Hey there! FWIW, I still daily my car. Saw your comment looking for help on my post and thought I could perhaps offer something of a response. Apologies if any of this stuff seems dumbed down, but I'm not sure how familiar you are with these systems...
There are a number of things that can cause a Code 66. It's a generic fault that basically means "There's something wrong with the turbo system" which isn't much help at all, but in my experience, I've found that code means that one of the turbos is either making too much, or too little boost while the secondary is doing it's pre-spool.
It helps to know how the TT system works, and it isn't that complicated once you wrap your brizzle around it. So here's a crash course that will miss some details, but here goes.
- Under ~4000 RPM, the ECU, BBOD, and associated solenoids and vacuum lines keep the secondary turbine (I'm calling it T2 from here on) exhaust inlet valve, and compressor outlet valve, closed. Boost pressure is modulated by the primary turbo (T1) wastegate.
- At 4000 RPM, T2's exhaust inlet valve is opened at about 25%, and the ECU monitors the T1/T2 boost differential pressure (BDP) with a sensor located on the firewall. The partial opening of the T2 inlet is what causes the VOD
- At >4400 RPM, if the T1/T2 BDP is at or near zero, and engine load is above threshold, the T2 inlet valve and compressor outlet valves are fully opened, and you're now in twin turbo mode. Boost pressure is still modulated by the primary turbo wastegate (at least on BG5 Rev C cars)
- When RPM drops below ~3600, and engine load is below threshold, the ECU closes the T2 valves, and you are back in single turbo mode.
Here's some things that you can do to try sorting it out...
- Check all vacuum lines on the car that come from the BBOD. Remove them one at a time, check them, and put them back before doing the next one. Consider replacing the vacuum lines, and if you do that, make sure you put any restrictor pills back where they should be.
- Remove the BBOD and check all the little lines and solenoids in there as well. Clean everything, and flush all of the little lines in there with brake cleaner, or Subaru Top Engine Cleaner. Again, consider replacing them if anything looks sketch, and if you do, note where any restrictor pills are, and ensure they go back where they belong.
- Check the turbos for excessive play in the shafts/bearings. They should wiggle just a tiny bit, but nothing excessive.
- Check that the wiring to the BBOD is in good working order
- Check the wiring to the BDP sensor on the firewall.
Something else that might be causing the issue is that you've installed a VF25 in place of the original VF18, without also replacing the VF19 Secondary with a VF26. They all look the same and both use a P18 turbine housing so they bolt in easy, but have dramatically different turbine and compressor wheel geometry, in addition to slight differences in compressor housing geometry, and therefore boost characteristics and flow capacities. This might be causing a BDP issue which would cause the ECU to lose its mind. Essentially, the VF25 primary can spool/boost much faster than the VF19 secondary, and the car is seeing that there's a difference in boost from one to the other.
I'll be on nightshift for the next couple days, but hit me up on insta @gtb_kate if you need help. I'll be away from my office for a bit, so that's an easier channel for comms.
Last edited by Phasewest; 06-06-2024 at 08:23 PM.
Reason: Clarity
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