Boost rises then falls

[Lucas03es]

My car has a B16g, and when at partial throttle, the boost will build between 5-10 psi, and then slowly fall back into vacuum without adjusting my foot. Any ideas? If it matters, the EGR is causing an idle surge because it is just unplugged and the blockoff plate should be here any day now. Thanks

 

[Griffeclipsed98]

what's your motor doing, is it still excellerating while your at partial throttle? and are your running a stock exhaust or pre cat, its hard to tell for sure. just running at partial throttle while your rpms are still riseing means your motor will still try to take on more airflow than the throttle will allow air to pass even if your boosting air flow still needs to keep moving past your throttle as fast as the motor is consuming it and it can very well reach apoint where the air flow wont be able keep up with the motor and you will see a boost drop. and adding to a problem of air flow they exhaust gasses needed to push the turbo need to be evacuated down the exhaust system. if there is enough of restriction anywhere in the exhaust line that could cause a bit of back preasure then your turbo will have a harder time spooling, it has to do with the volumetric efficency of all 4 of the intake/charge, motor, turbo, and exhaust systems. look to see where you might have a restriction and you might find your answer to the unsteady boost.

 

[Fabian]

unsteady boost could also be very easily caused by a shitty/cheap boost controller.

 

[housegsx]

When you get up to speed there is no more load or need to accelerate so your boost comes down. That's how it should behave at partial throttle. If you want more speed press the throttle pedal.

 

[Lucas03es]

The rpms are still building but slowly. Like in 5th at say 70 mph giving it gas to get to 80, it will build to about 7 psi and then fall back into vacuum while still accelerating. I have a stock o2 housing, 2.5" DP with a 3" catback. Also, the car has a homemade mbc, but I will be switching out for a greddy ebc once my friend pulls the harness for it out of his supra.

 

[ttetzner]

could be a boost leak, i had this happen to mine and there was a hole in the s pipe to the TB. It would hold boost then just give out and fall to nothing

 

[jrohner]

My car has a B16g, and when at partial throttle, the boost will build between 5-10 psi, and then slowly fall back into vacuum without adjusting my foot. Any ideas? If it matters, the EGR is causing an idle surge because it is just unplugged and the blockoff plate should be here any day now. Thanks

You are at partial throttle, boost won't stay the same as rpms go up at low throttle angles. This is normal, because load goes down as rpms go up, and load is what makes boost basically.


I don't believe there should be an issue with having the EGR just unhooked without being blocked off, unless the EGR valve is crudded up making it leak.

 

[Lucas03es]

I thought it was just a normal process having the boost fall like that, I just wanted to double check and make sure. The EGR plate got here today so tomorrow after work I will be putting it on and see how it works with my idle surge problem.