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Discussion Starter · #1 · (Edited)
To see original post, see SRT/Mo's quote of it.

To assist with searches, this turned out to be a BOV/MAF relationship of placement.

Having the recirc configured BOV about three inches Pre-MAF caused a strange turbulence that threw off the MAF readings, but not immediately, there was a delay in which it did this.

When looking at OBDII fuel trimming (if a piggy setup, won't be an issue if using speed density) one would deduce a vacuum leak is developing around that time, and not a delayed turbulance/MAF reading error.
 

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What I thought had been sorted out as my previous BOV being the culprit, turns out is something else.

Symptom: tons of + fuel trimming at idle, typical of a vacuum leak. Weirder part, it is time delayed. If I rev it, just slightly (another 500-1000rpm above idle) the fuel trimming goes down despite being in pretty much the same vacuum, seems all is well. When I let it go back down to idle, it runs rich from the older fuel trimming at the just earlier idle, trims that down, runs for about 8 seconds happy, then bam some kind of vacuum leak kicks in, slams it on it's ass and it has to trim back up to stoich. So much so, a "too lean" CEL kicks on.

Trouble shooting: After heaps of this'n'that, problem seemed to be rectified by taking the BOV out of the loop. OK, guess its the old BOV's diaphragm or some such thing.

Enter a new BOV. Problem is still there, still somewhat fixed when simply pulling the vacuum line off and capping it. :confused:

This issue is simply amazing.

Weird part 2: When allowing for an intentional vacuum leak (disconnect BOV line and not plug it), the car will idle higher (makes sense) and the fuel trims actually get a bit better. How the f*ck can this weird vacuum leak be so "big" (+20% LT +10-20% ST even with tuning the EMU to richen that area up) yet not increase idle (just makes the car idle crappy)?

What the heck could be making this time delayed fuel trimming/vacuum leak, and why is it pretty much eliminated with the BOV disconnected. Could I really have gotten another bad BOV? Doubt it as I went with a Synapse which doesn't even have a diaphragm. Could be so many other things I could attack and fix if it wasn't this super weird result when playing with the BOV.

Seen that before, check for leaks around your injector base to manifold.

Spray a little bit of brake cleaner around the base of the injector and see if the idle changes.
 

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The problem could be on the exhaust side in that you might have a leak before the primary O2 sensor. The car may think it's lean because of oxygen entering the exhaust stream. And the leak may be so small that it creates enough of this effect at idle, but not at speed where a small leak contributes less to the exhaust flow.

It can also be how your MAF if mounted in that airflow across it is non-linear at low flow rates.

Good luck and let us know what it turns out to be.
 

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Why does a vacuum leak that causes the idle to rise make sense? Every vacuum leak I've ever had kills the motor or it runs horrible. Is it different for a boosted motor?
 

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Why does a vacuum leak that causes the idle to rise make sense? Every vacuum leak I've ever had kills the motor or it runs horrible. Is it different for a boosted motor?
When a vehicle is running overly rich, an unregistered (Fuel ECU does not adjust for it) vacuum leak will lean out the A/F ratio and cause the engine to smooth out. This simply tells you that the engine's A/F ratio is too rich, the timing is set out of whack, or a combo of both.

K
 

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Discussion Starter · #7 · (Edited)
Thanks for all the suggestions.

Ki11bert, this is OE ECU, and then an Emanage Ultimate thrown in. Despite not using OBDII data when on the OE ECU alone, I think this problem was there. So it is something that was done during the kit installation (used Tnetics kit, price was right).

Mo, I think this problem was here before touching the injectors, as I would see a lean spike after pulling up to a red light (kit bolted on, but no EMS/injectors/plugs, just feeling it out), I attributed it to OBDII diagnostics and did not look at fuel trims so I can't be sure. We shot some brake cleaner at the injectors that had line of site anyhow, would have been much easier if I had one of your manifolds ;)

Teck, IMO a exhaust leak pre-o2 sensor would cause this issue sooner and would be loud, but it would be worth a shot to check it out I guess. What gives this the most credibility is despite good a/fs (sensor down stream in down pipe), it idles kind of crappy and an intentional vacuum leak raises idle and smooths things out. What goes against it is that both banks are trimming about equally, not just one.

