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Schizoid Boost

Thought I had it fixed------86 stock save intercooler & K27. Boosts to just under .8 with stock spring only when it feels like it, which is not very often and spends the rest of the time in the .6 range. I would suspect the wastegate, or a leak in the plumbing if it did not vary at all off of the low reading---but I'm stuck on what to look for. Spring failing when it gets hot?
Old 08-12-2010, 02:10 PM
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What boost are you set for? Loosing boost can be anything from a weak spark, to problems from wategate issues to boost leaks
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Old 08-12-2010, 03:32 PM
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So many places the air pressure could leak between the turbo and the intake ports including the bakelite injector blocks which eventually crack, you just have to take everything apart and check it all out.

Most people won't believe it until they see proof of it themselves but all the o-rings on the turbo outlet, charge pipe, intercooler, and throttle body absolutely suck in all respects and leak some air under boost even if they are brand new and seated flush with no gaps anywhere.
Try and find an oring setup on any turbo race car... it does not exsist because they leak.

Ther proof I have is from the 2 UMW K27HFS turbo rebuilds I've had mounted on the car that pumped more oil into the intake than BP could if they tried after only 1000 miles each while occasionally boosting no higher than 1-1.05 bar in first, second, and part of 3rd gear on I-95 entrance ramps.

When those turbos were blowing oil mist into the intake and oil smoke out the exhaust all the intake o-rings were new and seated perfectly and some oil sprayed out of the o-rings at the bottom of the charge pipe where it fit on to the K27 o-ring adaptor and also around the o-ring on the throttle body where the Garretson long neck intercooler fit perfectly over it.
If you have oil mist leaking out than air is going with it.

The best setup is multiple ply reinforced silicone hoses from a hose shop and good worm drive hose clamps or T-bolt hose clamps and not the cheap lightweight crap made for the ricer turbo market.
Old 08-12-2010, 03:50 PM
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Hello, You don't mention if you are using the factory boost gauge for reference.

If you are , Verify its accuracy with a mechanical gauge, Keep us posted.

Regards, Lou/ AERO DYNAMICS
Old 08-12-2010, 04:25 PM
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Thanks for the help---I'm running factory .8 spring. Had gone to a 1.0 but it was over-boosting so went back to stock. I see where the multiple points of potential leakage exist, and I do get a small pool of oil in the intercooler intake pipe---but wouldn't the low boost stay at the diminished level vs the up and down i get on the ,(factory), boost guage?

TG
Old 08-12-2010, 04:50 PM
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You shouldn't lose 25% of your boost from an O-ring leak. That would be one hellofa leak.
What conditions are you noticing this up and down boost level?
What are your ambient weather conditions?
Is your intercooler properly shrouded?
Do you have an aftermarket boost gage?
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Old 08-12-2010, 05:22 PM
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What I/C are you running? I agree with Jfairman that the o-ring adapters at the turbo discharge can leak in a unpredictable fashion, also the Ign. coil brackett (where the rear I/C mount is attached) can flex and cause leaks out of the discharge pipe upper o-ring, There are several possibilities for a boost leak but the fluctuating levels can help narrow the search, How about some pix of your engine?

Lou/ AERO DYNAMICS
Old 08-12-2010, 05:46 PM
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Just returned from another run. Engine oil up to normal temp--2nd gear roll on and wonderful .8 all the way to near red line. Backed off and did it in another 60 seconds or so---same engine oil temp reading---and right back to .6! Stayed there on 3 or 4 more runs before back home. Intercooler is a B&B--has been on car with previously mentioned 1.0 overboost as well as good .8 readings in the past. Do you think 1st boost is somehow deforming oil ring?

I appreciate the responses you guys have been coming up with----

TG
Old 08-12-2010, 06:17 PM
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O-rings are simple enough to change out. You may have one that is pinched or torn. When properly installed, secured and new they work fine. If anything moves around they will be the part to fail. Point is nothing should be moving around and if things are moving around they need attention.

