Boost leaks are a very common problem in turbo cars, especially ones with 10+ year old rubber pipes. The Supra's long intercooler piping path makes it fairly hard to find the leak and can be a daunting task. In this post I will try to describe all my experience diagnosing and fixing boost leaks. I've become quite good at it as all the local Supra owners bring their car to me when they have problems
Probably the biggest symptom is hitting FCO (fuel cut off) prematurely. Attempting to boost down the road only to have the car start jerking and the engine light come on is a huge hint that there is a problem. With an aftermarket boost gauge you should be able to hit roughly 11-12psi (near sea level) on the stock CT-26. Larger turbos will hit FCO sooner because FCO is based on volume, not pressure.
Another big clue that you have a boost leak is black smoke coming out of the tailpipe. This is caused by an excessively rich condition. The AFM is seeing XXX amount of air pass into the intake but somewhere in between the turbo and the engine there is a leak causing some of the air to be lost. Because the ECU does not know this, it is still adding fuel, this causes an overly rich mixture which is expelled out as black smoke.
Boost leaks can also cause bad gas mileage, lack of power, a whistling sound, a squealing/screaming sound, unstable boost and boost spikes. Depending on where the leak is you may have only one of these symptoms or you might have all of them.
The most common (99/100) cases the boost leak is caused by one of two hoses. Just feeling the hoses is not always going to show you a leak. Sometimes you must remove the hose and try flexing it around to find a crack.
The first hose that comes off the bottom of the turbo and heads towards the intercooler is probably the most common. It is the first to get boost and as such it receives the most. It also is close to the heat of the turbo and the exhaust manifold so it gets hard and cracks more quickly then any other hose. It's also the hardest to get to so it rarely gets replaced by previous owners. If you're trying to remove it, the easiest way is to remove the accordian hose and the hose clamp connecting to the turbo. Then get under the car and remove the hose clamp there and pull the hose down.
The second most common is the short section of hose that connects the 3000 pipe to the throttle body. This hose gets a higher peak pressure then any other hose, but only for a split second. As the throttle body slams shut the boost is quick to attempt to find a place to go. The throttle body is offering no escape so the boost quickly backs up. This boost slamming expands the 3000 pipe hose and as such it gets worn out quickly. I actually blew apart two of these hoses before going with an aftermarket piece. Because it has a ribbed section for the hose clamps you often cannot see or feel the crack in the hose until you remove it.
Now if you have the squealing/screaming sound under boost you can skip all the below suggestions. Your boost leak is probably on the exhaust side. As odd as that sounds, leaking exhaust gases coming from the engine to the turbo is considered a boost leak. The reason is any gases that escape the engine without entering the turbo is just lost power, that heat and energy could have pushed the turbo to spin more. The only way to fix this is to pull the turbo *big sigh*. This job really sucks, so anything you want to do on that side you will want to progress quickly. I.E. if you want to install an EGT gauge, a new turbo, change your oil easy, anything you want to do on that side in the near future. You will be removing the turbo and exhaust manifold to fix this problem.
If neither of those hoses are the culprit you can try to take the easy way out and simply tighten all your hose clamps. The stock hose clamps often loosen over time and in general cannot hold very strongly. A set of t-bolt (aircraft style) hose clamps will solve the stock hose clamp issue. You can pick them up from Napa for ~$4 each See a pic. The hoses need to be clamped on fairly tight in order to keep boost from leaking. I generally try to tighten them using a screwdriver and turning almost as hard as I can. If the hose clamp slips it's tooth then it needs to be replaced.
Also a visual inspection at this point could save you time later on. Inspect the intercooler carefully as they sometimes get hit with rocks and have holes or cracks in them. If you see a suspect area, put some watery soap on it and look for bubbles while the car is running. Another visual culprit is the stock plastic pipe on top of the engine bay just above and in between the AFM and strut tower. It's not uncommon for them to crack for whatever reason. You may have to remove it and hold it up to light to see.
If you still have a suspected boost leak there are a couple ways you can try to find it. The most popular is to spray some sort of liquid or gas (or soap) near all the intake hoses and look for a change in the car. The car might idle up, you might see bubbles in the soap, etc. I don't care for this method because it will only show you large leaks. It is possible for a crack in a hose to only show itself under pressure, just checking the car while it's just idling won't show you that particular leak. A better alternative is to make your own boost leak tester. You will need a source of compressed air with a regulator, a PVC end to fit in your accordian hose or first turbo intercooler pipe, and a tire valve stem (you can get used ones for free at most tire shops). Drill a hole in the PVC just big enough to pull the valve stem through. Attach the PVC to your accordian hose or first turbo intercooler pipe and pressurize the system to about 20psi. If the system holds the 20psi and doesn't drop off after a few minutes then congratulations you have no leaks. Otherwise try listening around for leaks or use some soapy water on all connections and rubber.
While it is a pain, if none of the above methods show the boost leak, I prefer to remove every hose and visually inspect them all. Any hose with cracks, holes, etc needs to be replaced. Try flexing the hose slightly in different directions, particularly around the hose clamp area and you might find a crack that was invisible before. Check and double check every hose, you certainly don't want to take all these hoses off again.
If you still can't find the leak it is possible your leak is a gasket on the intake plenum or manifold. This requires removal of the intake side, so you might as well have some new gaskets ready - throttle body to plenum gasket, plenum to manifold gasket, manifold to head gasket, ISC gasket, cold start injector gasket and EGR to plenum gasket.
At this point you've probably spent a few hours working on the car, if you still have not found the leak then you need to repeat the above procedures or consider that your problem may actually not be a boost leak. Check your engine error codes, if you get any code other then 34 then you could have other problems that are not boost leak related.
In some cases you'll find a rubber hose that is split near where the hose clamp normally sits as illustrated above. In some cases, if the rubber is still somewhat soft you can trim off the cracked hose and use the newly shortened hose. Try loosening some of the other hoses to give yourself some slack, spread the lack of space across a few hoses. Stock piping has quite a bit of play, you can usually trim an inch or two off and still make everything fit.
In the case of the 3000->TB hose you will have to purchase a new hose. The stock hose is ~$30 last time I checked and will blow out in a few years if you're running more then 10psi. What I did was run to a local rubber supply house (I didn't even know it existed until I asked around) and picked up some 5 ply hose. I was able to get a section long enough to replace the stocker for $5. You could also buy silicone hoses off the internet if you can't find anyplace local. You'll only save $5-10 that way though, but at least it won't blow out on you.
A solution for hoses cracked in the middle would be to consider putting a metal pipe in there. If you can cut out the crack and put in a short straight piece of metal, clamp the hoses onto the pipe, you should be good to go. You'd be surprised what you can do with metal pipes and trimming a hose just right.
Unfortunately in some cases you simply can't repair it and you're left with two expensive options. Either buy the hose from Toyota or buy a hardpipe kit. Typically the hoses run $50-150 from Toyota which is 1/3 the cost of a hardpipe kit. I much prefer the hardpipe kit option as it pretty much insures you will not have a boost leak problem again. You could try some local junkyards or some online junkyard searchs, but chances are the hose that cracked or blew apart on you will not last long if it's already 10+ years old.