Interesting Idle Vacuum Correlations PART 2
 

I've always felt that my idle vacuum was a bit too low.
It always came up a bit when I went to places with a bit less altitude, but there are other RX-8's here and their vacuum is higher. Even other MM turbo cars are higher, so its not the scroll, manifold or anything like that making back-pressure.

My current motor (and the one before it) ran perfectly and have good (or in the case of the current motor - crazy good) compression.
They are well-oiled and are totally normal in every way.
The idle is stable, smooth and regular.
There are no misfires, the ignition at idle is OE and the A/F is, too.

However, my idle vacuum is about 15" hg. It really should be at least 16 or 17 at minimum.

I tried going to a good set of stock coils (just to eliminate that possibility) and there was no difference from the BHR setup.

So, while charging my battery and flashing my PCM, I noticed something.
When I started the car, the current from the charger stayed in the 17 amp range!
When I stopped the motor, it dropped to 4 or 5.
Thinking that it could be something to do with my battery relocation, I moved the charger to the front of the car near the alternator and tried it again.
Same thing.
I tried a different alternator.
No change.
I spent several sessions with the motor chasing potential vacuum leaks with propane, carb cleaner and other solvents.
Its tight as a drum. No leaks.
I jacked up the OMP just to see the difference and, besides a ton of smoke, there was no change.

So, I am out of ideas.

Inspire me.

where are you measuring from?

Behind the TB where the VFAD used to be.
I verified it at the EVAP nipple like it says in the FSM.

that was my only guess :(

I gonna actually try pulling the alternator belt completely.
It might be an electrical load.
The only system I can think of that is only running when the motor is on that can pull that kind of current is the EPS.
I need to do some metered diagnostics to the wiring if it turns out to be electrical.

Today is good.
Late afternoon?

ponder this I shall.

Air bypassing the throttle?

Idle rpm are normal?

I'm not sure about the battery charger thing; maybe just a red herring. Did you try that on another car? Maybe it's just behaving differently with the running engine due to voltage. It's trying to charge what it sees as a 15 volt battery instead of a 12 volt?

No, that's a supercharger thing. :mdrmed:
No "idle bypass" or anything like that.

Yep, totally normal, smooth, stead and no dips or anything. Like glass.

Yeah, I dunno.
An alternator load is the only thing I can think of at the moment.
The FSM says either a vacuum leak (which I cannot find) or bad compression (and mine is around 130 per combustion chamber).
So, I'm still stumped.

When it cools off this afternoon, I'll pull the alternator belt and see what happens.

And is your idle lower than the folks with a better vacuum?

Nope.
A bit higher, actually.
I have it set for 850 with my AccessPORT.
When I drop it to 800 (OE idle) the vacuum is about the same.
When I raise it to 900, it goes up ever so slightly.

And is that with the air filter in place (guessing at a dirty filter)?

This is a new engine, so I would guess that there no defects after the throttle body that would impede airflow.

That's my two guesses. Next!.

Fresh air filter.
New intake with bigger tubing, actually.
No change.
TB - exhaust ports are brand new.
Last engine behaved identically.

I was hoping that the intake, being something, maybe, in common between engines was an answer.

Other than that, my old knowledge says the tune can affect the idle too. But I don't want to go there for fear of getting in way over my head!

Maybe take a step back and list all the things that affect idle vaccum and walk through them 1 by 1.

Intake leaks
Intake restrictions
air density
rotor sealing
AC load
Alternator load
water pump
oil pump
Idle RPM
What else?

What elevation are you located at?

About 1200 feet.

Try a AP underdrive pulley on that bad boy - runs for cover .......

If there is a high load on the electrical system, that would just make it worse.

did you not hard wire the eps? is there a fuse you can pull to isolate it in a easy way?

beers :beer:

I love simple suggestions!

Its nice, elegant suggestions like that which really float my boat.
It is so simple, I didn't think of it.

Thanks Swoope! That is what I'm looking for.
I'll go try that now.

FSM says theres a 60A fuse on the constant power and a 15A fuse on the ignition/switched power to the EPS control module

can you look at your PIDS/DTC

DTC B1318 is over/under voltage for the module
the PIDS might tell you something as well

C1099 - Abnormality detected in control current from EPS control module to EPS
motor.

There are no PIDs.
But the 60A fuse is a no-brainer - its right on the battery terminal.
Gimme a sec - I'm working on AP calibrations.
I'll run out and test it in a few minutes.

oh, lol duh.... this is why i have to touch things to make sense of them... i dont associate what i read with actual form/function very well sometimes