Inspar8r, if the vacuum leak is large and sudden, it can kill an engine, but if done just a bit slower the OBDII learns around it (or you tune for it), get back to stoich and you are now bypassing the OE DBW throttle body and hence the higher idle. Thus the 'drill a hole in the butterfly valve' trick some standalone users use to raise their idle around DBW.

Guys, I suppose it is not the BOV though removing the BOV from the equation changes the results dramatically. I suppose I have a lot to learn about vacuum/load dynamics and the BOV places some kind of parasitic vacuum load on the engine that exaggerates the problem, but is not the problem.

I am going to disconnect anything that has to do with tapping the manifold except the MAP sensor and see what happens, and I'll pursue your advice teck. I'm starting to suspect the brake booster or anything else that has a diaphragm and possible slow mechanical movement induced by vacuum. I am going to disconnect the PCV system too, that is a source of some un-metered air, I suppose if the complex flow dynamics of a PCV valve is compromised (they are not a simple check valve but also modulate flow based on load), it could be letting way too much air in at idle. I suppose if this is a vacuum driven faulty device issue on a speed density setup this would go un-noticed. Maybe plugging in the MSII will be part of the trouble shooting.
 

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Teck, IMO a exhaust leak pre-o2 sensor would cause this issue sooner and would be loud, but it would be worth a shot to check it out I guess. What gives this the most credibility is despite good a/fs (sensor down stream in down pipe), it idles kind of crappy and an intentional vacuum leak raises idle and smooths things out. What goes against it is that both banks are trimming about equally, not just one.
Think pin-hole leak in the exhaust. I'm assuming you've gone over the intake side so I'm just giving you another possibility.

Actually...when you speak of both banks. Do you really have 2 primary sensors from each bank installed? How is this configured? Is there any O2 sensors sequentially installed inline because O2 sensors can burn oxygen themselves changing the composition of the exhaust stream downstream for the next O2 sensor.
 

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Discussion Starter · #9 · (Edited)
No its a log mani with two bungs on either side, in their appropriate banks. You know me teck, I want to retain as much OBDII as possible, as if the car came boosted from the factory.

Well, embarrassing. I did not bother to check if the Tnetics BOV is cracked open at idle. I figured it was closed unlike the Synapse. I guess it must be cracked open like the Synapse. What appears to be happening is this:

I moved the MAF flange from pre-turbo to post turbo before the TB like most other piggy setups. The Tnetics BOV flange is a few inches up-stream from my new MAF position. The cracked upon BOV at idle must be causing turbulance causing the MAF to read much less air than what is really getting by, hence the major adding of fuel.

The strange part is the time delay involved here, where it idles just fine, then this turbulence eventually creeps up or something. The fix was putting tape over the BOV's output (could feel air flowing out of it at idle, guess the turbo is pumping even at that low of load). Runs great like when removing the BOV from vacuum. Maybe the time delay is ECU related, as supposedly ECU's do not look at MAF readings for cold start, tip-in and decel, or maybe it is just the dynamics of flow. With air rushing out of the BOV, it must be stalling flow at where the MAF's element is, while airflow to the engine is getting by around the element maintaining idle. I suppose clocking it or just the difference of 4 or more inches would have alleviated this.

So I bought a BOV for nothing (though I really wanted a Synapse anyhow), and IMO the easiest fix is a compromise to my goal. I really wanted to stay recirced to reduce noise and increase spool upon shifting, but rather than relocate the BOV or making some kind of baffles, I guess I will just buy Synapse's 'anti-stall' kit that removes this gap during idle.

I might just let my mild retardation take over and relocate the BOV, mess with some runners or baffles, or see if a Supra MAF fixes it... sucks.

Hope this thread is of some help for the other NA-Ts out there with their questions in regards to MAF and BOV placement.
 
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