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No boost, and Not enough boost.
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 4 Sep 2003 11:43:39 Share Post by Email
In Reply to: Ari, my sensor is brand new..., Joe98, Thu, 4 Sep 2003 08:50:30
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The circuit is just a wire from the sensor to the ECU. I'd have to dig up a manual, if I have one for that year to check the pin. It's just a wire, so all you would need to do is check continuity between the sensor and the ECU pin. Anything less than 2 or 3 ohms is just fine.

No need to remove the heater fuse, as you surmised.

Let's forget about the O2 sensor for a second, and go back to the problem; after all, that's what you're trying to fix.

You say you aren't getting boost. Are you getting No boost, minimal boost, or Base boost?

No boost - you can't even get the boost gauge out of the white zone. The white zone is vacuum. A non-turbo engine always runs in some vacuum. If you had no turbocharger, the needle would always be in the white.

Minimal boost - you can get the needle into the upper part of the white and just the beginning of yellow.

Base boost - you can get the needle to the middle of the yellow, but no farther.

By dropping off the W (Wastegate) hose, you proved that the wastegate actuator and turbo work just fine. With the W hose removed, there is nothing to limit boost, except the ECU cutting back on fuel or spark.

The main boost limit loop is through the BPC. If you apply boost (pressure) to the wastegate actuator, it exends, and that opens the turbo wastegate, reducing boost. By removing the W hose, there is no movement. The BPC connects the wastegate actuator to the pressurized intake manifold. If the BPC wants to limit boost, it allows all of the pressure from the intake manifold to make it to the actuator. If the BPC wants full boost, it bleeds off some of the pressure back into the intake of the turbo. Since there is a pressure bleed-off, it takes more pressure in the intake manifold to get the wastegate actuator to move.

Theoretically, the most the BPC can limit you to is Base Boost - half-way into the yellow. If your definition of "No Boost" is actually base boost, then you can be pretty sure the ECU/BPC is limiting you for some reason.

Yes, it could be a bad ECU, but I have to say that bad ECUs are very, very rare. But mechanics like to replace them because they're easy to replace. Base boost can be caused by vacuum leaks, bad plugs, bad DI, fuel pressure issues, a bad BPC. Unfortunately, those take troubleshooting by somebody that understands the system, not someone that will replace the part that's easiest to remove.

Now, if your definition of "No Boost" is less than base boost - needle stuck in the bottom of the yellow/top of the white at most, then there can be other things at work. Did anybody adjust the wastegate actuator? That rod length sets base boost. A bad(weak) spring in the wastegate actuator can cause less than base boost.

Don't forget simple stuff, like a clogged air filter, clogged fuel filter, old or wrong type of spark plugs.

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