slowguy

I was out driving around tonight and I was cruising at highway speeds. Everything was fine, pulling nice and hard, got up to about 80 then let off. Stepped on it again at 60 and it was like I have no boost at all (or very little). Didn't go out and hook up vag com yet but there are no cel light on, turbos are very quiet and spooling, shifting seems to be fine, all else seems to be normal. Any quick ideas before I start investigating further?

Just Me

Ideas? yep. Air/boost leak. As long as its not screwing up the mixture, they can run just fine that way, but power is down 40%

Grant

slowguy

Ok. That's where I'll start. I made a boost leak tester I found on a diy somewhere. I'll scan with vag com, then hook up the leak tester and see what happens.

I did have a hose blow off once that connects to the intercooler and it threw a code right away - and I heard a nice "POP".

slowguy

Pulled the codes today and found P1555 or 17963. Charge pressure - maximum limit exceeded. Need to do come research to find out more about this. First time it has happened.

SloopJohnB@mac.com

One excellent way to exceed charge pressure is for the boost pressure hose to the wastegate motor degrades and blows off or leaks.

Point is to check the hose to the wastegate motor first.

Happened on our 87 760T volvo....2.4 Liters and probably 350hp before it blew the #1 rod out the block....

slowguy

I'm still looking at diagrams so I can get a hold on how all this stuff works together so I know which pipe/tube does what. The n75 basically puts more or less pressure to the wastegate to control the amount of boost allowed into the intake? The wastegate is controlled by a small vacuum line attached to the wastegate?.. and the wastegate is on the hot side of the turbo? If I am understanding this correctly, then what do the diverter valves do if the wastegates in combo with the n75 control boost levels?

The wastegate in it's normal state (if you were to completely disconnect the n75) allows no boost or full boost to pass to the intake? The line to the wastegate is under a vacuum or pressurized?

SloopJohnB@mac.com

Yes..N75. Wastegate controlled by small PRESSURE line attached to the waste gate. Diverter valve allows boost to go around and back to the intake when throttle is closed to avoid compressor stall. Trick is that the controller to the waste gate is basically a spring loaded vacuum motor with a link to the waste gate...vacuum assists the spring in closing the wastegate, but pressure works against the spring until the spring force is overcome, thus opening the waste gate. Wastegates are often set at a basic boost level, say 7psi, then the N75 valve bleeds off the manifold pressure signal to the wastegate so it doesn't exceed base boost, enabling a higher than base boost manifold pressure.

The problem comes when the line to the wastegate controller is missing or leaking...the controller thinks the manifold pressure is zero so it keeps the wastegate closed and zing, one turbo or both if there's only one N75 valve overspeeds and manifold pressure goes way high.

slowguy

Took it out for a ride after resetting the ecu. Got on it a bit and running like it always has. Didn't go into limp mode again yet. I guess my next step will be blowing on the line that goes to the wastegate and see if I can locate any leaks. Because the code has only thrown once and didn't do it again after my last ride I am wondering if the leak is very small? I do have an inspection camera that I'll try and poke down in there to see if I notice anything obvious. If I blow on the line with my mouth, should the line to the wastegate have zero bleed-off? Will it hold pressure I put on it if it is not cracked or leaking?

SloopJohnB@mac.com

If the line was bad you would know it immediately. Sounds more like something else is cracked or leaking. And yes, there should be zero Leak in the line to the waste gate.

slowguy

After I sent the log to a very good tuner (who has been working with my file and tweaking to get it near perfect), he said the tripped code was caused by boost oscillation at lower rpm...which I posted in another thread. He said he fixed the tune and sent it back to me. I just have to flash it, go for a ride, and log it again. We'll see what happens.