Dear 914ers,

Something quite interesting happens with my 1.7 Djet 914. The fuel pressure drops randomly from 29 psi to 24 on idle and to 10 if I throttle it..
This can happen once every 3 days or sometimes 4 times a day...
The solution is to turn off the engine and start it again...
The fuel filter and the pump are new and the car can go 160 km/h without a problem. I´ve experienced this problem also with the old pump, that´s why I decided to replace it with a new fuel pump..

It makes me think that there is an electrical problem in the system, but I can´t confirm that yet as I haven´t meassured the Voltage on the pump when the issue happened.

The pump is in the front trunk.

What do you think that can cause this?

Cheers,
Bogdan

I would start with the rear fuse on the relay board. Remove it and clean the contacts. Check your grounds too.

Is it possible that air builds up in the fuel line?

There should be no air in the pressurized fuel line, just fuel.

Couple of thoughts.

That the fuel pump is in the front trunk means that it has been relocated, right? So is there anything specific to the custom install that would result in variable fuel pressure?

When the pump is on the fuel pressure is regulated by the fuel pressure regulator and held steady by a check valve in the fuel pump. I assume that you have a fuel pressure gauge attached to get the precise fuel pressure values you indicate.

Fluctuations in the fuel pressure from steady pressure to less pressure, indicate a bleed down in the system somewhere. Even if the pump is cycling on/off, the pressure should be held for awhile by the check valve in the pump and should not vary so much as to affect idle.

So where is it leaking fuel? Maybe a leaky injector or cold start valve? Maybe a failed/failing check valve internal to the fuel pump? Defective fuel pressure regulator?

Somewhere there was a thread about bench testing the fuel pump IIRC.

is the in tank fuel sock still there? Could be crud floating around that blocks the pick up. This is more a problem if the lines are reversed at the tank and the fuel is bypassing the sock completely. the return fitting in the tank is flush with the tank floor so it doesn't take much to plug it up.

Your fuel pressure regulator could have a problem and occasionally not seal on it's seat causing too much flow bypass to the tank. Fuel pressure problems related to engine speed/throttle position (ie: fuel use) are usually flow related. I've never had a problem with my 914's FPR, but have had issues like you're describing with failing FPRs on other cars. One of those times was on a dyno so the car wasn't even shaking down the road and pressure would randomly drop.

Perhaps you could test it by tapping it with a screwdriver or hammer while you are revving by hand?

Might want to check the fuel lines under the tank. After my pump relocation i encountered strange pressure drops due do the supply side soft hose collapsing under suction from the pump. this was in part due to a curve in the line that created a point where it was easier to collapse the hose.


I would think the fact that it appears be dependent on throttle would more likely point to a flow issue than an electrical one.

Hi guys and thanks for your ideas.
A few updates over the car.
I had this problem also when the pump was between the engine and the firewall.. I tought at that point that is due to the fuel filter, which was a bit dirty..
The tank sock is there and the tank is rather clean (I can see in the filter just a few tinny grains of rust)
All fuel lines are new: everything that was hard plastic now is copper and the rubber ones are new..
FPR is new..(the old one was completly stuck after 18 years of sitting stil)

From what you say above, I am also suspecting the FPR..I remember that I was doing an experiment to lower the fuel pressure and then it did not want to open at all. After a few more tryies it opend...

I was suspecting something electric, because when I start the heat blower I can here a change in the pump sound...

The temperature seems also to influence this problem, as it happens mostly when the engine is warm...

Well...now that you mention the above...I suspect a voltage drop. The fan and the pump I believe are on the same relay. Have you checked the relay? Is the pump up front now?

Copper fuel lines? not safe.

Jeff,
Please give me more details..

I've cleaned the fusess holder (relay board), replaced the fuses with new ones and put in a relay from the headlights.
On the highway I ride it ~140 km/h (80-85mph) and never gives me problems, but as soon I as enter the city or exit the city, the fuel pressure drops, I have to pull over and : contact on, contact off a few times to solve it..
The FPR is brand new...(2 months old...) I will try to repalce it as soon as possible...

ok..I'm puzzled again..I've just replaced (again) the fuel regulator...and the problem still persists.. after ~10 minutes of riding, the pressure drops...I have to stop the engine for a few seconds while the car is moving and start it again to regain full power...and then I can drive more ~minutes until I have to repeat the scheme...
I've checked the pump voltage while the pressure drops and is just below 13.5..
Is it an Amps problem..? something heats up and limits the current flow to the pump?
Or there is a problem in the brain, and it can not energize the pump relay properly..?

Kinked rubber line under the fuel tank.

Rubber hoses pinched anywhere, restricting flow? Under the fuel tank maybe?

Copper tubing is not good for automotive fuel lines. The soft annealed copper work hardens with vibration and will fail due to flexural stress, creating a safety hazard. The copper also absorbs heat, heating the fuel, which is not good. Replace with Type 6 Nylon per the original specification, or stainless steel available from many sources prebent just for our 914s.

You know I have had copper lines for ten years now...

You're just asking for trouble in that case...

--DD