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Happiest when Tinkering
 
gsmith660's Avatar
 
Join Date: Jun 2007
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Boost bleeds off at wide open throttle

Sometimes it does sometimes it doesn't any ideas?

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Old 09-01-2011, 07:08 AM
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It goes up to 6 psi then rapidly drops back to 0 psi or slight vacuum. It didn't used to do this do I have a leak somewhere? Could it be the wastegate? BOV? Has anybody ever seen this happen?
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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
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Old 09-01-2011, 11:31 AM
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Origional wastegate? Diaphram leak?
Old 09-01-2011, 04:41 PM
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Wastegate is not that old and is a Tial if it has a diaphram leak then I have a big problem.
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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
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76 Blazer also restored by me
Old 09-01-2011, 04:45 PM
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44 views and nobody has any ideas. Did I do something to offend?
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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
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76 Blazer also restored by me
Old 09-01-2011, 04:46 PM
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Quote:
Originally Posted by Benny771 View Post
Origional wastegate? Diaphram leak?
A diaphram leak usually raises it's ugly head as the WG not opening and an overboost situation.

I would look at whatever you're using for a BOV, whether the stock recirculation valve assembly or aftermarket gizzmo you may have installed. You're only getting about .5 bar boost, what's up with that?

As your boost begins to bleed off, how is the car sounding/running? Pouring black smoke out the tailpipe, anything like that? O'ring seal on IC "popping off" on boost?
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Old 09-01-2011, 05:11 PM
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My wastegate is a Tial and has a .5 bar spring my motor is a 2.7 with stock compression and the funny thing is it doesn't seem like I get a power dropoff when the boost goes away it is still pulling really hard and I was wondering if it might be something with the boost gage. I have a Synchronic blowoff valve all are new relatively speaking. I was wondering my turbo is from a 79 930 that was given to me 25+ years ago and I had it rebuilt then but it has sat on the shelf until now I made sure it was clean and spun up nice and was well lubricated before I put it in but I wonder if it has issues. I don't know it held boost real good until about a month ago and it started doing this I am going to look for leaks but it idles real smooth so I would think that would be rough since I should have a vac leak as well as a boost leak when the throttle goes shut I get about 22 in of vac. I do hear a whistle that I don't recall hearing before when at mild cruise and about 0 psi to 10 in vac. Thanks for responding Mark.
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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
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76 Blazer also restored by me
Old 09-01-2011, 05:31 PM
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When I first started driving mine the OEM hard rubber elbow from the AFM to the intake manifold moved off the AFM on the front side and you could not see it. Took me a while to find it and was causing the same symptoms you have.
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Old 09-01-2011, 05:43 PM
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Here are some pics of my setup. I hope it is a leak but I would think that I would have the typical rough idle and a vacuum leak as well.



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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
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Old 09-01-2011, 05:44 PM
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I will check that.

Quote:
Originally Posted by Rick Chip View Post
When I first started driving mine the OEM hard rubber elbow from the AFM to the intake manifold moved off the AFM on the front side and you could not see it. Took me a while to find it and was causing the same symptoms you have.
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76 Blazer also restored by me
Old 09-01-2011, 05:45 PM
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Sure sounds like you have a huge boost leak and should this be the case your car should be running very rich (black smoke coming out on boost). Try shooting some carb cleaner to all connections between the intake manifold to the compressor. When you car starts to change engine speed you have found the location.
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Old 09-01-2011, 05:54 PM
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I have a AFR gauge and it doesn't look like it is going pig rich and there is definately no smoke I have a cab and it would be noticed I am going to try the carb cleaner tomorrow I just think I should have a vacuum leak as well and my vac. is steady at 16 inches at idle (I have had it to 18 but seems to run better at 16).
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Last edited by gsmith660; 09-01-2011 at 06:13 PM..
Old 09-01-2011, 06:08 PM
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It doesn't sound like you have any symptoms at all...still runs strong, power doesn't drop off, AFR's aren't weirding out, the only thing changing is the boost "reading" on the gauge. Would that be a corrrect summation?

A bad boost leak post-turbo can cause rough idle, which gets progressively worse as you build boost (gets pig rich). But that in itself shouldn't cause your boost to drop off to zero...maybe diminish some, but still hold some level of boost while running like crap.

A boost leak pre-turbo...well, with CIS your engine would lean out big time.

Would be nice if your only problem is a faulty boost signal sender (are you using that funky orange rubber covered sending unit which is stock on 930's) or do you have direct vacuum line hookup somewhere that might be loose?
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Old 09-01-2011, 07:02 PM
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Thats what has me baffled AFR's when on it are between 10.5 and 11.5 it's EFI and my boost gauge is mechanical I ran a 1/8" hard line back through the tunnel and tapped in at the same location as the wastegateI will check for leaks around the boost gauge tap off to see if it is an isolated leak at the gauge.
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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
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76 Blazer also restored by me
Old 09-02-2011, 02:50 AM
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Ok I took it out for a spin I checked for vac. leaks with carb cleaner none found and if I get into it but don't open the throttle wide open maybe an 1/8 inch of pedal so just slightly the boost comes up and stays up and a 5th gear pull partial throttle maybe half to three quarters yields 3-5 psi boost all day long AFR's never drop below 10.5 even wide open. Is my 32 year old turbo dieing?
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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
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76 Blazer also restored by me
Old 09-02-2011, 04:49 AM
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If you lost boost abruptly at full throttle your teeth would be in the windshield. Sounds like the engine is just fine and the problem is the boost gage. I would verify this with another gage piped to the intercooler so you can see the spikes and all.
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Old 09-02-2011, 05:08 AM
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I changed the location that the gauge pulled off of to just below the throttle plate instead of off the intake the gauge survived a lengthy vacuum check so the piping is tight it held 25 in for several min. I may get another gauge and try that but don't have the time right now
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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
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76 Blazer also restored by me
Old 09-02-2011, 06:19 AM
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This is a turbo in a non-turbo engine application? Your signature says 76? I'm slightly confused... It happens.

Mark
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Old 09-02-2011, 06:55 AM
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Check your boost gage however keep in mind the symptoms you describe are those of a failing turbo.

Usually there is the more obvious "cloud of blue smoke" but not always - especially if the imbalance is focused on the compressor side. What happens is the rotor comes up to speed (boost) but then starts rubbing the housing and rotor speed drops (no boost).

You are looking at systems anyway so take the compressor inlet hose loose and look for signs of the compressor wheel rubbing the housing. Also grab the end of the shaft, hold it to one side and see if this causes the rotor to contact the housing.
Old 09-02-2011, 07:15 AM
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Quote:
Originally Posted by lucittm View Post
This is a turbo in a non-turbo engine application? Your signature says 76? I'm slightly confused... It happens.

Mark
Look at the pics above it will become clear

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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
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76 Blazer also restored by me
Old 09-02-2011, 07:30 AM
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