Pac996

A weak pump can be adjusted for till its too far out of the elctronic management of fuel and ignition for proper running.

If you had said its been warm and running the a/c I might of figured its the a/c cutting in and dragging the engine down. I imagine if refrigerant is low and or refrigerant oil it might cause a low enough rpm dragging it down. I'm not sure if you mentioned the a/c emblem is on or not when running the car. I'm a little confused why the a/c light comes on when hitting auto for using the temp for heat instead of a/c but eh. Just might be the a/c cutting on causing drag. The front right below the winshield has what looks like and a/c valves block. You have to lift the panel that also covers the airfilter area to see it. It's also the a/c servicing area for high and low pressure coolant lines. There are 134 coolant bottles that come with a pressure reader. It will read your low pressure line. Try seeing if the coolant reads low and tht might explain some drag in the system when the pump clicks on. When coolant is low the system will automaticly shut down when pressure drops too low then click back on when enough coolant has accumilated. Anyway try checking the coolant level since summer is coming.

Macster

In my cars with autoclimate control when I press the auto button the AC is activated. The snowflake appears on the climate control LCD. The air is passed through the A/C evaporator to dry the air then this cool air is routed through the heat exchanger to warm it up. The autoclimate control varies the flaps inside the AC system to direct the proper amount of air from the AC evaporator to the heat exchanger to get the cabin temperature to the desired temperature and maintain it.

I don't know about other cars but the autoclimate control in my Boxster (and Turbo) are fantastic systems.

Sincerely,

Macster.

Macster

Well, it takes time. A minute is too long but it can take more than a moment or two. If after a minute the idle speed hasn't gotten better and smoother (though it may not be as smooth as it was with the cap in place) this could suggest an air leak elsewhere. With the existing air leak and then this new one introduced by removing the cap the engine controller simply can't cope.

At this point it would be nice to have an OBD code reader/data viewer hooked up and examine the long and short fuel trims before removing the cap, after, and after putting the cap back on. The long term fuel trims before the cap removed if at the rich end of the adjustment almost certainly points to an air leak somewhere (or a low fuel supply or pressure problem).

All doable if the car's at a properly equipped shop.

Sincerely,

Macster.

Pac996

If this were the pre electronics days I'd be thinking vapor lock being approached or......I don't YET understand the fouling of the AOS or what ever puts coolant out into the engine the wrong way but if gizmo's aren't throwing an error I kind of think it increases the chance of corrupted standard data items of air fuel or spark with out an error code detected. Once again BUT I'm just a simple guy staying away from saying or thinking I hate the modern gizmo's.

I don't know if there is heat shielding that got messed up and heates your fuel. Some times symptoms like the click are just another thing thing out there with nothing to do with the problem but highly noticed when looking for symptoms. Good luck.

Pac996

Thanks Macster. I've been turning off the auto by hitting the fan adjust then turning off the a/c. Seems the heater works with out the dryer running too. Thanks for letting me know what's going on in there.

eggodynamics

Question- Have you taken the air cleaner off and inspected the butterfly valve on the throttle body? No way the dealership guys didn't do this but you never know.
+1 for bad gas.

kj217

yes, i did cleaned the throttle body,, 2000 Carreras have no idle control valve,,
if it is bad gas,, should i take the tank off and clean it then fill it again??

rb101

Not to hijack a thread, but my 99 kind of does the same thing. When the car has been run for 30 min - 1 hour, if I'm sitting idling at a light, the RPM's sometimes fluctuate +/- 200 RPM and then it settles down to it's normal idle. It does not do this when it cold or just warmed up and mostly happens in the afternoon. I have no CEL or codes, new AOS 3K miles ago, all new O2 sensors 1K ago and cleaned the throttle a few weeks ago. It's odd, but I'm not too concerned at this point. My thinking is the throttle valve actuator is wearing out after 108K.

Rick
99 996C4
87 944S

Macster

Bad gas is too vague a diagnosis I believe to justify removing the tank and cleaning it. Or even draining it by opening up the top and siphoning the "bad" gas out. Fooling around with a gas tank is dangerous business and unless you're quite confident in your ability to handle this safely I'd leave it for the professionals. Besides there are various hoses, connectors, wiring that if you're not careful you can damage and add further to your misery.

