Having a leak in your turbocharged car’s intake is like having a small hole in a balloon that you’re trying to inflate. Every turbocharged car seems to have a leak or two, and the only way to find it is to pump it full of air and see where it’s escaping.
For that we need some pipe caps to fit the various intake pipes.
The pipe caps shown above will fit a Subaru EJ20K - it may or may not be the same for an EJ20G. Remember when purchasing caps that you are interested in the outer diameter of the caps, and not the inner diameter that is listed on the tag. Bring a tape measure when you go shopping for caps.
A valve stem was inserted into two of the caps. I used a 5/8 drill bit, but if you don’t have a bit that big, you can carefully make a hole with a small dremel grinding bit until the valve stem fits. If your hole is pretty close to the diameter of the stem base you shouldn’t need any glue or sealant.
I elected to test my intercooler, manifold, and pre-turbo sections independently, which makes it easier to track down leaks as you’re not looking over the whole thing all at once.
Removing the intercooler can be easy or hard, depending on how your hose clamps are oriented. I make sure to allow access to the clamp on the intercooler side of the intercooler/turbo hose, which allows me to undo the two bolts holding the intercooler, slide it out of its hoses by pushing towards the firewall, and it’s free. That only works if you can loosen this clamp:
Out comes the intercooler and the pipe caps are installed on it, using the hoses from the car. With the air compressor’s regulator set to about 20psi, the intercooler was charged up. I used a spray bottle of soapy water and went over the whole intercooler looking for leaks, as they can sometimes have otherwise-invisible cracks in the aluminum. I found one leak on the Y-pipe hose underneath, which would have been impossible to find with the intercooler still in the car. Tightening the clamp up stopped the leak.
Leaks will look like this:
Next up was testing the manifold. You will lose pressure through the intake valves and into the cylinders, past the piston rings and into the crankcase. If that pressure has nowhere to go it can pop out your oil dipstick or worse; push oil out of seals. To prevent that you’ll want to remove the oil filler cap.
Another loss of pressure will be at the bypass valve (aka BPV). This is what that smaller hose on the corner of the intercooler leads to. There is also a (much smaller) hose leading to the BPV from the manifold. This line is what opens the bypass valve to let off excess boost pressure. When the throttle is closed, the manifold experiences vacuum (though the intercooler is still charged up with boost pressure with nowhere to go) and this small manifold air line opens the bypass valve with vacuum to allow that intercooler pressure to escape. Pulling that line off the BPV and stuffing a bolt in the end will seal it sufficiently for our test. Sorry, no pics of this part.
One more potential source of pressure loss will be the idle air valve, pictured below. There is a check valve in the manifold that should prevent any air escaping from this valve - you may have to pop the line off to verify there is no air escaping. I did have a leak here, but to verify other parts of the system were okay I temporarily sealed it off with a wine cork.
UPDATE: the IACV is now sealed. Here’s how it was done.
With the manifold charged up to 20psi, grab the soapy water and spray everything down. Pay special attention to where the manifold seals to the engine, as well as the small lines running to boost gauges and boost controllers. I found more leaks in these small lines, as seen here:
I added new zip ties to these small connections to seal the leaks.
Next up is the pre-turbo side of the intake. Realistically, this side of the intake system should never see boost, so it’s not strictly necessary to check it. Even if it is leaking, the only drawback is that unfiltered air will be drawn in through the leaks and won’t be accounted for by the MAF sensor, causing a very slightly leaner fuel mixture.
Take off the air filter (or air box, if you’ve still got one) and put the big pipe cap in its place.
Add the turbo/intercooler pipe to the turbo and cap the other end of it.
Cap off the bypass valve.
You’ll also need to stop air from flowing through the PCV lines coming from the plastic intake pipe. These are located in front of and behind the area where the manifold passes over the plastic intake pipe.
