What would cause the turbo to boost in normal driving... no pulling a hill, no pulling a trailer, no load. Sometimes will boost going down a hill.

details from OP comments below:

The check engine light was coming on, codes read that the waste gate needed replaced, we replaced it and now the turbo boosts when just driving normal speed on a straight road with no load or hills... is that normal? Has never done in the past. Why would it boost when going down a hill almost coasting? We are gauging boost when it kicks in like a jet engine starting up.

  • Welcome to the site. Is the "Check Engine" light on? Have you had the codes read? Is the engine running as it should; no surging? The problem might be caused by a defective/stuck waste gate or boost controller. – CharlieRB Jul 10 '17 at 14:22
  • What makes you think there's a problem? The turbo will produce some boost in normal operations so the diesel runs at maximum efficiency. – GdD Jul 10 '17 at 14:36
  • What does "boost going down a hill" mean to you? – cory Jul 10 '17 at 15:43
  • How are you gauging boost? – GdD Jul 10 '17 at 17:50
  • 2
    Did you think you were getting the correct boost before the wastegate failed - it may just be that you are now getting "normal" turbo boost behaviour. – Solar Mike Jul 10 '17 at 19:36

The operation of the wastegate is to open when the engine reaches it's target boot level to allow exhaust gas to bypass the turbo and prevent pushing higher boost levels. When the turbo is pushing less than the target level boost, the wastegate is closed, so all of the exhaust gas is flowing through the turbine, causing the turbo to spool.

Given that your truck has all factory parts, it seems this is when the turbo was designed to spool in order to give maximum torque at low RPM.

I believe your old wastegate was incapable of being held closed all the way. This was allowing exhaust to bypass the turbo, which would increase the amount of exhaust gas required to spool the turbo.

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