Really rich at idle/cruise, but runs better that way?

[Guest bastaad525]

A kinda side note, or side discovery, that I learned today while at the dyno...

 

The car runs REALLY rich at idle and during cruising, low throttle operation. Ranging from about 11 to 12:1. I've always known it was running rich, but now I have the definate numbers. But here's the weird part... it idles fine at this a/f ratio... a little misfire here or there, but otherwise, smooth idle, and drives fine on the streets driving normally... other than the stinkier-than-your-average-240 rich exhaust smell, you wouldn't know it was running rich. Additionally, I get no black smoke from the tail pipe, confirmed when I had my wife follow me around the block and down the street a few times. When I pull my plugs, they are dark, but not black, just a darker tan, almost like milk chocalate colored.

 

The oddest thing however, is that if I try to lean the car out at idle... say by tightening the AFM spring (I tried adjusting it a measley three more gear teeth tight... barely anything at all, and the idle got pretty bad), or clamping down on the fuel feed line to decrease fuel pressure (my fuel pressure does sit a little higher than normal at idle), the idle gets worse and worse, a lot more misfiring, and just generally lumpier. I can't even get close to getting it to run at stoicometric, which is where I thought you wanted fuel to be during normal driving conditions (if not leaner)... the motor starts running really badly by that point. This makes sense for why hooking my O2 sensor or TPS causes the car to run badly as well... hook up either one and the a/f ratio leans out... but the motor just hates it. This was even after replacing my O2 with a new one.

 

Why will my car only run well super rich like that? Should I just leave it be, or start fiddling with stuff or what? I wouldn't care much... but I really want to get rid of that last little bit of misfire, and with gas prices the way they are, getting some better mileage would be nice (averaging about 17mpg's now)... What do you guys think I should do?

[Drax240z]

It's a by product of the L28ET engine I believe. My car also exhibits similar traits. With the wideband on mine I leaned out the idle significantly from ~11.5:1 (where it ran smooth) to ~13:1 (where it is a bit rougher). I've come to the conclusion that these engines simply need to run rich at idle to be smooth.

 

With that in mind, you can gain a lot in the way of throttle response by leaning it out under light/mid throttle.

[Guest bastaad525]

Funny thing is throttle response seems great to me... of course it could always be better... so how would I lean out light and mid throttle w/o leaning out idle? I'm thinking of just letting idle suffer, hooking my O2 and TPS back up, and enjoying better driveability and gas mileage, and just tell people I have a big cam

[clint78z]

To set the idle mixture on the AFM use the bypass screw under the rubber grommet. I would shoot for about 13:1 at idle, getting it stoich will be a challenge. Increasing idle rpm helps too.

 

Drax on the SDS you can open throttle plates with the idle screw, then dial back the timing a bit. This will increase the amount of air required to maintain 900rpm. Fuel adjustments will be a little less sensitive. TimZ gave me that idea.

[Drax240z]

Hey Clint, that isn't a bad idea. I might give that a try in the next little while.

[Guest bastaad525]

My bypass screw has a metal plug over it... don't think I haven't tried getting that thing off! Is there some magic trick to getting it off? And I thought the bypass screw adjusted mixture across the whole rev range, not just at idle? And it won't idle nicely at ALL at 13:1... after 12:1 it gets really rough.

[clint78z]

If you look in your service manual if mentions the afm idle bypass circuit. It says that this is set from factory, but can be adjusted if co and nox readings are off. Basically it sets you AFR for idle. Since the amount of idle air is very small it has to bypass the afm door. It doesn't have enough pressure to open the door and meter it.

 

This will have almost no affect on the rest of the fuel map because the amount of air bypassing though this circuit is so small.

 

Set idle with that screw (a controlled air leak)

Shift the fuel map up and down with spring tension and your BEGI unit

[Guest bastaad525]

*sigh* more tuning...

 

 

yeah well... it's time to start looking at a wideband of my very own then... I can't afford any more dyno time that's for sure. Guess I know what to ask for for xmas eh? Til then crappy gas mileage it is.

