Boost Leak Riddle...

[german_cargo]

I don't have any engine codes but I'm down on power under full throttle, and I hear a faint vacuum cleaner sound under light throttle right when I tip in that fades away if I stay in the throttle. However -- It doesn't make the vacuum cleaner sound when I rev it at idle, and the mechanic I took it to said there were no boost leaks that showed up with his smoke test and didn't think any more diagnosis was necessary without a trouble code present until he replaced the boost pipes from the intercooler up to the intake that had oil residue between the joints.

I've done the "dog collar" mod with a plump new O ring and replaced the weeping boost pipes from the intercooler up to the intake and it made no difference. Someone suggested the EGR pipe is cracked and to check for soot, but I don't see any soot anywhere. I noticed recently that sometimes it makes the "choo" sound when I let off the throttle and sometimes it doesn't under similar conditions. The vacuum cleaner noise is 100% consistent though.

What kind of boost leak or vacuum leak shows up under minimum throttle while driving but not while revving in the driveway???

 

[JETaah]

Could it be that you have an exhaust leak somewhere ahead of the catalytic converter (cracked inner flex pipe) and that the converter is partially plugged?
I assume that this is a BEW engine, yes?
Try placing some thick or doubled-up washers in between the turbo outlet and the down pipe to relieve exhaust restriction and see if the power comes back.
Yes it will be a bit louder but may reveal if the converter is causing your problem at a higher load.

When you rev the engine without a load the ECU backs off the vanes so as to not create boost when it is not needed.

 

[BobnOH]

.................snip........................ and the mechanic I took it to said there were no boost leaks that showed up with his smoke test.................snip........................What kind of boost leak or vacuum leak shows up under minimum throttle while driving but not while revving in the driveway???

So perhaps it's something else, vacuum tubes, wiring or who knows?
Revving in the drive there is no load on the motor, nothing much happening with the turbo and such.
No stored codes? Not everything is monitored by the ECU.

 

[german_cargo]

Could it be that you have an exhaust leak somewhere ahead of the catalytic converter (cracked inner flex pipe) and that the converter is partially plugged?
I assume that this is a BEW engine, yes?
Try placing some thick or doubled-up washers in between the turbo outlet and the down pipe to relieve exhaust restriction and see if the power comes back.
Yes it will be a bit louder but may reveal if the converter is causing your problem at a higher load.

When you rev the engine without a load the ECU backs off the vanes so as to not create boost when it is not needed.

Thank you so much for this suggestion! I didn’t even think of the possibly of a plugged cat. It started happening right after a very intense collision with a brick on the road that cracked my oil pan open, so damage to the cat and / or exhaust pipe would actually make a lot of sense…

 

[snakeye]

The egr closes at full throttle I think, so might be egr related. I had a boost leak on my bew, at full boost it went away. Never found out what it was, the car rotted away before I had a chance to look at it, but I had air shooting down right underneath the turbo. Did you crawl underneath with the car idling? What kind of vacuum cleaner sound are you talking about? That's very vague. Is it the suction sound or the sound of the vacuum cleaner running?

 

[german_cargo]

The egr closes at full throttle I think, so might be egr related. I had a boost leak on my bew, at full boost it went away. Never found out what it was, the car rotted away before I had a chance to look at it, but I had air shooting down right underneath the turbo. Did you crawl underneath with the car idling? What kind of vacuum cleaner sound are you talking about? That's very vague. Is it the suction sound or the sound of the vacuum cleaner running?

Sounds like a vacuum cleaner running. Not like turbo whistling or air through a small nozzle. It sounds like there’s a small shop vac under the hood of the car that gets turned on and off when I apply and then let off light throttle but a lot quieter than an actual vacuum cleaner would be. The mechanic confirmed the noise was abnormal.

I haven’t crawled underneath with it running yet. Call me a whimp but I can barely get it high enough that my chest isn’t rubbing on the bottom of the car while I’m underneath and I’m claustrophobic. With it running I get super squirmy and afraid of getting burned and maimed. I suppose I should get some bigger axle stands.

 

[Zak99b5]

It started happening right after a very intense collision with a brick on the road that cracked my oil pan open, so damage to the cat and / or exhaust pipe would actually make a lot of sense…

How long did the engine run without oil after the pan broke?

 

[Tdijarhead]

How long did the engine run without oil after the pan broke?

My daughter hit an unmarked cut out in the road with her beetle. That accident drained the oil pan and gave her turbo a nice screech, fortunately I had a spare turbo on my shelf.

 

[BobnOH]

Had this happen as well. Daughter raked a pavement edge, ripped a hole in the pan, got on the freeway. It was winter.
Turbo survived, but crank and rod bearings spun, oil pressure no good, light on.
But we never got any unusual noise.
If it ran without oil for any length of time, you may want to spot check a couple bearings.