Guys,


A couple of years ago while the car was just idling in the yard I noticed the RPM’s beginning to fluctuate. Upon further inspection I noticed that the fuel pressure was swinging wildly along with the RPMs until the FP dropped below 30psi, and the car stalled. The car would crank right back up but w/in 2 minutes the fuel pressure would begin to do the same thing.
At this time I had an MSD electric fuel pump under the tank pushing fuel to a Bosch Mustang GT pump mounted in the frame rail closer to the engine. All was plumbed with -6 hose, and a 3/8’’ steel return line through the tunnel back to the tank. Both pump were relayed. With an electrical meter I noticed that there was a voltage drop both up & down as the pumps would begin to cycle erratically. This led me to change the 993 main engine wiring harness because it was bad anyway. The problem was still there. I realized that the relays were wired incorrectly as the wires were getting hot so I corrected that but still have the problem. I thought that 1 pump was overwhelming the other so I removed the Bosch pump and ran solely on the MSD. The wires were then cool to the touch and there was now a full 12 volts to the pump at all times but the unit would still begin to cycle after a couple of minutes.
I now decided to step up to a better pump, and bought a Aeromotive pump and 100 micron filter. Upon removing the previous pump and filter I noticed that the filter was clogged with trash from the tank that it picked up. Well I thought, problem solved with the new unit.
I installed the new pump running -8 supply line to the pump. I Tig’d a -8 fitting on the 914 supply flange nut, and screwed it back under the tank. By the way, the supply hole opening on the 914 tank is the same ID as the -8 fitting. No restriction there. Cranked it up, ran fine, but overly high fuel pressure. 60psi plus.
With all this installed, the car has run reliably for a year. This year we overhauled the engine. I finished the engine, reinstalled, and everything worked perfectly for about 150 miles.
I added the fuel pump controller thinking it would bring the Fpressure down some but it doesn’t so, I decided to add the regulator to bring the pressure down. After installing it is when the wild cycling began again.
My first thought was fuel starvation due to another clogged filter. The Aeromotive unit is cleanable and sure enough it was packed pretty good with trash. replacing it did not solve the problem. The fuel supply line is completely free and quickly empties the tank when pulled.
The fuel line is only about 2’ long, and the pump is mounted lower than the tank for a good gravity feed. The car currently starts every time but within a couple of minutes, it begins to experience fuel starvation.
I removed the pump per the pic, and laid it in a pan of gas with the regulator installed. I ran the pump off a battery charger for 30 minutes @ an adjusted 55psi, and it ran flawlessly. The needle on the regulator never moved. I did put a inferred temp gauge on the pump and it indicated 109 degrees after 30 minutes of use which I thought may be high, but it ran fine.
I reinstalled the unit back in the chassis, and the problem was still there.
The pump sound like it is cavitating or maybe vapor locking.
Give me some ideas, what am I missing?? A fuel delivery system is not rocket science. The car will not run dependably until this is fixed.
the pics show how all is set up now. This weekend i'm going to run the supply line into a gas can to see if that will run ok. if it does i'll suspect the tank, and if it doesn't I'll suspect the pump.

Any advise is appreciated.

Ron

Ok,
my setup is completely different from yours but here's what i have and it has worked great so far.


I used a stock 993 fuel pump. I bought a 993 racing pump (higher volume) too but never got around to install it.

The key is that on a 993 the hose from the tank to the pump is HUGE. You need plenty of diameter on the suction side.

I mounted my pump right under the tank outlet. McMark made a little box where the pump can be removed for inspection from the passenger foot-well.

I then used Chris Foley's SS fuel lines to deliver the fuel to the back and for the return.


The only problem i ever had with this setup was after a several 100 miles drive i got stuck on a 100+ day in traffic and i'm not running the steering rack cover and my oil cooler directs hot air straight onto the tank. So much so that my tank gets too hot to touch after a long drive on a warm day. I really need to add a shroud to vent the air somewhere other than inside the front trunk.
So i got vapor lock. Not enough to kill the engine, but you could hear the pump getting louder as it was pumping vapors mixed in the gas.
Obviously, that wasn't really the pumps fault ...


