Boost leaks...
- Thread starter IHI-RHC7
- Start date
Well, I think I figured it out.
Here's the port on the back of the throttle body that is connected to the pcv outlet. Yes, It's the big one in the upper left hand corner.
So our throttle bodies have a capped port on the top of them.
Rumor had it that under this cap was a screw which one could adjust the idle manually if the ISC couldn't do it for some reason. Well, I thought I'd see just what was under it.
Look at that, nothing at all. It's just a casting hole that was plugged. One 89 TB down the drain...:3d_frown:
So I thought I'd shine a little light on the subject to find out how much boost this port was losing, and it turns out that jetjock and I were both right.
Behind the big port on the back face of the throttle body, there is an orifice, but I never would have seen it unless i removed that cap on top of the tb.
In the back of that black hole is a smaller hole. I'd say it's still at least a 2mm hole, though. So it's hardly a .020-.030" hole. It's closer to .100"
Less of a big deal, but still a factory boost leak, nonetheless.
I tried to shine some light through the hole to highlight it, sorry for the fuzzy pics.
So, there it is, Smaller than .25 Inch, but still a formidible boost leak, in every GTE TB out there. The "metered orifice" is precisely the kind of boost leak that we try to hunt down and destroy every day. What's interesting, is because it leaks boost back in front of the turbo, behind the afm, this leak does not cause us to run rich, it just lowers the boost threshold of the turbo. When running anything less that full steam on the turbo, you will not notice this leak. But when the turbo is "running out of breath" capping this port will give the turbo at least a few hundred more rpm before it does. In our case, that means that @peak power, we used to have 10 PSI, which made 305rwhp. Now @5500rpm, we have 17psi. That should turn into one huge powerloss from that tiny hole. Again, had we been running 10 psi, we wouldn't have noticed anything. Had we been runing a bigger turbo, we wouldn't have noticed anything until maybe 20 psi, 30 psi, it's hard to say.
But with a stock CT, running more than 15 psi, You should do this.
Here's the port on the back of the throttle body that is connected to the pcv outlet. Yes, It's the big one in the upper left hand corner.
So our throttle bodies have a capped port on the top of them.
Rumor had it that under this cap was a screw which one could adjust the idle manually if the ISC couldn't do it for some reason. Well, I thought I'd see just what was under it.
Look at that, nothing at all. It's just a casting hole that was plugged. One 89 TB down the drain...:3d_frown:
So I thought I'd shine a little light on the subject to find out how much boost this port was losing, and it turns out that jetjock and I were both right.
Behind the big port on the back face of the throttle body, there is an orifice, but I never would have seen it unless i removed that cap on top of the tb.
In the back of that black hole is a smaller hole. I'd say it's still at least a 2mm hole, though. So it's hardly a .020-.030" hole. It's closer to .100"
Less of a big deal, but still a factory boost leak, nonetheless.
I tried to shine some light through the hole to highlight it, sorry for the fuzzy pics.
So, there it is, Smaller than .25 Inch, but still a formidible boost leak, in every GTE TB out there. The "metered orifice" is precisely the kind of boost leak that we try to hunt down and destroy every day. What's interesting, is because it leaks boost back in front of the turbo, behind the afm, this leak does not cause us to run rich, it just lowers the boost threshold of the turbo. When running anything less that full steam on the turbo, you will not notice this leak. But when the turbo is "running out of breath" capping this port will give the turbo at least a few hundred more rpm before it does. In our case, that means that @peak power, we used to have 10 PSI, which made 305rwhp. Now @5500rpm, we have 17psi. That should turn into one huge powerloss from that tiny hole. Again, had we been running 10 psi, we wouldn't have noticed anything. Had we been runing a bigger turbo, we wouldn't have noticed anything until maybe 20 psi, 30 psi, it's hard to say.
But with a stock CT, running more than 15 psi, You should do this.
Another case of my PCV plumbing working out right by accident 
!
Very interesting Jake thanks for taking the time to investigate this.
You might be able to get a small freeze plug or machine one up from bar stok to replug the hole.
!Very interesting Jake thanks for taking the time to investigate this.
You might be able to get a small freeze plug or machine one up from bar stok to replug the hole.
Wow, think about what a supercar the mkIII would have been back in the day with a beefier exhaust and a MHG and an ok oilsystem with thermostat and a simple oneway valve to prevent this leak, FFI and e few other things.
Stock boost level 12-14 psi. And NOT sub 10:1 a/f...
Properly engeneerd it would not have cost much more but it would have been a totally different car...a KILLER for its time. And to this day.
Well enough daydreaming :icon_bigg
Nice find, thought about going to our local saab dealer and getting a one-way valve thats stock on 9-5, that should do the trick.
