Something is running when the engine is off
 

Just bought a 2000 S with 88k km from the original owner. Car has sat for a couple years after getting wet, which killed the immobilizer unit. It's definitely a bit of a gamble buying a non-running car, but I feel pretty confident it will be worth while. The IMS bearing was addressed (proactively, I believe) about 6 years ago. Put a new battery in and the dash lights come on and I could close the top, but no sound when trying to start. I trailored the car directly to Porsche to have them program a new immobilizer and keys; that is where the car is at right now.
My question is, does anyone know what the sound is behind the driver's seat when the engine is not running? It's a bit of a humming and started as soon as I connected the battery and would not stop until I disconnected the battery, regardless of whether the key is in the ignition. At first, I thought it sounded like a fuel pump running, but then realized the fuel pump should be in the front. My other guess is something to do with the convertible top motor.
Once I get the immobilizer sorted, I'll probably be back with more questions.

Curious, what did you pay for this project car?

Could be the engine compartment fan

Paid $6200 CAD. Looks like the rear main seal is leaking, so I am planning on doing that and the clutch once the electrical is sorted. Top needs to be replaced as well. If that is all, then it's a good deal.

I was considering a fan. Is it conceivable it would come on without the engine running at all? Will they run for a bit once the engine is turned off?

Mine has run for a few seconds after engine is cut off

After I've done a drive and, if the fan is running before I shut down... turn off the ignition... remove the key... there's nothing that's going to be running. That fan is going off. Off as in STOP. Further... when that fan is running it sure doesn't sound like a "humming"... it sounds like a fan.

As others have offered perhaps the engine compartment fan. While normally it should only come on with the engine on and if the engine compartment gets too hot, with my 2002 it would come on after the engine was shut off but only if the engine compartment got really hot and I left the car unlocked.

Occasionally I'd return to the car shortly after parking it on a real hot day and as soon as I unlocked the car and turned on the key the fan would come on.

However, almost always -- after I no longer had a garage for the car -- when I left the car I'd lock it and even if the engine compartment was hot the fan would not come on, until I had unlocked the car and turned the key to start the engine.

Another thought is does the car have vented seats? If so my thinking is the vented seat fan is running.

There is the secondary injection pump which normally should be controlled by the key and the engine running.

There is of course the cabin heater/AC fan motor. There is also supposed to be a small electric motor behind the round grill on the passenger side which is there to circulate cabin air over a temperature sensor which is used by the climate control system to monitor/maintain air temperature in the cabin. (In my 16 years I never hear this fan.)

You have mentioned the top motor.

But the immobilizer really doesn't play a role in any of the the circuits/systems I mentioned.

Unless there are signs of water damage or rodent damage at the engine controller -- located on the bulkhead at the front of the rear trunk -- or signs of water in the doors my WAG is what you are hearing is a relay buzzing.

If the car was left to sit with water at the mobilizer who knows what could have happened to the wiring, the socket into which plugs the immobilizer.

You can remove a number of fuses -- note the fuse slot from where each one came from -- and by a process of eliminate determine which circuit is responsible for the noise.

Thanks for the replies. A buzzing relay sounds like a possibility, but from my memory of a couple days ago, it was more of a constant humming, much like any fuel pump I've heard. I did not investigate the passenger side as it seemed to be clearly coming from behind the driver's seat (which was actually out of the car at the time, so it wasn't below the seat). Didn't know about the option of vented seats, but this one does not have that. Judging by the uncertain responses, it seems that something unusual is making noise. The car is at Porsche for a few days until the immobilizer arrives, so if I have it back next weekend I will investigate further.

Good news. New immobilizer and the car fires up right away and is running smoothly. Still have the noise though even when the key is out of the ignition, actually two noises. One noise from around the engine area (I can't say if it is more driver or passenger side), which sounds like an electric motor and does not sound like a fan to me. Another sound in the front low down; again it sounds like a fuel pump and seems to be in the right location. Where abouts is the motor for the convertible top? My top is closed, but the "open" light is on. I couldn't get the top to move, but have just learned that the e-brake needs to be applied, so I'm assuming that is the reason it won't move. How long do you hold the button when you open/close? Will the motor stop or will it keep trying to open/close after fully opening/closing? I believe I may have stopped the closing too early when I initially closed it.

I uploaded a video on YouTube of the two sounds. I am fairly confident it has to do with the fuel system. It is burning quite rich (evidenced by the smell of gas when running). If anyone can provide guidance it would be greatly appreciated. This weekend I'll get under the car to see what I can see.

Talking to myself mostly it seems, but I just confirmed that it is the fuel pump. Pulled the fuse and both noises were eliminated. So now I am asking why is my fuel pump seemingly on permanently? I let it run for close to an hour and then started up the car no problem, so it isn't actually pushing fuel through the injectors, otherwise, I would expect it to be flooded.

Well, I did offer you need to pull fuses to ID which circuit is powering whatever is running. You have ID'd the fuel pump as the culprit.

If I have to make a guess as to why the fuel pump is running when the key is off I'd have to believe it is due to the ignition switch. It is bad.

You sure did Macster. Thank you. I was not meaning to convey any anger, more humor.
I will try to look into the ignition switch a bit. Electronic problems are my nemesis, so I may end up making a toggle switch for the fuel pump as a temporary fix. I'll order a new relay as well in case that is the issue.

If the switch is bad chances are it is powering more than just the fuel pump. If so this will run down the battery very quickly.

There is no looking at the switch. Well, I guess you can and maybe see something. I've seen bad ones and good ones and except for the obvious differences due to one being in service and one being fresh out of the box they look the same. If one goes to the trouble of pulling the switch out probably the best course is to just replace it with a new one.

You can try to wiggle the key as you turn the switch on and off and push or pull the key to try to see if you can influence the fuel pump running. If you can that's a good sign the switch is bad.

The fix is to replace the switch. Many replace just the electrical portion which fits at the back end of the switch. Porsche no longer sells the electric portion. The reason I was told is too often just replacing the electric portion did not address all the problems and there were too many "come backs" that had the tech then replacing the other portion of the switch. At any rate Porsche sells only the entire switch assembly, which includes the mechanical portion (which includes the steering lock). But Audi does, or used to, sell the electric portion. (Audi and the Porsche part are for all intents and purposes interchangeable.) I do not recall the Audi part number but a google search for boxster ignition switch should turn up the info.

When the switch in my Boxster was suspect I just had the tech replace the entire switch/lock mechanism.

If you end up replacing any part of the ignition switch you have to be careful. There is a lot of wiring where you'll be working and if you get too rough you can damage/break something. The key surround has an antenna behind it which is the antenna used to broadcast an RF signal to the RFID pill in the key and then receive its reply which is the RFID which if received correctly and recognized then allows the engine to crank.

Mess this up and the engine will not crank and you'll be suspecting the switch.

The ignition switch sounds like a logical culprit, except that I haven't noticed anything else receiving power. I'm guessing that if it is something other than the switch I could pull the switch out and the pump still might run. I'll carefully try that today.

I also came across a post blaming the crankshaft sensor on the fuel pump running, but I would assume that if my crankshaft sensor is bad I would have other problems with the engine running. I do have a camshaft sensor fault according to Durametric.