1974 914 2.0 fuel pressure question

[Lockwodo]

I checked fuel pressure yesterday with the engine idling. It was 29 PSI which is within spec, so that's good. I then shut off the engine and observed pressure drop. The pressure dropped from 29 to 25 PSI after one minute, and dropped further to 20 PSI after about 6 minutes, and was at 10 PSI after an hour.

Is this normal/acceptable, or do I have a fuel leak I need to chase? If I'm dealing with a leak, is the most likely culprit the fuel pump check valve, a faulty pressure regulator, or a leaky injector? I guess out of the three possibilities, I'd be most concerned about a leaky injector since the system maintains 29 PSI when the engine is running.

Thanks in advance!

[Dave at Pelican Parts]

It sounds fine to me. Is there anything else that makes you suspect some kind of fuel delivery problem? If not, don't worry about it.

--DD

[Lockwodo]

Hi Dave. The idle isn't smooth and sometimes it bucks and backfires when accelerating. Actually, it seems to run a bit differently each time I start it. But it has run great in the past, so it feels like some sort of intermittent issue.

So far, I've replaced all ignition wiring and plugs, coil, disty cap and rotor (it has an electronic ignition module), rebuilt the TPS, cleaned the ground connections (except the tranny to chassis, haven't gotten to that yet), replaced the K&N, and checked for vacuum leaks with propane.

I replaced the fuel filter and tested/set the fuel pressure at 29 PSI. The fact that the fuel pressure drops when the engine is shut down could be an indication of a leaky injector, right? So on my "to do" list is to pull the injectors and have them reconditioned and possibly replaced.

BTW after I replaced the fuel filter, I connected tygon tubing to the pressure side fuel line at the pressure regulator and ran it into a gas can. I bumped the fuel pump by turning the ignition on and off and saw air bubbles in the tubing. Despite how much I did that, there were still tiny bubbles there, so I suppose there may be an air leak in the fuel system, and if that's the case I would think it could cause missing.

I'm no where near giving up but I gotta admit this is frustrating compared to the American classic cars I'm used to working on!

Doug

[jess p]

The pressure regulator should not lose that much pressure after an hour. Overnight it should be at least 15. Check the bypass relief valve in the pump first and if the symptoms don’t improve you may need a new fuel pressure regulator.

[jess p]

One more thing. Did you replace the fuel filter? Did you see any rust? Fuel pressure regulators do not like rust particles.

[Dave at Pelican Parts]

A leaky injector should be easy to see. You can remove both injectors on one side from the intake and leave them connected to fuel rail. Stick them in a couple of jars, and jumper the fuel pump relay. (Put a thick wire between #30 and #87 on the socket.) Run it for a bit to build up pressure, and look at the injectors. Or let it run for half a minute or so, then pull the jumper and look in the jars. Should not be any fuel in there.

--DD

[Lockwodo]

Hi Jess P. Yes, I did replace the fuel filter and I haven't noticed any rust in the fuel line. I'll take a look at the old filter for evidence of rust.

Dave: yes, injectors are at the top of my to do list.

Today I noticed the following: I started the car from cold and it idled more or less smoothly for 10 minutes. Then, I depressed the accelerator pedal to see how it would rev up, and it started running pretty rough. When I let it drop back to an idle, the engine surged, almost died, surged, almost died, etc. I'm thinking the auxiliary air regulator is not be closing and that is causing the surging. Thoughts?

Thanks!

Doug

[jess p]

I don’t think will surge and die. If the AAR valve dosen’t close you would have a high idle. You can remove the AAR and squirt some wd40 into the AAR as they sometimes stick. As I recall the AAR closes even in cold temps after 3 minutes. If I were a betting man I’d say it’s the fuel pressure regulator and second the bypass relief valve.

[Lockwodo]

Hi Jess and thanks for the feedback! I checked the AAR a few days ago and it is closing all the way when the engine is warm. And I agree with you that an AAR valve stuck open wouldn't cause the surging at idle.

The fuel pressure remains a constant 29 PSI when the surging behavior is happening, and I replaced the fuel filter, so I don't think fuel pressure is the cause.

I'm not familiar with the "bypass relief valve" you mentioned and I thought I knew all the air system related components on this car. Does it go by another name? When looking into this using the 914/4 Tech Notebook, I found one diagram indicating that an EGR valve may be in the system. Is this what you're referring to? All the other information I've reviewed on the Tech Notebook does not show an EGR valve. Perhaps that was added in 1975 models?

I'm currently giving the PCV valve a much needed cleaning. I want to eliminate that as a potential problem and anyway it's routine maintenance that apparently hasn't been done in quite a while. And, I have a set of new vacuum hoses on the way and I'll install those.

Thanks, Doug

[Dave at Pelican Parts]

EGR was added in 1975 in California (and I think Massachusetts?) cars.

It sounds like the "bypass relief valve" that Jess is referring to is the valve in the fuel pump. If the pump develops too much pressure for even the fuel pressure regulator to handle, that valve will open and pressurized fuel will be diverted to the "R" port on the pump, joining with the return from the fuel pressure regulator and going back to the tank.

--DD

[Lockwodo]

OK, thanks Dave. The previous owner had a two port fuel pump installed. I verified that when I replaced the fuel filter. So, I'm guessing the pump doesn't have the "R" port and the line going back to the tank, right?

Oh, and I'm thrilled not to have an EGR valve by the way!

Doug

[Dave at Pelican Parts]

The two-port pump has no "R" port, correct. I don't think there's any valve in it, but I am not sure. I believe that the pump rotor itself acts as a valve to prevent (or restrict) flow back through the pump body.

--DD

[Lockwodo]

OK, thanks Dave. That might explain the gradual leak down of fuel pressure that I'm seeing when the engine is switched off.

Doug
Santa Cruz, CA