good boost in 1st, 4psi in 2nd
 

My car is sequential. 14 psi in 1st, but as I shift into second, the turbo only holds about 4 - 7 psi even to redline. The only way to get full boost is to let off to build up vaccuum.

Fix is to replace the one-way check valve into the pressure chamber. To test the check-valve, suck on the green side with your mouth and you should be able to pull air through the dark pipe.

Mazda part no. N390-13-995A.

Great to see solutions posted!

You might ask Dale Clark for a set of his Viton check valves ... Great upgrade.

Will this also be the problem if it boost 5psi on 1st only, and the other gears normall?

Anyone?

James - thanks for that recommendation. I went to ebay and bought 4 valves from Dale Clark. Wow, what a night and day difference with the OEM Mazda one-way check valve.

Boost is immediate to 14psi, drops to 8psi and immediately surge to 14 psi again.

So this is a plug for Dale's viton check valves: if you have slow boost build up after the first turbo or it won't build up past 4 psi once you are past 4000 rpm, replace all four of the check valves with Dale's viton. And for the price of one Mazda check valve, I got 4 valves.

You may want to check to see if the restrictor pill is still in the turbo or in the actuator line. I had that problem when I transferred a turbo from another car and used new silicone hoses (which didn't have the restrictor pills in place). All I got was around 5-7 psi and then full boost with the second turbo.

Another item to check is the turbo control solenoid right under the UIM with three lines going into it. That part gets heat damaged and won't give you full boost.

I have been searching for lots of boost issue info.

Could the check valve cause you to have no boost at all?

I found a broken check valve under the vacuum tank and replaced it with a used one from another car but I don't register any boost at all now. I thought it was my gauge and replaced it, now I see this and it makes me wonder. Thanks.

I am gonna hit up Dale either way about getting those viton valves.

Replaced the check valves with Dales viton, and it's fine now