As the title says I've been having a weird boost issue, I've searched high and low but no proper answer but a few things it could be.


On full boost, the boost fluctuates up and down according to my boost gauge e.g. up to 1.3bar then down to 1bar then back up to 1.3bar like its flapping as such but it doesn't happen every time. On full boost sometimes it will hit full boost and stay level or it will hit 1bar and stay level as such which I find strange as I wouldn't think a boost leak would do this.


Things that I think it could be is the:
Faulty N75
Cracked manifold
Cracked compressor side
Faulty wastegate
Oil Seg broken valve


Anybody able to point me in the right direction as it seems like a strange issue.

Change the boost control solenoid (n75)

Mine was fluctuating once it was heat soaked, was fine when cool

They are less than 30 quid on amazon

febi bilstein 45461 Pressure Converter for exhaust control system, pack of one febi bilstein 45461 Pressure Converter for exhaust control system, pack of one: Amazon.co.uk: Car & Motorbike

Craig830 said:

Change the boost control solenoid (n75)

Mine was fluctuating once it was heat soaked, was fine when cool

Click to expand...

Thanks dude, did you have any other symptoms?
& how did you find out it was the N75 valve in the end?

Check the resistance of the coil when it's cold and hot, as per the engine course manual on the bravo downloads section here, mine was out of spec when it was hot

Use MES and graph:
-intake pressure (mbar)
-throttle plate (%)
-boost pressure sensor on pipe before throttle body (mbar)
-n75 control (%)

A graph containing these will tell me if the ECU can control the valve properly, or maybe there is something else (boost cut).
Ben D had a similar issue... turned out he had a cracked exhaust manifold and a cracked turbine housing (both common faults on t-jets)

aurick86 said:

Use MES and graph:
-intake pressure (mbar)
-throttle plate (%)
-boost pressure sensor on pipe before throttle body (mbar)
-n75 control (%)

A graph containing these will tell me if the ECU can control the valve properly, or maybe there is something else (boost cut).
Ben D had a similar issue... turned out he had a cracked exhaust manifold and a cracked turbine housing (both common faults on t-jets)

Click to expand...

I need to get mes for my laptop, do I just need a standard license?

I just bypassed the n75 and only achieving 0.3-0.4 bar but no fluctuating, does that sound correct?

JackFi said:

I need to get mes for my laptop, do I just need a standard license?

I just bypassed the n75 and only achieving 0.3-0.4 bar but no fluctuating, does that sound correct?

Click to expand...

what are you using to read boost levels?
The instrument panel display is useless.

what gear, load, throttle plate %..etc etc?

aurick86 said:

what are you using to read boost levels?

The instrument panel display is useless.



what gear, load, throttle plate %..etc etc?

Click to expand...

Using a aftermarket boost gauge, yeah the display doesn’t show the fluctuating.

I need to get Mes installed but just wondering if the fluctuating went away running the the wastegate directly to the bottom of the turbo which it did but the only boosted to 0.3 bar, do you know what pressure the wastegate opens?

0.3bar sounds about right for wastegate pressure.

The pressure might be to low to be impacted by cracks.

Is the gauge analogue? hose to gauge or does it have sensor in the engine bay?..wires to gauge.
Is the gauge pickup in the intake manifold?

aurick86 said:

0.3bar sounds about right for wastegate pressure.

The pressure might be to low to be impacted by cracks.

Is the gauge analogue? hose to gauge or does it have sensor in the engine bay?..wires to gauge.
Is the gauge pickup in the intake manifold?

Click to expand...

I did think this as 1.3-1.5 is a lot more pressure than 0.3 but seems weird its not constant fluctuating.


The gauge is a stepper type with the vacuum line connected to a sensor which goes to the gauge. The pressure is read from the fuel regulator.

JackFi said:

I did think this as 1.3-1.5 is a lot more pressure than 0.3 but seems weird its not constant fluctuating.


The gauge is a stepper type with the vacuum line connected to a sensor which goes to the gauge. The pressure is read from the fuel regulator.

Click to expand...

You need to realize that there is a pressure ratio in the exhaust manifold (before turbo), and the pressure in the intake.
I remember reading about this at some point about... 2:1 to 4:1 in certain conditions.
Think about it like this: for 1.5 bar in intake it takes about 3 times more pressure in the exhaust manifold before the turbo. ..si 4.5 bar .
Combine that with ~350-400 degrees C.. and you have enough force to open cracks up... and force the ecu to overcompensate with the wastegate control.

