Psi drops when boosting

[gsxofftheline]

I have had boosting problems for a few weeks now and can't seem to figure it out. It pulls pretty decent in 1st and 2nd gears and in 3rd and on when I hit 12 psi it drops quickly to 7 psi and floats at that psi.

I fixed virtually all boost leaks, pressurized it with my freakin bike pump to 15 psi( would do it to 20 psi, but it was a bi***). Then it would bleed down 15 psi-0 in exactly 3 minutes, which I believe is pretty good. I DID hear air coming out of the oil dp and in the valve cover but if its holding that good it should still boost pretty good.

What I have done. Boost leak test and that passed, fixed exhaust leak at exhaust mani and turbo gasket. There is one leak at the dp gasket( will get gasket tomorrow) but that shouldn't really affect boost since its after turbo. I have tested the MBC and WG. Pressurized WG and it opens at 8 psi, then pressurized the MBC to WG and it opens at 12 psi. MBC is set to 12 PSI. It is a joe's mbc? I believe its called. It doesn't feel like I'm losing fuel, changed out clogged fuel filter.

I don't believe its exhaust leaks for before when it had leaks it boosted at 4200 rpms, now its at 3500 rpms. Psi just drops and I can't figure it out. Especially a few days before MOD. Anyone have any ideas? Thanks

 

[the_mork]

If it is a JoeP MBC then it shouldn't have this problem but if it is something different and it is ball/spring type there are some MBCs that don't have a bleed hole so when they open boost pressure flows around the ball and seat then gets trapped there holding the wastegate open longer than you would like.


As for the air coming out of your valve cover get a new PCV valve, you want an oem one the replacements at auto parts stores will leak brand new. Extremepsi.com sells them for a reasonable price.

 

[waltz]

i had the exact same problem, i just turned up my boost controller and problem solved

 

[kenamond]

I tend to agree that this might just be a boost spike masking itself as a proper 12psi MBC setting. One easy test is to get the car to 4k rpm in 2nd gear and use the brakes and gas together (called brake boosting) and go WOT and adjust the brakes to hold the car at 4k. Check your boost gauge.

If you have a boost spike, it usually blips past the desired boost and settles on whatever the MBC is set to. You go through 1st and 2nd pretty fast, so you might just be spiking to 12psi, and shifting to 3rd before it has time to settle, but in 3rd, you have to hit 95mph or so before you have to shift, and there's plenty of time for the spike to settle.

If it's not that, I'd check to see if the MBC is dirty or maybe you connected it to the BOV line instead of the compressor outlet nipple to get your boost signal.

Other than that, I'm perplexed.

 

[ttsrq]

I think kenamond is right and that your MBC is just spiking to 12psi, and then dropping down to 7 ( what its set at) try to raise your boost alittle bit and see if it holds steady at about 12, it might spike to 16 or so though.

 

[kenamond]

And if your MBC vaccuum lines are routed off the BOV line or are too long, that can make spikes worse. My Joe P IL MBC has just enough line to go from the compressor nipple to the top of the radiator shroud (where the MBC is covertly mounted) back to the WG nipple, and I don't see spikes.