Does this necessarily mean overboost?

I have a P1557 sending me into limp mode. I can't find a straight answer if this means overboost or not. I have run the N75 test in VCDS and can watch/hear the actuator move from full on to full off. I have put a vac pump and determined the actuator moves when it should. This turbo was completely dissembled and cleaned before installation. This is a malone stage 3 programmed ecu with a vnt15 turbo and a 3 bar map sensor. I have not been able to watch/log requested vs actual boost yet, as the laptop I use for this has a battery that lasts exactly 0 seconds if not plugged in. (batt on the way).

It would be helpful to know if this code definitely means overboost. I'm not sure how I could be over boosting if the N75 is cycling as it should, which it appears to be. Any thoughts? If it could be under boost, then checking for boost leaks would be my next step.

Check the vacuum line going to the N75?
Could also be a sticking VNT rod but you said turbo was cleaned. How long ago was the Turbo disassembled and reinstalled?

Also, the N75 might be clogged, or the tubing clogged with rusty crud from the "old" turbo. Just replacing the hose(s) won't necessarily fix it.

Have you actually addressed every solution presented in the Ross Tech Wiki or just blown them off for being to hard to understand? I lot of people do this and don't address what they don't understand.

No, the code does not mean overboost. Just like it says, it means there is a positive deviation from what is requested. Requested-VS-actual boost. you need to figure out what that means.

Ol'Rattler said:

Have you actually addressed every solution presented in the Ross Tech Wiki or just blown them off for being to hard to understand? I lot of people do this and don't address what they don't understand.

No, the code does not mean overboost. Just like it says, it means there is a positive deviation from what is requested. Requested-VS-actual boost. you need to figure out what that means.

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To me, positive deviation means that at some point in time, the ecu is seeing a higher boost pressure than it is requesting. Is there more to it than that?

Let me me more clear by saying this is a swap into an 874runner and some things have been deleted. The vac system is very simple now (perhaps too simple). The vac goes from the vac pump to the brake booster with a check valve. I was tapped for the n75 in between the motor and the check valve. No other connections. I have since made a change (last night, I'll detail that in a moment)

The turbo has run about 10 gallons of fuel since the cleaning. The actuator starts to move around 3inHg and is full travel at 17 or 18. The actuator moves freely, and can be seen doing so during the VCDS test.

My own internal logic figured that a problem in the vac system would cause negative deviation, as a lack of vacuum would cause LESS boost pressure.

I'm not ready to call it fixed yet, and I know I changed too many things at once to really know what made the difference, but we will see how it goes. Before this "fix" I hit limp mode up a certain hill every time. So far I have 2 hours of drive time with no limp.

I added another check valve at the T in the main line, and introduced the vacuum canister. (a google search said once that a bunch of people were running without it with no problem). This seems to give the ECU better control over the turbo, and I can even see it on my OBD boost gauge. I can only assume that having a buffered, more consistent supply of vacuum, makes it easier for the ECU to accurately modulate boost pressure.

I'm cautiously optimistic that this will fix it, as I feel like I have addressed all the other parts of the system, but I will keep this updated regardless.

Bolts-n-Hose said:

To me, positive deviation means that at some point in time, the ecu is seeing a higher boost pressure than it is requesting. Is there more to it than that?
Let me me more clear by saying this is a swap into an 874runner and some things have been deleted. The vac system is very simple now (perhaps too simple). The vac goes from the vac pump to the brake booster with a check valve. I was tapped for the n75 in between the motor and the check valve. No other connections. I have since made a change (last night, I'll detail that in a moment)

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I take the 'positive deviation' as to much boost also.

BUT it may be the difference between the requested boost and the actual boost is to much, not just more then the turbo can handle which will also throw a code.
I think it's a 4 pound difference for a certain length of time.

You are using the N75 ?

I know there are pictures of the hose routing here if you are only using the N75.
You must have the 3 ports connected.
One of the ports goes to the bottom of the air cleaner box so only filtered air gets sucked into the N75 as needed.
Dirt or rust can mess up the N75.

Bolts-n-Hose said:

To me, positive deviation means that at some point in time, the ecu is seeing a higher boost pressure than it is requesting. Is there more to it than that?
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Yes and no. The ECU is seeing positive deviation, but it's not telling you why.
It can be boost control, but it can also be loose piping or bad/dirty vacuum line or any of the other stuff in the wiki.

Overboost usually means the N75 is unable to vent a sufficient amount of vacuum fast enough, or the VNT vanes or the VNT actuator mechanism are stuck or worn such that they cannot return to the rest position properly.