Secondary Boost Questions
 

Hey, Looking for some advice on troubleshooting secondary boost issue.
Car is 93 Base with downpipe. Enfini turbos (if that makes a difference) and new vacuum hoses installed with engine rebuild (Pineapple Racing) about 9k miles ago. Otherwise stock. I've owned the car for a few months and added temp and boost gauges which is when I discovered the loss of boost above 4500 rpm.
Boost pattern is 12-5 then decreasing to redline. Otherwise the car seems to run good.
I decided to try to isolate the problem between the intake and exhaust side of the turbo controls by plugging the line to the charge relief valve and measuring pressure at the Y-manifold on the secondary side of the charge control valve. I did this by connecting the gauge line into the manifold port for the CR and CC solenoid pressure feeds -which I left disconnected to keep the CCV closed. Result was 5-7 psi after 4500 rpm. Since I didn't see ~10 psi I am thinking the problem is on the exhaust side versus intake. So next I checked the turbo control actuator by briefly going to WOT in neutral. The actuator rod moved an inch or so at high RPM so I assume it is okay (i.e. the secondary is getting exhaust flow) I'm probably missing something but it seems like this points to a problem with the secondary turbo itself? . Does my troubleshooting methodology make sense? Anyway would appreciate any advice. Thanks.

The solenoids, actuators, and check valves get cooked over time. Were they tested when you replaced your vacuum lines?

You can ID which ones to test based on your problems and comparing that to the control system in the FSM, or you can just test them all.

Here's the how to...

https://www.rx7club.com/3rd-generati...-stuff-802060/

Thanks. Yeah the invoice for the engine rebuild (done before I owned the car) includes the "solenoid pair bolted to the front of uim" which I assume is the wastegate and pre-control solenoids but doesn't mention any of the others or check valves or actuators and whether they were tested. So definitely could be problems with any of those parts. I was just thinking there was some more assembled testing I could do to narrow down the problem before taking everything apart.

It's probably something with the turbo transition that's not working right, either the TCA isn't opening or is sluggish or the CCV isn't opening.

Get a T fitting and some extra vacuum line, T into each part of the turbo control system with your boost gauge and go for a drive. Watch how it's working. That's the easiest way to narrow down what part of the system isn't working right.

Also, it's a quick check to see if your vacuum and pressure tanks are holding vacuum and pressure. Pop the line off to the tank and see if they release their vacuum/pressure, if not you may have a bad check valve or a leak in the vacuum line or tank. That's an easy thing to look at and a common problem.

Boost leak could also be a problem, they will do all kinds of weird stuff.

Dale

Thanks for the suggestions...particularly about checking the tanks. I discovered there is no check valve in the hose between the vacuum tank and the manifold! So that's the first thing I need to correct. Do you still sell viton check valves? I see in previous posts that you did in the past.

PM me about check valves.

I don't think there is a check valve between the vacuum tank and the rat's nest, the check valve is one of the ones hanging off the UIM. It supplies vacuum for a few different things.

Dale

Well I can follow the hose from the vacuum tank to the side of the intake manifold (via a tubing tee and there is no check valve so the vacuum tank must always be at manifold pressure. Unless I'm confused (which I haven't ruled out) I'm missing the one circled in the attached diagrams.
Thanks

if you're sure your vacuum chamber doesn't have a check valve connected to it then yes you should fix that. Vacuum is needed to supply all the major actuators.

Twin Turbo Diag:
Troubleshooting

The vacuum chamber's check valve is the one in the front of the UIM. The line from the vacuum chamber to the rat's nest doesn't have a check valve.

If you look at the diagram posted, the check valve for vacuum is circled.

There are T's in the metal vacuum spider. That circuit supplies vacuum for what needs vacuum and at the end of the circuit is the vacuum chamber, it just stores a large supply of vacuum.

Dale

Hey so the missing check valve was the problem. Boost pattern with valve installed is 11-8-10 slightly dropping off to 9 or 8 near redline. Happy to get some second turbo action!
Thanks Dale for the check valves and everyone for their input.

A couple of easy fixes could be the wastegate seeing full boost pressure or a collapsing turbo inlet.

If you're intent on troubleshooting yourself I would first familiarize yourself with the system. You will need to know when you should be seeing pressure and vacuum on each line and tee into the turbo control lines with a vac/pressure gauge to observe while driving. That and bench test each component. The only way to get through this is to be super methodical and test everything. You can verify the solenoids work by the above methods rather than tearing apart rats nest.

Good place to start:

Troubleshooting