AVCS Checks and Diagnostics

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AVCS Checks and Diagnostics

Postby bigBADbenny » Tue Oct 06, 2020 4:13 pm

AVCS checks, diagnosis and considerations.

If your car bucks and stumbles in first and second during take off,
and you can’t hold steady rpm in neutral at and around 2000rpm on a warm engine,
if during the test the car bogs and stumbles,
or you have misfire cel/dtc codes eg P0011 and or P0021 or misfires on bank1 or bank2 that’s not spark plug or coilpack related:

Diagnose or replace both avcs solenoids with genuine items.
But don’t fire the parts cannon before reading this faq and attempting diagnosis!
Its a 1.5 hour install.

Here’s my incrementally updated AVCS diagnosis guide: logging is the key.

Consider an AVCS check as part of any health check for your car, especially if you experience a noticeable stumble or hesitancy under acceleration.

What is AVCS?

AVCS, Active Valve Control System is Subaru’s implementation of VVT, aka Variable Valve Timing for either inlet cams only (single avcs, SAVCS) or both inlet and exhaust cams (Dual AVCS, DAVCS, also known as Quad AVCS).

The system uses signals from the cam and crankshaft position sensors, and oil pressure to control the variable cam angles.
Electronic solenoids aka OCV’s: Oil Control Solenoids, are opened and closed by the ecu to vary the amount of oil being flowed into the cam gear thus varying the VVT angles according to self calibration of the system and specific load and rpm tables in the Engine Control Unit aka ECU tune.

Basic diagnostic considerations:
Therefore the loom, plugs, cam and crank sensors may also be considered for inspection, see the FSM workshop manual link below for the factory procedures.
These generally involve voltage, resistance or load tests, but full instructions are in the FSM.

Does the car have oil and what is the condition of the oil and filter?
IME, the oil needs to be extremely low for avcs to stop functioning, very close to the point where the whole engine will cease to function.

Or you may have grounding issues, including engine ground, heads to body, battery ground et al, check and clean them all.
This is fairly obvious in logging as a broken up cam VVT angle signal or actual cam signal dropouts, as seen in logging AVCS VVT angles.

A dead, worn out battery is a potential cause of system-wide issues, its not simply a matter of having the correct voltage, the battery must be able to support the cranking load of starting the car, or the system load when the car is not running: if youre in doubt, take the battery to a specialist that can load test it.
Make sure to clean the battery clamps & terminals and replace the former if they no longer tighten, or are corroded.

About Check Engine Lights:
CEL/DTC codes usually indicate an issue with an affected sensor or system, to an issue upstream affecting that sensor or system, or if a group of codes, may be seen as symptomatic of an issue affecting the entire system, eg dying main battery, no oil, swarf in oil.
In the case of AVCS CELs, the description of the fault is "cam position over advanced" which some assume to mean "cam postion sensor issues".
In practice, the fault is fairly rarely cam sensor related, is easy to rule out (log cam sensors) or in the case of swarfed and sludged sensors, suggests a much larger issue, eg extreme service abuse.

Overarching diagnostic considerations:
If, during logging, either or both AVCS VVT angles remain at zero, especially after the heads have been rebuilt, it’s possible that the tiny oil supply galleries in the front cam bearing caps (that contain the OCV manifolds aka housings) may be clogged with excessively applied sealant, obstructing oil supply.

If either of your AVCS VVT angles are fixed at or above zero, and the gears were recently installed, suspect they may have been damaged due to incorrect internal cleaning of refurbished gears or simply incorrectly reinstalled.

In the former instance the refurbished cam gears should be able to be activated with shop air from a blower nozzle prior to installation.
In the latter case the specific reinstallation procedures must be used when offering up the gears: pins aligned with holes on the camshaft, do up the cam bolts by hand, use the specific tool to hold the cam gear when torquing the bolts, do not hold the camshaft itself.

Obviously, AVCS cam bolts must be used with the axial oil drain hole for each AVCS cam gear. Non AVCS cam gears do not have this hole.

Oddball diagnostic considerations:
Also theres 4 total types of union screw bolts, aka banjo bolts: two differently sized holes to presumably restrict oil flow, and bolts with or without filters, meaning the consideration is to not mix up the location regardless of whether removing filter or not.

