Hissing noise around 18inHg vacuum
01-20-05, 09:08 AM
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Hissing noise around 18inHg vacuum
Fellas
Some of you may or may not be aware that I am partway through troubleshooting my 7.5-5-5 boost pattern, so far with not much luck. Still got plenty of things to check out though.
ANYway I've lately been driving and paying much more attention to the sounds in my engine bay, and I've noticed this hissing noise, kinda like leaking boost, only I dont think it is.
Background Info: Vacuum at idle reads 18inHg
Let's say Im driving along nice and slow in 2nd gear, around 2000rpm, and I let off the throttle. Boost gauge shows around 20inHg vac. I lightly press on the throttle, get the vac to just past 17-18inHg and "psssssssssssssst". Let off the throttle and it goes away as my vac plunges back to 20inHg. Lightly press on the throttle again, and "psssssssssssssst" around 18inHg again. The hissing doesn't come on slowly, it's a sudden change from nothing to hissing.
I pull into the garage at home and pop the bonnet. At idle, there is no sound. I give it some throttle and "psssssssssst" nice and loud now the bonnet is popped, coming from the drivers side of the UIM (Now I'm an Aussie so that's the US passenger side). I can hold my hand there and feel the air blowing.
The other day I thought it might be a bad UIM gasket, but if the UIM is in vacuum, the air won't be leaking OUT of it. Thought it might be the hose from the air pump but don't see how that would be effected by UIM manifold pressure or throttle position at constant RPM. I wouldn't think it's a boost leak because it's kind of loud, at 2000rpm with only a *hint* of throttle, and I had the intake off the other day and couldn't find anything. That and the fact it's very audible while driving at 2000rpm, with a hint of throttle, enough to get me to 16inHg or so.
Any thoughts? I have not had the time yet to explore further, it's a bit dark outside.
Cheers Lads
Had to edit this, too many bloody typos
Some of you may or may not be aware that I am partway through troubleshooting my 7.5-5-5 boost pattern, so far with not much luck. Still got plenty of things to check out though.
ANYway I've lately been driving and paying much more attention to the sounds in my engine bay, and I've noticed this hissing noise, kinda like leaking boost, only I dont think it is.
Background Info: Vacuum at idle reads 18inHg
Let's say Im driving along nice and slow in 2nd gear, around 2000rpm, and I let off the throttle. Boost gauge shows around 20inHg vac. I lightly press on the throttle, get the vac to just past 17-18inHg and "psssssssssssssst". Let off the throttle and it goes away as my vac plunges back to 20inHg. Lightly press on the throttle again, and "psssssssssssssst" around 18inHg again. The hissing doesn't come on slowly, it's a sudden change from nothing to hissing.
I pull into the garage at home and pop the bonnet. At idle, there is no sound. I give it some throttle and "psssssssssst" nice and loud now the bonnet is popped, coming from the drivers side of the UIM (Now I'm an Aussie so that's the US passenger side). I can hold my hand there and feel the air blowing.
The other day I thought it might be a bad UIM gasket, but if the UIM is in vacuum, the air won't be leaking OUT of it. Thought it might be the hose from the air pump but don't see how that would be effected by UIM manifold pressure or throttle position at constant RPM. I wouldn't think it's a boost leak because it's kind of loud, at 2000rpm with only a *hint* of throttle, and I had the intake off the other day and couldn't find anything. That and the fact it's very audible while driving at 2000rpm, with a hint of throttle, enough to get me to 16inHg or so.
Any thoughts? I have not had the time yet to explore further, it's a bit dark outside.
Cheers Lads
Had to edit this, too many bloody typos
Last edited by jeffrored92; 01-20-05 at 09:11 AM.
01-20-05, 10:02 AM
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I'll post better info when I get to work in the morning, after I've had a play, because it's almost bedtime now
BUT with the limited testing I've done so far the air feels strongest very close to the UIM on the US passenger side, right in between the two nipples (one for boost gauge, other to the ABV/BOV). It's down a bit from the top, near a metal tube that connects to a hose returning to the stock air box.
It's not particularly close to the charge relief. I do know the 1" hose from y pipe to charge relief is loose, I can rotate it quite easily about the y-pipe end. At such low load though, would the secondary even be pre-spooling?
The air feels closer to the y-pipe coupler (which is still in need of a closer inspection) but even that doesnt seem likely to me, given that I'm at 2000rpm, a *touch* of throttle and about 17inHg in the UIM.
Possibly something to do with the Air Pump? I'm thinking this primarily because I know the turbo's can't be cranking out much pressure (if any) yet there is enough pressure to cause an easily audible hiss. I seriously need my FSM!!! Can't be bothered downloading another copy.
BUT with the limited testing I've done so far the air feels strongest very close to the UIM on the US passenger side, right in between the two nipples (one for boost gauge, other to the ABV/BOV). It's down a bit from the top, near a metal tube that connects to a hose returning to the stock air box.
It's not particularly close to the charge relief. I do know the 1" hose from y pipe to charge relief is loose, I can rotate it quite easily about the y-pipe end. At such low load though, would the secondary even be pre-spooling?
The air feels closer to the y-pipe coupler (which is still in need of a closer inspection) but even that doesnt seem likely to me, given that I'm at 2000rpm, a *touch* of throttle and about 17inHg in the UIM.
Possibly something to do with the Air Pump? I'm thinking this primarily because I know the turbo's can't be cranking out much pressure (if any) yet there is enough pressure to cause an easily audible hiss. I seriously need my FSM!!! Can't be bothered downloading another copy.
01-20-05, 10:07 AM
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Probably should clarify this. I'm in Aus so got myself a RHD car.
I'm talking the non throttle side of the UIM. My drivers side or Your passenger side.
I think I should just relate to sides of the car the US way to make it easier for you guys and save me some typing in the future.
I'm talking the non throttle side of the UIM. My drivers side or Your passenger side.
I think I should just relate to sides of the car the US way to make it easier for you guys and save me some typing in the future.
01-20-05, 10:13 AM
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remove the crossover pipe and have a look at both the cross over pipe and the couplers.
if the couplers are original, you should definitely replace them with some high quality silicon couplers and clamps. the cheap screw clamps often fail and even though they feel tight, they might not be tight enough to not vent boost.
the crossover pipe that goes between the Ypipe and the intercooler has been known to crack on occasion and is very difficult to spot the crack. perhaps see if you can borrow/acquire another and try replacing it.
if the couplers are original, you should definitely replace them with some high quality silicon couplers and clamps. the cheap screw clamps often fail and even though they feel tight, they might not be tight enough to not vent boost.
the crossover pipe that goes between the Ypipe and the intercooler has been known to crack on occasion and is very difficult to spot the crack. perhaps see if you can borrow/acquire another and try replacing it.
01-20-05, 06:54 PM
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Had all the pipes out last weekend, I had trouble getting at the lower clamp on the y-pipe coupler so haven't had that completely off yet. It looked OK while connected to the y-pipe but I'm aware I need to check around where the lower clamp may be digging into it. Tomorrows job is to take it off and inspect it properly.
Do you really think it could be boost leak at such low rpm and load? I don't see how the turbos could be producing enough pressure for an obvious leak under those conditions.
Do you really think it could be boost leak at such low rpm and load? I don't see how the turbos could be producing enough pressure for an obvious leak under those conditions.
