No fuel pump sound when starting

Shawzy · 09-08-2002, 07:32 AM

After (successfull!!) DIY rebuild of 915, put the engine back in yesterday.

Went to start it and first thing i noticed - no fuel pump sound when key was turned to 'on'.

I tried to start it anyway, and after a bit of sputering she started up and then stalled. i started again and all was fine. i drove it last night for 1hr and all was fine.

so - fuel pump works fine i assume or i wouldn't have gone very far!! I assume she can't be fed fuel via gravity or capillary action - heehee

if the fuel pump is fine, why don't I hear it when starting? before my engine drop I ALWAYS heard it buzzin' away before turning the key to crank.

i have some ideas but not having looked over elec diagrams i'm only guessing.

IDEAS?

thanks

Superman · 09-08-2002, 08:22 AM

Your FP is obviously running while the engine is on. So, turning on the key and lifting the sensor plate will make the starter run.

There is a circuit that should cause the FP to run, before the engine starts, while the starter is running. Some cars also have a circuit that runs the FP for a few seconds when the key is turned "on," before the starter or engine are engaged. Whatever systems you car should, they should be working in order to ensure good starting.

marcesq · marcesq's Garage

Are we talking about a 2.7, 3.0 or 3.2, because on the 3.2 the FP starts when the car is cranked.

Shawzy · 09-08-2002, 08:47 AM

OK-

When I dropped the engine I noticed a blue connector that was not connected - there are 2 identical connectors that combine into one larger cable. One attaches to the bottom of the lift plate area, the other closer to the throttle linkage side.

The one not connected was the one under the lift plate. I thought it was for a cold start sensor, what looked like in the Bentley like a little strip of metal that heats up and changes how the lift plate operates (either allows it to lift for cold start, or other not sure). I don't know if this connector is in fact for this cold start sensor.

i of course re-connected this cable. could this be the culprit i wonder? I'm not sure how the sensors are incorporated into the starting procedure - if one is faulty perhaps the circuit doesn't engage the fuel pump before starting. i will pull it and see what happens.

is the starting circuitry 'smart' enough to know whether to activate the pump before starting? or is it an 'always on' feature and therefore i should hear the pump every time before starting?

Shawzy · 09-08-2002, 08:49 AM

sorry marcesq - 3.0 SC

marcesq · marcesq's Garage

I believe (but not a 3.0 expert) that the FP gets power when the key is on and in the crank position.

What is funny is that the car is running. Open the hood turn the key on and listen right over the smuggler's box just to make sure, otherwise, I would defer to those who know the 3.0 system better than I.

Good Luck.

Superman · 09-08-2002, 09:02 AM

The thing behind the sensor plate is probably a sensor plate sensor. It senses whether the sensor plate is lifted. When it is lifted, the switch makes the FP run. During starting, the sensor plate is not really lifted. It is lifted by the air sucked into the engine by the engine running.

Prior to when the engine fires up, during starting, something else needs to run the FP. There is a circuit that runs the FP when the engine is being cranked. That's the reason for the extra wire going to the starter.

In addition, some cars have a circuit that runs the FP for just a few seconds when the key is turned on, even before running the starter.

These systems, that deal with the fuel pump relay, work the way they do so that in the event of a collision, the FP will not spray gasoline over a hot, wrecked car. No, the FP does not run simply when the key is "on." It runs basically only when the engine is running or trying to start.

Shawzy · 09-08-2002, 09:20 AM

"No, the FP does not run simply when the key is "on." It runs basically only when the engine is running or trying to start."

I beg to differ unless there's something I am missing.

In the past, just before starting the car, I would distinctly hear the fuel pump humming when my key was in run or position II (one before crank).

Now, it APPEARS the fuel pump only runs when the engine is running or when cranking.

So long as i won't run into difficulty when on the road, i'm not worried.

Shawzy · 09-08-2002, 10:12 AM

I think I've got it!!

Thanks to this thread and my bud tbitz, i think i've figured it out.

as mentioned in above posts, when i dropped the engine i noticed a connector below the lift plate that wasn't attached.

i thought this was for a cold start sensor, but now believe it is a safety switch mech for the fuel pump. when the lift-plate lifts (do to air entering engine when cranking or run), switch contacts open and the fuel pump is allowed to run. as the supe said, this is a safety feature to ensure fuel pump can't run if engine is off. good idea, but ironcially the fuel pump circuitry allows the pump to run period when the key is in run and this connector is left detached!!

in my case, the connector was off (for who knows how long) and so the fuel pump was always running when the key was in the run position (i think even my wrench thought this was normal). i thought this was the norm as well, but i have now been corrected. who knows if the same circuitry exists on older cars - i hope yes!!

my car would not have been very safe in an accident (god forbid). glad i now have it connected....

i hope others read this thread to ensure the fuel pump behaves as it should.

thanks again everybody

dean · 09-08-2002, 02:12 PM

I think you've got it also. The fuel pump also runs while cranking

stormcrow · stormcrow's Garage

Is your P-car a 78 or newer? The setup on my 78 is as follows:

When you turn the key to the on position, the fuel pump should not run. When you turn the key to the start position, the fuel pump will run via the fuel pump relay and the flow switch. When the engin starts the air flow switch works in conjunction with the fuel pump to keep the fuel pump running.

Should the engine stall or stop for some reason with the key in the on position, the fuel pump should stop as a safety. This will stop fuel flowing if the car has been damaged or should a fuel line rupture causing the engine to stall.

Steve

Shawzy · 09-08-2002, 07:10 PM

mine's a 79 and now works exactly how you stated stormcrow

before I re-connected the loose wire i found dangling by the CIS (i.e. air-flow switch connection), my fuel pump would go on whenever the key was 'on' regardless. NOT GOOD. that means every time i wanted to roll up the windows, reset alarm, close sunroof etc with the engine off, the fuel pump was pumping away needlessly. I am so glad i discovered this and corrected it - piece of mind when on the road too.

i don't know why the heck the electrical gurus at porsche made it possible for this to happen. I would have thought they would have made it impossible for the fuel pump to run at all when this cable was disconnected, not the other way around. that seems more sensible to me, although perhaps a faulty safety circuit could leave you stranded without a fuel pump!