I am having a very specific issue here. Pump no longer holds pressure after shutdown. Original location, original three port pump. It still pumps great when running, but no longer holds pressure. Need to fix that and quick. Do I have to replace it or can it be repaired?

I would suggest you test the injectors, including the cold start injector.
I would also suggest you test your Fuel Pressure Regulator.

If all of those are good, then you might need to replace the pump.

https://members.rennlist.com/pbanders/DJetParts.htm

This is after shutdown. I will have to check the injectors. I don't smell gas. I think I will pull the line to the rail, plug it, and see if it holds pressure. Hate to pull all those injectors if that isn't it. I figured it might be a check valve or something in the pump. A friend said that was a simple fix in a 911. Not sure what year.

"Failed Check Valve: The check valve keeps the pressure to the injectors and CSV up after the fuel pump is shut off to prevent vapor voids from forming and speed up starting. A failed check valve may cause some hot starting problems. There has been one report of a failed check valve causing the fuel pump to return all fuel to the tank, with no supply to the injectors. See the URL below for a test procedure."

And that describes my issue (all of a sudden I have extreme hot start issues). I don't have the rich issues I would get from a stuck injector.

Sadly the link below pbanders mentions is dead.

Hmm seems that "check valve" is debated too. Powered up the pump this morning with no start. 24 lbs at the gauge. Slowly drops over > ten minutes but still reads greater than 15.

Blocked the whole rail after my fuel pressure gauge
Blocked the csv
Blocked the return after the regulator

No change.

Very slow pressure loss. 1 hour later still have 5psi.

Yesterday, when hot, pressure left immediately after shutdown. No pressure on start (vapor lock).

Crawled under the car and covered it all with insulation tape. Anything I could reach. Drove around a bit, got it warmish. Got home, turned it off, came back 5inutes later, 29psi and holding. Going to work out for an hour and look again then get it real warm.

I really think it needs pressure to keep it from boiling and loss of pressure at the pump.

After an hour, pressure back down to 0. Nothing in the pump. 10lbs first key turn. 15 second. 24 on the third )which is normal for the car not running. Started right up when I tried.

Hmmm. Time to go for a hot drive and see what happens on hot shutdown.

Maybe I need to install a dash fan down there on a timer.

Update, went and ate lunch. Came back to it 15 minutes later, still at 22lbs.

Very interesting.

Engine was nice and hot. Parked. Pressure was 24 by the time I got the hood open (engine off). But by 5 minutes later it rose to 30. Hmmm...Click to view attachment

The way to check your injectors is to remove the nuts on both, pull them both out, put them into glass baby food jars that fit in that area.
You are looking for a stuck injector that is dribbling fuel after the car is off.

It’s a fairly simple test, leaving all fuel lines connected.
Same test for the cold start injector, which does leak sometimes.

Rich

Or you just remove them from the system like I did without having to pull them.

The rate of change was unaltered by closing off that portion of the system. And again, I don't have a richness issue.

So I tried to start it after it sat for 10 minutes (still at 30 psi). When I turned on the pump the psi dropped to 19 and the car sputtered but didn't start. Checked the pressure and it was 15 now. Complete vapor lock now, pump unable to generate more than 15 psi. Eventually got enough flow to start it (took 30 seconds). Things have improved since yesterday with the new insulation but it still just isn't enough. Yesterday it wouldn't generate any pressure at all when locked up. And that was immediately after a stop (in the time it took to pull a plug).

I think this is some boyles law stuff. The fuel needs to be under pressure down there where it is hot or it boils. Interesting that the pressure up top was 30 but obviously before or in the pump something had changed. Maybe I need to insulate that damn gas filter.

I had a 3 port fail this way with the pressure holding feature.
3 ports have this built in.

I was able to open the pump... and replace the rubber to freshen the holding pressure feature...


symptoms before repair were loss of pressure when sitting with engine off... over 20 minutes or so.... no leaking injectors...
lots of cranking to rebuild the pressure before it would start... 20 seconds of cranking instead of 1 second....

ultimately I didn't trust that pump even after repair... so installed a 2 port pump and put the 3 port into my travel spare bag.

Why do you need to have the pressure hold when the engine is shut off?

Maybe its something I don't understand.

when I replaced my fuel pump, (Bosch 044) it came with a check valve that I removed.

I did not see a need to have pressure on the injectors when the engine is not running.

It always drops to zero when I shut it off. never have a problem starting up.

have since replaced that pump with a quieter pump (Bosch 69133) that does not have a check valve.

still starts right up on first turn of the key.

Is your pump under your engine or has it been relocated? Gas under pressure has a higher boiling point.

It's not the hot injectors and rails that are the issue, that can be and is repressurized quickly. It's the pump that can't do it's job with fumes.

Well, the factory moved the pump up front under the tank. I would get the parts to do that move so you can access it from inside your frunk. PMB even moved my pump for my 1970 6 that way. I am sure that someone must have pointed out that the factory did this on 75 and 76 cars.

I remember that my 74 2.0 back in 1979 would stop working and just die in traffic and if I came back to it in 30 minutes it would run fine.

What do you have to lose but a little time and money. Cool fuel is the idea.

