Strange boost/fuel cut ST165

K

Killtodie

New member
I’m having a strange issue with boost/fuel cut. Doesn’t occur on street driving, but does happen when I’m on the highway, and it seems to get worse as I drive.
At first it doesn’t’ happen till I’ve been at full boost for a few seconds, it will cut. The more I drive, the earlier it cuts. Later when I’m at half boost, or medium acceleration, and then it will get to a point where slight acceleration triggers the cut.
I have verified that I have no boost leaks, engine is stock, turbo is a twin scroll CT26, tubular headers, 3” DP, stock boost, stock boost gauge hooked up.
I know that the gauge has a built in fuel cut when it hits over. I don’t have a real vacuum gauge. Could it be that?
Spark plugs are new, about 3k miles, doesn’t feel like a blowout. Happens on 87, and 93 octane
 
F

freddie

New member
I had a similar problem with my 165. It did not appear until the engine came up to temp. Did all the checks fuel cut, fuel pump, vac leaks, check turbo and waste gate. every thing was operating normally.

Then I removed the throttle body and pulled it apart. The idle valve section at the bottom of the TB., one end has an electrical solenoid and a coil spring on the other end in the middle is the by-past valve which is supposed to rotate open and close to let manifold pressure drop and or idle air by-pass. The shaft would not turn properly because it was gunked up with black carbon. I cleaned this with AFM cleaner, made sure the shaft was turning freely then cleaned the TB also with the same stuff, and put it all back together again.

No more problems, car idles and runs little more smoother and no more jerking when I lift off on the accelerator.

Merry Christmas
Freddie
 
K

Killtodie

New member
Are you referring to the T-VIS?

Forgot to mention, my check engine light also comes on during this, but doesnt stay lit after.
 
underscore

underscore

Well-known member
It doesn't matter if the CEL turns off, the ECU should still store the code. If you don't have a proper boost gauge at least borrow one and verify what the car is hitting before it starts to cut fuel.
 
D

d1alltrac

Active member
It's normal for the car to go into safe mode after hitting fuel cut. If you don't cycle the ignition it will cut fuel every time the turbo starts to spool, be it 0.1psi or 10psi.

It's not uncommon for an st165 to hit fuel cut with no mods other than a 3" dp. There's really not much that can be done as far as diagnosis until you get a good boost gauge. The fact that it starts during highway driving isn't surprising, higher gears put more load on the engine and the turbo spools faster. You could be experiencing boost creep which causes the initial fuel cut, then you're just in safe mode after that. There could be interference issues with the wastegate depending on what downpipe you have, which could cause boost creep.

Get a real boost gauge and go from there, the factory one is garbage.
 
K

Killtodie

New member
d1alltrac":bz2633q3 said:
It's normal for the car to go into safe mode after hitting fuel cut. If you don't cycle the ignition it will cut fuel every time the turbo starts to spool, be it 0.1psi or 10psi.

It's not uncommon for an st165 to hit fuel cut with no mods other than a 3" dp. There's really not much that can be done as far as diagnosis until you get a good boost gauge. The fact that it starts during highway driving isn't surprising, higher gears put more load on the engine and the turbo spools faster. You could be experiencing boost creep which causes the initial fuel cut, then you're just in safe mode after that. There could be interference issues with the wastegate depending on what downpipe you have, which could cause boost creep.

Get a real boost gauge and go from there, the factory one is garbage.
Click to expand...

Thanks, I suspected most of that. I'll hook up my spare boost gauge, and for now disconnect the stock gauge.
There are instructions on how to bypass the pressure sensor for the stock gauge, I'll do that at a later time.
 
K

Killtodie

New member
Finally disconnected the gauge, still does the same thing, same code too.

Why would it still tell me its over pressure if the sensor is not getting any vacuum? I just unplugged the line going to it, if I unplug the sensor it puts the boost needle to max.

Should I unplug it, or unplug the vacuum?

Next thing is I'll install that real boost gauge.
 
M

Mafix

New member
st165s have more than 1 fuel cut. you have the boost pressure sensor and the afm voltage. I had this issue myself. solved it too. there is a silver box under the radio. it has 3 yellow connectors on it. bolts to the floor. take that out of the car and throw it in the trash. buy whatever standalone ems suits your fancy. make it work.
 
underscore

underscore

Well-known member
I could've sworn the sensor for the gauge and the one for the ECU are different, but I could be wrong.
 
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