No fuel pump buzz. SC won't start. Related?

jjcollier · 07-10-2005, 01:31 PM

I think there is a fuel pump issue here. When I go to start the car, the fuel pump does not buzz. Is there something simple that I am overlooking? In addition, I pulled an injector and all I got on an attempted start was one sorry squirt, then nothing.
Please advise if the absence of the pre-start buzz is a starting point for this car that will not start.

fred cook · fred cook's Garage

Check the fuel pump relay fuse in the front fuse box. If the ends of the fuse holder/fuse are corroded badly enough or if the fuse has gotten hot enough to weaken the tension on the fuse, the relay will not get power and thus not run the fuel pump. If that is not the problem, try putting one of the other relays in place of the fuel pump relay (the red one). They are all wired the same way, the red fuel pump relay is just a heavier duty unit.

Good luck!

Nitrometano · Nitrometano's Garage

Check also the safety switch. Is located in the rear part of the CIS air sensor plate. Is a blue connector. Try conecting and disconecting him. I have this problem some months ago with a 1979 911SC and that was the problem.

jjcollier · 07-10-2005, 02:12 PM

Dumb question: So it is definitely odd/wrong that my fuel pump is NOT buzzing at start up?
Thanks too for the above.

jjcollier · 07-10-2005, 02:19 PM

In regard to the sensor plate, if I push up on that when the key is in, the fuel pump buzzs. I have looked (inside the airbox?) for a blue connector and really see no connectors in or around it.
Please advise.
Tried the relays and there is no fuse corrosion. Seems those are okay.

Nick9143.0 · 07-10-2005, 04:17 PM

I had a similar problem today so I'm also look for advice. The cat is an 81 911sc and it was very hard to start when hot. I noticed that when I turned the key to the on position I could not hear the fuel pump run and then stop. I would like some suggestions as to what to check.

Nick

fred cook · fred cook's Garage

The connector for the "safety switch" is on the back side (towards the firewall) of the fuel distributor. If the fuel pump runs when the air plate is lifted, then the safety switch is probably ok as is the fuel pump and the fuse/relay. You might have a problem with the fuel accumulator. It is mounted on the left side of the engine compartment next to the fuel filter. If it is not holding fuel pressure, the engine won't start quickly as there is not enough fuel pressure to open the injectors. Try reaching around behind the fuel distributor to find the safety switch and remove the connector. The fuel pump should run when the key is in the run position. This will allow the fuel pressure to build and the engine to start. If this works, you will likely need to replace the fuel accumulator.

Good luck!

jjcollier · 07-10-2005, 05:22 PM

Good advice on the accumulator. I've been through that one before with my last 911. Definitely related to hot weather. It would turn and turn and turn and turn, then finally start. Not cool, but the new accumulator fixed it instantly.
I now know where the switch is, but damn is it hard to unhook without the engine out. Is it a top squeeze, side squeeze? Tough!!
I will try it again later, but ran out of time.
Due to the warm but not HOT, early evening stall, it doesn't feel like my former accumulator woes, but let's see.

Christien · Christien's Garage

Quote:

Originally posted by jjcollier
Dumb question: So it is definitely odd/wrong that my fuel pump is NOT buzzing at start up?

Yes, most definitely. If you know what to listen for and that sound is missing, that's 99% for sure the reason the car won't start.

jjcollier · 07-10-2005, 07:22 PM

Would accumulator problems cause the fuel pump not to run when the key is in the run position? If not, what causes the fuel pump not to run at pre-start if it will run when the sensor plate is moved by hand? Is it likely electrical or mechanical?

vash · vash's Garage

your fuel pump will not run with just the key (if it is like mine. 81SC) it needs the air to push up on the air sensor bar. that is what fred cook is saying. it is a safety mech, that prevents the fuel pump to keep spraying when the car is not running, like in a car crash.

nicks problem sounds like a fuel accumulator or fuel pump check valve prob. CIS pressure testing will make it show up.

jj, does your car start and run?

Hladun · 07-10-2005, 07:32 PM

Quote:

Originally posted by jjcollier
Dumb question: So it is definitely odd/wrong that my fuel pump is NOT buzzing at start up?
Thanks too for the above.

