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'74 2.0 fuel pressure and vaporlock problem

I'm having an issue with my fuel system that has something to do with either the pressure regulator or the check valve in the pump.

I inherited my fathers '74 2.0 914/4 back in 12/08. The last inspection sticker dated from '94. It was garaged during it's hibernation, but it was not prepped for such a long sit.

I had to replace nearly the entire fuel system. Tank, lower lines, strainer sock, pump, filter, (upgraded to metal) tunnel lines, injectors, injector seals and upper lines were all replaced. The only original fuel system parts still in the car are the fuel rails, pressure regulator and the cold start valve. I also replaced the points, plugs, plug wires, dist. cap and rotor, as well as timed it.

It ran great for a couple of months after getting it running again, but it started to get cranky about starting when warm, then it started not starting at all (luckily it was parked at my house when that time came). I put a pressure gauge in line with the CSV to check and set the main pressure regulator. Cycling the key resulted in the pump turning for a second or two, but no pressure. I unhooked the line that goes into the regulator and checked for flow, none.

So, this past weekend I got motivated to get it running again. I decided to try swapping out the FP relay with a new spare and discovered that there is a main engine fuse right next to it (I had replaced all the fuses in the cabin way back when). It was pretty nasty (not blown, just oxidized), so I replaced that too.

Now when it's cold (70f or so) I can hear the pump spin and some gurgling from the tank and it'll start. Pressure gauge reads 28-29psi and life is good. Let it warm up and then shut it off, the pressure bleeds off to 0 in less than a minute and it will not start back up. It takes a couple of hours before it'll start again.

I guess my question is this, what should I look for or test to see why the pressure is dropping so fast, setting up a vapor lock. I read somewhere that he check valve is there to keep pressure on the system to prevent a VL situation. And before anyone asks, the pump was moved up front a long time ago. The replacement pump is a rebuild though. Could the regulator be at fault (it does hold a constant 28-29psi when it running)? If the check valve in the pump is wonky, is there an inline CV that I can use?

Any suggestions or ideas are greatly appreciated.

Whitt
Old 07-07-2010, 08:31 AM
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The pump impeller is the check-valve on the pump end of the circuit. The fuel pressure regulator is the check-valve on the other end.

I would look for kinked fuel lines under the tank. They can collapse and prevent fuel from flowing.

The pump should not be vulnerable to vapor lock if it is under the tank.

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Old 07-07-2010, 07:05 PM
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Flow isn't the problem, unless it's hot from running, which it will do if it's cold (not been running). The rapid pressure loss seems to be the root issue, and I'm unsure of how to test either end without replacing parts (I'd rather avoid spending a boatload on parts that don't need replacing).

I did do some thinking, and it seems that iff the regulator was leaking and the check valve in the pump was good, there should at least be liquid fuel in the line prior to the regulator. Every time this happens I pull the line on the pump side of the regulator and there's nothing but fumes. I've even tried cycling the pump with the line unhooked (yes, extended and run outside of the car to a can) and the pump runs, but no flow. Only after it cools to ambient will it get flow and pressure.
Old 07-08-2010, 06:56 AM
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Flow isn't the problem, unless it's hot from running, which it will do if it's cold (not been running). The rapid pressure loss seems to be the root issue, and I'm unsure of how to test either end without replacing parts (I'd rather avoid spending a boatload on parts that don't need replacing).

I did do some thinking, and it seems that iff the regulator was leaking and the check valve in the pump was good, there should at least be liquid fuel in the line prior to the regulator. Every time this happens I pull the line on the pump side of the regulator and there's nothing but fumes. I've even tried cycling the pump with the line unhooked (yes, extended and run outside of the car to a can) and the pump runs, but no flow. Only after it cools to ambient will it get flow and pressure.
Old 07-08-2010, 06:58 AM
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I checked the lower lines. No kinks found.
Old 07-09-2010, 04:18 AM
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Just some suggestions:
1) How is the pump? You mentioned it was "moved up front"... which, iirc, would mean it would have to have been a replacement. I *thought* that the rear mounted pumps were "suckers" whereas the front mounted pumps are "pushers".

2) How is the condition of the relay board itself? On my '76, the underside was completely cracked up like a dry lakebed. Its possible that, exposed to engine heat, the board warps a bit and connections begin to fail (the circutry is inbedded in the plastic).

When the car is hot, and you try to restart it, do you hear the FP relay "click"?
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Old 07-14-2010, 01:59 PM
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I'm not 100% sure it originally was in the engine compartment, but when I started working on it it was up front. The one I pulled off was completely locked up, so I had to replace it. The only one I could find at the time that matched it was a rebuild I got from AA. It's the one with 3 ports. Worked great for a couple thousand miles, then this problem.

Small update though, now when I cycle the key, I do hear the relay click, but now the pump isn't making noise, no flow or pressure even when cold.

Is it possible to convert this system over to use the 75-76 pump? If so, is there a parts list or how-to?

I'll check the relay board the next chance I get. Also, sorry for the delay getting back on this thread.
Old 08-06-2010, 11:56 AM
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No relay click means no fuel pump power. You might try swapping out the relay with one of the others on the board. They are the big round ones (you could have as many as 4 on your relay board depending on options).

My car went through a similar situation. Replaced both pump and relay board. No issues since.
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Old 08-06-2010, 02:54 PM
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Got it runnning. Turns out it was

1) a failed fuel pump motor (fixed by using the old motor from the original pump that had a bum pump).

2) Wiring fault at the FP plug (I already said I hear the clicks but not the pump). Had continuity on the ground from the pump to the relay board, but not on the hot side. Fixed by making a new wiring harness that I spliced in behind the right side running board.

3) Two failed injectors on the driver side (odd that both on one side would fail at the same time). Fixed by working with ones that I never sent in for the core charge. Shot brake cleaner in them and carefully worked with the needle until they were free enough to operate (dad let it sit for 14 years before I got to it). Once I reinstalled these, I had fire in all four jugs.

4) Plugs had some carbon buildup, but not bad, replaced anyway.

5) Replaced fuel filter just for giggles while I had easy access up front.

6) Installed inline check valve just after the pump on the line going to the engine.

Still getting pressure drop off after shutting down (takes about 5 seconds to reach 10 psi, after that about 10 minutes before 0 psi). After putting the aux check valve on the line, I'm pretty sure it's not flowing back through the pump. Could this be a sign of a failing pressure regulator? It does hold a steady 30 psi when it's running.
Old 09-20-2010, 06:24 AM
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