Hi, just looking to get some other opinions on possible causes of my series 1 rx8 going very lean.
extra info:
2004 MT 6spd
rebuilt engine
catless midpipe
new plugs coils wires
new starter motor
all new seals inside engine along with rotor housings
new oil pressure switch (prob not relevant)

basically the car starts right up cold and hot, and starts off rich while warming up. then balances to 14.7:1 above 70c. but after a while the idle afr goes to 15.0:1.
vacuum reads 15-16hg, but when afr goes lean it goes down to 14-15hg at idle
Also upstream o2 sensor seems pegged right at 1.0v which is rich, and downstream o2 (catless) sits at 0.0v, which is lean. Shouldnt upstream o2 average .45v? and downstream o2 close to it aswell?
ill attach a couple pics of my readings which engine running at idle warmed up. Basically at idle engine is running really lean however once i start driving everything levels out and runs like buttered cream.
At first i though there was a vacuum leak, due to lowish vacuum/high ltft. But i went to a shop and paid them to smoke test it for leaks and they couldnt find any. on intake side anyways. Im starting to suspect maybe a dead upstream o2 sensor though? or a leak in exhaust manifold?
Just looking to get some professional opinions to make some sense of this data. Im at a bit of a loss right now as to why it leans out like this at idle but runs great while driving.

 

unless the pcm is throwing lean error codes then no, the conditions are within the normal expected operating range

Yes, it's really that simple.
.

 

So running almost 19%+ fuel trim is normal for this car? I always assumed for most vehicles it should be as close to 0 as possible. Of course plus or minus a couple % nothing's perfect. But ok thank you

 

That's a good point and I did overlook reviewing all those images that I'm not so familiar with doing as opposed to a proper log data output, but it wont trigger until 25% or so as I recall

did you consider the possibility of a vacuum leak causing that given those low values?

Maybe try spraying some carb fluid around and seeing if the rpm suddenly jumps up ...

 

Those numbers are pretty similar to what i am running with no issues or codes on a fresh engine in a S1 MT @ approximately 3,500km (other than decat codes below).
I also thought i was chasing a vacuum leak but did not find one. The highest LTFT i have seen was not quite as high as yours at 15.
I did have a decent hitout today of about 120km which settled on 12 LTFT. Car runs smooth. I am no longer looking for such issues. Just get the occasional stupid 139 or 420 catless codes that i will continue to clear until i get some tuning software sometime next year.
19 does seem a little high though, without suggesting throwing parts at it, if you happen to have spare O2 sensors lying aroung or maybe just due to change them and keep others as spares, that might give you info.
Good luck and please post outcome.

 

1V on the front O2 sensor is meaningless, it's an impedance wideband sensor.

Sounds like a vacuum leak. Your MAF rate is just about 20% low for 850rpm, so it almost guaranteed.

 

Ok yeah initially I thought it was a vacuum leak too but I've checked all vacuum lines along with oil and fuel injector seals and no luck finding it. The shop I took it too also hasn't been able to find one but I haven't looked on the exhaust side of things yet. I'll try taking it out on the highway more to see if trims come down been mostly city driving it so far. I know that's not great for it.

 

Sometimes it's as silly as the MAF o-ring in the intake tube and it's hard to nail down. But the data doesn't lie, you're reading below the air requirement for 850 rpm and the computer is adding fuel to compensate for unmetered air and bring AFR back to stoich.
​Is the intake stock?

 

Yeah it's stock intake. And I did suspect the MAF a few weeks ago and checked it. I pulled it and cleaned it with MAF sensor cleaner. And yeah the rubber o-ring had a small tear in it. But I gave it a coat of high temp gasket maker around the o-ring and seems to be fine.

 

Here's a picture of the seal before I repaired it. It wasn't good but not completely torn.

 

Another common source of vacuum leak is the VFAD line. There is a nipple out of sight under the throttle body, and a line that plugs into the intake "snorkle". There are a few connections along it that could have been forgotten or damaged during the engine out work.

If it were me, I'd replace that o-ring. FIPG combined with a gasket is a bandaid, it will eventually fail again. Usually it's one or the other, not both. But you can at least spray that area with carb cleaner or something similar from the outside while the cat is running to see if it changes the idle.

 

I think the vfad line is ok but I can check it quick. What would be best way to test it, just pull the line from airbox intake and plug a vacuum gauge to it while running? And it should be pulling same vacuum as engine correct?

 

Yeah the gasket maker was just a temp fix I can't find. the o-ring locally and to order one will cost me upwards of 50 bucks. Im in Canada btw lol. I was thinking maybe a plumbing shop might have an o-ring in same size but haven't had luck finding one locally yet.

 

i have also problem with high ltft in all rpm ranges. always +21 or +22.

bought the car non running. injector wiring was mixed, i fixed the wiring, put all back together. i drive for 50 miles. it was always 22. no change at all. stft close to zero at idle, cruising between -10 -18

and i have power kick around 6500rpm. like really powerfull delayed vtec.

new o2 sensor, new coils, wires, plugs.

anyone has idea?

 

Vacuum leak or poor aftermarket intake. Your AFR is fine, but the car is having to add fuel to make it that way, which means unmetered air is getting in somewhere after the MAF.

 

So short term trim is instant correction which is reading -15. That means the afr is reading rich if I'm not mistaken. And the long term fuel trim is the computer correcting that by leaning it out +22. So the engines running +7 overall which is good. But maybe injectors aren't hooked up right? How does it idle I'd be more interested to see that.

 

@Loki
@shawnstasiuk

idles perfect. starts around -10 stft. goes lower with time. sometimes jump again to -10. when it goes to around zero i am hearing clicking noise around ssv, vfad area.

today ltft drops to 19's. maybe i should just drive the car and look what happens? i searched for vacuum leaks but i dont have any experience with it. i use starter spray but couldnt find anything. weekend i will check that better.

another suspect is tune. car has a tune from a unkown company which not exist anymore.

 

Definitely looks like a vacuum leak. Your airflow is in the 4s g/sec, should be 5.5ish. So about 22% of your air is getting in somewhere after the MAF... and the car is adding 22% LTFT to compensate

Check the VFAD nipple under the throttle body, it's not visible, you have to feel for it.

Don't worry about STFT, especially while driving, it's subject to many variables. Worry about LTFT