1994 855T boost cutting in and out

[azureblue]

No vacuum leaks. I made a close inspection and made sure everything is clamped down, too. All hoses, including intercooler, (and the intercooler itself)were replaced when the PO blew the motor and had it replaced by a reputable volvo specialist.. PCV valve job was done last summer by a volvo shop. The brake booster line is solid, too and I put clamps one the hose at the joint under the air box - and the brakes work great.. The bad TCV hoses were replaced and the rest of the hoses including intercooler hoses are all good. I have sorted out leaky turbo hoses before, on a SAAB 99 and a 240T and 740T, and this doesn't act like that -... This acts like an intermittent electrical connection, not that sort of "soft" fail of a bad hose. Almost feels like a spark plug cutting in and out, but the turbo gauge says otherwise. I can watch the (stock) gauge go up to the middle at the edge of the red band / boost, then stop. Then a little while later, it will come up to a little boost, then, Again, without rhyme or reason - go way up into R boost territory where it used to be. And then decide no more boost.. The idle is great and the idle RPM control system works as it should... I thought the Connector for the TCV was dirty, but cleaning it didn't change anything. And just to aggravate me, I took the car for a two hour 70mph highway cruise, and it behaved perfectly - full R spec boost. Almost got a ticket trying to make it misbehave, too.

Maybe carbon buildup on the wastegate causing it to stick? Would a new front O2 sensor do that? The previous "replace front O2 sensor" code went away when I replaced it and wiped the codes.. Sorry to keep bugging the board about this, but this seems to be one of those thing that isn't one of the usual suspects, nor revealed by an OBD code read - at least so far.. Tomorrow, I'm going to wipe the codes again and take it for a ride, then get a OBD2 print out and see if that shows any change. But thanks for the advice....

[azureblue]

So to wrap it up - the OBD 2 test came out clean. And the car runs great, so I'm going to hope it stays that way. My best guess is the waste gate flap was sticking.. I may hang a gris gris bag on the rear view mirror, just to be safe...

[erikv11]

That's good to know (and great for the car!), that so many of the lines are replaced.

You didn't say in your post that you did anything to fix it though, just that the trouble is gone. Did the problem just go away, or did you do something to the car since your post last night?

[azureblue]

That is the aggravating part - I did nothing to it since it started acting right yesterday afternoon. Fingers crossed...