850 Turbo Zero Boost

[Nevada1906]

Hello all.

I hope someone can help me with this. I'm about out of ideas.

My 95 850 just passed 240,000 miles and this is what's wrong:

1 - The engine idles smoothly with a barely-perceptible fluctuation but stumbles at low RPM and quickly smooths out as RPM increases. IT ONLY STUMBLES IN DRIVE. In Park or Neutral in spins smoothly up and down the tach.

2 - No codes and no CEL.

3 - According to the factory turbo gauge, the turbo is producing no boost at all. The needle gets near the top of the VACUUM area and abruptly stops moving, even if I redline the engine. It won't cross into the BOOST area of the gauge, like it's hit a wall. The impeller spins freely, though, but I can't hear the turbo working and it seems to just be sitting there.

4 - Decelerating to a stop causes the engine to nearly or actually stall out.

5 - I just replaced ALL the vacuum lines in the engine bay with brand new hose and checked the routing 4 or 5 times to be sure it's correct. I also ziptied all the ends to the nipples to make sure they stay put.

6 - I've pulled the bypass valve off the turbo to inspect it, and the diaphragm is intact with no holes or tears. The vacuum port is clear, so I put it back in the car.

7 - The wastegate actuator does move, but not much, and not until the engine's past 3,000 rpm or so.

8 - I've pulled the intercooler hoses to check them for cracks, and they're all solid (Mishimoto silicone kit I installed last year). I took extra care to reinstall them all with aerospace clamps - and I made sure all the clamps were on square and between the retention ridges on the pipes. There are no breaks in any of the metal IC pipes.

9 - There doesn't seem to be anything internally blocking the intercooler.

10 - I replaced the flame trap last weekend.

11 - There's no smoke from the tailpipe, but the turbo oil seals are leaking.


I'm 99% certain I've eliminated any and all vacuum leaks, but still no boost. I figure these are the possibilities:

A - My wastegate is stuck OPEN.

B - My BCS is fried, but isn't throwing a code.

C - There's a blockage somewhere in the circuit that I haven't found, but I can't imagine where it might be - I've taken the entire intake system apart twice in two days.

D - Something else is wrong.

I'd really appreciate it if anyone out there could throw me a line on this one.

Thanks.

[abscate]

The turbo control valve could be stopping your boost. They are hard to test with simple equipment

Revving the engine won't make boost, the engine is smarter than that. It has to be under load and requested by the ECU

[Nevada1906]

If the Boost Control Solenoid was bad, wouldn't it throw a code?

[abscate]

It can go bad without setting a code.

Resistance spec us 29 ohms, no error bars given in VIDA

If it's got 100k on it, it's tired

[tryingbe]

Does the turbo spin by hand? Is the compressor wheel still attached to the turbine wheel?

[Nevada1906]

Okay, thanks.

At this point, I can't think of anything else that'd be the problem.

[tryingbe]

I have a good 16t if you need a turbo.

[Nevada1906]

Yeah, I can rotate it with my finger. I'm pretty sure the turbo itself is good, it's just not being instructed to boost.

[Nevada1906]

Thanks for the offer, but I'm going to use my existing unit.

[Ozark Lee]

Another place to look is the compressor bypass valve. The diaphragm is fairly thin and if it tears the bulk of the boost will get returned to the intake. If you have a vacuum pump with a gauge just hook the pump up where the vacuum line goes in and see if it will hold a few pounds of vacuum. If it won't pull a vacuum then that is at least a big part of the problem.

...Lee