thstone

I have a similar problem: 1999 C2 with rough idle at all temps (cold, warm, and hot). No CEL. I've replaced the coil packs and spark plugs. Throttle body is perfectly clean. Rough idle is unchanged with and without MAF connected. Disconnected battery to reset ECU, no change. Again, no CEL.

Local indy checked everything, could not find any problems. They are competent, having found vacuum leaks on my previous Boxster. They gave up.

Have appt on Friday at local dealer. Will report what they find.

Imo000

Did they do a leakdown test? Rough idle could easily be compression related too.

Nedster

Imo if you read those two dealer reports i posted in my second last post you can see they did both compression and vacuum leak test it seems.

thstone

Dealer says that the idle control valve has failed and needs to be replaced. Will have to order part so repair won't happen until next week.

Nedster

Glad they found the reason. Do i even have that on my 2004?

Macster

No.

Macster

The misfires and lower compression on bank 2 are clues.

I would suspect a VarioCam Plus solenoid/actuator is acting up or the Plus (variable valve lift) is faulty. My money is on the Plus.

The DME will attempt to adjust fueling to bring the O2 readings into line and this can go far enough the engine can misfire. The engine can misfire too if the valve lift is not what the DME commanded. Fueling, ignition timing, and valve timing are all selected with the proper valve lift being in effect.

The lower compression numbers on bank 2 cylinders suggests the cylinders are not filling as completely as those on bank 1.

There is a road test that can be done -- it takes a Porsche diagnostics computer and two techs to do the test -- that verifies the VarioCam Plus variable valve timing and lift are working properly.

However, I trust temp sensors were checked for reasonable readings? An air intake sensor or a coolant sensor that reads goofy for awhile at cold start or after can cause problems.

I have to point out that tearing into the VarioCam Plus system is an expensive undertaking so one wants to be sure to eliminate other possible causes of the symptoms before well, undertaking any work on the VarioCam Plus hardware.

Nedster

Thanks Macster, my dealer is still consulting other dealers for ideas on this, i will definitely run your suggestions by them.

How does VarioCam Plus act during start up and open loop operation? My idle is the worst at the beginning. It just doesn't stay high when i first crank the engine. It drops the same instant i start the car to the point it almost kills the engine, the whole car shakes for a bit, and then it shoots up. Don't understand what could tell the engine to drop the RPM's right away while the engine is still cold! A faulty Temperature sensor maybe, but i would've thought they would check that before they did compression test!

This is great info from in any case, i really appreciate it. Thanks!

Macster

Briefly at cold start the VarioCam Plus selects low valve lift.This reduces pumping losses and reduces fuel consumption and emissions at this time.

In open loop mode my understanding is low lift is selected under some low rpm low speed low load operation conditions. The same reason: to reduce pumping losses and improve fuel economy.

There are various things that can cause the rpms to rise then fall as they do in the case of your car's engine.

Too many to cover here. Besides I'm sure the techs' list is even longer, more complete than mine. Many (probably all but one) do not apply in the case of your car.

But a lot can be eliminated because what your engine has is a failure of some kind limited to one bank. This rules out a lot of common failures and this includes any related to the intake air temperature (with one exception I mentioned in an earlier post and which I won't cover again here) or a coolant temperature sensor which I suggested in an earlier post forgetting that only one bank had the signs of trouble.

The failure limited to one bank should -- but I'm not a trained/experienced Porsche tech -- should have the focus on what bank systems are separate.

Fuel lines are one. But this does not look to be a fuel line problem.

What it looks like is a VarioCam Plus problem.

Since the engine is cold the 02 sensors are not working. Thus the DME uses fuel/timing maps. These are based on the engine being at a certain level of operational goodness.

So the engine starts. The DME works the sensor numbers (MAF, intake air temperature, coolant temperature to name a few) and comes up with fuel injector pulse widths.

The pulse widths apparently agree with the needs of cylinder of one (the "good") bank but not the other bank.

Now how can the two banks differ in the amount of air they receive and thus react differently to the same amount of fuel?

Well one answer is the intake valves of the problem bank not being in the state the DME has commanded.

Now cam timing differences can be detected. The DME selects an advanced timing setting and then uses the camshaft position sensor along with crankshaft position sensor to know the cam timing is where it should be.

But there is no real way to know the valve lift is as it should be. There are no sensors which measure this directly.

There are ways to confirm to a certain confidence level the lift is as it should be.

Once the O2 sensors are hot enough to work the DME can use their readings to know something's amiss and adjust fueling at the problem bank to try to bring the 02 sensor readings of that bank into spec. But it probably won't succeed. The effects of the low lift failure is so severe a condition the DME can't adapt to it.

There is the possibility if the engine was run long enough -- I am *not* advising you do this -- there would error codes related to the rich or lean threshold of one bank being reached.

However, I was told when my Boxster's VarioCam solenoid/actuator failed the cylinders can misfire before the DME reaches some rich or lean threshold.

Please do not use the above as a club to beat your techs over their heads to get them to look at the VarioCam system.

But do discuss with them and get their input. If not VarioCam what then could account for this behavior, the misfires?

What can be done with reasonable effort and cost to eliminate other possible and reasonable explanations for this behavior and other signs of trouble?

Nedster

Figured i would just fill you in on the latest on this issue. I have been persistent with the dealer on resolving this issue, and they took my car in again. They can't seem to get any codes out of the car other than old misfire, so they disregarded the complete variocam solenoid theory. They think it is flywheel that is causing my rough idle when started cold. I was skeptical at first, and still am a bit, but did some reading and apparently a bad DMF can cause misfires and rough idle. DMF should be in on Wednesday.

Barn996

At this point, what has been replaced by the dealer, but hasn't fixed your problem?

washington dc porsche

MAF or Air Filter?

Nedster

Nothing, they replaced nothing. They acknowledged that there is an issue, but whatever testing they did with their diagnostic tools did not produce any definitive results that would point to an issue with a specific part. How they pin pointed the flywheel this time around, i don't know. I do have that loud clank if i shut the car off without holding the clutch in, which as far as i know can point to a DMF, but other than that, i have no idea.

alpine003

I'm not very fond of dealers or mechanics just throwing parts at it and guessing on my dime. I hope things work out for you.

Nedster

Even the flywheel will be replaced under either the third party warranty that i have or they will pay for all the repairs/parts. I bought the car from them and i explained to them that obviously the car came with this ongoing issue, and they agreed. All these tests and diagnostics has not cost me anything.