 

 

I may not even have to mess with the AFM at all... the reason it runs so rich at cruise is because I run with my O2 sensor disconnected... which as I've said, I do because connecting it screws the idle up pretty bad. But, if I reconnect it, and then use the bypass screw to richen idle? That might work... or will the O2 just keep trying to correct my rich idle and lean it out? Hmmmmm

 

 

Okay so... lets say I get a wideband of my own... I know I gotta keep idle in the 11-12:1 range, and I'm guessing as soon as I get off idle I need to aim for about stoich mixture?

 

I think this may be what Cygnusx1 was saying he did, he mentioned that he had adjusted his bypass screw full rich... and then tweaked the AFM for good driveability. Maybe I'll just do the same, adjust the screw full rich and then tighten the AFM until idle smooths back out... should end up with a much leaner fuel curve during normal driving.

[Guest bastaad525]

Well... just for the hell of it, I tried hooking the O2 sensor back up. And again, the car starts to run like crap... really really rough idle, popping exhaust. But, I figured... that may just be that it's idling too lean, but maybe the overall driving would be better, as the O2 should lean it out there too, right? So I took it for a drive... nope... light/mid throttle response was HORRIBLE... at lower RPMS the motor was vibrating really badly... more of a loping wobble than a vibration. So, I thought... you know I never did check that little LED on the ECU last time I tried the O2 sensor, so I got a mirror and checked it.

 

Sure enough, the LED was blinking... at idle, it would blink maybe once every 5-10 seconds or so... I forget if that is too much or too little. So it's definately getting signal from the O2, and I assume that hooking it up is leaning it out because that's exactly how the car runs if I tighten the AFM much more, and since the LED is blinking, and we know the ECU tries to correct to stoich from the O2 reading right? I certainly don't think hooking it up was causing it to run any richer than it already does w/o it. I tried with the TPS connected and disconnected and it made no difference whatsoever. The motor does not want to run right unless it's running rich. This was with a relatively new Bosch sensor.

[clint78z]

I would definately do more investigating into your fuel system, and basic torubleshooting. I am quite sure something is off there. What you need is a $20 multimeter from Radioshack, and a factory service manual. Your engine should be at least able to cruise at stoich. My guess if the extra fuel is masking a problem of its own. If you get this problem sorted out, I bet you will see major HP improvements along with driveability. You need to tap into the O2 wire and see if it is showing rich or lean, you can do your basic tuning with this info if go in small steps.

 

Idle mixture screw on AFM is for idle, O2 sensor should be able to correct for cruise, and AFM spring for higher throttle, and use the FMU to tune for full boost.

[Guest bastaad525]

Your engine should be at least able to cruise at stoich.

 

yeah it should... but it won't... with the O2 connected it runs very roughly even while cruising, until I get past maybe 2500rpm then it seems to smooth out and feel fine.

 

The odd thing about the O2 sensor thing, I'm not the only one who's had this problem... I forget who they were now as this was months ago, but I've read posts from two other guys on here who had the exact same problem, and were running with their O2's disconnected. Both had tried buying new sensors and still got crappy running with them hooked up. And then what about what Drax says? Why would the motor need to idle rich at all? And how would it be it needs to idle rich but suddenly under any cruising it should run at stoich? What kind of problems could the extra fuel be masking? The engine is rebuilt, and I've checked time and again for vacuum leaks but found none. I mean like 20 times I have checked very thouroughly, by spraying EFI system cleaner while the car was running, as well checking ALL bolts on everything, and checking all rubber parts for crackign. I've cleaned all my EFI connectors a zillion times and now have them all coated nicely with dielectric grease. Not long ago when I was having lots of electrical gremlins with my Sentra I had borrowed a multimeter for a few days and yeah while I had it I did a basic 'check up' on the Z's EFI... the AFM, head temp sensor, and O2 sensor voltages, as well as the TPS adjustment, all checked out per the Haynes manual. With the O2 connected, it goes back and forth between rich and lean (I forget the voltages now), as it's supposed to, this is according to the voltmeter and the ECU via the little green blinking LED.