Gauge seems like it's installed ~right.. but personal preference move it to some other line.. even drill and install a new nipple.
I don't like the fact the sensor is directly connected to the PFR; a leaking sensor can put AFR's out of acceptable range.

Use MES graphing for proper fluctuation monitoring.
Not sure how familiar you are with the t-jet... you might just be reporting the overboost function. Video or it didn't happen.

aurick86 said:

You need to realize that there is a pressure ratio in the exhaust manifold (before turbo), and the pressure in the intake.
I remember reading about this at some point about... 2:1 to 4:1 in certain conditions.
Think about it like this: for 1.5 bar in intake it takes about 3 times more pressure in the exhaust manifold before the turbo. ..si 4.5 bar .
Combine that with ~350-400 degrees C.. and you have enough force to open cracks up... and force the ecu to overcompensate with the wastegate control.

Gauge seems like it's installed ~right.. but personal preference move it to some other line.. even drill and install a new nipple.
I don't like the fact the sensor is directly connected to the PFR; a leaking sensor can put AFR's out of acceptable range.

Use MES graphing for proper fluctuation monitoring.
Not sure how familiar you are with the t-jet... you might just be reporting the overboost function. Video or it didn't happen.

Click to expand...

Thanks for the insight into the pressure on the manifold side of things, so actually it is a lot of boost so can force the cracks open easy.


Yeah the gauge has been working fine for the last 4 years, I know people don't like connecting it to the PFR as it could cause issues if the line is leaking or becomes disconnected, I actually replaced the tubing where it is T'ed off just incase it was leaking which it isn't.


I've owned T-jet engined cars for over 9 years but first ive had this issue. Its not the overboost for sure, the boost gauge like 'waves' up and down, if that makes sense?


I will try and get a video later of the boost gauge.

Sorry I haven't got around to doing a graph yet, I tried to get a video of the boost gauge but it didn't come out very well.


Would the on screen 'boost gauge' go up & down if this was the issue?

I had the same issue,pedal to the floor and turbo was on and off boost upto a certain rpm,turned out to be the oil segregater.
I took the cap off and and checked the orange diaphram looked ok no holes or cracks.cleaned it with foam mr muscle 5 times it was ok for a day.nxt day decided to cut the thing open and see what was actually inside,turns out there is only 1 flap inside and that is to allow oil to drain back into the air intake before the turbo.any how I ****ed that off and fitted some pipe work a 1 way check valve and a catch can in place of the oil segregater. Plus stops oil going threw turbo upto inlet manifold and I don't oil burn after long engine breaking and accelerating at the bottom of a hill.i used to hate that big puff of blue smoke after engine breaking on a hill.that **** is embarrassing
Happy days

Good afternoon, I have attached a MES graph, I hope I got the right information for you aurick86?

Don't know what happened to my photos in the last post but hopefully they work this time.

I’m trying to understand the graphs a bit more, I’m looking at it and seeing no fluctuation? Can anyone confirm, my boost gauge was only showing peak 1.1bar but I’m seeing 1.3-1.4bar on the graphs aren’t I?

JackFi said:

I’m trying to understand the graphs a bit more, I’m looking at it and seeing no fluctuation? Can anyone confirm, my boost gauge was only showing peak 1.1bar but I’m seeing 1.3-1.4bar on the graphs aren’t I?

Click to expand...

Yah... that is what i'm seeing. no fluctuation, no boost cut (can see that in throttle angle).

-Depending on the boost gauge sampling rate you might be loosing some data ...so the reason you're not seeing the same boost levels.
-The boost gauge might just not be very well calibrated.


Is the DeltaT in seconds? so 10-12 seconds to get full boost? that's not great, BUT depends at what rpm you start at. For example: if you start at 2000rpm, it will take much longer than if you start at 3000rpm.
-RPM and Vehicle Speed would help; So a few runs with what you have selected now, and a few with "Intake manifold pressure", "RPM" and "Vehicle speed".

I'd suspect the boost gauge/sensor: electrical connections (so worse when engine is bouncing about under load); or maybe it's just bad.
You could try fitting a restrictor in the hose to the sensor; the smaller hole will act like a capacitor on an electrical circuit.. it will smooth out the pressure to the sensor.


Was the gauge acting up while you were taking these graphs?