Leaned out the mixture, exhaust smell + idle get's worse???

[Guest bastaad525]

Just as the title says... on some advice from a fellow member (in regards to a slight high end misfire I'd had) I adjusted my AFM flap spring tension a few teeth tighter, to try to lean out my midrange. First thing I notice (which I already knew would happen as it's happened every time before) is that the idle gets a bit more rough. But worse, as I was driving around this morning, it just seemed to me that the exhaust fumes were MUCH worse than usual. Now I can't say I've ever noticed that before, though usually, when tuning the AFM, once I noticed the idle get more rough, I'd put it back to where it was.

 

Anyways, this whole thing still baffles me, as #1, I know for a FACT the car idles pretty rich, as confirmed on a wideband O2 when I have dynoed... so I can't understand why the car doesn't want to idle at like stoich or whatever... mine is happy around high 12:1 air/fuel ratio.

 

#2... I thought that strong smell of the exhaust was always partly or wholly due to how rich the car runs overall, so why would leaning it out make the smell WORSE? I mean my throat and lungs were burning, and when I got out of the car my eyes were watering... I don't think it's EVER been that bad. That was with only leaning it out like 3 teeth from where I had it.

 

Otherwise it seemed no worse for the wear, if anything the throttle did seem a little more responsive, and the car ran smoother at anything but idle... so I want to leave it there but... damn that smell is gonna kill me!

[proxlamus©]

That's weird... very weird..

 

When tuning airplanes with carb's... I know during the run up.. we rev it up to 1,700 RPM and start leaning the mixture out.. and we lean the mixture out untill I start to see the RPM's drop... then I add a bit more fuel back till it's at peak RPM or 1,700 RPM. Then I add 3 half turns of the mixture screw or basically just add some more fuel to be safe and not lean out too much....

 

Just like when tuning your AFM... at IDLE, finger the AFM flap and move it around rich or lean. You will notice when you lean the mixture, the idle RPM increases and the engine seems to run very very smoothly.

 

Now REV the engine and hold the engine at 2,500 RPM and do the same thing... when you add more fuel, the idle will sputter and run rough. Make it LEAN and the RPM's will run much much smoother and literally seem to rev up to 3,100 RPM!

 

You goal is to keep it a little bit rich, but have an excellent idle and good higher RPM range... and do this by moving the AFM wheel.... try leaning it out 4 teeth. Once you move, and tighten the spring/wheel now do the same steps over again.

 

With the now tighter spring, try using your finger to make it lean.. if you can't tell a difference, your a bit too lean. If you lean it by hand and the idle picks up a little.. not too much your perfect. Now try the same thing under 2,500 RPM or even 3,500 RPM and see how it does.

 

The stock EFI loves to idle rich... so don't forget if you lean it out at idle.. it will run even more lean at higher RPM's... so you have to be very careful. Make it rich at idle.. and a little lean at higher RPM's.

 

your FMU will be sure to compensate for the extra fuel needed under boost.. since that is what they are designed for.

 

As for the smell.. very strange.. maybe your running even richer?!

[Guest bastaad525]

well I'll tell you what this AFM is VERY sensitive.

 

When I first started adjusting it when I installed the turbo motor a couple years ago, I thought it was running rich, again because of the strong smell. So I marked the wheel to know where I started from, and started to lean it out. Idle would change noticeably after only a few teeth adjustment... idle was at it's smoothest at 2-3 teeth clockwise/tighter than stock. So I thought... okay maybe it's already TOO lean, so I tried going the other way... starting from the original stock setting... any tightening/leaning out from that point just made the idle progressively worse. So I set it back to where it idles smoothest, 2 teeth tighter than where it was before I touched it.

 

Fast forward a bit to my first dyno runs with a W/B, I was watching the a/f ratio and at idle, the car was running VERY rich, mid-high 12:1. I was like WTF?!?!?! so I asked the guy if he'd mind me tweaking with the AFM while it idled/cooled down between pulls, he said sure. I tried leaning it out again, and it seemed that as soon as the a/f ratio started to go up, the car started to idle more and more roughly.

 

By the way, air/fuel ratios are also rich at cruise, which I noted when he first was getting it rolling on the pulls before he'd WOT it. And yeah, it's also pretty rich at WOT/full boost.

 

i was trying to lean it out off boost to help with gas mileage and the smell, as well as cure a light misfire I had experienced a few times under heavy load (which seems to have been cured anyways by my lowering the gap of the spark plugs). I DEFINATELY leaned it out though... hard to mess that part up, the spring for the AFM is pretty tightly wound... so you gotta push on it with a decent amount of force to tighten, and of course to loosen, you're more just kinda letting it 'slip'.

