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Discussion Starter #1
Hi guys,

I have a strange phenomenom with my 147 JTDm 16v:
When accelerating at part load (constant throttle 20-30%) the boost gets very high, sometimes above 1.5 bar, and then instantly it falls down at a given rpm (mostly above 3000 rpm). The result is an unpleasant chance in acceleration while the accelerator pedal is in a constant position. This due to the exhaust backpressure that falls down with the boost, increasing the engine power while injection rate is more or less constant.
Also I noticed if a press the accelerator a bit more while the boost is building up, the boost immediately falls with 0,5-1 bar.
I suspected it was a result of the intake flaps and/or a sticking VNT mechanism.
So I made some logs, and to my suprise it doesn't look like that:



As you can see the flaps are fully open already at +/- 2200 rpm. And, a sticking VNT mechanism would cause raising boost with decreasing VNT %.
It looks like it is really programmed like this, because the VNT% also makes a sudden fall that results in the actual boost fall.

This only occurs at part load, at full load there are no problems at all. Boost is nicely kept constant through the rev range.

Anybody also have this experience? Or know what this is and why ?
 

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Hi Joel, do you have your EGR completely switched off?
 

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you're still on std map right?

Could be a egr problem as Yanko stated.

edit , did you check your maf? older jtd would have a lumpy ride in part load due to worn maf sensor
 

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It could also be caused by not finetuned boost management maps. At certain (rather low - see the table below) injection, the boost is managed in open loop by VNT duty cycle maps. In higher load, ECU switches to the fedback closed loop. It can be that the part with overincreasing boost is still under the open loop control and then when closed loop switches on, ECU founds the pressure to be too high and drops VNT to lower the boost down.

Code:
M_fuel thresh. switch-on closed loop comtrol, M_fuel = f(N) (10x1)

fuel qty(engine speed)/mm3

		rpm									
		1300	1500	2000	2500	2750	3000	3250	3500	3750	4500
-	0	61.00	47.00	36.00	25.50	20.50	16.00	13.00	14.00	14.50	17.00
 

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Same here, fully closed egr.
It switches to exact boost at 2750rpm or 50+% throttle.
Also searching for solution.
 

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As I wrote above - IMHO, too high values in VNT open loop duty cycle maps which don't match the pressure level requested in boost setpoint maps.
 

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Discussion Starter #9
Thanks for your comments!

Yan.ko what you're saying makes a lot of sense. I did not have this problem with my JTD 8v.
But I can imagine boost in the open loop load range is increased on this euro 4 engine for emission reasons.
However, they should have placed the switching point lower in the load range, or stretch the closed loop to the entire range. I think my downpipe made the matter a bit worse since that results in more boost at the same VNT %.
My car has a Squadra Massimo remap and fully blocked EGR by the way.

But, to solve the problem I think the preset VNT% has to be lowered in the open loop range. The only question is, who can do that. I will ask Squadra.
 

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The exact same thing happens to my Brera 2.4. Only on half throttle. I just changed the turbo and it's still doing it. I guess I will have to drive at full throttle all the time! Egr permanently closed and dpf removed/remapped etc.
 

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Thanks for your comments!

Yan.ko what you're saying makes a lot of sense. I did not have this problem with my JTD 8v.
But I can imagine boost in the open loop load range is increased on this euro 4 engine for emission reasons.
However, they should have placed the switching point lower in the load range, or stretch the closed loop to the entire range. I think my downpipe made the matter a bit worse since that results in more boost at the same VNT %.
My car has a Squadra Massimo remap and fully blocked EGR by the way.

But, to solve the problem I think the preset VNT% has to be lowered in the open loop range. The only question is, who can do that. I will ask Squadra.
This problem has nothing to do with euro 4 emission standards. Or with the switching point for that matter.
All ECUs work in open loop mode at low loads/rpms. What made your problems much more noticeable is disabling EGR and replacing your downpipe. Because both mods have great influence on VNT%.
Tuning VNT maps would help, but finetuning VNT maps is a painful work and most tuners just alter the "top end" of the VNT maps, because it takes too much time to finetune whole maps.
 

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Discussion Starter #12
This problem has nothing to do with euro 4 emission standards. Or with the switching point for that matter.
All ECUs work in open loop mode at low loads/rpms. What made your problems much more noticeable is disabling EGR and replacing your downpipe. Because both mods have great influence on VNT%.
On my JTD 8v I also had these mods and there transition from open to closed loop was fine.

I still believe the difference comes from the euro 4 engine, having much more EGR rate in the bottom load range that has to be compensated with higher VNT%.

Tuning VNT maps would help, but finetuning VNT maps is a painful work and most tuners just alter the "top end" of the VNT maps, because it takes too much time to finetune whole maps.
Yes, also with my Squadra remap I am quite sure this part of the map is standard.
We'll see what is possible regarding the software.

An alternative could be to lengthen the actuator arm, resulting in less VNT closing angle with the same duty cycle. But ofcourse that also has its tradeoffs... (more lag for instance)
 

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I only started noticing this drop in boost with partial acceleration after i changed the exhaust system for a more straight through system. But i find it hard to believe that it could be the cause since the CAT was still there. I have since changed the turbo and removed the CAT and it still drops boost on partial acceleration. Anything else worth checking?
 

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I only started noticing this drop in boost with partial acceleration after i changed the exhaust system for a more straight through system. But i find it hard to believe that it could be the cause since the CAT was still there. I have since changed the turbo and removed the CAT and it still drops boost on partial acceleration. Anything else worth checking?
I have no such problems on my car with that exhaust system (and decat)... :thumbs:
 

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Must be something else going on here!
Yes, I think so mate. Just to clarify: I have the same F1 remap, same straight through system and decat (with a few other toys) and have no such problems.

Maybe it is related to the 'boost flutter' issue you posted about previously? The problem you're having will most certainly be down to a mechanical issue.

Hope you get it sorted soon mate.:thumbs:
 

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Yes, I think so mate. Just to clarify: I have the same F1 remap, same straight through system and decat (with a few other toys) and have no such problems.

Maybe it is related to the 'boost flutter' issue you posted about previously? The problem you're having will most certainly be down to a mechanical issue.

Hope you get it sorted soon mate.:thumbs:
I just had the turbo changed for a brand new one and its still there! :cry:
 

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Not two cars are the same... If you do the same mods on two cars you cant expect completely the same behavior...
This is down to manufacturing tolerances. Maybe your turbos are set-up differently (VNT lever length can be a tiny bit longer and you can experience described problems...)
As you can see some cars have this problems with stock maps and no mods...

And that is why tuning is for. Any decent tuner can solve this problems, or at least minimize them, in reasonably short time...

my 2c
 

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Yan.ko is 100% right on this.

JTD 16v uses a quite "liberal" open loop mode, that is, it remains in open loop (direct read of VNT maps) for a long time... I always had this problem, and it got a lot worse once I've installed the GTB2056V (that spools significantly faster than the stock vnt17).

Solution?!: I've made a complete VNT tune for all IQs (on the map), 5, 7.5, 12.5, 15, 20mm ...and so on.

My doubts at the time were either to stick with stock boost maps, or to decrease them a bit. The advantage would have been to increase slightly the VE as we don't need AFs in the figure of 30+. The problem using lower boost figures is, of course, responsiveness when "hammering" the throttle ... I've kept the stock boost maps for IQs up to 60mm3.

Luis
 

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Since I've been tuning VNT from the beginning, I developed quite simple method ... Log iq, rpm, requested and real boost, vnt opening accros whole rev and iq range, import all data into excel, shake a little, do some black magic and voilà ...


Some old data before tuning shown on the picture
 
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