zak911

I've gotten a lot of good PPI advice from the threads on this forum. One of the recommended tests was a "cylinder deactivation test" using the PIWIS. The tech would singly disable each cylinder and note the engine RPM drop. Car passes test if all RPM drops are equal.

In discussing a potential PPI test with a Porsche dealership (I specifically mentioned an 08 turbo), the shop foreman didn't think that it could be successfully done on a newer Porsche as they have an electronic throttle which would return the RPM to the proper setting. He even yelled the question to one of his techs while we were on the phone.

Is this test not viable on a newer car with electronic throttle? or does the throttle get deactivated during the test? or can you note the RPM drop prior to the electronic throttle reacting? or what? The shop foreman is also interested in the answer.

Thank you

Michael-Dallas

Yes, I believe you note the RPM drop before it returns to idle speed. Consider that cars w/ throttle cables have some sort of idle-air control valve to control the amount of air in at engine idle; e-gas (or electronic throttle) cars should have similar tech. When I owned a 300ZX Twin Turbo years ago, the tech hooked up CONSULT (Nissan's version of PIWIS) and pushed a button on the menu; it executed the test and printed out the test results on a receipt so the tech did not have to write down any numbers or do anything. It was really that simple.

Here is more info on the power balance test: http://www.the-crankshaft.info/2009/...ance-test.html

///Michael

TT Surgeon

Never heard of this test, wonder how accurate it might be?

zak911

Does anybody know for sure if and how this can be done using a PIWIS?

Macster

I plan on asking a Porsche tech. It should be doable but the steps I do not know. And it may not be doable. It would be a big oversight in my opinion if the Porsche diagnostics tool and engine controller didn't support this test. It is a very useful test.

Oh, may not get to ask until after the 1st of the year. Planned on taking my Turbo in for an oil/filter service and asking then but just don't have time in my schedule this week.

Sincerely,

Macster.

Macster

Ok. Asked Porsche tech about this test this AM.

He says "not needed". (Supported but not needed.)

Why not I ask? Cause the engine controller constantly measures each cylinder's contribution to the total engine output, says tech.

I'll make this as brief as I can and just relate what I learned with out the back/forth...

The engine controller with the crankshaft sensor can and does measure the acceleration imparted to the flywheel each and every firing pulse.

If the engine controller detects an out of range acceleration condition (and over a span of time) it will set a misfire error code and light the CEL. It may in fact flash this CEL.

Now, the misfire can be due to spark, fuel, or electric/mechanical problems.

Spark may be a bad plug, bad coil, or cylinder fouled with oil (or coolant).

Fuel can of course be a bad injector, leaking, blocked, or just heavily deposited injector.

Mechanical can be a bad lifter, burned valve or if plug misfiring due to oil a worn piston, or a failed ring (compression or oil control). Or it may be a bad VariOCam actuator.

Electrical problem may be bad MAF or a bad sensor somewhere. (Can also be due to rodent infestation.)

And of course there are numerous combinations of the above possible causes.

The above is all well and good -- and I appreciate the time the tech took to go over the above with me -- but I have had time to think about this since talking with the tech this AM and have some thoughts on this.

The CEL may not be on. Now it may not have ever been on. Or the CEL may have been extinquished just before a prospective buyer is to view the car.

(And of course the CEL bulb must be present and not burned out, which means when the key is turned on the light should come -- all warning lights should come on -- and all should go out either seconds after the key turned on or after the engine has started.)

In the case of the possibility the CEL was just turned off the engine controller should be queried for some info that relates to this.

To query for this info probably requires access to a Porsche diagnostics computer.

The info is: Distance Traveled While MIL Is Activated. 0 to 65535km. Reset to 0 if diagnostics info cleared or 40 warmup cycles without MIL activated.

Number of warmup cycles since diagnostics error codes cleared. 0 to 255. If 255 count freezes.

Distance since diagnostics trouble codes cleared: 0 to 65535km.

Also, and this is important, now the above info may have been reset when a battery was disconnected. Or it may have been reset on purpose to mask a possible problem with the car or engine.

Which? Who knows?

Now if the above info indicates the CEL has not been cleared in a long time, if the number of warmup cycles is high or even at 255 (it does not wrap to 0 but stops at 255) this is a pretty good indication the CEL has not been cleared in a long time.

OTOH, if it is a low number...

One possible way would be to subject the car to a thorough test drive. A test drive that seeks to within reason give the engine every opportunity to manifest a misfire condition and the engine controller every chance to catch this and flag it with a CEL.

Thus I could still a good reason to subject the car to a cylinder deactivation test if one encounters a car that has recently had its CEL and error codes cleared.

The test would be quicker to identify a bad/weak cylinder than an extensive test drive. Also, the question becomes how sensitive is the engine controller with its misfire detection scheme? It can't be too sensitive cause people would be flooding the dealer with false alarms cause by overly sensitive misfire detection.

OTOH, if a cylinder weak enough to cause no engine rpm drop or less than any other cylinder one would think the engine controller would catch this by the change of acceleration of the flywheel whenever this weaker cylinder's firing pulse occurred.

Sincerely,

Macster.

zak911

As always, excellent input, Macster.
Thanks