Box4VIR

Starter986

Macster

Too bad there is no AFR, timing, MAF telemetry to view plotted on the chart along with the torque and HP.

With my 2.7l 2002 Boxster the fall off in power approx. coincides with the RPMs at which the VarioCam system is activated. From memory the system is activated at around 1400 RPMs to improve low speed operation (but I don't recall when it is deactivated) and then again at around 5200 RPMs to help with high speed operation.

You of course have a 2.5l car. I do not know the behavior of the VarioCam system in that engine but you should.

If you can clear the codes, even if none are present, and if you can then just road test the car. Nothing fancy just drive it normally. See if any active codes appear. Now you might get the O2 codes but with a code reader connected you will know this and continue to drive the car the idea is by doing so you give the DME time to recognize a problem (if there is a problem) with the VarioCam system or possibly with the MAF.

Afterwards check for any active or pending codes pertaining to the VarioCam system or MAF or really anything. Write them all down and make sure you get the freeze frame code and data too.

Have to mention the absence of the #2 O2 sensors can be an issue. The DME uses the readings from these sensors to fuel the engine to try to get the #2 O2 sensor readings it expects to get from working converters and sensors.

With the sensors removed and if the DME has not had the ROW "tune" applied (which eliminates the DME from monitoring the # 2 O2 sensors) the absence of these can mean the engine is not running optimally. This may not account for the drop off on power but can still result in the engine not running as good as it could.

What do the spec Boxster owners do about the converters and #2 O2 sensors?

Box4VIR

Thanks for the reply! I'll connect the O2 sensors, the shop I'm working with is convinced thats not it but starting from everything working seems like a good idea. We have reset and road tested the car multiple times, no codes of any kind. I've tried two new MAF sensors, same results.

The power loss is due to the car leaning out. We compared it to another Boxster and the fuel maps in open loop are different. Correct me if I'm wrong here, when the car is floored its running in open loop right? So its only going off of a pre-set fuel map, so its not the operation of the MAF or O2 sensors but the fuel map is wrong. If it has the wrong map the car doesn't know anything is wrong and therefore isn't throwing a code. If this line of thinking is correct than why would the ECU have the wrong map?

One other odd thing is the car had a problem a couple months before this dyno where it simply would not run at all. Also throwing 0 codes, it would start and run but if you gave it gas it would lean out and stall. If you unplugged the MAF though and forced it to run in open loop it ran great. After enough ECU resets it magically fixed its self. I'm starting to think the ECU is bad.

Sveach756

Might be silly but are you sure your o2 sensors are plugged into the right sensors and not swapped front to rear harnesses? The fact that it died last month when in open loop is interesting. It's a shot in the dark but maybe worth a check.
Shawn

Macster

Open loop mode may not be the mode the controller enters under hard acceleration.

Open loop is a cold start condition under which the controller fuels the engine by using other means to determine the proper mixture other than the #1 and #2 O2 sensors. These are cold, as are the converters and they don't work. Under normal conditions after a cold start after just a brief idle time the engine controller switches to closed loop mode. This is signaled by the cold idle speed dropping to near hot idle speed and in the case of my 2002 Boxster the secondary air injection pump shutting off. I verified the controller went into closed loop mode at this time by viewing the operating mode via an OBD2 code reader/data viewer/logger connected to the OBD2 port.

However, I have to admit while I am pretty sure I also monitored the operating mode under hard acceleration I do not recall if the controller switched from closed loop mode to open loop mode.

Once the sensors/converters are up to temperature then closed loop mode is entered. In this mode the engine controller uses the #1 and #2 O2 sensor readings to properly fuel the engine and to monitor converter performance/operating efficiency.

If one presses the gas pedal hard enough the controller can go into a mode that allows it to ignore the #2 O2 sensor readings and deliver a richer mixture conducive with more power. The controller still relies upon the MAF and the #1 O2 sensors for fueling it is just the leaner fueling is no longer required as the #2 sensors are no longer being monitored..

