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Discussion Starter #1 (Edited)
I've had this car behaviour since I bought it, it didn't seem a big problem.
It's a 166 2.4 JTD 20V, same engine as in the 156.

I've checked inlet hoses (good), fitted new maf, map and vnt regulation valve (just in case, all genuine parts not from eBay), checked vacuum pipes (cracked pipe on the outlet of the vacuum pump, fixed), totally blanked EGR with metal plate (not in the ECU yet) repaired swirl bar with brass kit and replaced every filter and oil in the car.
I still have this strange behaviour, flooring the pedal on 1000rpm, the car slowly builds boost, at 1300rpm it starts to pull some, at 1500 it reaches full boost (1.3 bar) then sudenly it drops to 0.8-1bar (the power suddenly goes down) and it starts doing boost again, to achieve again full boost at 1800-1900rpm, and it stays over the rev range.
The vnt actuator moves freely when plugging/unplugging the vaccum pipe, although it needs around 2 seconds to fully "suck" the actuator back into its place.


Check this pic for the detailed boost drop:
boost drops.png



This is on a full run, check the vnt %, may it be a stuck VNT mechanism/actuator not moving freely, therefore causing overboost?
graph.png


Or is it meant to be like this?
Regards :happy:
 

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It doesn't look like over boosting. My 1.9 regularly boosts to 1.5bar. You seem to have investigated the vanes and vacuum. If it isn't Smokey then it could be an under fueling issue.
At least this problem will save a few miles on your clutch.
 

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In the second graph the boost generally follows the desired boost curve, but keeps over/undershooting, while the VNT control goes in the opposite direction, trying to control it. This looks like sticking vanes to me, as you say. I think they sometimes work freely at idle, when you test them, but stick when loaded. I had a minor version of this which cleared up after some harder driving (with injector cleaner). Failing that an in tank turbo cleaner might help.

Incidentally, the two seconds will be just the time taken by the vacuum pump to restore the vacuum in the accumulator after you unplugged the pipe.
 

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Discussion Starter #4
Flooring the pedal with no boost makes very little smoke, just like any other diesel, can't be seen on the mirrors. The fuel pressure holds on at desired pressure over all rev range as far as I remember.
Underfuelling? How? It does that every time you release the pedal, no matter the rpm or the speed.
When pushing the accelerator on high rpm the drop is much quicker and barely noticeable.
Just wait form the turbo to go down and then push it again, it always does that even on lightest acceleration, faster or slower.
So I guess that yes, the vnt has a small point where it stucks a bit.
I must say I have a very small exhaust manifold leak, few bubbles when spraying soaped water, I think it doesn't affect on this behauviour maybe I'm wrong?
 

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Sticking vanes on my turbo turned out to be slight surface rust on the face of the chamber, not carbon. Depending on where the rust is, that could lead to odd behaviour. Testing with a vacuum pump (Mityvac or clone) on the actuator would show whether the VNT is working linearly or sticking.

The rust was easily sanded off once dismantled, but it wouldn't be fixed by aerosol. If you have an exhaust manifold leak already...

[EDIT -see http://www.alfaowner.com/Forum/alfa...lean-how-anywhere-159-1-a-9.html#post14300937 ]
 

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Discussion Starter #6
Thanks. Yes, that happenned on my 156, it kept overboosting for no reason, the rust was blocking the vnt vanes.
However removing the turbo on this engine is not as easier as the 156, hehe...
 
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