 

So that's what I"m really getting at... if the EFI checks out, what other problems do I need to start looking at that are possibly being masked by the rich running? I've done a full tune up recently, so that base is covered (within the last 4 months - fuel filter, clean K&N air filter, new plugs/wires/cap/rotor, adjust valves, adjust timing to stock, idle set to ~800rpm, no PCV system so no valve to change, no EGR valve to screw anything up, running '75 intake manifold). I'm very baffled by this as well, had come to accept it for the longest time, but the dyno testing brought it back to the forefront. I could drive it like this the rest of it's life really, though the fumes are probably shortening that time span a bit but really would LOVE if I could bump up the gas mileage and get better driveability to boot... but I've been over this problem 1000 times with a fine toothed comb and really don't know what else to try.

 

 

Would any of this make sense to have anything to do with the ZXT motors super low compression? or something to do with the cam profile?? Those are the only reasons I can think of that the ZXT motor would need to run so rich like this....

[cygnusx1]

Baast. I would connect the O2 and try to tune out the richness by using the AFM screw and spring just to see if it gets you anywhere. Mark everything first so you can go back. I have a 900rpm idle and factory timing. Actually what I do is set the throtte stop screw close to 900, screw in the AFM screw (rich) and then turn the AFM spring back and forth from lean to rich until I get a feel for how far I can go with the AFM spring. Then I go as lean as I can go without getting the bumpy idle on the AFM spring and then go like 1-2 notches richer from the leanest. Then I recheck the idle and start over if needed. I dont have a clue as to the AF ratio. I just get it idling smooth and it seems to cruise ok in that range. For reference, I can get a fairly smooth idle for about a 8-10 tooth section of AFM spring wheel. Within that section of AFM adjustment I can get the idle to rise and lower about 150rpm or so....oh and you can make black smoke come out of the tailpipe and you can also make it go away by turning the AFM wheel. When you have the AFM overall set too rich, after about 10 seconds of idling, rev the motor and you will get a black puff.

[Guest bastaad525]

But isn't the O2 going to keep trying to correct for any adjustments I make? One reason I don't want to go messing with the AFM is because if I go messing with it now I might affect the WOT fuel curve and w/o a WB I'm just shooting in the dark and asking for trouble. The idle is pretty smooth as it stands right now, so call me chicken but I"m a little leary of screwing with it. I get a range of maybe 6 teeth, either 3 rich or 3 lean from where it was initially set (I'm assuming that was the factory setting). Any more than that and idle suffers considerably and noticeably.

 

I'm a little mixed up on exactly what you're recommending though... so plug the O2 in (which is causing the motor to lean way out, presumably around stoich as the LED on the ECU is blinking on and off as it should), and then adjust the idle mixture rich so to smooth the idle back out, and then also adjust the gear as lean as possible while still maintaining that smooth idle? And again, isn't the O2 sensor just going to keep trying to correct my corrections? If I'm just forcing the O2 to cooperate and get it running smoothly, is that not the same thing as just running with it disconnected? It runs smoothly as it is, minus the slight misfire, no bogging, good throttle response, good color on the plugs, and hauls booty on the boost. My last ZX used to run super rich, got about 10-12mpg's, black plugs, no smoke from the exhaust though, but you knew it was running rich, bogged really bad and had crappy throttle response, very noticeable and not at all how this one is running now.

 

Cygnus what kind of mileage do you get? Doing primarily what kind of driving? I do about 50/50 highway/street and get about 17-19mpg's.

 

I'll see if I can't get that stupid plug off the idle mixture screw and then I'll try your suggestion... like you said I guess I can mark everything so no harm it playing with it a little. I would FEEL better having the O2 connected, but I do wonder if having it connected is actually going to make the car run better at all.

[wheelman]

Baastadd525,

 

What plugs are you running and how are they gapped? When you rebuilt everything did you install a new ignition system, like an MSD or something?

