Losing Fuel Pressure after shut down.. FPR or Injectors?

[Alex Mc]

This is the third installation of a hard start problem on my 325i M20.

I put a gauge on the car this weekend and monitored the pressure after running the engine up to temperature. Sure enough, on my second test (problem was intermittent) the rail came down to 20psi and over the next 45 minutes basically to zero.

FPR or Injectors is what I'm thinking, but is there a way to test the FPR? Is there any other component that could lead to a loss of pressure after shut down?

I had the thought that a leaky injector would cause fuel to pool in the cylinder, which would sebsequently lead to a smoke cloud of sorts when the engine finally fired up. This has never happened. So learning toward FPR?

Finally. Any problem with an AutoZone FPR? Half the price of OEM and it's not exactly a complex component.

Huge thanks to all who have helped me track this down. The collective know-how of R3vlimited is why I frequent this forum. Cheers.

[e30Rooster]

Idk if the fuel return lines have a check ball in them like on mustangs but that could be it.

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[oldmann]

Should be able to pull the vac. line at the fpr (plug the line) and get an increase in fuel pressure.

If you have a leaking injector, pressure should drop off within a few minutes.

Failure of the built in ck valve in one of the fuel pumps may be an issue.

Start the engine , get the pressure up, shut it off and clamp the return line shut and see how long it will hold pressure without dropping off too much.

20 psi while running sounds low, steady running pressure for my m42 (different motor I know) is 40 psi.

Also try clamping the return line while running & ck pressure, fuel pump (pumps?) may be weak. Pressure should jump up over 60 or so.

[Alex Mc]

I worded that poorly, I meant that FP fell to 20psi (5-10 minutes) after shutting down.. while running at idle it was over 40psi.

[jlevie]

Since you had normal pressure in the fuel rail while the engine was running, the loss of pressure with the engine off is most likely not the FPR, but rather the high pressure pump. The check valve in it is leaking. But even with a loss of pressure while off, if the pump in in good condition and the filter isn't clogged the rail pressure should hit normal within a few turns of the engine. So it would sound like the pump is worn in addition to a leaking check valve.

[oldmann]

With it's running try to duplicate what happens at normal shutdown (drop in rail pressure) with the vac. line. If you can I'd suspect the FPR.

[Alex Mc]

Since you had normal pressure in the fuel rail while the engine was running, the loss of pressure with the engine off is most likely not the FPR, but rather the high pressure pump. The check valve in it is leaking. But even with a loss of pressure while off, if the pump in in good condition and the filter isn't clogged the rail pressure should hit normal within a few turns of the engine. So it would sound like the pump is worn in addition to a leaking check valve.

I haven't replaced the fuel filter but have one on order.

The car is a '91 318is so I've only got the one pump in the tank.

Do I have to replace the pump in order to replace the check valve? Could you not install that valve downstream of the pump (assuming the pump is in good shape)?

[dnguyen1963]

Fuel line check valve part # 16 149 068 988.

[jlevie]

I haven't replaced the fuel filter but have one on order.

The car is a '91 318is so I've only got the one pump in the tank.

Do I have to replace the pump in order to replace the check valve? Could you not install that valve downstream of the pump (assuming the pump is in good shape)?

You can insert a check valve, but go back and read what I said about a good pump...

[oldmann]

This is the #16 149 068 988 for a retrofit part, It goes inline at the fuel pump outlet. Looks like a piece of fuel line, with 2 clamps.

This was originally a fix for the E34, BMW dealer part, ask to have them ck availability of this part, most counter guys don't know what it is. I've never seen this part except at the dealer, less than $20.

[Alex Mc]

You can insert a check valve, but go back and read what I said about a good pump...

Jim, I did see that but was also not sure of the fuel filter's condition. Also, a check valve would confirm where I'm losing fuel pressure.

I'll put the new filter in and see what difference that makes in terms of cranking effort, and procede with a new pump if nothing changes...

[Alex Mc]

I realized that if I just let the car crank long enough it will start everytime, which has me totally convinced that the fuel pump is simply on its last legs.

I ordered a new (VDO) fuel pump from Pelican Parts today, along with a new fuel filter.

[Alex Mc]

New VDO fuel pump, new Interstate battery, new Bosch fuel filter and new rubber fuel lines. The problem has not gone away.

Key clue: Engine starts perfectly when cold, long start when warm

I'm going to run a FP gauge under the wiper for a few days starting this weekend.

Any additional thoughts out there? Thanks.

[Earendil]

New VDO fuel pump, new Interstate battery, new Bosch fuel filter and new rubber fuel lines. The problem has not gone away.

Key clue: Engine starts perfectly when cold, long start when warm

I'm going to run a FP gauge under the wiper for a few days starting this weekend.

Any additional thoughts out there? Thanks.

Coolant Temp Sensor. Based on what you've replaced so far, and the difference in cold vs hot starts, that'd be my guess. The suckers are cheap too, so unless you know the age of the one that's in there, why not?

[jeffnhiscars]

Also try clamping the return line while running & ck pressure, fuel pump (pumps?) may be weak. Pressure should jump up over 60 or so.

In case someone decides to try this it strikes me as HIGHLY dangerous and likely to cause something to blow with potentially disastrous results. We are looking at full pump pressure (not the lower FPR pressure) on a running engine.