[RB26powered74zcar]

My last turbo motor with TWM ITB's / Tec2 did the same thing when I tried to get the idle any leaner than 13:8.

 

It would choke me out, and burn the hell out of my eyes while sitting at idle, so your results arn't that odd...

 

I just went back to 13:5 and lived with it. My issue had alot to do with the whole intake set up.. (TWM's, etc.)

[Guest bastaad525]

so why would a leaner burning engine smell WORSE then when it was running more rich?? I mean to say, from a technical standpoint, what element is it present that becomes MORE present when leaning out the mixture, that is causing irritation to eyes and throat?

[Sean73]

I think you leaned it out to the point that you developed a misfire at idle.

 

Misfire = unburned fuel, and more fumes.

 

I pulled this little blurb off a Google search:

 

# A Rich mixture will increase HC and CO (but lower NOX) emissions

# A Lean mixture will reduce HC and CO emissions but NOX goes up (unless mixture is very lean)

 

If you're running without a TPS, this can also have an adverse effect on idle. The TPS is supposed to command the ECU to run rich at idle. If the TPS is disconnected, or not functioning, then you have to adjust the AFM extra rich, just to get a good idle, but unfortunately this might alter the rest of the fuel map to be too rich.

 

Same deal on running without an O2 sensor. At cruise, the ECU wants a 14.7 A/F ratio. Without an O2 sensor, the ECU doesn't know how to lean out in cruise mode, and you have excess fuel going out the tail pipe.

 

My stock Z31 ECU always runs a little on the rich side at idle. I solved most of my fumes problems by sealing up the rear of the car w/ new gaskets & weatherstripping, and adding a longer exhaust tip.

[Guest Loose_Screws]

Did your boost by chance increase after running a leaner mixture? If so, maybe the RRFPR is dumping yet more fuel with the higher boost. Can you change the 'rise' setting on the RRFPR? I have personally never fooled around with one, but it's a thought.

[Guest bastaad525]

hmmm well... you could be right on all counts Sean this is all just further testament to my growing impatience with the stock EFI.

 

1. yes I do have a light misfire at idle. It's intermittent in frequency, not REAL bad. And it does clear up once I get it under load, even at cruise, it seems to go away completely. At any rate, it's been there from day one after I did the swap. I have tried MANY things to try to get rid of it, but have only been able to lessen it a bit. Oh and yeah leaning it out did cause the misfire to get a little worse/more frequent.

 

2. yes I do run with my tps disconnected. BUT, connecting it does NOT get rid of the misfire. What connecting it DOES do, is make the idle worse, AND gives the motor a nasty bog (more like a bad hiccup) every time throttle is applied from idle.

 

3. yes I do run with the O2 sensor disconnected. BUT, connecting it only makes the idle MUCH WORSE. I have even spent like $80 to buy a nissan brand O2 brand new, but it did not solve the problem. I hook it up, car warms up, green LED on the side of the ECU starts blinking, and the car starts idling very very roughly, and in general drives like crap.

 

4. I put the AFM spring back to where it was originally, before I had ever adjusted it. Idle is still a bit more rough than usual (richening up like 2-3 teeth from that 'stock' position gives it the smoothest idle), and the exhaust smell is cut WAY down. Much better

 

I have tried all possible combinations of running those two sensors.... connected/connected, connected/disconnected, disconnected/connected, and the motor definately idles and runs the smoothest if I leave them both disconnected. And as I mentioned above, I have spent much time and money chasing the problem on BOTH sensors with no luck, so finally after about a year I gave up trying to figure it out. Other than the misfire at idle the car runs great, though gas mileage sucks. The exhaust smell usually, is only as bad as everyone elses... that is to say, I only get it on decel after hard acceleration. Normally during just cruising I never smell a thing.

 

Oh, and as far as I could tell boost did not change after adjusting the mixture.

[Guest 1seymour]

i have similar problems.

 

regarding adjusting the afm:

too tight = poor idle and fouled plugs - gas smell from exhaust.

too loose = good low-midrange response but sputtering at high end

 

i found that disconnecting IAC helps keep the car from fouling plugs at idle.

 

i have to have A/C on to increase idle to keep from dying when running the afm tight (a/c doesn't work but turning it on raises the idle).

 

my setup: 1983 280zx turbo 3.1 stroker, stock efi, 3 inch intake, free flowing exhaust through a flowmaster turbo muffler.