boost turns to -.50 vacuum after 5k rpm... help

[iriehachi]

Hello all, I've recently got my brother's 240sx. ever since i driven it for the first time, it makes boost from 3500 rpm all the way to 5000 rpm but right at 5000 to 5100 rpm, the boost turns to vacuum. The engine still revs past it but very slowly and it feels as if i'm driving a really weak N/A car.

I thought that the internal wastegate was getting stuck open so i've replaced the turbo with an IHI one. Its doing the same thing. I thought maybe the power fc could be set up wrong so i've even initialized it (now runs really rich and rough) but still does it.

I've checked all the couplers but none of them are ripped. I've even "stuffed and blocked" the blow off valve thinking maybe the blow off may be leaking but it didn't work either.

What can this be? Help!!!!

Could it be the power FC? in the past, i've experienced major boost leak before but never have i seen .9 bar of boost turn into -.50 bar of vacuum.

[driftbros]

Sounds like a boost controller or wastegate problem to me. Let me know what you are running and I give it a go.

[iriehachi]

I have a redtop sr20det with greddy intake manifold, stock exhaust manifold, ebay front mount intercooler, greddy type s blow off, oil catch can, tomei 740cc injectors and power fc running a map sensor.

I had a gt2871 r turbo on there but i thought the wastegate was bad (because it was making negative boost at 5k just like now) so i swapped it out with an IHI t25 turbo. Its still making negative boost at 5k. This is whats strange. If i half throttle it to bring the boost down to like .4 bar, i can rev it to 6500 rpm but once i floor the throttle all the way, just like that, the boost goes away and then comes vacuum plus no power.

Another strange thing is, say that i floor it. The car will make boost at 3500 rpm and go all the way to 5k then the boost dissappears and turns to vaccum. I can still hold the accelerator down and the rev still goes up very slowly but even with vacuum when i let go of the accelerator, the blow off valve goes off... kkssshhhhhh.

[iriehachi]

I had a greddy profec b boost controller but because of the losing boost at 5k issue, i pulled it off and installed a ghetto ebay manual boost controller. I don't think the boost controller is the issue. its got a metal ball being pushed by a spring plus all of the ports have been teflon tapped before being screwed in place.

to test to see if i had a vacuum leak somewhere, i pulled the vacuum line coming from the hot pipe to the boost controller and when i did the idle went up. when i placed the vacuum line back onto the port coming from the hot pipe, the idle went back down to normal.

could the power fc be causing this. I'm thinking about pulling everything off and going stock. getting a stock intake manifold, maf, stock ecu and stock injectors to see if it works.

[driftbros]

You say that you are getting vacuum above 5k rpm. This is based on the MAP hooked up to the PowerFC? Since your BOV is making the sound of releasing pressure, I think you may have boost, your MAP is just not reading it. The reason your car is falling on its face could be the FC isn't in the right fuel or ignition map.

I would say that you need to get a stand alone boost gauge and test the readings from your PowerFC and MAP sensor.

[iriehachi]

i have a separate boost gauge hooked up. this reminds me of another thing. The boost gauge reads higher than what the power fc reads. just a lil bit though. like .2 bar


Modified by iriehachi at 1:05 PM 8/26/2009

[driftbros]

Where are you sourcing from for the boost gauge and MAP? Where is your BOV? I'm intrigued. I need to figure this one out.

[iriehachi]

there are 2 air ports beneath where the throttle body is located on the greddy intake manifold. one of the ports is where the vacuum hose for the fuel pressure regulator is attached to. I used the other one for the power fc MAP sensor and "T"ed into it for the boost gauge.

[iriehachi]

finally figured out what was wrong. The stock black rubber / plastic turbo filter tube was caving in on itself when the turbo made boost. I pulled off the filter and tube and just like that, no more vacuum at 5k rpm.

Now, i need to figure out how to get rid of the initial stumble when i try to take off from a complete stop. I'm sure its power FC related. I wonder if a good tune would take care of that?