Even if you are careful, these hoses, connectors and so on are getting on in years and are brittle, dry, and will not take kindly to be handled as you make your way to the tank and back out again.

Just how much work are you prepared to do to resolve this symptom? How much can you do? Without at least a code reader, data viewer to read codes, clear codes and view OBD sensor data in real time you're shooting in the dark.

Even with above you have little better than candle light to pull back darkness over what's going on.

Sincerely,

Macster.

Macster

In your car's case I believe it has a old style throttle. But you cleaned the throttle body. Was the butterfly operated by a cable from the pedal and lever or was there a black box on the side of the throttle body? If the former, then the throttle body has no electro-mechanical throttle valve actuator but instead relies upon an idle control valve for idle speed.

Many have reported removing this ICV and cleaning it thoroughly has seen any idle variation symptoms gone.

Sincerely,

Macster.

rb101

It's a 99 C4, e-gas, no idle control valve.

Rick
99 996C4
87 944S

Macster

Ok. E-Gas it is. My info is E-Gas first appeared in MY 2000 cars.

Regardless, it has what it has and if it has a throttle valve control module then this could be wearing out.

108K miles is alot of miles. My car has 230K+ miles and the throttle valve control module still works ok.

But different cars. Different throttle valve control modules. In both our cars' the throttle valve control module runs self-tests before every engine start to verify mechanical integrity.

Then during driving the throttle valve control module is continuously monitored: Throttle valve position; throttle valve ranges; throttle valve end stages; and voltage are all continuously monitored.

My point is that one would have to believe that if the device were worn enough to allow the air flow to vary enough to cause the engine speed to vary by a significant amount this would have an error code active.

Can't say if this will help or not, all you can do is try it you want to. Turn off all electrical loads. Headlights, etc. Remove the key from the ignition.

Following proper procedure disconnect the car's battery.

After a minute or so -- seconds is long enough but a minute's better or you can wait longer if you want -- reconnect the battery.

Turn the ignition key to on but not to engine start position. Do not touch the gas pedal. Wait at least 10 seconds for the throttle valve control module to recalibrate its adaptation values.

If it doesn't help could be the throttle valve control module but I'd not advise you to replace it on the strength of result of the battery disconnect/throttle valve control module recalibration I mentioned above.

Sincerely,

Macster.

kj217

to share the same experience,,
i have uploaded a video of the symptom,,

http://www.youtube.com/watch?v=4Taz4CJiSnw

Macster

When the rpm drops can you hear any other noise? I'm thinking some parasitic load kicks on. A/C compressor. Electric fans.

That kind of a period -- about a minute by my watch -- between rpm drops doesn't follow a vacuum leak induced rpm drop.

My WAG is a fuel supply problem but a real odd one (if I'm even correct).

While engine running if you could arrange to listen to the fuel pump even going so far as to raise the front of the car and get safely under the car and with a stethescope pick a good spot to listen in on the fuel pump to see if you can hear the pump slow down or stop momentarily I think might have some value, be worth the effort. It might be of some value to listen at the gas tank filler opening with the cap removed to hear any sounds from inside the tank.

If there are any fuel hoses -- rubber like -- watch them to see if they twitch or move or collapse or do anything at all when the rpms drop.

If you are prepared to do the work you could set up a fuel pressure gage and watch the fuel pressure while the engine's running and see if the pressure changes.

Or if you don't want to mess with a fuel pressure gage -- and these can be messy -- then with an OBD code reader/data viewer connected the car's OBDII port watch the short term fuel trims to see how they behave before, during, and after the rpm drop.

I know I know kind of an all over the map series of suggestions but really this is a process of gathering info and trying to find something that provides a clue as to what's going on.

Refresh my memory: Since this symptom pretty easy -- I take it -- to reproduce -- why haven't you taken the car to a Porsche dealer or a good indy shop for this symptom?

Also, you were able to reproduce the symptom with the MAF disconnected at the wiring harness, yes?

Sincerely,

Macster.

redridge

when you unplugged the maf.... does it behave the same way? If it does, you may have a bad maf, even though you cleaned it. You really need to start eliminating hardware to properly troubleshoot.... maf, throttlebody, gas, pump, filter etc, etc... but do it one at a time, surely its one or more on the list.