Weird electrical problem (fuel pump), Driving me effing nuts Options

[campbellcj]

This is driving me crazy. After I drive for 5-10 minutes, the fuel pump completely cuts off and the car dies. But then after the car is shut off for a minute or so it fires right back up.

I know it's not the pump itself since I swapped in a new one last weekend (I figure it's good to have a spare in my track box anyway). I can't find any simple wiring or fuse issues. Doesn't seem to be a general electrical issue as I still have ignition and accessory power when it happens; the Bosch CD still whines etc.

It's been impossible to troubleshoot since so far I can't get it to happen idling in the garage, and it won't last long enough to probe around with a voltmeter when it does happen.

This is a six with a totally redone/custom fuel system; Webers, Holley red pump regulated to 3.5psi, AN-6 lines everywhere.

Anybody seen something like this before?

[john rogers]

What brand of fuel pump is it? I'd take a volt meter (turned on) with me and take a ride and as soon as it happens jump out and test the voltage at the + connection at the pump. If there is any power then it is the pump although they usually need at least 10.5 volts to run.

[94teener]

Make sure your pump & filter connections (on the suction side) are sufficiently tight. Otherwise, you may be sucking a little air with the fuel until it builds up enough to momentarily starve the engine. this can happen even though you may not be leaking fuel while the engine in running or shut off.

Phil

[iamchappy]

Temperature sensor number 2 in the cylinder head.

[914GT]

Just a thought here, but maybe check for a partially-clogged screen in the tank. The pump draws on the fuel in the line and feeds the engine for a while, but eventually runs out due to blockage. Then the fuel line refills waiting for a while, and the process repeats. Carefully pull the hose off the pump coming from the tank (have a clamp on the line). Gas should freely flow out the line.

The other thing you could so is swap the relay out with a known good one to rule-out a flaky relay.

[nyum96]

I agree with the fuel tank screen. I had the same problems and bought a new pump. If its not electrical, it can be the screen. Mine collapsed and all the rust particles clogged it.

BB

[Spoke]

I had the same problem with my 914/4. I drove for a while then the car died. Waited a minute then started up and ran for a minute, then died. This repeated until I got home. 4 miles took 1 1/2 hours, but I didn't have to have it towed. (IMG:style_emoticons/default/mueba.gif)

Turns out the problem was massive amounts of crunchies in the gas tank had clogged the fuel delivery openings. Cleaned out and sealed the tank and never had the problem again.

I can't tell if your problem is electrical or fuel related. If you think it is electrical, you need to measure the voltage at the pump as you're driving. One way to do this is to run a temporary wire from the pump to the interior through the center tunnel. Connect your voltmeter inside and drive away. This may sound goofy but intermittant problems are almost impossible to find after the fact. Good luck.

Spoke

[campbellcj]

Thanks for the ideas guys. It's got to be something simple that I just haven't spotted yet.

I thought of a supply problem (clogged filter screen or whatnot), or also the air leak on the suction side, however neither of those would stop the pump from running, right? I figured the pump would still run even if sucking air? It's a loud sucker so you can clearly tell when it is not running.

The tank (cell) is new so there sure shouldn't be any rust or gunk; although I can't rule out much at this point.

I think I'll go ahead and rewire it with a dedicated (toggle) switch and relayed circuit. I'd been meaning to rewire the ignition and starter circuits as well, so I guess now is the time.

[DJsRepS]

How about the rivits in the relay board. I read that it could be an sorce of intermitent troubles. All the connections are like rivits and corosion and vibration can mess them up. I soldered all mine from the back side. Had to soak in gas over night to remove the tar like stuff from the factory. It did not cure my miss but it took the process by elimination down an notch. Swap the round pump relay for an headlight motor realy. Check FI harness grounds and terminals. Mine had a couple of green colored copper wires holding a terminal on.

[campbellcj]

Looks like I found the problem. Sometimes it's the simplest darn things.

I checked the flow from the cell and it's fine (in fact I darn near took a 100 octane shower.)

Then I re-checked the pump wiring. Turns out it was either a loose fuse and/or loose crimp connections at the fusebox.

I went on a successful (i.e. no stalls, no tickets, no wrecks) test drive, peeking at the fuel pressure every so often and it held steady at 3.5psi. It sure helps to have that guage as well as a removable engine lid.

Sooo...this prolly means I should rewire the car with one of those 'modern' fuseboxes and aircraft style switches. Not sure the OE setup can handle the vibration and g's, especially after 31 years.

[brp914]

In case your problem isn't solved, my 6 had similar symptoms once. It turned out to be the CD. I would whine alright before starting the engine, but after the engine starts you cant hear it anymore - thats when it would cut out, but by the time you pull over it would resume its whine and restart. The infuriating thing was that it died a slow death, taking months before it finally crapped out completely. By then the pitch of the whine would vary. Also put it on a scope, it could only muster 50V output rather than hundreds.

Even if your problem is solved - I mean if you're gonna keep a spare fuel pump - I would suggest keeping a spare CD. The Permatune CD does not whine which I kinda dont like. There are places that rebuild Bosch CDs for ~$125

[campbellcj]

brp -- great advice -- BTDT! Now I have a spare fuel pump and a spare CD in my track pit box.