911SC Bad Fuel Pump or Wiring?

rsiegel · 07-01-2006, 08:55 AM

I've been chasing an odd intermittent no-start problem for months. Yesterday I had the good fortune to have the car die completely (finally!) and do the no-start thing right in my garage (yes!). It had spark and would turn over and then die if I squirted in starting fluid, so it nailed it to a fuel delivery problem. I hooked up a fuel pressure gauge at the fuel distributor, and propped open the flap to get the fuel pump to run, and there was zero pressure. Just to be sure, I pulled the fuel pump relay and jumpered pins 30 and 87a and loosened the fuel pump line at the top of the filter. Nothing -- zero pressure.

I jacked up the car and exposed the fuel pump, and again jumpered across the relay. The pump clearly was turning. In fact, I'd forgotten to tighten the gas line at the fuel filter, and the engine compartment was now soaked with gas. I tightened it and let the gas dry, and then the car started right up.

What do we think? Do fuel pumps die intermittently like this, or if I replace it, am I still likely to have the problem because it's wiring and not the pump?

I know -- the next time it happens, check the voltage at the pump. But right now it's running (drat!).

--Rob

CliffBrown · 07-01-2006, 09:54 AM

Rob

I would start with a check of the electrical ground straps and connection points (a good cleaning never hurts) and the fuel pump electrical connections all the way upstream.

I couldn't tell from your great troubleshooting description (really) if you could hear the fuel pump running when you lifted the air sensor plate and jumped the pins.

It's times like this that we wish Porsche had installed another dash warning light (like the alternator light) to indicate when the fuel pump was not receiving 12v. It would be an easy DIY. Would be even better if we had a fuel pressure gauge.

I'm a old pilot and a proponent for better instrumentation in cars, like aircraft mfg install.

Joe Bob · Joe Bob's Garage

CIS needs a real high pressure and when they go they go.....some will have HOT start issues but that is just the check valve in the fuel pump neck that fails allowing residual fuel pressure to bleed off ($18 part)...when you price a new one...sit down....$180+ last I checked.....

I agree that you should check grounds and such before pulling it and get a pressure gauge. If you have a Haynes or a Bentley manual it will tell you where to hook up and what values to look for.

My guess is that if it's running...it's probably losing it's oomph and needs either Viagra or a new unit.

rsiegel · 07-01-2006, 10:17 AM

To answer Cliff's question, the first time that I lifted the air sensor plate and then jumpered the pins, I THOUGHT that I heard the fuel pump, but I was listening at the engine compartment, not at the front of the car, and I now believe that what I was hearing was... well, I don't know what other electromechanical gizmos are clicking and whirring when you lift up that plate, but something else makes noise. The fact that there was no gas coming out of the fuel filter implies that the pump was not, in fact, running.

Once I had the car in the air, and I jumpered the pins the SECOND time (and then put the relay back and lifted the plate), there was no question that the pump was running because gas gushed out from the open line (my bad).

I now can tell the difference between the two sounds from the engine compartment when the plate is lifted up, and further know to listen for the fuel pump at the front of the car.

Cliff's suggestion about wiring a light to the fuel pump (so I see that I'm getting voltage there) is a great troubleshooting tip. I should've thought of that. Thanks.

--Rob

CliffBrown · 07-01-2006, 11:25 AM

Quote:

Originally posted by rsiegel
The fact that there was no gas coming out of the fuel filter implies that the pump was not, in fact, running.

--Rob

Rob

I asked about the pump because I wanted to determine if there was the remote possibility of a fuel supply problem (tank strainer or clogged line). I can't assume that the pump was running.

It just helps the rest of us if we know exactly what happened during each step of the troubleshooting.

Sounds like you're doing a great job so far.