wstrohm

1994 FD, 84 K+ miles. Recently completed vacuum hose job; converted to Viton. Car runs fine except at idle; only about 10" Hg vacuum at 720 rpm; idle has random miss typical of a vacuum leak. Last time vacuum was measured was about a year ago, when it was 16" Hg. Have been looking for a problem, can't find a loose or split hose, but have not yet removed UIM again, so cannot see a lot of stuff.

In chasing this problem, I finally noticed one hose connection shown both in the "System Diagram" of the fuel system on page F-7 of the 1994 RX-7 workshop Manual, and also on the "Vacuum Hose Routing Diagram" on page F-10. (This is the diagram that has been blown up & used as a model on various websites giving "how-to" instructions for the hose replacement job.)

My question is about that hose in particular. As described by Dave Booth: "The last one is a 6 mm hose that comes off the downward-facing elbow on top of the turbo. It tucks behind a short water hose and heads back along the top of the engine well below the solenoids, then turns sharply downward and connects to a pipe on the engine block." (I did replace that hose with a 1/4" ID Viton hose and hose clamps.)

According to the System Diagram, that hose connects from the Y-pipe output from the turbos to a point just at the intake port of the #1 rotor housing. (There is no such connection into the rotor #2 housing.)

Unless there is a lot of restriction in this connection somewhere not shown, this amounts to a vacuum leak. It feeds air from the turbo outputs ahead of the throttle butterfly directly into the intake port of #1 rotor housing. This makes no sense to me.

Does it make sense to you?

Thanks!

---- Bill

wstrohm

Here's a pic of that hose colored in on the vac hose diagram. Any help would be appreciated!

---- Bill

dgeesaman

iTrader: (7)

Yeah, as far as I can tell that's all it does, but it is normal. IMO, it's just there to dampen the pressure through the intake.

Dave

wstrohm

Dave,

But... but... at idle, it's a vacuum leak, no? I still don't get it...

Is there at least some restrictor somewhere? Did I replace the hose and miss a restrictor in the old hose? Or is it a "calibrated leak" by means of an accurately drilled small hole in either that elbow at the Y-pipe or at the pipe on the block? But then why would the hose be a 6mm rather than a 4mm? Would you please elucidate? Thanks!

---- Bill

dgeesaman

iTrader: (7)

It's not a vacuum leak - the only thing making the pressure different on both nipples of that line is pressure loss due to the intercooler, and flow over the throttle butterflies.

So really they are two points in the same pneumatic 'chamber' which is the intake path. And by connecting them it would dampen any local pressures perhaps generated by the intake ports. When the UIM is in vacuum, the y-pipe nipple is also in vacuum (with only a slight difference in value).

The hoses that could cause the leak would dumping to the outside air directly. So an mis-connected hose, or a hose clamp out of place, etc would be the more likely candidate. I recall seeing a procedure by Steve Wynveen on the fd3s.net site that documents how he plugged the intake at the end of the elbow and applied pressure to locate the leak. Something creative like that could be your best approach if pulling the UIM doesn't show any obvious mistakes.

Dave

DaveW

DaveG - you're usually right on, but in this case I don't think so. There is no significant vacuum prior to the throttle plates (except for intake restrictions). The vacuum is created by the rotors trying to pull air past the throttle plate restriction. The only time these 2 areas are at similar pressures is when the throttle is quite open.

So, I guess the question is: how much restriction must be in the line in question for it to not screw up idle speed, mixture, etc.? And what IS it intended to do?

Is this possibly a part of the PCV system?

dgeesaman

iTrader: (7)

But you are referring to idle only, where the throttle plates are nearly closed and creating maximum restriction. I've never put a boost gauge at multiple locations along the intake to quantify the magnitude of them, so I'll take your word for it. I do know that with the elbow not connected to the intercooler, there is still very significant vacuum felt there. I expect that the air filter also is a significant restriction.

In boost or maybe even mid throttle, I think the plates are open enough that the restriction nearly disappears.

The experiment for this is simple then: if you suspect this line might be the problem, pinch it while the engine is running and see how it affects the idle vacuum.

[edit: mine is connected with the 6mm viton from HighTempSilicone, and it did not affect my idle vacuum]

Dave

wstrohm

Two Daves... my point was the point made by Dave W. The only problem with my car is at idle, and that is when the throttle butterfly is closed and vacuum should be at somewhere around 16 - 18" Hg. The engine is currently showing 10" Hg. This hose, if not restricted, would "short" air upstream of the throttle to the intake manifold, and reduce vacuum. I am wondering if possibly there was a restrictor pill in the old 6 mm hose that I did not see. There is no indication of such in the FWM, though. So maybe there is restriction in the Y-pipe or the engine intake port itself which minimizes the effect of the leak.

Probably my leak is somewhere else... just wondering why that line is there at all.

I cannot pinch the line as it is currently inaccessible, and making it accessible would disable the engine so that it would not idle (I think). Dgeesaman, if you used a hose with no restrictor and it caused no problem, I guess my problem is somewhere else.

Thanks for the inputs!

---- Bill

dgeesaman

iTrader: (7)

My guess is they put it there to stifle any resonant behavior in the airflow. It's possible that at the right rpm, the frequency of the rotors passing the intake ports would resonate and create a standing wave - by putting that small short-circuit line there, it would dampen that behavior.

[edit]
This has happened at Raccoon Mountain hydro plant, where they have 500MW hydro turbines pumping water up and downhill. The vibration when a standing wave resonates the structure is deafening and actually shakes the powerhouse. That's saying a lot, considering the powerhouse is a cavern cut out of the solid bedrock of the mountain.

Another possibility is that it dampens the pressure pulsation created when the throttle butterflies are suddenly closed.

Dave