1996 855 Turbo - Diagnosing low boost

Post by **ZionXIX** » 13 Feb 2015, 19:00

So one morning last week, I woke up to a low turbo boost when I started the vehicle. At idle, the needle sits 3/4 of the way into the negative region. when accelerating hard, the needle will move no more than 1/4 of the way into the positive region. I have read many great posts about turbo problems on this site but i still need some troubleshooting help.

Im thinking there is a leak somewhere but i just cant seem to find the darn thing. I replaced the wastegate actuator hose and the CBV hose as they were transitioning into a powder form and felt like jelly. I have not replaced the compressor house hose because its one tough SOB to get to. I read in other post that the TCV has a high failure rate. I thought about trying ipd's TCV replacement. Is there a way to test the TCV to make sure its working right?

Any help is always appreciated.

Thanks.

Post by **Ozark Lee** » 13 Feb 2015, 19:16

Is the performance seriously degraded? Many times low boost indications on the boost gauge are a result of a vacuum leak that isn't even associated with the turbo vacuum lines. One other place to look is at the BOV. They are at the age where the diaphragm is rupturing on most of them.

A quick way to check is to hook up a vacuum pump with a gauge (like a Mitytvac) and see if the BOV holds a vacuum.

...Lee

Post by **ZionXIX** » 13 Feb 2015, 19:29

I really have not noticed a decrease in performance but I drive pretty cautiously anyways. I assume you're talking about the CBV, does the car need to be running when I test it?

Post by **Ozark Lee** » 13 Feb 2015, 22:03

ZionXIX wrote:I really have not noticed a decrease in performance but I drive pretty cautiously anyways. I assume you're talking about the CBV, does the car need to be running when I test it?

Blow off valve or compressor bypass valve - it is essentially the same thing with different names although technically the compressor bypass valve is more accurate since it vents back to the turbo intake. No, you don't need the car to be running to test it. Just pull a small vacuum (3 or 4 PSI) and see if it holds. I plug the gauge straight into the nipple with a short piece of vacuum line.

...Lee

Post by **ZionXIX** » 15 Feb 2015, 07:08

Ok, so i dont have a mityvac but i did have syringe lying around. I pulled the plunger all the way back and it did indeed hold a vacuum. I held it in place for 30 seconds and it still pulled the plunger back to its original position every time. What else do i need to start looking at?

Thanks.

Post by **Ozark Lee** » 15 Feb 2015, 08:40

I would start looking for vacuum leaks starting at the line from the gauge. The line from the gauge is a hard plastic line that can degrade over time but the joint where it transitions to the rubber vacuum line near the fan shroud is a more likely place for things to go bad. The PCV vacuum lines are always trouble but they usually will trigger a CEL in fairly short order. Check the rubber elbow at the end of the intake manifold and then move on to all of the lines from the vacuum tree.

I suppose it could be an intercooler or intercooler hose problem but I think you would notice that with the accelerator pedal more than you describe.

...Lee