Occasionally built up swarf or sludge on the cam sensors has been seen to cause AVCS issues.
Again, this is fairly obvious in logging as a broken up cam VVT angle signal or actual cam signal dropouts.

However, eg the cam position sensors are relatively difficult to access and more commonly the issue is caused by AVCS OCV or cam gear issues.
Here’s how I accessed my cam position sensors, to replace the o-ring seals: viewtopic.php?f=10&t=36503

As regards GR STi, there was an OCV part number update around 2010, but regardless of your car always make sure your OCV part numbers match your heads, ECU, model etc, eg via a VIN parts search.

General diagnosis:
The following is generally for cam positions over CEL’s (check engine lights) aka DTC’s (diagnostic trouble codes) P0011 &/or P0021.

Log AVCS VVT left and right angles with Btssm or Romraider or a shop scanner thats compatible with SSM at 1700-2000rpm in neutral and in gear on the freeway, look for VVT angle delta, oscillation, lag or inactivity.

If you only have a generic workshop scanner, look for these parameters and use them in live view.

No logging tools, car bucks and stumbles: BLIND TESTING

If you don’t have SSM logging, one failing AVCS OCV can sometimes be diagnosed by idling a car in neutral til warm, then vary the rpms very slowly between idle and around 2000rpm, pausing and holding certain revs for a minute or so. This can trigger the fault mode on one AVCS bank, causing a stumble or inability to hold the desired rpm. I have tested this on my single AVCS GT-B, not sure if it works for dual AVCS issues.

My theory is that slight differences in the left and right AVCS oil feed pressures, combined with the self calibrating nature of the AVCS system cause one side, usually the LHS, to become overdriven and eventually fail.

AVCS failure stages:
The beginnings of AVCS OCV failure is seen as left to right angle lag.

The intermediate stage is one side wildly oscillating whilst the other side operates correctly.

The terminal stage is one side becomes inactive at which point the AVCS system deactivates completely.

All stages can be encountered independently and without triggering a cel.
Meaning taking a look at AVCS operation should be considered a part of regular maintenance and health checks.

If either issue is revealed, swap AVCS oil control solenoid side to side to see if the issue follows the affected solenoid.
If so, replace both OCV solenoids.

This is a blind test to check if the issue is OCV related, or AVCS oil supply or cam gear related.

If the issue doesn’t follow, check the OCV oil supply banjo bolts for blocked screens. The filters can be very deep in the banjo bolt hole and or broken up.
Use a sharp toothpick to fish them out. The specific banjo bolts that contain filters have a raised nipple on the bolt head. The filters can be deep inside the head, try fishing it out with a toothpick (push in, pull out) or, if the filter is broken up, use straight fine tweezers, or create a vacuum venturi by blowing compressed air across the bolt hole.
The difficult banjo bolts to access on a complete engine is the one behind the LHS timing cover.
Online guides have shown that extracting this banjo bolt is possible if the nipple on the bolt end is filed down and the cam cover is gently pryed with a bar, otherwise its easy with the timing gear off for service.

Very rarely, an AVCS cam gear may be jammed with debris, or via incorrect installation, usually in this instance it will be locked at a particular angle as revealed by logging.

Go for the easiest tests (logging) first, if all results are negative, eg you have AVCS inactivity which does NOT follow the OCV side to side swap, pull the crank and cam sensors to check for contamination by metal particles on the cam sensor face or check for oil supply issues at the banjo bolt filters. Generally, the banjo bolts that contain filters are the bolts with a raised nipple on the head.

The FIx
My solution for a failed OCV is to start with installing two new AVCS solenoids and then to periodically swap them side to side (eg at 50k mi) or when significant angle lag becomes noticeable in logging.

The AVCS solenoids are relatively inexpensive and can be installed in well under two hours with fairly basic tools.

The avcs issue seems less prevalent in prefl libs, perhaps this is due to the turbo & avcs oil feed being located on the right rear head, and the exhaust avcs solenoids being fed directly from the heads via the avcs ocv manifolds, however:

For dual AVCS prefl 2.0t, check if the exhaust AVCS solenoid O-rings (#1 in 5 in the diagram below) or the exhaust AVCS oil control solenoid manifold gaskets (#17 below) or the exhaust cam position sensors are leaking, the loom can suck oil all the way back to the ecu plug, causing multiple issues.

