aowolf

About the car: 1999 tiptronic 166,000 miles.


What the car is doing: when going along at slow speeds (under 40mph) the throttle is inconsistent. For example if the car is at 1800 rpm it will randomly jump to 2000 rpm with no change in throttle input, it will then quickly return to 1800 rpm. This is most notable going up hill. The car feels low on power.

What I have done: I've changed the oil air separator, the spark plugs, cleaned the throttle body and idle valve. Within 4000 miles I changed the air filter.

It almost feels as if the MAF sensor is going bad but, it has always coded when their was a MAF problem in the past. I've noticed that I can't get the oil cap off while the car is running and wonder if this means there might be an air leak?

Macster

Unable to remove the oil tube filler cap while the engine is running is a sign the AOS is bad. I would hazard a guess the replacement AOS is bad or perhaps there's a problem with its installation.

aowolf

Could be be something besides the AOS causing the oil cap to be stuck? The car stopped smoking as soon as I replaced it. And the car runs far worse if I create a leak in the line from the AOS to the throttle body. It's possible that I broke the plastic flex tube to the AOS but I haven't seen any new oil leaks.

Macster

My experience with the first AOS was it created so much low pressure in the crankcase the oil filler tube cap while I could unscrew it I could not remove it from the tube while the engine was idling. I could remove the dipstick but heard the obvious sounds of air being drawn in which given my experience with other cars was out of character, as was of course the inability to remove the cap.

By stuck if you mean you can unscrew the cap but can't remove it that's a symptom associated with AOS trouble.

But if you turn off the engine and can't remove the cap, even after a few minutes to give any low pressure time to dissipate then something is physically preventing the cap from coming off and that's not an AOS caused symptom.

When you create an air leak at the AOS hose that connects to the intake just behind the TB and the engine reacts that's to be expected. If the oil filler tube cap is hard/impossible to remove to due to low pressure in the engine from a failed AOS there's no leak (or a very tiny one past some seals though some owners have reported hearing a howl which is air being pulled past/through the rear main seal) so the engine is being fueled properly until you create an air leak.

If you broke the tube oil leaks may not appear right after maybe not for some time and maybe not every since the AOS may not be working properly and it is not removing any oil to leak at that point. Also because the engine is "sick" you may not be using the engine in a normal fashion that would have oil leaking.

You need to double check and triple check the AOS installation and all hoses/fittings to be sure you cracked nothing and to put it succinctly you didn't screw something up during the AOS repair.

It is a case of the last thing touched first thing suspected.

aowolf

I reinstalled the AOS this morning and doubled checked all the connections. There was some oil in the J tube going to the intake, and a small air leak on the same tube. I fixed the leak. I played around with the oil cap, oddly after first starting the car today it wouldn't budge then after a few minutes I was able to get it off. At this point it goes on and off fine with the engine running. I couldn't feel any pressure in the dip stick.

The car however is still has a consistent throttle wavering while driving uphill. I should have mentioned earlier that the weaver appeared suddenly. I came out of a building to drive home when I first noticed it.

If I plug the AOS and drive around for a few miles, can I confidently eliminate as the cause of the wavering?

Macster

I'm not sure how you would plug the AOS and I can't recommend you do this even if you can figure out a way.

A consistent throttle wavering while driving uphills raises the possibility of a fuel supply problem.

Are you sure the fuel tank has plenty of fuel?

There is also the possibility of water in the fuel. Someone a while back posted about some issues with his car -- Oh, it was starting issues -- and he found the fuel tank had a lot of water in it. Maybe your car's tank has just enough to cause the symptom you are reporting.

How many miles is on the fuel filter?

One test you might consider is disconnecting the MAF at the engine wiring harness. Be careful to avoid static discharge into the MAFconnector or into the wiring harness connector.

Clear error codes even though there are none (the CEL is dark) to reset learned fueling values to their defaults, then drive the car normally and see if the same behavior is present.

If not then connect the MAF, clear the codes (reset the learned fueling values) again, and drive the car normally and see if the symptoms return.

While I do not expect this to happen just so you are prepared if at any time the engine starts acting up worse, makes noises, etc. shut off the engine ASAP and do not start it again. You are through diagnosing the wavering symptom. You have much bigger fish to fry. Flat bed the car to a qualified Porsche tech for his professional assistance.

aowolf

Pelican's AOS guide says you can plug both ends leading to the intake to see if performance temporarily improves.

