Navara over-boost

This 2007 D40 Nissan Navara had been suffering from a loss of power under certain load conditions.

A brief history of the vehicle shows a replacement turbo had been fitted, to resolve a complaint of noise, verified by an air leak from the housing.

Since the turbo replacement, the customer had complained of the condition mentioned above.

I had been called to investigate this concern and my initial diagnostic interrogation had recorded DTC’s P0234 and P0238, both relating to turbo over boost conditions. I advised that the vehicle or the turbo was returned to the supplier of the replacement part, for inspection, as the most likely cause of the problem was the calibration of the re-manufactured unit.

After the owner had the vehicle inspected by the turbo specialist, it was returned to the garage for further diagnosis, as the specialist had claimed the turbo was good.

So I carried out another diagnostic scan, after the systems had been cleared, and found the same DTC’s had repeated.

I then embarked on a full turbo boost pressure and control system analysis. I wanted to test and identify the fault in such a way that I could show proof of the cause. I was still quite certain the turbo was at fault.

So I coupled up my wps500 to the intake boost hose, via a hose insert I had stored in my tool box. Luckily it fitted! The boost pressure sensor,  in this case a MAP sensor,  was a little difficult to access and I wanted to compare real-time pressure with serial data. I was able to confirm the data related accurately.

I also set up my 2nd wps500 into the wastegate vacuum capsule and connected a voltage probe to the wastegate duty solenoid. I figured I had all the signals I needed and so routed the cables into the cab, so that I  could monitor the scope during a road test.

I got the driver of the vehicle to drive me, so that we could replicate the conditions for system failure.  Sure enough the fault occurred and I was able to save a capture.

You can see that the boost pressure exceeded the 2 bar, (including atmosphere ), specification. In case I had read the specification incorrectly, I noted the boost pressure sensor specification showed a maximum capacity of 2.5 bar including atmosphere.

As you can see the actual boost pressure exceeded 1.7 bar above atmosphere.

The following image illustrates the wastegate control against the actual boost.

Turbo boost control bad

This capture illustrates the over boost condition and shows the proof of the wastegate control failing to regulate the boost. The result was another replacement turbo, which did rectify the fault. Sometimes saving money costs money!

Again, unfortunately I don’t have a scope trace of the good turbo operation on this vehicle.

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