aaddpp

Hi, I am trying to understand the inner working of the 928. I know the fuel dampeners have vac lines hooked up to them, but what exactly does the dampener do. Also is the vac neg pressure related to rpm - i.e. the higher the rpm, the more vac applied to the dampener?

Thanks,
Dave

WallyP

All eight of the injectors open at once, which means that the fuel flow starts and stops very abruptly and very quickly. The dampeners absorb the pulsations and smooth the fuel flow.

The vacuum lines on the dampeners have no effect on fuel flow or fuel control. They simply allow any fuel leakage from the dampener diaphragms to go into the intake manifold rather than spilling over the engine and setting it on fire.

The fuel pressure regulator controls the fuel pressure in the rails. The ECU controls fuel flow by controlling the open times of the injectors. If the pressure is higher, the same open times would put more fuel into the engine.

The vacuum line to the regulator trims (slightly adjusts) the fuel pressure according to the intake manifold vacuum. Lower vacuum (more open throttle) increases fuel pressure for a higher mix and more power; higher manifold pressure (less throttle opening) lowers fuel pressure for a leaner mix and better mileage.

On later cars, the fuel pressure variation is only meaningful in "open loop mode", as the oxygen sensor/ECU control the mixture to near-stoichiometric in "closed loop mode". The WOT switch, which actually closes near 60% throttle, kicks the ECU out of closed loop into open loop.

Ducman82

my dampaner decided to start leaking in to the vac lines. so i used epoxy and s screw to block it so the car will not flood when sitting for awhile after driving it. i hate long cranks and starts.

aaddpp

Wally, thanks for the reply I have a much better idea of what is going on in there now.

Appreciated,
Dave

aaddpp

Follow on.

Is it possible to have a situation where a dampener would not leak noticeable amounts of fuel unless the vac line was attached, and then when a vac line is attached the dampener would leak sufficient amounts to flood the engine?

Ducman82

when the dampener leaks, it leaks at 30 psi. mine had what i would call a "small" leak in to the lines, but it was enough to flood the engine after i let it sit for more then a few min when it was warm.

Jerome Craig

Wally, what a great example of clarity in writing, no technical mumbo-jumbo....just the raw facts!

Jerome

WallyP

If you have a pinhole in one of the diaphragms, it will leak all the time that the fuel system is pressurized. This will not do too much while the engine is running except to reduce fuel economy by a bit.

When you shut the engine off, the pinhole will continue to leak into the vacuum line, and thus the manifold, until the fuel system pressure drops to near zero. With the engine warm, this will make starting very difficult until the engine gets cold - after that, the engine will usually crank well.

So, if your problem is:
- Easy starting when cold;
- Hard starting after the engine has sat non-running for a few minutes;
check all three of the vacuum lines for traces of fuel. If you find none, suspect a leaking injector.

aaddpp

Thanks Wally,

I wrote in a previous thread that I found the vac tube disconnected while doing some post purchase service. After hooking it up and running for a short time, the car would not start (while warm). Never had a chance to test cold b/c I had to push it back in the garage and catch a flight. At no time prior to the reconnection of the vac line to the pressure dampener did I detect the smell of fuel, and there was no evidence of fuel having leaked from the rear dampener.

I was sort of hoping that the leak, if there was one, was right at the threshold for a leak to occur, and that the vac line negative pressure was what put it over the edge. Based on what you are saying it sounds like this is unlikely to be the issue....