Best Bob B

Yes, absolutely. That is what I need to do but my time is limited. I need it functional and reliable at the end of may and I am super busy at the moment. I want to do the full tangerine kit but I just don't think I have time. Thus the insulation. I am gone for the next week. During that time I need to figure out which way to go.

And I have a full tank of gas. Sigh.

interesting read @emerygt350 .

i think you are right.
its the section of line that pops out of the firewall and feeds to the pump that is the cause of vapor lock issues in 914s with original fuel pump location. thats the bit of line where the fuel boils. it is subject to heat radiation from the mag fan casting. due to conductance all the heat in the engine flows to that fan shroud and radiates from there after shutdown. while the engine is running however the mag casting is cool due to all the air flowing through it. its after shutdown it becomes a heat sink.
that fuel line sits about 6 inches away from that heat sink.

most fuel pumps don't work so well in suction. they are more powerful on the other side.
so the feed line is a kind of combination of gravity flow from the tank and the weak suction of the pump. thats why its a bigger line than the return.

your speculation that its gasping at vapors is fairly sound i would say.

your tests indicate its not back flowing through the check valve in the three port pump.
thats a little spring loaded pin with a rubber tip on the end of it. most are perished and don't hold pressure anymore if they are still on the car. as others describe above.
if they are losing pressure straight away after shut down thats because their check valve is kaput. thats not the case with yours. test proves it.

i have pulled apart two three port pumps completely.
the top three port section is where the check valve is and you can get that off without going deeper. and you can carefully take it apart if you are curious. the critical piece is the pressed tin crimped cover on the end which holds the spring in. its easy to damage getting it off and then trying to get it back on.
if you want to try its all here.

http://www.914world.com/bbs2/index.php?sho...350416&st=0

but i don't think its your problem.
your injector ring is holding pressure.
i think your logic as to what is going on sounds right.

i have the fuel pump in original location.
have resisted the unoriginal modification.
but i now have metal fuel lines in the tunnel and almost to the pump.
and i have wrapped them with a very effective reflective insulation wrap.
have had no problems with vapor lock since doing that.
interestingly i never experience vapor lock before doing the conversion to metal lines.
only after. so the metal lines were worse than the plastic originals in terms of heat absorption. insulating the metal lines got rid of the problem.

possibly that more modern pump builds pressure almost instantenously.

anyway. if you are not having problems then nothing to worry about.
whats that old saying - don't fix something that ain't broke.

with the originals the idea was to try and hold pressure in the injector ring to give you a fast start. mind you if you left the car for a week or so, all the pressure would be gone.
in original condition when new the system could hold sufficient pressure for up to 24 hours to give you a fast start. that would have helped with cold starts in north american winters. as the system ages it would lose pressure over shorter time periods.

i've got mine working tip top again in the 74 L jet with the fuel pump under the engine.
it holds pressure for an entire working day pretty much. or it did 3 years ago.
probably still pretty good now. if i do a same day start in the car at present it barely even turns the starter motor before the engine fires. leave it for a week and its about 2 or 3 cranks.

@emerygt350

i was watching the full unedited version of the interview with the mclaren mp4/4 F1 car designers last night on youtube before going to bed.

towards the end there was a new added section.
a discussion about the fuel system.

it seemed the mp4/4 suffered from fuel vaporisation problems late in races due to the circuit the fuel ran in, which meant it had to be drawn up from the bottom of the tank bladder and over the engine and to the rear of the engine. this also increased the temp of the fuel in the total circuit. sometimes the fuel in the tank would start boiling.
and the car would suffer starvation as the main pump began to cavitate.

the solution was pretty good. they built a little catch tank of sorts under the quick fill fuel cap and pumped first to it and pressurised at a low pressure of 0.5 bar. this was enough to keep the fuel feed to the main pump liquid. they made a check valve themselves out of a ball bearing and the spring from a ball point pen.

the pressure was quite low. 0.5 bar = 7 lbs. it was done with tiny electric fuel pump.
way less than the pressure in a 914 injector circuit or an F1 turbo engine.

the comment was made in the video by the engineers that pumps only like to pump and not suck fuel. a good flow is required for feed. a good flow can be simulated by a small amount of low pressure. so the main fuel pump which generated much higher injector pressure was fed by this low pressure line. the tank itself and return circuit remained unpressurised.

one way around the old 914 problem is not necessarily to go the high pressure route from the tank to the engine as per 75 cars but simply to induce a small amount of pressure in the feed line from the tank with a tiny pump?

i dunno? just a thought. it may be enough to keep the fuel from boiling in the critical part of the fuel line after the tunnel and before the fuel pump. a low pressure feed.

https://www.ebay.ie/itm/293759097396

such pumps are very small. easily accommodated discretely under the tank.
and with less stress on the fuel lines through the cabin if you don't have the steel lines in yet. or you don't feel comfortable with a higher pressurized line through the cabin (which i must admit is how i feel re high pressure line through the cabin - i know that is probably not rational given i have steel lines in now......but).

Very interesting! I have had stupid old v8s that run steel lines across cast iron manifolds too. The 3 pounds was obviously enough to keep it liquid.

Engines do like cold gas. I am going to take another look at my lines when I get home.i already have dei on the lines from the firewall to the pump and the pump and other have insulation wrap.

Maybe a little more in the right spots ...