Please help me troubleshoot a fuel problem

Jamie79SC · 02-10-2003, 06:37 PM

Well, for the first time in several years, the '79 911 SC left me stranded. Could one of the pros please help me efficiently get to the bottom of the problem?

On Saturday evening I was cruising down the highway, about 50 miles into a trip when the engine quit cold. There was no sputter or stumble, the lights stayed bright and the tunes kept playing - the engine just stopped.

I took the car out of gear and coasted to the side of the road. On trying to restart, the engine turned over fine but didn't start at all. There was no smell of gasoline in the exhaust pipe at all which lead me to think fuel as the the culprit.

I replaced red fuel pump relay (and the brown one next to it for good measure) still no joy. AAA kindly flatbeded me home for the night.

With the car at home, I tried lifting the air plate with the ignition switch on and no fuel pump sound or smell...

So, I think I've isolated the problem to fuel (no smell, no fuel pump sound) and have elinated the red relay (got a brand new one today - still no joy). So, what is the next logical step in isolating the pump, wiring, or???

Thanks guys!

Nathan · 02-10-2003, 06:52 PM

Is there gas in the tank?
Why I ask because I remember a thread that had similar symtoms as yours and all it was gas tank empty. Fuel Gauge not reading right.
Just trying to help

Jim Sims · 07:14 PM

Check fuse 16 (25 amp) although if this is blown the fuel pump should still run while the starter is engaged so I doubt that's the problem. The black wire from terminal 30 of the fuel pump relay base harness goes to the fuel pump. There is also a brown ground wire running from the fuel pump to a ground point. Check for voltage on the black wire with the key in the on position and the air meter plate lifted (or it's air flow switch connector loosened). Check to make sure the ground wire is connected. If there is voltage to the fuel pump and the ground wire is in place then the fuel pump has failed. Cheers, Jim

Jamie79SC · 02-10-2003, 07:50 PM

Nathan,

Good question! I should have mentioned that there is penty of fuel - about 3/4 tank - based both on guage reading and on miles driven since full.

Jim,

Great instructions! I have my working orders. I'll let you all know.

-Thanks guys!

Jamie79SC · 02-10-2003, 08:24 PM

Jim,

BINGO! The sixth fuse in from the left is toast. Unfortunately, the insulation on the red wire leading to it looks a little toasted as well. I'm a little uncomfortable about putting in a replacement, but we'll see if the replacement doesn't blow right away. Fingers crossed.

Jamie79SC · 02-10-2003, 08:36 PM

Alright - she's running fine. No blown fuses for now.

I imagine I should be concerned that the insulation right at the terminal looks like it's been hot. What's the consensus on how to put my mind at ease on the state of my twenty-four year old wiring?

masraum · 02-10-2003, 08:39 PM

Um, you could get good and liquored up. That would probably give you tons of bravado!

Hockey fan · 02-10-2003, 09:08 PM

Jamie,

Congratulations on getting your car going. There must have been something in the Pacific Northwest air this weekend as my car is dead as a doornail in the driveway now. Seems like fuses are the easiest first check.

Jeff

Jim Sims · 08:06 AM

Corrosion (oxidation) of contact between fuse clip and fuse. Corrosion increases resistance which makes heat which promotes more oxidation which in turn increases resistance more and eventually the fuse gets hot enough to melt and blow. This heat will also conduct into the wire. The wire may also be loose where it is clamped to the fuse terminal with a small screw. I would clean up the fuse contacts with a fine emory paper or a fine metal brush and check the tightness of the screw clamping the wire. Install a new fuse. If the fuse blows again you have a short in the wiring or perhaps a fuel pump motor which is drawing too much current. Cheers, Jim

Jamie79SC · 02-11-2003, 07:41 AM

Thanks Jim!

jason2guy · 02-11-2003, 08:19 AM

same thing happened here. now that you have discovered the problem you have to fix it. the wire is gonna kep getting hot.
you can either replace the 10 fuse block (correct fix), or you can do what i did. i actually swapped the wires right next to the fuse in question with the next fuse block location wires. see its actually the little (brass?) block that is creating the problem over time. so by swapping with a new fuse block location it remedies the situation. just a thought

Superman · 02-11-2003, 10:22 AM

Glad the problem is identified. I'd add only this: Old, tired fuel pumps draw way more current than fresh new ones.

Rick Lee · Rick Lee's Garage

I had the same symptoms when my fuel pump died. It happened in the middle of nowhere on a very hot day and it totally sucked. Some vendors told me they had never sold a Carrera fuel pump because they last forever. Well, not mine.

ChrisBennet · ChrisBennet's Garage

My Carrera fuel pump died too. I was told by a Bosch guru that the dying fuel pump heats up the DME relay.
My DME relay showed some heat damage but hadn't failed. (The Carrera DME relay is really two relays in one package, the actualy DME relay and the fuel pump relay.)
-Chris