The car is a daily driver and I've been driving it for a few weeks since the problem started. Its been refueled 2 or 3 times. There is at least 70k on the fuel filter. I'll go test the MAF now.

aowolf

Ok, tried the MAF same rpm jump happening every few seconds going uphill. Whatever is "kicking in" adds quiet a bit of power nearly instantly, I can't add that much power that quickly, in the same gear, by stomping on the gas. Maybe its an injector issue?

Macster

It is not a common injector failure mode to change its behavior so dramatically then revert to normal operation. Generally they leak or they do not. They have a bad spray pattern or they do not. They work or they do not.

And one acting up would I think be hard pressed to account for the burst of power you report. So you are looking at more than one injector failing in an uncommon way.

The cause of the symptom is a common failure. One such common failure mode could come from fuel supply or fuel pressure.

To state the obvious sufficient fuel supply and at the right pressure is a factor of the fuel pump, the fuel filter, and pressure regulator, ignoring of course the fuel lines and tank venting/vapor lines.

Now if fuel pressure was tapering off the DME would adjust by increasing the amount of time the injectors were open. Then if the fuel pressure suddenly recovered this might account for the burst of power.

However, without extra air the extra fuel just makes the mixture overly rich so I would think the engine would just stumble not perk up.

Which then leaves the intake system, specifically the throttle. Now if your car had e-Gas I'd suspect that but IIRC the 99 is pre e-Gas.

About the only thing I could think of the account for the behavior would be a binding throttle cable/throttle/butterfly valve. It binds then unbinds and opens wider and you feel this with a burst of power.

However you report you can't replicate the burst of power by stomping on the gas pedal which sort of eliminates the binding theory.

Too bad you do not have an OBD2 code reader/data logger to log or at least view OBD2 like short term fuel trims, MAF, timing, and some other things before, during, and after the behavior appears.

At 71K miles the fuel filter of your car is past its change interval based on miles.

Do not underestimate what the fuel filter has to deal with.

When my 02 Boxster's fuel pump failed the "fuel filter" which was the fine plastic mesh that was part of the lower portion of the fuel pump housing/enclosure had a goodly amount of its surface area filled with fine particles of I have no idea what material, just I guess the normal particulate matter than gas tanks collect. The tank proper was quite free of particulate matter the tech reported.

With the earlier cars and their inline fuel filters I could envision a situation where the filter becomes full of this particulate matter.

In the case of my car's "fuel filter" there was plenty of mesh still free to allow fuel into the pump's intake port.

As for the AOS, if you have instructions for blocking off the AOS and are comfortable trying that go ahead.

You can try blocking off the AOS.

You can inspect/eliminate as best you can the possibility of a binding throttle.

But you should seriously consider replacing the fuel filter if the AOS and throttle do not pan out.

Or if you do not want to throw parts at the symptom take the car to a qualified tech and have him make a diagnosis.

aowolf

It seems like a wise a idea to change the fuel filter anyway so I'll get started on that. I wonder if someone in the Seattle/PNW area has a decent OBD scanner! I'll happily supply beer and

sjfehr

I had your exact same symptoms last summer; never did figure out exactly what caused it, but it wasn't a dirty throttle body.

I suspect it was a vacuum leak around one of the vacuum connections on the intake manifold, as I was unable to replicate it after wrapping the two most likely connectors with a bunch of electrical tape. In my case, it was worst going up hills at low rpm in a high gear, which is a situation I don't see much in Chesapeake VA.

aowolf

Ill try that next! I know a bit of oil got into the intake from the bad AOS, maybe it wore away at some the seals? (Can oil do that?) I took the car to a solid Porsche shop in the Seattle area. We ran it on a Porsche systems tester and found no codes. They thought it may be the transmission slipping (I'm not so sure) but we topped off the fluid and the car is still doing the same thing.

sjfehr

Mine started up at some point well after AOS failure; I didn't really notice it until a road trip. AOS replacement disturbed and I suspect loosened the vacuum connector right after the throttle body that connects to the AOS, and it just won't stay on tight anymore. A little electrical tape seems to have done the trick.

Some people have had this issue from a dirty throttle body and reported cleaning it fixed it, but my '04 has a redesigned throttle body and cleaning did nothing at all.

aowolf

Electrical take on the tube from the AOS fixed it... I think I read somewhere else that the factory tube is a little too short and doesn't quiet fit into the place. I did replace the O rings on it but it must not have made a perfect seal.

powderhound

Good to hear. It's nice when we get to hear the end of the story. Especially a happy ending.