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Low boost on 92 740 turbo

VB242 said:
Is the waste gate plumbed to the nipple on the turbo outlet?
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yes. and the CBV is running to the T under the intake manifold.

the wastegate in the turbine housing is sealing when the actuator is NOT engaged - at rest, basically.. I verified when I took the turbo out yesterday. so i wondered if the actuator could be faulty; but since it's just a mechanical part, I don't know how it would fail in such a way as to always leave the wastegate open once the car is running.
 
When your engine is cold. Remove the intake hose to the compressor and check the turbo shaft for play. It may be you have a worn out turbo as suggested. You are looking for very little play. If it moves around enough to impact the housing. It's done. You should have a small amount of side play and a small amount of play lengthwise.
 
dl242gt said:
When your engine is cold. Remove the intake hose to the compressor and check the turbo shaft for play. It may be you have a worn out turbo as suggested. You are looking for very little play. If it moves around enough to impact the housing. It's done. You should have a small amount of side play and a small amount of play lengthwise.
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I checked the play on the bench yesterday and both the turbine and comp wheels appeared fine. very little play and both spun freely.
 
confirmed that the compressor shaft doesn't have play.

would there be any benefit to hooking up a boost controller and trying to boost the PSI? I.e, if the boost controller is adding more boost but turbo is still loosing boost, might it confirm a fault wastegate actuator or CBV or boost leak? or is that just asking for parts to break?
 
Pull the CBV and make sure it isn't torn.

I guess you could wire the wastegate arm shut and go for a drive and see what happens.

We have a new high pressure smoke machine at the shop for testing for intake/exhaust leaks. They can be tricky to find.
 
I'm thinking it's the wastegate actuator. CBV spring and rubber looks good.

I plumbed in an MBC and tried turning up the boost, but couldn't get above 5lbs. however, disconnecting the wastegate actuator hose - disabling it - results in overboosting, 10, 15lbs. so i think the actuator is somehow dumping boost around 5lbs. i think I can maybe, maybe hear it opening just as I hit boost, but it's hard to say.

it could still be a boost leak? although if the car will boost to 15lbs, I suppose a leak isn't present.
 
Well, you know from the factory these cars are low boost cars. The stock boost on my B21FT was only 6.5psi and even when you put the intercooler kit on it with turbo+ the main part of the available boost is 8.5psi. When turbo + is enabled it goes to 10.5psi.

One thing I wouldn't do is to plumb the boost gauge in with the hose that goes to the actuator. I would connect the boost gauge to another port on the intake plenum. A good manual boost controller should be able to adjust to raise the boost quite high. Although the control the mbc has is determined by the spring that is inside it. Just like the spring in the wastegate actuator.
 
dl242gt said:
One thing I wouldn't do is to plumb the boost gauge in with the hose that goes to the actuator. I would connect the boost gauge to another port on the intake plenum. A good manual boost controller should be able to adjust to raise the boost quite high. Although the control the mbc has is determined by the spring that is inside it. Just like the spring in the wastegate actuator.
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the boost gauge is plumbed into the stock 'turbo' line under the cluster.

I have the mbc plumbed between the wastegate actuator and the outlet on the compressor. i thought that was the only way to get the proper pressure signal to the actuator?

regardless, like I said, it appears that turning up the mbc didn't break the 5lbs boost threshold. i'll play around more tomorrow.
 
mschultz373 said:
the boost gauge is plumbed into the stock 'turbo' line under the cluster.

I have the mbc plumbed between the wastegate actuator and the outlet on the compressor. i thought that was the only way to get the proper pressure signal to the actuator?

regardless, like I said, it appears that turning up the mbc didn't break the 5lbs boost threshold. i'll play around more tomorrow.
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Have you tried connecting the waste gate directly to the turbo outlet? How is the MBC connected? The waste gate should come off the port closest to the adjustment knob and the turbo outlet to the bottom of the MBC. I can almost guarantee your problem lies with the MBC, either it's defective or plumbed incorrectly.
 
yeah, the wastegate was directly connected to compressor housing outlet prior to MBC install. then I was getting ~4lbs of boost.

with MBC installed between wastegate actuator and outlet, despite increasing the boost on the MBC, I still was topping out around ~4lbs.

I then tried T-ing the MBC pressure line off the CBV line and put a nipple on the comp housing outlet. there I topped out at ~5-6lbs of boost despite MBC settings.

then, simply removing any hose to the wastegate actuator I top out around 10lbs of boost. that's with the nipple on the compressor housing. last week, when I was running 'stock' with a line from outlet to actuator, removing that line from the actuator would result in 15lbs or so of boost. I didn't push it beyond that.

here's a picture of the plumbing currently - MBC T'd off CBV line, nipple on comp housing outlet, line to actuator disengaged in photo:
<a href="https://ibb.co/72js6DG"><img src="https://i.ibb.co/xJXxRbM/DSC-0032-1.jpg" alt="DSC-0032-1" border="0"></a>

so the actuator works, but for whatever reason, I'm unable to 'set' the pressure in the lines correctly to get it to actuate only at higher boost levels.

the hallman unit is new and just arrived friday. could be faulty but doubt it would be D.O.A.

as an aside - on one of my pulls, the oil dipstick came out of the tube. that was running the nipple on compressor outlet and T'ing the MBC off the CBV line. my turbo is also leaking oil. unsure if those would factor into this low boost issue.
 
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The CBV line should be coming off the manifold, this is not the place to be hooking up your waste gate. This is after the throttle plate, it should be plumbed off the compressor, before the throttle plate. Where are you located? Maybe someone could stop by and see what you've got going on.
 
the CBV line runs to a T under the intake manifold which splits for the CBV and to the egr vac regulator by the ABS/OBD boxes. is that wrong? if so, that may be the problem.

plumbing MBC between compressor outlet and wastegate will NOT raise the boost. so the MBC could be faulty. or there's some other issue, which I lean towards since I'm getting 3lbs less boost than stock level running no MBC between comp outlet and actuator.
 
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well with everything plumbed correctly, the MBC is still not raising boost, BUT disabling the wastegate actuator results in overboost. so my conclusion is that the actuator is faulty and needs replaced. yay or neigh?
 
mschultz373 said:
well with everything plumbed correctly, the MBC is still not raising boost, BUT disabling the wastegate actuator results in overboost. so my conclusion is that the actuator is faulty and needs replaced. yay or neigh?
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A spring inside the wg actuator holds the wg closed, air pressure on the diaphragm inside the wg from the turbo pushes it open and lets the exhaust bypass the turbine. If the spring in the wg is holding it closed with no air pressure then it seems as if the wg is working properly.
 
I'll look into the MBC being faulty then. seems the only logical thing? could be cat or exhaust restriction. or my paper air filter... thanks yall.
 
yeah pretty fun!

my buddy and I went out for a few rips and I think I got a few backfires pulling fairly hard out of the hole. my idle is lumpy as hell so i'm thinking the car is running rich and so the backfire makes sense. but should I concerned about damage to anything? it really seemed to happen only out of the hole in 1st on hard pulls.
 
mschultz373 said:
yeah pretty fun!

my buddy and I went out for a few rips and I think I got a few backfires pulling fairly hard out of the hole. my idle is lumpy as hell so i'm thinking the car is running rich and so the backfire makes sense. but should I concerned about damage to anything? it really seemed to happen only out of the hole in 1st on hard pulls.
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Check the oil and make sure it's not milky looking. Look for evidence of coolant being forced out of the overflow reservoir. These would indicate a blown head gasket.
 
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