P0402 after cleaning the air intake

[Exige255]

Hi all,
A few months after replacing the 4th cylinder glow plug, I had to replace it once more.
I've decided to do it by myself and had to remove the air intake and the air throttle.
The elbow was very clogged, while the valve was pretty clean.
I cleaned up both, and, after mounting them back, the P0402 error popped up.
I've recently cleaned up the MAF sensor and used the Liqui-moly 5168 spray to clean up the air intake on the engine head side but the error is still there.

From the OBD scanner, I can see the MAF sensor value increase while accelerating.
The engine runs smoothly when in idle, but it lacks power and the revolutions don't go over 3000.
If I clear the code, it comes back after a few minutes.

Does somebody have an idea on how to fix it? Or something I should check?

Thank you

 

[KJowner]

I'd guess the EGR is stuck open, the amount of carbon build-up may have been masking the issue.
The EGR system is terrible and best disabled.

 

[Exige255]

Thank you for your suggestion.
I've also thought that cleaning up only a part of the circuit could have caused a difference in pressure

 

[Exige255]

Another chapter of this neverending story.

I've disassembled the intake again and then the EGR valve.
I don't know if it's the expected behavior, but the throttle is always open no matter the revolutions. It only closes for a few seconds when the engine is turned off.

I've also cleaned up the MAP sensor. It was entirely clogged.

The EGR valve looks good. I don't think it's stuck open.

What else could cause that issue?

 

[KJowner]

The diesel doesn't have a throttle, the butterfly flap is used to restrict inlet air by the ecu to reduce emissions.
A blocked MAP can cause issues, the ecu relies on it to control fueling so if it's blocked you will loose power.
The MAF is only used to check EGR function so if this or it's wiring is faulty the ecu will log a fault in the emissions system, it will also do this if the EGR is stuck open or closed or has been disconnected.
Depending on where you are in the world it's generally recommended to get the EGR and flap disabled, you can see the black muck they create in the engine.

 

[Exige255]

Unfortunately, I think disabling the EGR is not an option since I'm in Italy. Every two years we have the car revision and they control the emissions.

The MAF sensor is working, since I can see the value increase when I accelerate. But, according to this video

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, 5 grams is too low for a 1.6 (the value goes up to 8-9ish when they fix the issue), and I also have 5.
It should be much higher.

 

[KJowner]

Check for a boost leak from the air hoses, that would mess up MAF readings.
You could also speak to your fellow Italian

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