Do not like the idea to block the TB port because mostly when driving I`m not boosting. Comments plz.
suprafredde
Stock boost level 12-14 psi. And NOT sub 10:1 a/f...
Properly engeneerd it would not have cost much more but it would have been a totally different car...a KILLER for its time. And to this day.
Well enough daydreaming :icon_bigg
Nice find, thought about going to our local saab dealer and getting a one-way valve thats stock on 9-5, that should do the trick.
Do not like the idea to block the TB port because mostly when driving I`m not boosting. Comments plz.
suprafredde
Shorting the block still sets the ISC to a high idle on later model years, I've done this on my '89 and a few others for one reason or another over the years.
Thanks for the research, too - I'm likely going to plug mine just to see what happens.
Thanks for the research, too - I'm likely going to plug mine just to see what happens.
Grim, when you cap it, you'll know if you've altered anything because the idle should drop 200 rpm or so. It stayed low until I pulled the EFI fuse, then it corrected within 5 minutes or so.
Since you're running an upgraded turbo, you might not notice a difference, but I did notice much better fifth gear response on the freeway.
Jetjock, I believe the difference between the years is the idle screw, but I'm not sure how it plays into capping the port. The pre 89 TB may have a secondary port in the throttle throat, and the screw adjusts that flow, or the screw may adjust the flow through the pcv port. I'm not sure, since the 87 tb is back on the car now. (We had previously swapped my port matched intake manifold with throttle body from my 89, so the factory 87 TB is now back on the car)
It would be nice to have some in depth insight to the inner workings of the pre 89 TB, but that will have to wait for another day. Just out of curiousity, why did you cap off your port, jetjock? I hope you have one hell of a seperator
Since you're running an upgraded turbo, you might not notice a difference, but I did notice much better fifth gear response on the freeway.
Jetjock, I believe the difference between the years is the idle screw, but I'm not sure how it plays into capping the port. The pre 89 TB may have a secondary port in the throttle throat, and the screw adjusts that flow, or the screw may adjust the flow through the pcv port. I'm not sure, since the 87 tb is back on the car now. (We had previously swapped my port matched intake manifold with throttle body from my 89, so the factory 87 TB is now back on the car)
It would be nice to have some in depth insight to the inner workings of the pre 89 TB, but that will have to wait for another day. Just out of curiousity, why did you cap off your port, jetjock? I hope you have one hell of a seperator
This is a very interesting thread as ive learned some from it. I like the idea of putting a check valve there and im also gonna do the leak test.
Great Thread guys :bigthumb:
Great Thread guys :bigthumb:
is this the problem i have been having i recently boosted my ct26 to 15psi and it spikes 15 then settles at 10-12 with a EBC after it goes over 4000rpms i have shimmed the wastegate and i dont see any leaks on the intercooler pipes is this what the problem could be?
I was wonder about this issue for a while but I could never find any threads on it. However I pretty much came to the same conclusion. The peice that's pressed into the throttle body should be a one way valve whether it actually is or not. (I think it was at one point, but it just doesn't work anymore...) The problem with this is that it can't be replaced like a regular pcv VALVE because it's pretty much part of the TB.
Yes it is a boost leak. Even at 6 psi, no pressue should be going from your intake manifold BACK into your turbo, that makes so sense at all.
BUT ANYWAY... I fixed the problem by putting an aftermarket pcv valve in line between the TB and the pipe that connects to the valve cover ports. This fixes the problem but you have to make sure you get a high quality valve. If you get a Fram valve from Kragen it will not do you any good because those are not meant for turbo applications, the valve will open both ways if enough pressure is put on it. What I did was order a pcv valve for a 94 supra that had a 2jzgte. These valves work really good and they're only like 6 bones. You might need to be a little creative to put it in line if you want to keep the stock pcv chrome pipe, but it's possible...I did it. :naughty:
Yes it is a boost leak. Even at 6 psi, no pressue should be going from your intake manifold BACK into your turbo, that makes so sense at all.
BUT ANYWAY... I fixed the problem by putting an aftermarket pcv valve in line between the TB and the pipe that connects to the valve cover ports. This fixes the problem but you have to make sure you get a high quality valve. If you get a Fram valve from Kragen it will not do you any good because those are not meant for turbo applications, the valve will open both ways if enough pressure is put on it. What I did was order a pcv valve for a 94 supra that had a 2jzgte. These valves work really good and they're only like 6 bones. You might need to be a little creative to put it in line if you want to keep the stock pcv chrome pipe, but it's possible...I did it. :naughty:
An old thread. I'll point out again the PCV port on the TB isn't the same size all the way through as it appears on the outside. There is an orifice internal to the TB. It's really a non-issue as far as boost goes. Not to mention it's there for a reason and shouldn't be prevented from flowing in that direction.