Linky to the pump install ...

http://www.914world.com/bbs2/index.php?s=&...st&p=906806


Ignore the hose that says "NOT FOR FUEL INJECTION" That was just for testing, it was replaced with the correct hose.

Andy,
The pic of the pump running in the tank, the pump is running flawlessly. But, then again, the hose and pump are almost submerged.
Once the car is started and run a minute or two, the pump starts struggling to maintain steady pressure. Growling like mad. It's either starving for fuel. (Which I believe first) Vapor locking, or Cavitating.
This weekend I'm going to disconnect the supply line from the pump, and run a simple hose (-8) to a gas can, 12'' of hose max, and see if the problem still persists.
my feeling is that if runs fine off the can, the problem is in the tank. If it still runs bad, then I lean toward blaming the pump.
The downer on this is that I have drove the car for a year w/no problems, and now I'm back to square one.


Ron

Like i said, on a 993 the inlet hose is HUGE. I forget the diameter, but it looked like 1/2" or more!

If the pump gets noisy, that means it's not getting enough fuel.

Have you checked to bottom of your tank? Any debris? Is the sock clogged? Are you running a filter before the pump? If so, did you try to bypass the filter for testing?



I'm sorry i can't be more helpful. Your setup is very different from mine ...
Andy

I also used a stock 993 pump and the stock regulator on the passenger side fuel rail. It's worked flawlessly for 11 years now. I mounted it above the tank because I didn't want to have to drain it before changing either the filter or a bad pump.

Click to view attachment

Click to view attachment

I'd do a rigorous analysis of the flow and regulation of your "complex" parts including the regulator. Assuming there is no blockage in a filter that's likely where your issues are. Just because a pump will come up to pressure doesn't mean it'll flow enough gas.

I've had this problem before (on another car that had the pump below the tank) and replaced the pump anyway. The problem was solved.

Good luck!

I don't have any experience with these pumps, but, it would seem that if the electrical connections are good, the only thing that will cause the pump to struggle to maintain pressure is supply of fuel or a regulator that is failing.
Tom

Don't be sorry about being helpful. this is were we solve problems and gain tribal knowledge.

Ron

If I recall the fuel filter resolution has to be 1 micron for Porsche fuel injection, certainly not bigger than 3 microns. If that's the case your injectors might need to be cleaned. I have a place that does it and replaces all of the rubbers fro $20 each. I have mine plumbed in -8AN as well. It matches the original 993 fuel-line ID pretty darned close.

The professional cleaning work includes a spray pattern analysis and flow report done on a Bosch FI machine. And they return them with new o-rings installed.

Good luck!

Ron,

Good catch on the 1 micron filter.
I have a Canton 1 micron filter mounted on the firewall right in front of my engine. My fuel plumbing is -8 from tank to 100 micron filter,then pump, then -6 line straight to the back of the car,then 1 micron Canton filter(which has been changed), then through the regulator, and up into the fuel rails.

I am going to run the car this weekend off a gas can on the ground. If the car runs fine that way, there's gotta be something restricting flow from the tank to the 1st filter inlet.

Thanks for your thoughts,

Ron

I like the regulator after the fuel rails. That way the entire injector bank is at the pressure you set, and the return line is a low pressure branch that dumps back into the cell. Usually into a surge tank. Do you know that your fuel rails are up to pressure? Is there a separate diaphragm for the fuel return? Hmmm.

I used -6 for my return line but -8 for the supply. I don't think that would matter in this case... except WOT, but it is smaller than factory.

Not related to your problem... just thinkin'.

Here is my $0.02:

You show the pump and a suction line working correctly in an open container of fuel. (not the safest thing, but I understand)

I would try using the same suction line and the fitting from the bottom of your tank just as your setup exists in the car for your trials of running from a can of gas. It should work. If not, remove the tank fitting from your suction line and try. If it works then, its a restriction in the fitting.

I have collapsed suction lines in the past. Those hoses can partially collapse and cause some flow restrictions (especially if they are bent when installed).

Good luck and I hope you find your culprit.




On another note, please consider having your tank cleaned out. It makes sense if you are plugging filters that you need to get rid of all the foreign stuff.

I too would recommend changing to a back pressure regulator (regulator after the fuel rails) to ensure that both see the same staging pressure.