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ST185 no boost, runs fine

PostPosted: Tue Aug 02, 2022 10:29 pm
by gtapexmickey
So I have a 1989 narrow body GT4 that about a year ago suddenly wouldn't boost. I felt no specific change or anything happen. Just would not increase boost pressure. Gauge is working and reads vacuum fine. But absolutely no boost pressure. I jumped the gun on a kinegawa turbo because I wanted to upgrade anyway. Didn't solve the problem. I thoughtaybe the waste gate actuator or possibly the T-VSV, so I got the forge one that bypasses the vsv. Still no boost at all. I smoke tested to find a possible leak but I don't see anything. I spoke to a few people and they said it may have skipped a tooth in timing, but I'm not sure. Haven't done a compression test or popped the timing cover yet because it's a bit tight there. figured I'd make a post to see if anyone has any insight before I did.

Re: ST185 no boost, runs fine

PostPosted: Tue Aug 02, 2022 10:56 pm
by underscore
Do you have an aftermarket guage or are you using the stock one? It sounds like the sending unit or the guage itself isn't working properly if you didn't actually feel a difference when driving. Get a mechanical guage on the engine or hook an air line to the pressure sensor with the ignition on and gently apply some positive pressure and see if it starts reading anything.

Re: ST185 no boost, runs fine

PostPosted: Tue Aug 02, 2022 11:10 pm
by gtapexmickey
underscore wrote:Do you have an aftermarket guage or are you using the stock one? It sounds like the sending unit or the guage itself isn't working properly if you didn't actually feel a difference when driving. Get a mechanical guage on the engine or hook an air line to the pressure sensor with the ignition on and gently apply some positive pressure and see if it starts reading anything.


I have both the stock as well as a mechanical, both read vacuum, both go up to about 2 psi on WOT near redline, which is about normal for most non turbo cars. I have like no power, probably because I have no boost

Re: ST185 no boost, runs fine

PostPosted: Wed Aug 03, 2022 1:17 am
by grip-addict
Recommend checking:
The wastegate actuator to make sure the flapper door is shut at rest
That the flapper door opens at the psi you want it to
The exhaust bolts/studs between the head and the turbo are all tight and that there is no leaks from there
That the comp/turbine wheel can freely spin and that you didn't leave a shop rag in there like someone that looks suspiciously like me did in the past...

Re: ST185 no boost, runs fine

PostPosted: Wed Aug 03, 2022 5:04 am
by underscore
Ah I see what you mean now. When you swapped the turbo did you unbolt the manifold from the head? The gasket might be leaking if it's not an intake leak. On that front I'd check the two vacuum lines going to the PS pump, they're set up in such a way that they like to fall off and they're a bit hidden (you can see the ends with the wheel off) so it's easy to miss if they're leaking.

Re: ST185 no boost, runs fine

PostPosted: Wed Aug 03, 2022 10:12 am
by gtapexmickey
grip-addict wrote:Recommend checking:
The wastegate actuator to make sure the flapper door is shut at rest
That the flapper door opens at the psi you want it to
The exhaust bolts/studs between the head and the turbo are all tight and that there is no leaks from there
That the comp/turbine wheel can freely spin and that you didn't leave a shop rag in there like someone that looks suspiciously like me did in the past...


Waste gate is closed, even still I'd build boost in higher rpm. Never took the manifold off so I don't think I messed up the manifold to head connection. I had this condition prior to replacing the turbo

Re: ST185 no boost, runs fine

PostPosted: Wed Aug 03, 2022 4:33 pm
by underscore
My thinking is if the manifold is leaking and you didn't take it off then it will still be leaking. I think the left side under the alternator is the usual spot since it's hard to see.

Re: ST185 no boost, runs fine

PostPosted: Wed Aug 03, 2022 5:23 pm
by grip-addict
does it "feel" like it's boosting at all? I'm guessing not but it's worth asking.
does the wg spring take sufficient pressure to move? If it just blows open with the slightest breath of wind, then that could be the cause
is your wg flapper door binding up or can it freely move?

Re: ST185 no boost, runs fine

PostPosted: Thu Aug 04, 2022 12:12 pm
by gtapexmickey
grip-addict wrote:does it "feel" like it's boosting at all? I'm guessing not but it's worth asking.
does the wg spring take sufficient pressure to move? If it just blows open with the slightest breath of wind, then that could be the cause
is your wg flapper door binding up or can it freely move?


No power, feels slower than a stock 5sfe. Waste gate moves with no issue, actuator hold it's shut, it is a brand new turbo

Re: ST185 no boost, runs fine

PostPosted: Sat Aug 06, 2022 12:08 pm
by Gert
gtapexmickey wrote:I spoke to a few people and they said it may have skipped a tooth in timing, but I'm not sure. Haven't done a compression test or popped the timing cover yet because it's a bit tight there.

Good point.
When the turbo is fine and the wastegate too I should check compression and cam gear timing.

Re: ST185 no boost, runs fine

PostPosted: Sat Aug 06, 2022 4:46 pm
by underscore
Just to cover all bases, your CEL works and there are no codes right?

Re: ST185 no boost, runs fine

PostPosted: Tue Aug 09, 2022 2:57 pm
by alltracman78
HIGHLY unlikely to impossible an exhaust leak would cause that much loss of boost. Unless you had a giant hole.

Check engine and ignition timing. If your engine is running hot it will retard ignition timing. Or if it thinks there's knock. Which can effect boost.
Also make sure your cat isn't clogged, if you have one. Checking compression is a good idea too.