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Old 01-10-2019, 03:22 AM   #1
namre
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Default Engine bucking at ~2500 rpm

hello. i have an engine bucking/hesitation at around 2500 rpm esp. in 2nd or 3rd gear when trying to keep rpm stable. so no issues under acceleration or wot with boost. steps went through:
- tuning checked. no symptoms. afr seems stable at that moments. so no oxygen sensor error or tuning error expected,
- fuel pressure and pump checked. pump is replaced and pressure is fine,
- plugs were changed and gaps are checked,
- vacuum or turbo related leakages checked,
- ignition coils checked,
i got nothing else to go forward but problem persists. only question mark left is if the 280 degree (stage 3 i guess) cams can cause this kind of issues since normally i have a rough idle and inefficiency at low rpms.
any clues?
thanks.
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Old 01-10-2019, 08:58 AM   #2
rtv900
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your running 280 @ .050 cams on a street vehicle?
how's that working out?

I would think a major lean condition would cause bucking, or just ignition drop, but it sounds like you checked all those things
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Old 01-10-2019, 09:14 AM   #3
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Quote:
Originally Posted by rtv900 View Post
your running 280 @ .050 cams on a street vehicle?
how's that working out?

I would think a major lean condition would cause bucking, or just ignition drop, but it sounds like you checked all those things
+1 for this guy...

280 degree came on a street vehicle is just simply a bad idea, too much duration is never a good thing for low rpm cruising
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Old 01-10-2019, 09:25 AM   #4
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Quote:
Originally Posted by WRXXXED View Post
+1 for this guy...

280 degree came on a street vehicle is just simply a bad idea, too much duration is never a good thing for low rpm cruising
so would it be the cause?
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Old 01-10-2019, 09:30 AM   #5
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Quote:
Originally Posted by namre View Post
so would it be the cause?
It might be, from the checklist you went through it sounds like you hit all the right usual check spots. I wouldnt know what else to check but cams might be the cause, if you keep the rpms around 3.2k while cruising it might be better, ejs dont usually like to stay below 3k for wot pulls. Thats also how a few friends got rod knock. (foot to the floor under 3k)
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Old 01-10-2019, 11:04 AM   #6
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Quote:
Originally Posted by namre View Post
so would it be the cause?
I could see that being at least a concern in 'low' rpm operation, and 2500 is low when you have a big cam like that.

Basically just envision your semi boosted cylinder full, and then your piston starting on it's way back up with the cam still open for another 50-80 degrees depending how it's timed.
At ultra high rpm (what the cam is made for) that's fine, the intake charge will have enough momentum to keep carrying itself in while that piston starts back up.
But at low rpm it won't, so now you are pushing some of that air AND fuel back up and out the intake valve into the manifold. So who knows how much fuel you are losing. Maybe your fuel is coming in late or swirling in such a way that a heavy rich mix goes out and leaves a lean one left inside.

Basically who the hell knows. All anyone knows is that huge cams are for racing, not street driving. Just because they can provide a great peak number doesn't mean they are good
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Old 01-10-2019, 11:52 AM   #7
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thanks for comments. i know cams were bad call.
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Old 01-10-2019, 01:34 PM   #8
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Your wideband only measures " burnt fuel".

So with the reversion RTV900 talked about your readings are skewed as to what the actual combustion Lambda is.

You might try and forget about the usual target lambda and try and find its happier spot.
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Old 01-10-2019, 02:08 PM   #9
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Quote:
Originally Posted by ripman View Post
Your wideband only measures " burnt fuel".

So with the reversion RTV900 talked about your readings are skewed as to what the actual combustion Lambda is.

You might try and forget about the usual target lambda and try and find its happier spot.
sorry, do you mean that I tune this spot until it stabilizes no matter what the afr will be?
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Old 01-11-2019, 05:35 AM   #10
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Quote:
Originally Posted by ripman View Post
Your wideband only measures " burnt fuel".

So with the reversion RTV900 talked about your readings are skewed as to what the actual combustion Lambda is.

You might try and forget about the usual target lambda and try and find its happier spot.
this came into my mind also unless i misunderstand you. my concern is that is it bad to have jumps up/down in fuel table according to specific areas since the numbers are always sequentially increasing or decreasing? does it hurt injectors?
this is valid for a specific spot of off target lambda for example.
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Old 01-11-2019, 01:59 PM   #11
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If it is a lean misfire you should be able to hear it popping a bit

You can add fuel to an area of the map without having to do it one cell at a time.
You could try altering the fueling in the area where you are having problems.

I your MAP sensor ok? Have you done a MAP calibration
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Old 01-12-2019, 03:01 AM   #12
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Quote:
Originally Posted by ripman View Post
If it is a lean misfire you should be able to hear it popping a bit

You can add fuel to an area of the map without having to do it one cell at a time.
You could try altering the fueling in the area where you are having problems.

I your MAP sensor ok? Have you done a MAP calibration
I dont hear any poppings.
I have already calibrated the map sensor.
The point I was trying explain was that lets say I have adjusted fuel to 50% at 2500 rpm but itís about 10ish at 3000 rpm and onwards and works perfect. So what happens then? Should I detoriate them as well?
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Old 01-12-2019, 01:47 PM   #13
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You are getting better information on the Link Forum than you'll find here.
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Old 01-14-2019, 08:49 AM   #14
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Quote:
Originally Posted by ripman View Post
You are getting better information on the Link Forum than you'll find here.
yeah thanks them as well but i may have some mechanical problem somewhere cause playing with fuel didn't help. even if it works then my afr will suck on the other hand. therefore it's the cams or something else (o2 sensor, ethrottle).
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Old 01-17-2019, 11:50 PM   #15
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Post your MAF, primary open loop fueling and AVCS TGV Open maps. I run the same cams, no problems here.
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Old 01-18-2019, 12:35 AM   #16
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Quote:
Originally Posted by slow.wagon View Post
Post your MAF, primary open loop fueling and AVCS TGV Open maps. I run the same cams, no problems here.
as far as im aware there is no maf, avcs also cancelled. do you mean the fuel table?
as an update: there was a calibration error in ecu lambda sensor. we have changed the fueling according to what it says on the gauge. it's better now. only in 2nd gear if you force to drive at 2500 constantly little bucking occurs. i guess this is normal?

