by JiPe38F » Sat Oct 06, 2018 11:34 pm
Hi gals and guys
I live in Grenoble, France. So be kind with my schoolish english... I purchased recently a Subaru Forester 2.0 l atmospheric, with 4EAT automatic gearbox. 2007 model 160 mkm. Nice car, I like it when it runs correctly, which is not the case most of the time. I had an issue with the auto gearbox : fault code, "power" lamp flashing. I finished to understand that there is a leak in the torque converter clutch (which cancels the slip when rmp is above a threshold in 4th gear). Torque converter to change... Some say that this issue could be caused by a too high oil lever in the gearbox, which creates spikes of oil pressure. Thanks to the guys (subaru ?) which serviced the car before I bought it.
I have another issue. Some times, more and more frequently, under 3500 rmp, my 158 hp forester behaves like the first car that I drove, decades ago, a Citroen 2cv : a vintage flat twin with 600 cm3 engine and 20 hp ! You can imagine the fuss with an auto gearbox : if it arrives in 4th gear, I accelerate, which makes the gearbox to shift down to 3re gear, the rpm go over 3500, and then my car accelerate like a dragster, until I release pressure on my foot, the gearbox shifts up to 4th gear, and so on ! WTF as would say your cousins from over the pond ! I began changing the ignition coils, resetting the spark plugs to 0.8 mm gap (they were nearly ok). Same as you said, everything went correct for a few km, time for the ecu to learn the engine parameters, and again the problem...
Lurking here and there on the net, I ended coming on your forum and this thread. Great ! I think you got the cause of the problem. I have read the "romraider" thread on the subject, with the solution proposed by Sasha in Russia. Now, let me add my two cents to the discussion.
Just a word about my personnal history. In a previous life, as was a robotics engineer. So I had to cope with some hydraulic robots. Fact is that sometimes, these machines get a parkinson-like behaviour, their hands shaking like those of a person with this deadly disease. Most of the time, the cure is simple : flush the oil off, clean the oil circuit, refill, purge air bubbles, and everything goes smooth again. The reason is that there are close loops including software - electronics - hydraulic valve - hydraulic cylinder (linear or rotating) - position sensor and, when oil gets old and/or dirty or when there is air in the hydraulic circuit, the loop characteristics change, which brings the closed loop to unstability.
Well, a bell rang in my brain. How does a acvs work in my EJ204 engine ? On the front of each intake cam shaft, the pulley contains a rotating hydraulic actuator, with a 50° angle range. It is fed with oil through an hydraulic valve, which is commanded by the ecu. In order to give the right advance angle to the shaft, the ecu reads a position sensor on the back side of the intake camshaft. I suppose that it calculates the difference beetween the angle measured by this sensor and two times the angle of the crankshaft, also monitored with a sensor. This value gives a measure of the hydraulic actuator's position, from 0° to 50° inside the pulley. The point to understand here is that this works as a position closed loop, like in the hydraulic robots ! You see where I'm coming to ?
I think therefore that when the oil gets dirty, the hydraulic circuit beetween the hydraulic valve and the pulley gets clogged with shavings from the engine wear, or if the oil pump introduces some air bubbles in the high pressure oil pipes, the closed loop characteristics get out of normal range, and the loop gets unstable. If the angle value is in mid range, the parkinson oscillation of the actuator inside the pulley can be unnoticed. But, if the angle value is around the maximum, this oscillation can cause the hydraulic actuator to get into mechanical limit, which causes a shock (steel on steel impact, brutal oil pressure spike). Now, the reason given by Sasha, our russian expert, that this shock is interpreted by the ecu, through the click sensor as "too much ignition advance", giving as a consequence a change to "retard ignition", this not modifying anything and... ultimately the ecu getting to maximum lag ignition delay, seems to me a good logic path. As for my car, giving the loss of torque on the engine, Citroen 2cv like behaviour, and some rattle noise which my wife once remarked, asking "why does your car make this noise ?" !
So, what are, in my opinion, the fixes for the problem ? I think that I will concentrate my efforts on the closed loop of the avcs system. First, change the oil in my car. It's true that it's getting more or less dirty now. I have seen worse on other cars, but now I understand that I must keep it super clean. I will fill it with 5w30 grade, as now, ignoring the risks, I used standard 5w50 to complete the oil level. If this doesn't fix the issue, I will open all the hydraulic components of the closed loop : pulleys on the intake camshafts, electro-hydraulic valve, clean all this, flush a detergent liquid inside the circuit, get off the steel shavings with a magnet. I will also open the crankcase under the engine (I hope it's possible...), clean all the crap inside it, see if there is a magnet with steel shavings stuck on it, get them off. Ultimately, if this is not enough, I will think of an issue that I had with a motorcycle : the ignition position sensor was an hall effect sensor. Taking it off, thinking it was to change, I discovered that it had steel shavings stuck on it by magnetic effect. I just cleaned it and put it in place again, and the bike got a nice engine run with super accelerations ! I can do it on the position sensors on the intake camshafts and crank position sensor.
Nice program, isn't it ? I was thinking of changing the torque converter, which implies to separate motor from gearbox : some say that on subs, the easiest way to do it is to take the engine out ! Well, once the engine is out, it should be easier to do all this service to the avcs loop, isn't it ? What do you think of my servicing program, guys and gals ?