mel_t_vin

Folks, am remotely assisting a friend with a boost-related issue in TX. Car is an unmodified '89.

Upon cold start, morning or afternoon, all is well. After driving ~30 minutes, with engine [well] at operating temperature, boost onset becomes more and more delayed, and maximum boost pressure becomes less and less, until finally there is no boost pressure at all, and engine simply operates under natural aspiration. Not all of a sudden, but a gradual process over 10-15 minutes. Interestingly, the next morning all is well at cold start [full boost], until this process is repeated. Doesn't matter is ambient temp is 50*F or 80*F.

Am thinking substrate in the catalytic convertor or the muffler is expanding due to heat, restricting the exhaust, and [gradually] preventing the turbo charger from generating positive pressure. Does this make sense? Is anyone aware of, or has had experience with, a heat-induced, exhaust-related boost issue?

MUSSBERGER

Diverter valve sticking or an internal leak?

http://forums.pelicanparts.com/911-9...-question.html

mel_t_vin

DV sticking...when things get warm? Suppose that's possible.

Funny thing is, she runs like a swiss watch, with full boost, upon cold start. And the waste gate audibly vents in first and second [pfttt], no issue. Strange...