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Discussion Starter · #1 ·
I've created a topic about a cold start shake of my Giulietta a few months back.

At first I thought it was the multiair unit going bad since the shaking only happened for a few kilometers. So I went ahead and did a multiair reset using my OBD cables and strangely, the issue went away for a while.

But.. recently, the problem started to reappear. Every morning (especially when it's very cold outside), a few seconds after I start the engine, the car starts to shake and the acceleration feels stumbly. While trying to find out what exactly was going on, I went ahead and put the car in dynamic to see the boost pressure, and there it was! During those "shakes" the turbo doesn't go over 0.5 bar. Not even at higher rpms.

So my guess is that the turbo somehow is failing when it's cold but I'm not really sure where to start checking.. I don't have any fault codes and the car runs fine after "warming up". Any suggestions?
 

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The turbo on the MA is a standard type with no variable geometry, it is controlled by a wastegate which is opened by the boost pressure itself after being diverted by the control solenoid.
So for it to only happen when cold the solenoid must be opening wastegate if boost is low or the fuelling and rpm is being restricted by the ECU which in turn holds back boost. The turbo itself won’t be the cause as it would be happening all the time.
My old MA used to give similar symptoms on first cold start of the day I never checked the boost but it was laggy and a bit lumpy for 30 seconds or so. Never had any faults pop up and new oil seemed to improve it for a while so I just put it down to characteristics of the engine. Maybe MA solenoids a bit slow to work with stone cold oil that’s a bit thicker when old?
 

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How much is a new solenoid ? Does the solenoid control the valve to atmospheric pressure ? If so and the solenoid/valve is misbehaving and not opening when cold then the full boost pressure will go to the wastegate keeping it open, giving a closed loop no boost scenario. I wouldn't like to count the number of cycles for these solenoids.

You're testing the boost on the road, yes? Where does the 0.5 bar come from ? I think the maximum boost is between 2 + 2.5 bars (absolute not relative)

Are you getting maximum boost on the dashboard gauge when the engine is warm ?
 

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The solenoid vents back in to the intake pipework, it’s normally closed and only opens when boost needs controlling to open wastegate. So I wouldn’t suspect it to be sticking it’s either being commanded to open or the fault lies elsewhere if the issue is actually a fault rather than a characteristic.
 

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Discussion Starter · #5 ·
The turbo on the MA is a standard type with no variable geometry, it is controlled by a wastegate which is opened by the boost pressure itself after being diverted by the control solenoid.
So for it to only happen when cold the solenoid must be opening wastegate if boost is low or the fuelling and rpm is being restricted by the ECU which in turn holds back boost. The turbo itself won’t be the cause as it would be happening all the time.
My old MA used to give similar symptoms on first cold start of the day I never checked the boost but it was laggy and a bit lumpy for 30 seconds or so. Never had any faults pop up and new oil seemed to improve it for a while so I just put it down to characteristics of the engine. Maybe MA solenoids a bit slow to work with stone cold oil that’s a bit thicker when old?
Hmm, you might be right. I'll change the oil soon and i'll retry resetting it to see what will happen. Thanks.

How much is a new solenoid ? Does the solenoid control the valve to atmospheric pressure ? If so and the solenoid/valve is misbehaving and not opening when cold then the full boost pressure will go to the wastegate keeping it open, giving a closed loop no boost scenario. I wouldn't like to count the number of cycles for these solenoids.

You're testing the boost on the road, yes? Where does the 0.5 bar come from ? I think the maximum boost is between 2 + 2.5 bars (absolute not relative)

Are you getting maximum boost on the dashboard gauge when the engine is warm ?
Yes, I'm getting maximum boost when the engine is warm.
 

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The solenoid vents back in to the intake pipework, it’s normally closed and only opens when boost needs controlling to open wastegate. So I wouldn’t suspect it to be sticking it’s either being commanded to open or the fault lies elsewhere if the issue is actually a fault rather than a characteristic.
I THINK we're saying the same thing, internal pipework is at atmospheric pressure and I think the logic is flawed. You want the valve to open until the boost is too high. If the valve doesn't open the boost simply opens the wastegate and you get the symptoms described, no ?
 

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No it’s closed in direction of wastegate normally so that boost pressure down the hose from compressor housing does not do what you describe above. Once solenoid energises it opens the port to wastegate diverting boost pressure to open the waste.
 

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If it turns out not to be oil related I would check for error codes when the problem is manifesting. As the car is running fine most of the time, the pending codes might be getting cleared.

Monitoring the key sensor values as well might point to the possible guilty one which could be causing the turbo to wind back when cold. What ECM do you have in the car, bosch, magneti, CF5/CF6 ?
 

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Discussion Starter · #9 ·
If it turns out not to be oil related I would check for error codes when the problem is manifesting. As the car is running fine most of the time, the pending codes might be getting cleared.

Monitoring the key sensor values as well might point to the possible guilty one which could be causing the turbo to wind back when cold. What ECM do you have in the car, bosch, magneti, CF5/CF6 ?
Magnetti CF5
 
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