DME not activating variocam?
07-01-2011, 02:51 PM
#16
Race Director
Thanks for your input macster. I never thought about the Dme adjusting fuel injection.
I don't think I understand why the cam timing would be out with a failed variocam though, doesn't variocam just retard ignition at certain revs for performance gains?
I would like to know what actually fails in the actuator.
I don't think I understand why the cam timing would be out with a failed variocam though, doesn't variocam just retard ignition at certain revs for performance gains?
I would like to know what actually fails in the actuator.
VarioCam is a system (under DME control) which allows the DME to vary the intake cams' timing. VarioCam Plus is timing variation along with lift.
The intake lifters have a low lift and a high lift portion and the intake cam lobes have a dual lobe configuration that depending upon which portion of the intake lifter is activated produces high (normal) lift or low lift.
The cam timing would not agree with what the DME expected, either from sensor readings or the o2 sensor readings would not fall into the acceptable range.
The VarioCam timing is there to make the engine a multi-purpose engine. Under some operating conditions the cam timing (and in the newer cars cam lift) is adjusted (retarded) to improve low rpm response and reduce emissions.
At other rpm points (and there are other things other sensor inputs the DME considers as well as rpms) the timing is adjusted (or returned to its normal timing) to help the engine make more power. Then at even higher speeds (above 5K rpms IIRC in at least the case of my 02 Boxster) (and again under certain other conditions) the cam timing is retarded again to improve steady speed cruise fuel economy.
What fails in the actuator? I wish I knew.
But I have a guess. The actuator is just a fluid valve system and relies upon oil and oil pressure to function. It is like the valve body in an automatic transmission (in some respects to be sure but also to be sure the valve body in the automatic transmission is a much more complex system). After so many miles so many triggers of the actuator and of course the underlying valving that controls this the valves/bores wear out.
Anyhow, I have the one out of my Boxster's engine but the actuator is just a rather big aluminum housing with a cam chain rail/guide fixed to it (and of course under control from the innards of the actuator and I might add the guide rail facing had no signs of any wear). The actuator based on what I remember thinking when I looked at it, does not appear to even be something one can disassemble with any ease. The tech told me there are no servicable parts inside which is why they are simply replaced when bad.
Maybe it is time I pulled the old actuator out of the trunk and brought it up to my office to have a closer look...
Sincerely,
Macster.
07-01-2011, 03:23 PM
#17
Three Wheelin'
On the 3.4L the cam mechanism is welded together.
The actuator looks to be operated by a port that is opened/closed by the solenoid which allows oil to increase/decrease tension on the pistons and chain and cams. There is a plastic piston for that port which seems to have a spring under it as well.
The actuator looks to be operated by a port that is opened/closed by the solenoid which allows oil to increase/decrease tension on the pistons and chain and cams. There is a plastic piston for that port which seems to have a spring under it as well.
07-01-2011, 03:39 PM
#19
Race Director
I have pics of mine. I'll upload them later today after work. The housing appears to be welded together. I see no signs of wear. The rail covers are unmarred. I have a pic of a new actuator with new rails and covers and pics of my old actuator with 240K+ miles on its rail covers and I can't tell the difference. There appears to be no discernible wear of the rail coverings.
One rail's piston comes out and allows the piston/rail assembly to come from of the housing I see no signs of wear on the pistion's obvious sliding bearings. There is a plastic (composite plastic) rod in the spring in the rail's piston and the rod's ends show no signs of wear. In one pic one can clearly see the number of the part on the end of the rod.
The other rail's piston is while movable is not removable but like the other shows no signs of wear from the exposed diameters and both pistons exhibit no scary amounts of play when in the housing.
If one didn't know the actuator I have on my desk didn't work as far as I can tell there is no way to know it doesn't work by looking at it. In fact there is no way to tell how many miles the thing has on it. Those composite plastic rail covers look like they are essentially just out of the box new.
Sincerely,
Macster.
One rail's piston comes out and allows the piston/rail assembly to come from of the housing I see no signs of wear on the pistion's obvious sliding bearings. There is a plastic (composite plastic) rod in the spring in the rail's piston and the rod's ends show no signs of wear. In one pic one can clearly see the number of the part on the end of the rod.
The other rail's piston is while movable is not removable but like the other shows no signs of wear from the exposed diameters and both pistons exhibit no scary amounts of play when in the housing.
If one didn't know the actuator I have on my desk didn't work as far as I can tell there is no way to know it doesn't work by looking at it. In fact there is no way to tell how many miles the thing has on it. Those composite plastic rail covers look like they are essentially just out of the box new.
Sincerely,
Macster.
07-01-2011, 03:44 PM
#20
Three Wheelin'
Reminds me of and looks like a shift solenoid pack I just replaced on a PT Cruiser. The transmission stopped shifting, but when I had the sealed part out, it looked fine to me. The new one looked the same, but the car started shifting again after it was installed.