Here (front):
and here (behind):
For this to be a one-man operation you’ll need to connect these two ports to each other with a length of hose. If you’ve got a friend to help, just get them to stick their fingers over the openings to seal them.
One more area to be capped is the idle air valve line (from the other side this time), again capped with the wine cork. It may be easier to remove the hose if you also remove the bolts holding on the hard line:
Now everything should be capped and ready for testing. As I said, this side shouldn’t really see boost pressure, so 20psi is too much for testing here. 10psi is more appropriate. Soap-spray everything connected to this part of the intake.
I found one hose clamp on the flexible piece of intake piping could use a little tightening, but otherwise the pre-turbo side was leak-free.
Well, that’s it. Time to put everything back together. Make sure you don’t forget to reconnect the PCV lines and the bypass valve manifold line - these spots aren’t easily seen and may be forgotten. Also reinstall your oil cap.
One step I didn’t take, but one that may be wise, would be to reconnect the intercooler and do one last check with everything installed - from the pre-turbo piping all the way to the manifold. This would allow you to test the intercooler connections that were replaced by caps when we tested the manifold and pre-turbo section, ensuring that every connection is leak-free. I just made sure to tighten those clamps and called it a day.
One more test you may want to do (since you’ve already got things disassembled) is to check the bypass valve seals. This can be done by either pressurizing or putting vacuum on the manifold reference port to see if any air leaks past the seals. I used my vacuum pump/brake bleeder to pull vacuum, checking that the valve opens and that the vacuum doesn’t slowly drain out. If you’re using pressure, be sure not to use too much or you can blow out the seals. You won’t see the valve open with pressure.
Well, that’s it. I found fewer leaks than I anticipated, and I haven’t seen any performance increases from fixing the small leaks I found. However, now I know my boost is healthy! This is also a great project for familiarizing yourself with how all of the turbocharging components work together.
See part 2 of this project, sealing the IACV valve leak.
Cost breakdown:
$10 - pipe caps (Home Depot)
Incidental supplies:
valve stems
wine cork (comes free with wine purchase)
pipe clamps
soap
Thanks to Mechie3, author of this guide at NASIOC, for the inspiration!
For that we need some pipe caps to fit the various intake pipes.
The pipe caps shown above will fit a Subaru EJ20K - it may or may not be the same for an EJ20G. Remember when purchasing caps that you are interested in the outer diameter of the caps, and not the inner diameter that is listed on the tag. Bring a tape measure when you go shopping for caps.
A valve stem was inserted into two of the caps. I used a 5/8 drill bit, but if you don’t have a bit that big, you can carefully make a hole with a small dremel grinding bit until the valve stem fits. If your hole is pretty close to the diameter of the stem base you shouldn’t need any glue or sealant.
I elected to test my intercooler, manifold, and pre-turbo sections independently, which makes it easier to track down leaks as you’re not looking over the whole thing all at once.
Removing the intercooler can be easy or hard, depending on how your hose clamps are oriented. I make sure to allow access to the clamp on the intercooler side of the intercooler/turbo hose, which allows me to undo the two bolts holding the intercooler, slide it out of its hoses by pushing towards the firewall, and it’s free. That only works if you can loosen this clamp:
Out comes the intercooler and the pipe caps are installed on it, using the hoses from the car. With the air compressor’s regulator set to about 20psi, the intercooler was charged up. I used a spray bottle of soapy water and went over the whole intercooler looking for leaks, as they can sometimes have otherwise-invisible cracks in the aluminum. I found one leak on the Y-pipe hose underneath, which would have been impossible to find with the intercooler still in the car. Tightening the clamp up stopped the leak.
Leaks will look like this:
Next up was testing the manifold. You will lose pressure through the intake valves and into the cylinders, past the piston rings and into the crankcase. If that pressure has nowhere to go it can pop out your oil dipstick or worse; push oil out of seals. To prevent that you’ll want to remove the oil filler cap.