Last edited by namre; 01-18-2019 at 02:11 AM.
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Old 01-18-2019, 12:20 PM   #17
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If your running the BC 280 then you will be fine. I know as a fact they are fine on the street. What turbo are you running? it sounds like a lean condition to me, or vacuum leak. How did you do a pressure/smoke test on the engine?
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Old 01-20-2019, 10:32 AM   #18
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He posted a resolution on the Link forum.

It was simply bad tuning, despite saying tuning was good.
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Old 01-20-2019, 10:50 AM   #19
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Quote:
Originally Posted by stevieturbo View Post
He posted a resolution on the Link forum.

It was simply bad tuning, despite saying tuning was good.
thanks for the update. but you may have also mentioned why it seemed fine. it was not on purpose. afr values were crap in ecu due to miscalibration.
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Old 01-20-2019, 10:53 AM   #20
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I understand it was not on purpose, but having sensors or inputs calibrated incorrectly is still bad tuning.

And when you had the visual gauge when tuning too....this should have been spotted very early and it is fortunate it didnt lead to engine damage.

I know Link will say to use their own CAN Lambda and they're probably correct. But it is fairly overpriced for what it is, and it lacks a display for in the car. Which is always handy to have.
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Old 01-22-2019, 08:03 AM   #21
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Quote:
Originally Posted by stevieturbo View Post
I understand it was not on purpose, but having sensors or inputs calibrated incorrectly is still bad tuning.

And when you had the visual gauge when tuning too....this should have been spotted very early and it is fortunate it didnt lead to engine damage.

I know Link will say to use their own CAN Lambda and they're probably correct. But it is fairly overpriced for what it is, and it lacks a display for in the car. Which is always handy to have.
i have had a bucking at very slow speed after releasing gas pedal and putting my foot back on. i could not find an issue on my afr at that time. is this normal or again a lean condition? it understand it's natural for subarus from following thread.

Quote:
Originally Posted by ShadowIMg View Post
I think you guys are misinterpreting, either that or I am, as my spec.B will do this same thing:

Start car moving in 1st, go up to 10mph or so, remove foot from throttle.

Car will start to shudder/buck - every Subaru I've driven will do this. My understanding is it's backlash in the trans itself from shifting from the engine leading to the transmission dragging the engine.

Pretty much just if you're going to be in 1st or very slow in 2nd and have the foot 100% off the gas, press the clutch in. Not much else to do about it.

Bushings would change what will happen, but instead of the kind of "gummy" movement you get from it, you'll just get a very direct harsh vibration from the trans transmitted directly to you instead.

Last edited by namre; 01-22-2019 at 09:01 AM.
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Old 01-22-2019, 12:56 PM   #22
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Problem could be too little fuel, too much timing, poor idle control settings, poor overun fuel cut settings or quite a few other things.

But it is not normal and should not be like that

Nobody other than the person driving/tuning it will be able to tell if it's lean...as you have the information in front of you. But forget targets, just give the engine what it wants to be happy.
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Old 01-23-2019, 12:16 PM   #23
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Quote:
Originally Posted by stevieturbo View Post
Problem could be too little fuel, too much timing, poor idle control settings, poor overun fuel cut settings or quite a few other things.

But it is not normal and should not be like that

Nobody other than the person driving/tuning it will be able to tell if it's lean...as you have the information in front of you. But forget targets, just give the engine what it wants to be happy.
i think i have found where problem is coming from. i don't know how to explain it since i can not upload files or picture here.i will try to explain more in detail in linkforum. but basically it occurs when ignition idle control locks out and kicks in within a moment. since acceleration position is too sensitive you can momentarily release pedal and put back in. when you accelerate 0.1% there is no problem idle ignition is locked out. but whenever you release the gas idle ignition control suddenly kicks in and tries to lower rpm with retarding the ignition to say -15 degrees.
so if you make the idle ignition control kick in by releasing gas, under some rpm, under some speed limit then you are in retarding position. if you touch back the gas pedal just afterwards then buckling starts since idle ignition tries to adjust the idle and ignition table makes something else.
workaround to my mind would be putting 0 to speed lock out. but it creates other problems like floating or high idle at low speeds.
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Old 01-23-2019, 12:35 PM   #24
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Again....the problem will be the tuner. All aspects of tuning need to be right including all throttle position calibrations as well as when/were idle control etc will activate. And how much control it actually has to add/remove air or timing etc in order to alter idle speed.
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Old 01-23-2019, 01:06 PM   #25
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Quote:
Originally Posted by stevieturbo View Post
Again....the problem will be the tuner. All aspects of tuning need to be right including all throttle position calibrations as well as when/were idle control etc will activate. And how much control it actually has to add/remove air or timing etc in order to alter idle speed.
fair enough. maybe if two tables (idle ignition retard according to idle target rpm and main ignition) have the kind of synced values it settles down. but i am not sure how to do it in practice since they have completely different dimensions.
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