99' Mazda Miata P0172- fixed rich 02

Hello everyone-
New to forum. Been lurking for a while. 20 year tech. Trying to up my game with scope usage. I have a verus but by no means is it my "go to" tool. Trying to use it more and more so please understand that other then watching many of Pauls videos- I'm still a rookie! So here is what I have:

1999 Mazda Miata-
212K on the clock-
Recent engine build by another shop- (seems strong- 21" of vacuum at idle)
Has an aftermarket intake (with a reman maf) along with a header and high flow catalyst (that doesn't appear to be working well)

Car runs great. You would never know there was an issue except for the mil and P0172. At idle b1s1 voltage is pinned at exactly .9v. As soon as I bring the idle up a hair it starts switching. Give it a vacuum leak and O2 voltage drops down to .2v. Rear O2 when finally warmed up is sitting right at .8v so I'm fairly sure that I have a true rich condition.

What I have checked:
Fuel pressure is 62psi-
Verified cam timing-
There are no vacuum leaks-
Made a block off gasket for egr just to rule that out.
Throttle position is zero at idle
STFT: -18
LTFT: -20
Coolant temp: 181f

I checked the reman maf with the scope. At idle its right at 2.2v. Verus says it should be around 1.8v. Is that enough to cause an issue? Not sure if its the aftermarket intake causing this? I also attached a cell phone pic ( because the greenhorn couldn't figure out how to snap shot it while learning on the job) of my throttle snap and it does not really look like what's in Paul's book. Any thoughts? Thanks for takin' a look.

what does the maf read grams/sec at idle?

Hey sir, thanks for posting!

The MAF sensor reading does sound high at idle, but this one probably won't run well without the MAF connected, huh? The waveform itself doesn't look bad, I like the initial peak and the flow ramp.

Is there a fuel pressure regulator to check on this engine? Thinking of fuel leaking into the vacuum hose, dumping raw fuel into the intake. Can't recall if this setup is return less or not...

Its a return less system. Mazda specs out 65psi at idle. I will get the maf g/s reading tomorrow. One observation I have which I doubt plays a role is the B1S1 has been moved towards the rear about 3 feet from its OE location as a result of the aftermarket header. I see a lot of these cars with way more mods with no issues. Most are asleep for winter at this point. Also did a wot pull and O2 voltage was a steady .8v. I have pinched off the purge valve hoses... Injectors appear to be stock Mazda parts. Thanks for the input everybody

You've changed how that engine breaths with that aftermarket intake and exhaust. The question is, how much? It could also be the MAF.
It could be worse with a factory MAF.
O2 is fine, good job so far brother.

Maybe you could try to manipulate the maf with a potentiometer. Set it at 1.8 volts at idle and see if that changes your o2 reading. Just a thought. Have not tried it myself on a maf curcuit.

Inge Jeppesen wrote: Maybe you could try to manipulate the maf with a potentiometer. Set it at 1.8 volts at idle and see if that changes your o2 reading. Just a thought. Have not tried it myself on a maf curcuit.

Oooooooh, I like it! I can't say I've tried that, either, but I kinda want to experiment now.

I thought about going that route. Maybe backing out the wire and use a "aa" battery as a 1.5v source and see what happens. Any change would help me to think I'm heading in the right direction. Never got a chance to revisit the car today. Man- I hate snow tires!!

quick update on this:
Provided maf with 1.5v- no change- O2 pegged at .9v
Looking at data again- I realized I missed loop status- at idle with quickie launch scanner it said "OL drive". As soon as it came off idle O2 began to switch- trims would dial back. Happened to look at status while moving it around and noticed as soon as I put the clutch in- loop status went to closed and O2 began switching again. Jumpered the upper clutch switch which I thought was only for cruise release (which this car does not have) and all is well with the world again for the Miata. Not a fix yet- but I have some direction. Iat, coolant temp and tps are where they should be. What else can play a role in making this thing go open at idle? There has been so many different paws on this car- anything is possible. I had enough time to find and verify all grounds and they are good. My guess- ultimately- this guys goal is to have the car good enough to pass NYS inspection as its only real duty appears to be autocross and track days. I need my smoking gun!

Well, if "anything is possible" perhaps I can toss out another 3-0 curve ball for you. You noted the go-fast bits on the car like intake, header, high flow cat, so I can imagine to fellow installing these parts may have tossed in some sort of aftermarket ECU tune to make best use of those parts, while at the same time adding to your headache.

I was watching a Mr. Danner video recently where he said something to the effect of "If you have a car with a tissue box in the back, drive a bit slower when you try to duplicate the concern. Big subwoofer box, perhaps be a bit more aggressive." You see headers/high flow cat, etc., think ECU tune. Perhaps it is one of those piggy-back tuners where it is adapted into the ECU itself and clearly visible. The presence of an additional wideband O2 sensor is pretty common on those cars with aftermarket tunes. Might even be some higher flowing injectors on there as well which could be a factor. If it is one of those ECU reflashes, it might be a little harder to tell if the stock ECU tune has been altered.

Strangely I found a thread in a Mazda MX5 one make forum that detailed what seems to be a mirror image of the condition --- rough lumpy idle when warm with the clutch pedal depressed eventually followed by a P0704.
That car had two clutch pedal switches the owner stripped and cleaned them both and the condition was cured.
From another source the Mazda switch appears to be an NC type.

Thanks guys- first thing I checked was for any kind of piggy back tuner and the harness looks surprisingly clean. Almost positive there is no flash tuning for this year. I also checked for the classic installation of a spark plug anti-fouler on the front O2 but that usually results in a lot of + trims. As its snowy and icy here now, and the car has a worn out set of r compounds on it my guess is it will be hanging out for a while so hopefully I will have more time come post holidays.