 

My thoughts are that if you have some weakness in the ignition system it may be the factor that causes your car to prefer to run rich. Also do you know if the ECU controls the timing or spark energy in any way? What is the stock timing setting? How far is it advanced at full advance? Have you tried adjusting the initial timing advance after setting the EFI up "correctly" to smooth out the idle?

 

Just some thoughts and more variables to explore but it seems strange that those turbo motors should prefer to run rich at idle.

[Guest bastaad525]

Yes I run an MSD 6A, stock coil, NGK 7.5mm plug wires, timing is set to 20 degrees initial with TPS disconnected, not sure what full advance is, I'm running NGK plugs, heat range six, I think they are BPR6EY 11's, I know I need the 6ES's but damn if I can find them anywhere! I dunno what's the difference the S's and the Y's? I have them gapped at about .050 to take advantage of the MSD. ECU is a stock turbo ECU, I know it controls timing. Spark is VERY bright and strong a the plugs with the MSD. I used to run 17* BTDC timing, to play it safe, then advanced it up to 20* (stock setting) after adding the FMU and setting it safely rich, the idle did smooth out a bit.

[wheelman]

Forgive my ignorance but are the NGKs you're using stock spec plugs? I looked at NGK's docs and they recommend closing the gap and using a colder heat range when the compression ratio is increased. The turbo increases the CR and based on what you report about how the engine acts when you lean it out a colder plug and/or smaller gap might work better. I'd also try closing your gap to .040 with your current plugs and see what happens. You shouldn't get any pre-ignition at that setting and it might eliminate your mis-fires.

 

Wheelman.

 

P.S. Be sure to say so if you've already tried these changes.

[Guest bastaad525]

yeah I've tried running them as small a gap as .035 and it did not get rid of the misfires. About running the colder heat range, I don't think that would matter for my misfires at idle... you're right compression is higher with the higher boost, but compression ratio at idle is unchanged, no matter how high I turn the boost. So stock heat range plugs should work fine and should not be the cause of my misfiring at idle and low rpm/low throttle driving. I haven't noticed any misfires at high rpm, either on the street or on the dyno. I'd think I'd be fine running the larger gap with the MSD.

 

This all contributes to my suspicion that the misfiring may actually be due to partially clogged/dirty injectors, as has been suggested to me, this may be creating a bad spray pattern and too much solid droplets of fuel aren't getting burned in the combustion process, but rather, spit out the exhaust where they suddenly combust and cause my misfire-like symptoms.

 

I picked up a can of that Seafoam stuff today and will try it out, not expecting much to happen though if the Techron or Neo fuel system cleaner didn't fix it.

[Guest 1seymour]

i did replace my injectors with 6 used ones. the newer ones were a lot cleaner and the car does run better with them than the old ones.. but the afm issue is still there.

[Guest jjohart]

I may be one of the folks who unhooked my 02 as a band aid correction for crappy restarts/leanitis,e t c. Only when I looked, as in had a air/fuel gauge put on, did I see I was running rich, EXCEPT at idle.

If only the change to a cooler plug were a point and shoot solution. I too have little interest in F---ing with the AFM..oh, I mean my mechanic would love to charge me, since I am more than a little mechanically inept.

I was just wondering (a little bit of this is in my own post of late) if there are any other quick tricks to try and make it run leaner/stoich. I remember putting a clothespin on the fuel return line, before I unplugged the ECU, and the car would run fine! Is there any line I could similarly clamp a bit and bring the fuel level down from rich? I don't smell anything at the back end of the car, but there's a stove top of smoke residue all over the muffler and I've been getting 2-5mpg less.

Any backwoods home spun suggestions apprec., but I AM NOT GOING TO PULL THE AFM, I don't care how primitive it is, one spring turn the wrong way and boom!

I'm also wondering if reducing the AFPR I had put on would help, or is that for fuel pressure alone?

Thanks

John-83 280ZXT

[zguy36]

I don't agree that the L28et has to run really rich at idle. I just finished putting in a megasquirt and Innovate wideband and it idles great at 14.5-15.0 I am not a fan of the stock fuel management, since it is very inconsistent.