Details on page 2.

Edited by Scubaboo
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What do you mean by cut out for a second? Like all power gone or revs like in neutral?

 

When I hit overboost it was like hitting a brick wall (scared the shit out of me) and I also got a check engine light/overboost code. I was in the middle of tuning and Dave had the max limit set low to protect things. Once it got that turned up I didn't see any overboost issues.

 

I've had the wastegate close on me when waiting too long to hit boost while in the throttle. That was a good loss of power, but not like the overboost issue I had, that was a much worse reaction. Once I got out of boost range and hit the gas again everything was back to normal.

 

Can you continually replicate the issue?

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All power gone. Like an over-rev fuel cut. But power comes back almost immediately.

 

Yes I can replicate the issue. Full throttle and by around 4-5k rpm and 17-18psi it will cut for a second, then power is back like nothing happened.

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Yes still on stock fuel pump.

 

I had that parameter engaged on Romraider and it was at 33%, but it peaked at 100% at some point.

 

I'm not all that familiar when it comes to cars that are boosted and such, but could the fuel pump be too weak? Maybe you need a more powerful fuel pump?

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Sounds to me like a fuel supply issue as well. I've also experienced the overboost cut, and it really is violent.

 

I'd check your fuel filter (and maybe spark plugs) for debris and go from there. If you can log with RomRaider, you can log injector duty cycle and ignition timing to confirm if the ecu is commanding something to change/cut.

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My experience in trouble shooting, a weak fuel pump has a tendency to show up in knock at first as lean (DAM). If it goes undiagnosed, it then cuts out like ignition issue.

 

A stock tune shouldn't be hitting 17-19psi.

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Coco, plugs are new, changed them out maybe 300kms ago. OEM NGK's.

 

I've gotten Romraider installed and working but havent graduated to logging yet lol! I did the best I could at the time, having it on while testing.

 

Dave, I had FLC as a parameter on display and got -1.5 min/.75 max? I forgot to put feedback knock correction on. But I had cyl 1-4 roughness monitors on display, by the end of testing doing full throttle runs replicating the cut maybe 6-7 times, I had a total of only 2 misfires, 1 misfire on cyl 3 and 1 misfire on cyl 4?

 

Driving home from testing I was watching the monitors and there was no more additional misfire counts.

 

I checked my screenshots of Torque from when I first got the car and my boost peaks were only 14psi.

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Can you tell throttle position/percentage when it happens? What do the fuel trims look like when if falls on its face?

Do you record a log of it happening? Can you view all the items in a graph? Makes it easier to identify events.

 

How many miles on the car now?

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Looks like you have romraider figured out, just need to give it a name and click the "start file log" button for a log. You can change the save destination in the settings if you want. Looks like you're only getting 4 data points per second with Slow-K which isn't fantastic. Not sure if you have to use that with your setup, but I think I'm normally closer to 20 or 30/sec with one of the other options.

 

 

I'd be interested to see a log of wastegate duty cycle and boost. It might not be fuel cut because of over boost, but the ecu might be trying to reign in the (what looks like overboost compared to your other screenshots) spike by either hitting the max WGDC value in the tune, or just opening the WG to slow the rise in boost.

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I was wondering about Slow K, is it because I have a lot of parameters checked or is there a way to select a better connection? I'm connected with a Tactrix 2.0 if it matters.

 

I'll try to get the rest figured out so I can make a proper log.

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More parameters will definitely slow it down. I use SSM-CAN, but I know only certain methods work with certain market ECU's, so not sure if everything carries over. I've had it glitch sometimes where it reads slow (4-5 q/s) so I unplug/replug the tactrix and it jumps to a normal higher speed.
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Decided to inspect ecerything first before trying to log. Found a dangling vacuum line to turbo.

 

EU1Gxfu.jpg

 

Putting it back together and zip tieing the crap out of it, hopefully that's it.

 

Yikes, that vacuum/boost line is part of the boost control. The wastegate will never open (and overboost) with that disconnected.

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Yikes, that vacuum/boost line is part of the boost control. The wastegate will never open (and overboost) with that disconnected.

 

Yup looks like that was exactly what it was doing!

 

Thank god Subaru programed boost cut for this scenario!

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I have this same issue, right as it hits full boost in 3rd gear it cuts power. I have my car up on the ramps right now and am waiting for the exhaust to cool before I check to see if I have the same issue. Thanks for posting, hopefully thats my problem too!

 

Sent from my SM-G970U using Tapatalk

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So I checked under the car and couldn't find anything that stood out. Hmm anyone else know if overboost is causing this issue and what might cause that?

 

I get good boost at WOT in 3rd as it is spooling to full power, but right between 3500-4500 rpm it falls on its face. Only in 3rd and sometimes 4th gear. If I let off the throttle halfway, then the power picks back up.

 

I have a feeling it is related to the car being under full power. Right at max torque is when the power is cut.

 

Thanks for any further help. Here is a pic of the underside by the turbo.d5f1f49dec8c162d312cdd042f0057b6.jpg

 

Sent from my SM-G970U using Tapatalk

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Have you checked for any codes? I think a malfunctioning boost control solenoid can do this too, and that throws a code. Mine did not have any codes.

 

Do you have a boost gauge? Check what the boost pressure is when it falls on its face.

 

If it is overboosting at cutoff with no codes, and you cant find any obvious disconnected hose, do a boost leak test.

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Have you checked for any codes? I think a malfunctioning boost control solenoid can do this too, and that throws a code. Mine did not have any codes.

 

Do you have a boost gauge? Check what the boost pressure is when it falls on its face.

 

If it is overboosting at cutoff with no codes, and you cant find any obvious disconnected hose, do a boost leak test.

I am not throwing any codes, and do not have a boost gauge yet. Just got the car 6 months ago.

 

I guess I will investigate a boost leak now. As far as I can feel, it feels as though it is making the full 14 psi right before it shuts off. Thanks for the feedback.

 

Sent from my SM-G970U using Tapatalk

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