Can't explain the difference between the open mode maps of the two controllers other than this may arise if the two controllers are from different MY or different sized engines. Or one controller is bad and a section somehow got corrupted. Not sure but I would think the controller firmware would have some mechanism to detect corruption of these maps (a CRC perhaps) but maybe not.

Do you know the history of your car? Could the DME have been modified by a previous owner applying some aftermarket "tune".

A bad engine controller is I guess a possibility. It was quite common to in the OBD2 section of my Boxster references to for various error codes the DME being at fault was one possible cause for the error. However, in every case the DME being at fault was at the bottom of the list. Still doesn't mean it can't be at fault here.

Any signs of water around the DME? Any signs of rodent infestation? Gnawed wires, hoses? Litter on top of the underbody plastic panels? On top of the engine?

mikefocke

Brett posted on PPBB:The O2 sensors do just that, sense the amount of O2 in the exhaust gas relative to the amount of O2 in ambient air. Perfect combustion of a perfect mixture of air and fuel (around 14.7/1 air/fuel ratio) leaves behind only CO2 and water as products of combustion. All the oxygen gets consumed in the combustion and combines with all the carbons and hydrogens. If there is not enough fuel (lean mixture), then all the fuel gets burned leaving some oxygen left over. Conversely, if there is too much fuel (rich mixture), then all the oxygen gets burned leaving behind extra hydrocarbons (fuel). Now an oxygen sensor outputs a voltage between 0 and about 1 V depending on the difference between the amount of oxygen in the exhaust and the amount of oxygen in normal air. If there is a lot of oxygen in the exhaust (lean mixture condition), the sensor outputs close to 0 volts. Conversely, if there is no oxygen in the mixture (rich condition), then the output is close to 1 V. These O2 sensor voltages are read by the computer. This is the feedback loop that tells the computer how the engine is performing with regard to air/fuel mixture. It's impossible for the computer to hold the exact perfect air/fuel mixture constantly, so the way mixture control is designed is for the computer to continually adjust the mixture from very slightly rich to very slightly lean and back again using feedback from the pre-cat O2 sensors. This means that the pre-cat O2 sensor signal will oscillate back and forth from high to low to high to low voltage as the computer adjusts the mixture. In a normal running engine at idle the signal goes from low to high voltage and vice versa about every 1 second, with a transit time from low to high (or vice versa) being about 200-300 milliseconds. This transit time is important because as an O2 sensor ages, the transit time gets longer, and eventually it can get too long such that the computer will call it a malfunction and signal a check engine light and fault code for a slow responding O2 sensor. O2 sensors need to respond to mixture changes quickly so that the computer can keep up with the proper mixture adjustments. So the bottom line is that the pre-cat O2 sensors should oscillate between about 0.2 to 0.8 volts regularly (about every 1 second at idle) in a healthy engine. The post-cat O2 sensors are identical to the pre-cat O2 sensors (same voltage outputs). They are there only to monitor the performance of the catalytic converters. So, as discussed, the pre-cat sensor signals are oscillating between 0.2-0.8 volts. Once the exhaust gasses pass through the catalytic converter, most (all, in theory) excess fuel (hydrocarbons) will be combusted thus reducing hydrocarbon emissions. The cat uses oxygen in the exhaust to combust the fuel. So what you end up with in the exhaust after passing through the cat is a gas mixture that is reduced in hydrocarbons and reduced in oxygen relative to the mixture entering the cat. The post-cat exhaust gas mixture should be CONSTANTLY low in oxygen if the cat is doing its job of burning excess fuel. Therefore, the post-cat O2 sensor signal should be a constant lower voltage reading (not oscillating). So, if the post-cat O2 sensor is seen to oscillate just like the pre-cat O2 sensor, that means that the post-cat sensor is seeing the same gas mixture as the pre-cat sensor meaning that the catalytic converter isn't doing its job of burning excess fuel. The computer monitors the post-cat sensor and compares it to the pre-cat sensor. If the signals are similar, it assumes the cat is bad.

