How about a nice game of: Connect The Hose...

[My_Q45]

Hey,

I have just finished a plenum job, started up the car & heard a vacuum leak. I was unable to find a connection. The leak was coming from a metal line in the back driver side of the plenum. It's one of the very small lines that run under the plenum & the back of the connection veers off to the passenger side. It's to the right (looking from the front of the engine) of the piece that connects to the plenum with a solenoid & a brown connection harness. I hope this description is detailed enough so someone can tell me where it connects.

Thanks in advance

[Q451990]

Any chance you can take a picture and post it here?

Heath

[Qdog]

I believe that would be a vacuum line to a fuel damper, on the passenger side fuel rail. It's illustrated on page EF & EC-6 of the service manual. BTW, can anyone explain the function of the fuel dampers, why are there two, and can they be tested or cleaned safely ? My fuel rails are still off the car awaiting injectors....

[AZhitman]

Qdog - Email szhosain about some 300ZXTT rear rims (4) he saw for sale in LA.

[Q45tech]

The fuel "pulse" damper gets its signal from a vacuum line just after the MAF.....There is no vacuum on the line until high load and rpm. It is measurable at 4,000 rpm [2" H2O] and increases to 7" H2O at 6,000>7,000 rpm WOT.........this is a tiny amount.

{{warning when you change the air/filter resistance in front of MAF you change the vacuum depth but the MAF alone is half the resistance so with a cone filter the vacuum might only drop to 4" H2O.}}

The damper acts as a variable restriction on the fuel rail to smooth the pressure as each injector opens.

Lots of fuel is circulating [in rail] at idle/cruise but little is allowed in the engine.

If you could view the MAF signal [before conditioning] it is not smooth^^^^^^^^^^^ made into ----------------------.

Same with rail fuel pressure VVVVVVVVVVVVVV, it drops minutely as each injector opens then recovers as each closes before the next one opens.

Injectors are measured at 43.4 psi but PSI REALLY varies 45-40 psi........Normal analog fuel pressure gauges are damped [the reading is smoothed /averaged] so you can't see the fulcuations.

Humans can't see 100 ----psi changes [at 6,000 rpm] per second.

Hopefully I am explaning this: Every one of the 50-100 air gulps per second as passed thru MAF creates a 50-100 momentary increases in fuel pressure to compensate for that fuel that flows out of the rail..........a fine tuned symphony.

There is a mechanical time lag, but this is compensated by the air distance from the vacuum port [after MAF], the vacuum hose line distance] to the damper [its reaction time] and the air path from the MAF to the intake valve.

The system is very important to optimize fuel flow above 3,000 rpm at WOT.

[Q451990]

So basically the pressure regulator is the "in" door and keeps the input pressure regulated, and the damper is the "out" door and holds some fuel in the rail to maintain pressure and cut down on pulsations during injector openings? The higher the vacuum (e.g. WOT, the more the damper closes - holding more fuel in the rail. Without the damper wouldn't the fuel pump be working much harder to keep fuel in the rail since the gas could flow much more easily back down the return line?

Heath

[Qdog]

Thanks, Q45tech, amazing. Do the dampers ever fail, and if they did, how could you tell ? Is it safe (or useful) to give them a direct shot of gumout type solvent while they're off the car ?

[My_Q45]

Yep

That was it. I didn't see that hose because it went under the plenum. I tugged on it a few times before & it didn't want to move. I followed it the best I could & found an unattached end.

I still have a very bad idle, but that's probably from the 2 failing injectors.

Thanks for the help guys!

[Q451990]

I still have a very bad idle, but that's probably from the 2 failing injectors.

That would do it... hope they're in the 1&2 spots so you don't have to pull that plenum again.

Heath

[Q45tech]

You got it backwards: the damper is the rail input and the pressure regulator is the rail output [feeds the return line].The pressure regulator [with 18", HG of plenum vacuum] yields 34 psi in the rail and as the throttle is opened the vacuum drops and the pressure goes up >>> 9"HG = 38-39 psi till at 2" HG it is 43.4 psi on average.