One thing to consider is that once these O-rings fail they do not cure themselves. You get one good run and then lose 0.2bar on all susequent runs. Let the car sit and the process repeats. If the O-ring has failed the process should not repeat. If something is loose and the O-ring is sliding up and down it should not need to sit overnight to re-seat itself but would pop every time boost comes on and re-seat when off-boost. Yes strange things can happen but this is not typical.
I would change out the O-rings and see if you spot anything.
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Old 08-13-2010, 05:51 AM
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Quote:
Originally Posted by RarlyL8 View Post
.....Is your intercooler properly shrouded?....
not to hijack, but maybe related:
Brian how does the shrouding on the i/c figure into a boost lost?
I'm curious because I'm in the planning stages of an i/c for my 77 (under the stock tail).
thanks - Bill K
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Last edited by bkreigsr; 08-13-2010 at 06:45 AM..
Old 08-13-2010, 06:33 AM
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Also curious to that question----my shroud consists of the B&B aluminum "plate" that came with the I/C to replace stock untit. I do not have any kind of compression gasket between this plate and the engine lid.
To answer one of your questions Brian, conditions for driving have been 72 to 85 degrees, 15% humidity on each drive. Temp difference is due to starting in Denver and travelling up into the foothills where it cools quickly.

TG
Old 08-13-2010, 06:42 AM
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I asked that question as it relates to ambient weather conditions.
Our weather here in AL has been near 100F and high humidity for 67 days now. If your intercooler is not shrouded in these conditions it will heat soak very quickly, to the point of drastically reducing power and affecting boost.

Your environment is much better so this should not be a problem. The altitude may be a consideration though. If you are running rich under boost the altitude will make the situation worse. Pig rich will also affect boost levels and power. Are your AFR's in good shape?
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Old 08-13-2010, 07:44 AM
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AFR's are good now but had a big problem with them until Bosch brain was replaced. In that process rest of fuel mgt stuff really had a good going through- (this was the episode where multiple trips to emissions station resulted in a dropped transmission when the tester forgot about that pesky third pedal). A little more info--previous to Bosch problem where duty cycle was not operating--I had removed the cat and .8 was working great. Stock cat and exhaust are back on the car and at the same time this boost problem has appeared.

TG
Old 08-13-2010, 08:25 AM
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Quote:
Originally Posted by j8tg View Post
Engine oil up to normal temp--2nd gear roll on and wonderful .8 all the way to near red line. Backed off and did it in another 60 seconds or so---same engine oil temp reading---and right back to .6! Stayed there on 3 or 4 more runs before back home.
Just for fun, try to duplicate the above test, but roll the window down and listen for the waste gate as you roll into boost. Can you hear the wastegate open at 0.8? Then at 0.6? Is the wastegate stock?

When it consistantly won't boost high, stop the car and put your hand by the wastegate outlet and see if there is leakage from a stuck WG valve.

Does the car run differently each time, or about the same? What are you using to measure boost? The max boost pressure could be fine, but the gauge or sensor could be wacky.
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Old 08-13-2010, 12:12 PM
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"Stock cat and exhaust are back on the car and at the same time this boost problem has appeared."

Now that you said that it could be the cat is old, used, and somewhat clogged up and making exhaust backpressure after the turbo to the point it is lowering boost spool up.

That probably doesn't explain the boost lowering after one run unless all the heat buildup in the cat is causing crap or something inside it to expand and choking the exhaust flow more and increasing backpressure more that second time you floored it and lowering the boost.

Take the thing off after you pass emissions with it if possible and try again.
Old 08-13-2010, 12:38 PM
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Using stock gauge. No muffler on the wastegate so I can hear it open at both .6 and .8. Seat of the pants test and expression on wife's face,(easily amused),is performance is better with increased reading on gauge.

Going after wastegate this weekend. Will disassemble, install 1.0 bar spring and see what effect that has. Also checking and re-seating turbo to intercooler plumbing. Will report results. Cat only has 15k on it so should be good.

Thanks,

TG
Old 08-13-2010, 01:09 PM
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OFF THE BOOST PIPE NOW...
 
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Get a good aftermarket boost gauge as mentioned. You're guessing until you do.

Quote:
Originally Posted by j8tg View Post
Using stock gauge.

TG
Old 08-13-2010, 01:53 PM
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What does your ass tell you ?? Seat of the pants is a better indicator to an oem boost gauge. I can feel a 5 hp diferance in my DD when i remove the air filter, you should be able to feel the diff in 8 1/2 pounds of boost from 11 1/2 pounds.
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Old 08-13-2010, 02:37 PM
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