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A Tata with a Quadrajet BS IV (90 ps) diesel engine was brought in for low power. When starting the engine in a no load condition the turbo boost pressure is 950 mBar. When the engine accelerates the boost pressure dropped. Regarding this issue the service manager said the turbocharger is not working properly, so it was replaced.

After installing a new (re-man) turbocharger the boost pressure increased and the maximum is now 1.3 Bar. The service manager says this is too low, the minimum spec is 1.7 Bar.

Before replacing the turbocharger again are there other things that should be checked?

  • 1 Bar = 1000 mBar. I believe you mean 0.3 Bar and 0.7 Bar instead of 1.3 and 1.7... I used to work on a dyno, I know how confusing it is :D – Arturs Bolsunovskis Jul 22 '16 at 16:56
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My first thought, based on the boost dropping as you rev the engine before the turbo was replaced is that there is an inlet restriction. Something like a dirty air filter or damaged inlet piping. That could also cause the replacement turbo to fail to deliver full boost.

One quick check for this would be to remove the compressor inlet pipe from the turbocharger. That would also bypass any sensors upstream of the compressor – for instance a mass air flow (MAF) sensor if the engine has one, so it is possible that the ECU could set codes, or put the engine in a reduced output (limp) mode as a result of the loss of the sensor.

As rpmerf mentioned a restriction on exhaust flow could also cause low boost. This could be caused by a blocked catalytic converter. It can be a real pain to open up the exhaust experimentally, but if the car has an O2 sensor or some other port in the exhaust system you can measure the exhaust back pressure to get a feel for how restricted the flow is. Ideally you'd compare the value to a similar car with a working turbocharger.

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How is the turbo pressure regulated in these cars? If there is a linkage that actuates the wastegate maybe it needs adjustment.

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Addition to dlu's answer.

Look for restrictions in the intake and exhaust. You can try bypassing the intercooler. For the exhaust, you can temporarily set up an open pipe out of the turbo. either way, something is restricting the turbo from keeping up.

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Also if air ducts or hoses transferring compressed air from turbo charger to inlet manifold (via intercooler or whatever) are not tightly secured, can cause the compressed air to leak past through them and therefore resulting in lost boost pressure.

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