Help tracing a boost leak?

okay xpost from my build thread for help. Been tracking a boost leak on this for a while now.

I found the source of one boost leak. The power steering idle up valve was broken off the pump and left dangling by some past mechanic... where it eventually snapped off the connector to the manifold. So I was able to fix that at least.

After fixing that, I found more issues though.

Firstly, and maybe I just don't understand how this is supposed to work, but...



Any pressure I put into the intake is lost unless I plug this hose up here. This recirculates back into the intake after the MAF but before the turbo. If I plug that hose, I can build pressure with the boost leak tester. I'm not sure what this is supposed to be, but I'm relatively sure it isn't supposed to be dumping boost out of it.


It seems to be connected to the AC idle up valve and then runs back into the mess under the manifold somewhere.


When I pinch this hose I don't hear/feel any hissing, so I presume the boost is escaping from somewhere else.

Any ideas? Or is this 'normal' with the motor not in operation?


This device, whatever it is, is shown to be leaking also via the smoke test. Anyone know what it is or where I'd get a replacement?

I can also hear a hissing sound coming from the EGR valve as I build boost. Is that normal or does it need to be replaced? can it even be replaced? seems the part is discontinued and ebay shows nothing. if the EGR isn't really needed I might be tempted to pull it off, fill it up with JB weld, and stick it back on, rather than getting a block off plate, in hopes the smog inspector is none the wiser. Is there any reason not to do this?

Last bit of bad news is the gas tank is leaking somewhere up near the top, and I can't find any info on replacement gaskets I'd need to take it down and look at it.

This device, whatever it is, is shown to be leaking also via the smoke test. Anyone know what it is or where I'd get a replacement?

This is your EGR valve vacuum modulator (Toyota P/N 25690-88380). This particular part was only available in the US on ST165s & ST185s and SW20L (MR2s) with the 3S-GTE engine - so getting an identical new part may be problematic.

When I pinch this hose I don't hear/feel any hissing, so I presume the boost is escaping from somewhere else.

The diaphragm valve to the right is your A/C idle-up diaphragm assembly, of which the smaller vacuum lines come from your A/C idle-up VSV (vacuum switching valve) located on the upper firewall on the far passenger side of the engine compartment. This diaphragm valve is Toyota Part No. 88606-20210, and is only used on 3S-GTE engines due to the fact of how it works. Most normally aspirated engines do not use this diaphragm valve in the A/C idle-up loop, as the intake is always under vacuum, so when the A/C VSV switches when the A/C is on, air only goes into the intake manifold to help increase engine idle (and top end speed for that matter) just a little. In a turbo, the diaphragm valve needs to close when the intake manifold goes to positive pressure (under booster). If your diaphragm valve doesn't close when the intake manifold is under pressure, it would be a source of a boost leak. With the engine not running, you could blow into one of the larger hoses connected to this diaphragm valve, and then blow into the other larger hose. You should not be able to push air through either hose when blowing into them. If one allows free air flow, your A/C diaphragm valve is defective. In this case, disconnect both larger hoses from this diaphragm valve, plug both, and try to determine if you still have a boost leak.

PS: You might want to try blowing air into each port of the A/C idle-up VSV on the passenger firewall as well with the engine not running. It also should not pass air in either of the ports with the engine not running. If it does pass air in either direction, that could also neutralize the A/C diaphragm valve function. So best to check both valves on this idle-up circuit.

93celicaconv":2jce1eua said:

The diaphragm valve to the right is your A/C idle-up diaphragm assembly, of which the smaller vacuum lines come from your A/C idle-up VSV (vacuum switching valve) located on the upper firewall on the far passenger side of the engine compartment. This diaphragm valve is Toyota Part No. 88606-20210, and is only used on 3S-GTE engines due to the fact of how it works. Most normally aspirated engines do not use this diaphragm valve in the A/C idle-up loop, as the intake is always under vacuum, so when the A/C VSV switches when the A/C is on, air only goes into the intake manifold to help increase engine idle (and top end speed for that matter) just a little. In a turbo, the diaphragm valve needs to close when the intake manifold goes to positive pressure (under booster). If your diaphragm valve doesn't close when the intake manifold is under pressure, it would be a source of a boost leak. With the engine not running, you could blow into one of the larger hoses connected to this diaphragm valve, and then blow into the other larger hose. You should not be able to push air through either hose when blowing into them. If one allows free air flow, your A/C diaphragm valve is defective. In this case, disconnect both larger hoses from this diaphragm valve, plug both, and try to determine if you still have a boost leak.

PS: You might want to try blowing air into each port of the A/C idle-up VSV on the passenger firewall as well with the engine not running. It also should not pass air in either of the ports with the engine not running. If it does pass air in either direction, that could also neutralize the A/C diaphragm valve function. So best to check both valves on this idle-up circuit.

Click to expand...

Well the good news is that system appears to be working as intended according to this page of the manual.

The problem is that there is air pressure coming from *both* large hoses in roughly equal amounts. The "air cleaner" side of the circuit is getting boost on it (assuming I plug the port where it's supposed to dump back into the intake) Again that's this port here. I need to plug it or the manifold loses pressure through this port just as fast as I can pump it in at the turbo. The only thing I can find in the manual that looks possibly related is here in the FI section page 3 (colors added):


So... what is the thing circled in red, and why does it go to the intake and TC VSV? Could that be the source of the leak?

I'm going from memory since my car is a lot different now, but doesn't that go to the idle air control valve?

It would make sense there might be a boost leak there when the car is off since the idle air control valve is probably slightly open. I don't know if the ECU commands the IAC to be closed under boost.

Your best bet might be to plug the IAC intake tube and check for Boost leaks then.

Yes, the item circled in red in the IAC (Idle Air Control) valve mounted on the throttle body. And yes, when the MAP (labeled as the Turbocharging Pressure Sensor in this diagram) senses positive pressure in the intake manifold, the ECU closes the IAC valve.

Magroo is on a good track regarding troubleshooting. By removing the vacuum tube on the IAC valve and plugging it (only reason to plug the end of the vacuum tube is to prevent dirt/dust from being pulled into the tube and getting into the intake air duct downstream of the air filter), you can then run the engine and put your finger over the IAC valve tube connection to ensure idle speed drops when covered (it should). Then you could boost the engine and see if air comes out of the IAC valve tube connection (it should not). If air comes out through the IAC valve tube connection, your IAC valve is not moving properly or your ECU has failed and is not controlling the IAC valve properly. Alternatively, you can plug the IAC valve vacuum tube connection and then check if you have boost leaks anywhere. If you don't have any leaks in this case, pretty sure you will have isolated the problem to one of 2 causes. Then you can diagnose the ECU output to the IAC valve to see if the ECU is controlling properly or not - if the ECU is controlling properly, time to remove the IAC valve and clean it up to get it to operate again - if the ECU is not controlling properly, time to check your ECU - potential capacitor failures.

Thank you for the help, both. I wasn't sure if it was supposed to be open when off, but if it is then what I'm seeing could just be normal. I'll need to put it back together and give it a few more running tests.

P.S. I found an allegedly compatible EGR vacuum modulator to replace my leaky one, but it has an extra port on one side. I'm guessing I can just plug one of them, but does it matter which? or does it matter which side connects to the manifold? Thanks

Adding to what 93celicaconv said, don't forget the wiring between the ECU and IACV. Dirty connectors and broken wires love to cause problems that aren't immediately obvious.