Misfire CELs can be related to AVCS.

Generally misfire CELs are calculated by the ECU and primarily generated from anomalies between spark timing events versus crank speed and afr input.

Log per cylinder misfires or per cylinder roughness.

If misfiring, check spark plug condition and swap around coil packs on the affected cylinder to check if the fault is cylinder or coil pack related, or coil pack loom plug related (usually broken plug clip). This is a blind test for coilpacks.

Generally a single cylinder misfire = coilpack or sparkplug issue.
The barb that holds the coilpack loom plug on is known to fail allowing the plug to back out causing continuity issues and misfires.

Blind test the suspect coilpack in another cylinder to see if the issue follows, and check the loom plug barb is intact on the coilpack. Replace or use a ziptie to hold the plug on the coilpack.

Bank 1 or 2 misfires: diagnose avcs

All cylinder misfires: loose crank pulley.

Get an LV Learning View) whilst you’re at it.
LV will reveal inlet leaks, IAM (low IAM = failsafe mode).
Regularly health check your car, including reading the spark plugs, and doing inlet pressure tests.

Its always a good idea to consult your cars FSM workshop manual to check the actual diagnostic procedures.

I’m always updating the info here as I do more research. Some items I need to clear up below.
Can skipped or stretched timing cause an AVCS CEL or issue?
What are the causes of gradual VVT angle lag, that eventually leads to one bank failing completely?

Let me know if you have any information to add!

Link to FSM workshop manual & parts info:

FAQ: Subaru FSM Service Manual Parts Procedures Diagnostics • club.liberty.asn.au


Some recent reference material taken from this post on FB Subaru Mechanics group:

https://m.facebook.com/groups/163728227 ... ef=m_notif

FA 2.0 BRZ gt86 avcs vvt ecu faq here:

https://www.armstrongfamilyblog.com/arm ... tion-p0018

Dual avcs exhaust solenoid, gaskets and manifold (scroll pic down if you can't see the business end(:
30E969D6-5D06-428F-BF3D-EC127582641E.jpeg (93.28 KiB) Viewed 1459 times
Last edited by bigBADbenny on Tue Dec 15, 2020 5:55 am, edited 2 times in total.
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Re: AVCS Checks and Diagnostics

Postby bigBADbenny » Sat Oct 10, 2020 1:36 pm

Recent updates!
Still to add:
H6 alvs & avcs.
FA avcs/ecu issue.
Snapped cam cap bolts.
Cam journal clearances.
Effect of vacuum leaks in heads affecting avcs oil drain/scavenge.
Use of an external oil supply & opr/s to feed ocv’s.
Links to related FAQs and DIYs.
Eg plug & loom oil contamination.
Cam position sensor R&R.
R&R guides for OCV’s and banjo bolts.
Seperate posts for some of the avcs faq info, eg misfires.

Got anything to add? Help me organise the info in the FAQ?
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Re: AVCS Checks and Diagnostics

Postby Jesse » Mon Nov 02, 2020 11:55 am

This is a awesome write up, but im having a issue with my 2013 wrx that seem to fall between the lines i have a p0021 code on my accessport it says avcs intake left is working for the first 10-20 mins of driving then it stops and stays at 0 or 1. And thoughts?
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Re: AVCS Checks and Diagnostics

Postby bigBADbenny » Mon Nov 02, 2020 12:01 pm

As described above in the faq.
You have initial sync but the system is unable to maintain it, gives up entirely so the engine isn’t hideously out of whack.

You can log avcs vvt angles plus tuning parameters on an open/closed loop drive with freeway cruise, post a link and I’ll graph it for you.

At this point you may want to swap solenoid sides to see if the issue follows or not (blind test for solenoid vs bank issues).
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Re: AVCS Checks and Diagnostics

Postby cowwws » Mon Jan 18, 2021 12:39 pm

Thanks again Benny for another great write up - I finally got around to removing and cleaning out my exhaust AVCS's and removing those pesky little filters. I had previously replaced the top AVCS but it turns out the exhaust ones were the real trouble makers.

No more annoying stumbling around 2300 rpm - happy days!
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