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Old 01-10-2019, 04:22 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, 09:58 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, 10:14 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, 10:25 AM   #4
namre
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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, 10:30 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, 12:04 PM   #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, 12:52 PM   #7
namre
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thanks for comments. i know cams were bad call.
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Old 01-10-2019, 02:34 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, 03:08 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, 06:35 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, 02:59 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, 04:01 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, 02:47 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, 09:49 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 01-18-2019, 12:50 AM   #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, 01:35 AM   #16
namre
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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 03:11 AM.
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Old 01-18-2019, 01:20 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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