The fuel pump should not buzz when the key is switched on. The erroneously called fuel pump relay is actually a safety relay that allows energizing the fuel pump only when the engine is CRANKING. Once the flapper switch opens (indicating airflow to the engine) the fuel pump relay is de-energized and the fuel pump continues to run on the ignition switched system. If the flapper valve closes (no airflow in say a crash) the relay is energized and the fuel pump stops. The fuel pump can be made to run all the time the ignition switch is turned on by removing the fuel relay and jumpering contacts 87a and 30.

vash · vash's Garage

wow, got in just seconds before hladun. btw, he is the better explainer.

Nick9143.0 · 07-11-2005, 04:23 AM

Just to be clear on how the fuel pump is supposed to act. I thought the pump would run for a couple of 2 secs with the key on but engine not running to pressurize the system. Once that happened the pump would shut off until the air sensor moved indicating a running engine. When I go to start the car now key on engine off I don't even get the pump to run at all. Starting the car takes longer now and it runs very unevenly for 10 secs or so then everything is normal. I thought the accumulator only held pressure in the system with the engine off, so it wouldn't come into play. Would a bad accumulator cause the pump not to do its initial run? I had guessed it was a sender problem rather than a mechanical problem.

Nick

Westy · 07-11-2005, 06:14 AM

jj, welcome to our SC CIS world. First of all, if I ever give you any advice on this system, don't listen to me. Instead, make some friends who have the CIS equipment (guages) to check your system. It just gets too costly to randomly replace things. As for the cutoff switch, lay a drop mirror on the right side of the motor, then hold a mirror at an angle right above the back of the airbox. You can see everything quite clearly, including the wires you want to disconnect. But this is only if you refuse to listen to my advice about listening to me.

vash · vash's Garage

nick, i also DO NOT hear the fuel pump when i turn my key on. my car starts and runs perfectly. i never heard of the "two second" rule.

Gunter · Gunter's Garage

Hladun: Do not yank on the "Sensor Plate Contact Switch". All the electrical connections for WUR, AAR, CSV etc. are secured by a small wire clip. You need to open this clip with a small picking tool. If you just force the plug off, it will break. Now you have the plug disconnected and when you put it back on it will not stay connected. Bad mistake!
If you want to learn about your SC and CIS, get the Bentley SC Repair Manual and other related books.
As others have stated, the pump is not supposed to run with ignition on.
But, when you lift the sensor plate in the airbox, with ignition on, the injectors give off a little hissing meaning: the pump runs.
Regarding hot-start-problems, loss of pressure in the system can be caused by a leaking check valve on the pump, a bad accumulator, leaking CSV or injector and.........................?

Hladun · 07-11-2005, 09:00 AM

Quote:

Originally posted by Gunter
Hladun: Do not yank on the "Sensor Plate Contact Switch". All the electrical connections for WUR, AAR, CSV etc. are secured by a small wire clip. You need to open this clip with a small picking tool. If you just force the plug off, it will break. Now you have the plug disconnected and when you put it back on it will not stay connected. Bad mistake!
If you want to learn about your SC and CIS, get the Bentley SC Repair Manual and other related books.
As others have stated, the pump is not supposed to run with ignition on.
But, when you lift the sensor plate in the airbox, with ignition on, the injectors give off a little hissing meaning: the pump runs.
Regarding hot-start-problems, loss of pressure in the system can be caused by a leaking check valve on the pump, a bad accumulator, leaking CSV or injector and.........................?

Read my post, I never said to yank on anything.

Gunter · Gunter's Garage

Sorry, it was JJcolier who yanked on the Sensor Plate Switch.
Good luck, Mr. Know-it-all.

jjcollier · 07-12-2005, 11:52 AM

I gave a little pull, but gave up once I realized it was not going to come off that way. Tricky to get to that one. Regardless, it does not sound as if that is the problem.
I am double-checking now to see if I have spark. I pulled a spark plug and it was wetter with gas than I recall them being in the past, so I hope this means that fuel is not the issue (despite not having the familiar pre-start buzz).
Thanks again for the feedback.