The boost pressure fluctuates (somewhat rapidly) in my 1994 850 Turbo.

I’ve posted this question on another board and had little luck. A co-worker informed me that the individuals on this board are very knowledgeable and sometimes provide better and or more input. Here’s hoping. :)

If I pin the throttle, it will build full boost, however, as soon as it reaches full boost, it drops to near atmospheric. From there, it continues to build to full boost and then drops off again (this time not quite to atmospheric, each drop off is less than the preceding one).

This process continues through redline and is not in sync with gear changes.
It has been suggested that this might simply be a leak. I've looked around and not found anything; I have sprayed carb cleaner around all the connections and observe no change in engine speed.

I can't imagine how a simple leak in the system could cause this. I've had leaks before and they simply resulted in not building full boost, or taking a long time to do so. This is a pretty rapid fluctuation, say between 1 cycle a second (1 Hz) and 1 cycle every two seconds ½ Hz.

Is there a way to check the bypass valve and the waste-gate?

Any help would be much appreciated.

I have checked and replaced all the vacuum lines. I am having trouble determining what these vehicles use as a blow off valve. As I understand it, they use a recirculation valve. Is this the same as the EGR, or is it separate? Is there a way I could de-activate either the waste gate or the recirculation valve and see if that stops the problem? Obviously I would need to be very careful not to overcharge things if I did this.

Thanks,

Jay

If you like your head gasket, do not disable the waste gate. Too much boost has also broken wrist pins.

The EGR is separate. The BOV has an internal diaphram that might be cracked. Or, check your engine codes and look for an overpressure error that will not turn on the CEL.

Klaus
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The more I learn, the more I forget. So why learn?

Other than the blow off valve not working correctly, please check all of the vacuum lines around the turbo. The hoses get old and start cracking and the only way to visually check them is to take them off, one at a time so you know where they go back, and bend them lightly. You will probably have to replace one or two.

Klaus
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The more I learn, the more I forget. So why learn?

maybe it's just me, but if I was willing to go through all the trouble of taking them off and checking them, I'd skip the checking and just be replacing them. It's the same amount of work, and vacuum line is cheap.
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1998 V70 AWD Turbo 195k+

Sorry, I attached the following to the wrong message the first time:

I have checked and replaced all the vacuum lines. I am having trouble determining what these vehicles use as a blow off valve. As I understand it, they use a recirculation valve. Is this the same as the EGR, or is it separate? Is there a way I could de-activate either the waste gate or the recirculation valve and see if that stops the problem? Obviously I would need to be very careful not to overcharge things if I did this.

Thanks,

Jay

It almost sounds like an intercooler pipe is split or a leak on the air charge side rather than a turbo control issue where it's happening out of sync with going through the gears.
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Wallie

Finally a new clue.

I got a 5-3-5 (turbocharger control valve signal) code today. I'm going to try putting the control valve from my 1996 850 turbo on and see if it solves the problem.