rich_russell

My BG5B Legacy GTB's developed an odd problem over the last few days, I think it started to happen a few weeks ago, but has been getting worse.

On the primary turbo (<3000rpm) performance is normal.
Over 4500rpm (ie both turbos), again performance seems normal.

However if you accelerate from say 2000rpm, the revs climb to around
3500, rise quickly to 4000, then drop back sharply to 3500, and climb
more slowly. Once above 4500, it takes off again. So all acceleration
is lost for a few seconds until it starts climbing again.

This to me smacks of a problem with the changeover from single to twin
turbo operation - maybe a big leak on the secondary side (so all the
boost pressure is lose when the valves open), or either the exhaust or
intercooler valves not opening at the right time.

Or maybe it's the clutch slipping as the primary turbo creates enough torque, then the ECU opens the valves to bring in the secondary turbo - the torque drops (as is normal on twin turbo Legacies), and the clutch bites again. Suddenly the engine sees a big load, bogs down and the revs drop. Then it starts climbing again, but the turbo(s) have lost boost.

I've taken the intercooler off and checked all pipes for obvious
splits or leaks, removed both bypass valves (dump valves) and checked
them (both seal perfectly, and open when a vacuum is applied to the
small pipe). Intercooler valve moves smoothly, and the actuator for it
shows no signs of leaks. Not sure how to test the exhaust valve.

There are no ECU codes stored (I was hoping for the dreaded 66 at least!).

In 1st gear and 2nd gear, if you accelerate hard, it just flies
through the transition region and the revs don't drop. I guess this is
due to there being enough boost being produced by the primary turbo to
keep it going during the transition, or if it's the clutch, it somehow stays engaged.

Any ideas please? I'm off to Scotland in two weeks for a holiday, and
really could do with sorting it by then...

Rich.