Box4VIR

Thank you so much for this information! People that really know these details are hard to come by. The car was bone stock when I got it so I doubt it has an aftermarket tune but it could, its been a track car for awhile now and I've had it completely apart several times and have never noticed any water or rodent damage. Do you know if the car changes its controlling approach during the cam roll over? For instances does it run in open loop during this period, or read the rear O2 sensors? The car has headers, an open exhaust and a de-muffled intake tube. Its possible any original fuel map it might be relaying on during cam rollover is to lean for the freer flowing engine.

Macster

As far as I know -- and I observed this with my 2002 Boxster -- when the cam timing was supposed to change I never noticed the engine controller changing operation modes. The idea was the change in timing was to be as transparent as possible with no outward sign of anything major (or minor) going on. The engine's behavior was so smooth -- more than one passenger referred to the engine as being sewing machine smooth -- that I'm sure Porsche wanted to avoid any risk to this behavior.

Not sure what muffler there is with the intake tube to remove. There might be a plastic screen just downstream of the filter box and just upstream of the MAF. This needs to be present as it is there to massage the air flow to make it more linear for the MAF. All kinds of weird behavior can arise if this is removed, or as I found out, if any trash (leaves) get in the air box and end up plastered to the screen.

Starting with 2000MY the intake was equipped with a resonance flap controlled by the engine controller which activated this at around 3K RPMs (and deactivated it at around 2800 RPMs). This flap was there to change the effective length of the intake runners which helped improve cylinder filling at higher RPMs. For this there was a bit of what we used to call coming on the cam (or with 2-cycle racing motorcycles, coming on the pipe). The engine manifested a bit of extra, well, enthusiasm (?) as RPMs climbed and the resonance flap was activated.

Have to say too based on my 16 years 317K miles of time with my 2002 Boxster I found the engine to be a real marvel. For 2.7l it had impressive torque, response at the bottom end, took mid RPM usage in stride, and even at 317K miles pulled hard all the way to red line. A real nice piece of internal combusting hardware. I almost tear up just thinking about it it was so wonderful.

This was due to level of engineering the engine received from the air intake on the side of the body to exhaust tip. The engine and all its supporting intake and exhaust hardware was a total package designed to flow air -- which essentially is what a reciprocating internal combustion engine does -- effectively and efficiently.

Now I know you have a 2.5l engine and while I have no direct experience with this engine a number of 2.5l engine owners have reported that it is one sweet engine and some prefer it over the 2.7l engine.

To put on a set of header, remove the converters, fit an open exhaust, de-muffle the intake, you may have (almost certainly have) upset the air flow through the engine and probably (based on what you are experiencing) for the worse. All is not lost though. What you have done to the engine's "tune" may be addressed by a proper tuning session on a chassis dyno with the proper equipment to monitor the exhaust gas and other telemetry to come up with a custom tune to make the most out what changes you have done and restore all the HP that has been lost maybe even add a bit more HP.

You can continue to search for a magic bullet to cure what ails the engine and you may find it, honestly I hope you do and soon, but keep in mind that tune thing I mentioned.

Box4VIR

Byprodriver

Check fuel supply & pressure. Lot's of old pumps failing, sometimes slowly.

Porschetech3

My guess would be you have a failing vario-cam solenoid. One that is drawing too much power when activated at times. The 12v power that goes to the vario-cam solenoids also goes to the MAF sensor and other sensors with 12V. If the vario-cam solenoid is pulling too much power it can drop the voltage on the MAF down a volt or two and would be reading a lower OUTPUT ie. less air flow indicated=less fuel delivery commanded.NOTE: the 12v to the MAF, vario-cam is actual battery voltage and can be as high as 13.5v at RPM...

I would suggest you put a simple volt meter directly to the MAF 12v wire and good ground and road test to above 6k to monitor values when the vario-cam activates.. If you see a considerable drop in voltage as the vario-cam activates, then OHM test the vario-cam solenoids. Spec I believe is 13ohm?

NOTE; if this is the case, the reason there is no codes set is 1. You are in open loop 2. even though the MAF is giving "false " information, the DME has no way to tell or "double check" the MAF information..and believes the information to be somewhat plausible.