Another loss of pressure will be at the bypass valve (aka BPV). This is what that smaller hose on the corner of the intercooler leads to. There is also a (much smaller) hose leading to the BPV from the manifold. This line is what opens the bypass valve to let off excess boost pressure. When the throttle is closed, the manifold experiences vacuum (though the intercooler is still charged up with boost pressure with nowhere to go) and this small manifold air line opens the bypass valve with vacuum to allow that intercooler pressure to escape. Pulling that line off the BPV and stuffing a bolt in the end will seal it sufficiently for our test. Sorry, no pics of this part.
One more potential source of pressure loss will be the idle air valve, pictured below. There is a check valve in the manifold that should prevent any air escaping from this valve - you may have to pop the line off to verify there is no air escaping. I did have a leak here, but to verify other parts of the system were okay I temporarily sealed it off with a wine cork.
UPDATE: the IACV is now sealed. Here’s how it was done.
With the manifold charged up to 20psi, grab the soapy water and spray everything down. Pay special attention to where the manifold seals to the engine, as well as the small lines running to boost gauges and boost controllers. I found more leaks in these small lines, as seen here:
I added new zip ties to these small connections to seal the leaks.
Next up is the pre-turbo side of the intake. Realistically, this side of the intake system should never see boost, so it’s not strictly necessary to check it. Even if it is leaking, the only drawback is that unfiltered air will be drawn in through the leaks and won’t be accounted for by the MAF sensor, causing a very slightly leaner fuel mixture.
Take off the air filter (or air box, if you’ve still got one) and put the big pipe cap in its place.
Add the turbo/intercooler pipe to the turbo and cap the other end of it.
Cap off the bypass valve.
You’ll also need to stop air from flowing through the PCV lines coming from the plastic intake pipe. These are located in front of and behind the area where the manifold passes over the plastic intake pipe.
Here (front):
and here (behind):
For this to be a one-man operation you’ll need to connect these two ports to each other with a length of hose. If you’ve got a friend to help, just get them to stick their fingers over the openings to seal them.
One more area to be capped is the idle air valve line (from the other side this time), again capped with the wine cork. It may be easier to remove the hose if you also remove the bolts holding on the hard line:
Now everything should be capped and ready for testing. As I said, this side shouldn’t really see boost pressure, so 20psi is too much for testing here. 10psi is more appropriate. Soap-spray everything connected to this part of the intake.
I found one hose clamp on the flexible piece of intake piping could use a little tightening, but otherwise the pre-turbo side was leak-free.
Well, that’s it. Time to put everything back together. Make sure you don’t forget to reconnect the PCV lines and the bypass valve manifold line - these spots aren’t easily seen and may be forgotten. Also reinstall your oil cap.
One step I didn’t take, but one that may be wise, would be to reconnect the intercooler and do one last check with everything installed - from the pre-turbo piping all the way to the manifold. This would allow you to test the intercooler connections that were replaced by caps when we tested the manifold and pre-turbo section, ensuring that every connection is leak-free. I just made sure to tighten those clamps and called it a day.
One more test you may want to do (since you’ve already got things disassembled) is to check the bypass valve seals. This can be done by either pressurizing or putting vacuum on the manifold reference port to see if any air leaks past the seals. I used my vacuum pump/brake bleeder to pull vacuum, checking that the valve opens and that the vacuum doesn’t slowly drain out. If you’re using pressure, be sure not to use too much or you can blow out the seals. You won’t see the valve open with pressure.
Well, that’s it. I found fewer leaks than I anticipated, and I haven’t seen any performance increases from fixing the small leaks I found. However, now I know my boost is healthy! This is also a great project for familiarizing yourself with how all of the turbocharging components work together.
See part 2 of this project, sealing the IACV valve leak.
Cost breakdown:
$10 - pipe caps (Home Depot)
Incidental supplies:
valve stems
wine cork (comes free with wine purchase)
pipe clamps
soap
Thanks to Mechie3, author of this guide at NASIOC, for the inspiration!
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