Purs like a kitten...

[cozye]

FWIW, I've got mine set at 6 teeth richer on the AFM, and timing set at 15 degrees. It's pretty drivable set like this. Only temporary in my case until I find the root cause. I don't think the injectors are the source of your issue though. I still have the nissan injectors on mine and I had them cleaned and flow tested so there aren't any issues there.

If you suspect the FPR, you can disconnect it and go for a drive. I did that as well. It helped some, but again, it was just a test. I get 37psi with FPR disconnected, 32psi at idle with it connected

[FastWoman]

Problem is, I'm afraid to advance my timing any further than the spec 10 deg BTDC. I still get infrequent intake pops, unless my AFM is adjusted AT LEAST out as far as it is now. The 36 mpg tells me I'm running really lean -- to be confirmed very soon. I may ultimately advance the timing a tad beyond spec, but not until I have the mixture right.

BTW, you forget I don't have Nissan injectors installed right now. I'm running brand new Sorensen injectors. They were sold to me by an Ebay vender who sells 280Z stuff and were represented as being compatible with the car. The hoses were a bit short, so I was a bit suspicious. Perhaps my next step should be to pull the fuel rail and flow test the little guys.

Just like you, I found the AFM's main spring tension about right at the original setting, per the article on Atlantic Z. I've checked/measured everything else on my EFI. As far as I'm aware it's not an ECU issue, because my spare ECU works the same way. The easiest thing for me to do right now is to flow test. If my injectors, fuel pressure regulator, and fuel pump are delivering to spec, then I'll be suspicious of the pulse duty cycle delivered to the injectors by the ECU and will start looking for cold connections on its PC board.

Anyone know what the spec flow rate is on these things?

[FastWoman]

Well, I've pulled the #1 plug. Yes, it looks lean, but not terribly so. There are still light HC deposits on the body. Insulator looks a bit whiter than I would like. However, it looks like I'm in the ballpark. In addition, I wiped down the rim of my exhaust tip before leaving, and wiped it down upon return. There is a TRACE of black soot -- not much, but enough to tell me I'm not out in the left field of lean.

I suppose 36 mpg at mostly 45mph straight cruising isn't all that surprising. Fuel usage is proportional to the square of velocity, so my 36 mpg at 47 mph (best estimate) would be about the same as 19 mpg at 65 mph. If you temper that with the fact that maybe 1/4 of the driving was in-city, occasionally with moderate-heavy acceleration, I suppose I'm not too far off the mark.

I'm reminded of when I drove my little '75 Celica approx 35 mph down an icy Texas highway for hundreds of miles. I got 45 mpg on that trip! Wow, was I impressed!

I still think I might need more fuel pressure. I'm not comfortable with the AFM adjusted that light.

[jonathanrussell]

It feels a bit like you are running in circles. If it were me, I would buy and install an afm that is correct for your car and set to factory specs (new, rebuilt, etc). Then, see how the car runs. If it runs poorly, don't adjust the afm to mask the real problem. Instead begin diagnosing the real problem. Then, once you have everything working the way it should, adjust your old afm so that it works like the new one and can serve as your backup.

[ZCurves]

It feels a bit like you are running in circles. If it were me, I would buy and install an afm that is correct for your car and set to factory specs (new, rebuilt, etc). Then, see how the car runs. If it runs poorly, don't adjust the afm to mask the real problem. Instead begin diagnosing the real problem. Then, once you have everything working the way it should, adjust your old afm so that it works like the new one and can serve as your backup.

EXACTLY what I was thinking! Remember, Time is Money!!

[FastWoman]

... but alas, some of us have more time than money.

When the AFM is adjusted to specs (correct spring tension, flap movement, voltage output), the engine runs way too lean. I already know that.

The only thing I haven't checked out is the fuel flow rate (having assumed it would be correct from new injectors). I think my next logical step is to milk the fuel rail, so to speak.

[Z train]

Have someone take a picture of a correct AFM and then set yours accordingly.

[cozye]

oYou can't take a pic of the correct AFM settings. There isn't any mark on the gear. I can tell you for sure mine wasn't tampered with and I'm experiencing the EXACT same issues as her. I've also done a lot of the same test with the same results. I'm sure she marked the original setting on her AFM so she can put it back to spec. The problem is that the thing pops through the intake like crazy at anything below 2500 rpm so she has to do something to be able to drive it somewhat and feel it out.

As far as advancing the timing. I'd prefer to have mine on 10 too. I can tell you that the more advance is supposed to be better for a lean mixture, as it takes longer for the lean mixture to burn so in theory the advance should give it more time. I've tried mine on 15 degrees for a few days, I'm thinking about cracking it back down to 12 or so just to see.

I'm getting about 19mpg at mostly 70mph driving with this setting.

I have a spare ECU, AFM and throttle body of a low mile part out coming. I could have sworn this was an ECU issue, but you've tried that so I'm not so sure now. I've resistance tested my AFM 10 times it seems, and under a load did the voltage output. It tests fine. The only thing I can figure is that maybe the temp sensor is off some, although it seems to test ok too.

The flow rate on the injectors I got before cleaning and testing was about 225cc. After, they are all getting right at 240cc.

[Z train]

oYou can't take a pic of the correct AFM settings. There isn't any mark on the gear. I can tell you for sure mine wasn't tampered with and I'm experiencing the EXACT same issues as her. I've also done a lot of the same test with the same results. I'm sure she marked the original setting on her AFM so she can put it back to spec. The problem is that the thing pops through the intake like crazy at anything below 2500 rpm so she has to do something to be able to drive it somewhat and feel it out.

As far as advancing the timing. I'd prefer to have mine on 10 too. I can tell you that the more advance is supposed to be better for a lean mixture, as it takes longer for the lean mixture to burn so in theory the advance should give it more time. I've tried mine on 15 degrees for a few days, I'm thinking about cracking it back down to 12 or so just to see.

I'm getting about 19mpg at mostly 70mph driving with this setting.

I have a spare ECU, AFM and throttle body of a low mile part out coming. I could have sworn this was an ECU issue, but you've tried that so I'm not so sure now. I've resistance tested my AFM 10 times it seems, and under a load did the voltage output. It tests fine. The only thing I can figure is that maybe the temp sensor is off some, although it seems to test ok too.

The flow rate on the injectors I got before cleaning and testing was about 225cc. After, they are all getting right at 240cc.

FWIW-you're getting the same MPG as i'm getting with my 810 Wagon(L-24E/auto/3.54s)

[cozye]

FWIW-you're getting the same MPG as i'm getting with my 810 Wagon(L-24E/auto/3.54s)

Yeah, from my understanding, 19-20mpg is pretty much on target for a 4sp at that rpm.

Fastwoman. I forgot to mention RE: your soldering on the ECU. I did inspect mine over real well with a magnifying glass. I reflowed all 35 pins on the ECU connector, a couple of those guys looked a bit cold/suspicious. It didn't help. There was an old thread on one of these forums where I guy had some poor solder joints on his ECU, specifically the connector solder joints and it helped him. It's a possibility and no harm in trying, just don't get your hopes up. Especially since you've tried a 2nd ECU.

I have been thinking about this problem non stop for weeks. I am beginning to wonder if it is in fact an AFM problem. I've ruled that out with testing a few times, but I wonder if the test procedure is robust enough to detect these minute problems. Basically you are just looking for a resistance test from the various pins. The static resistance from the reference resistor and air temp sensor are easy. The resistance test from the potentiometer leaves a bit open to interpretation. It jumps around a lot as it moves, and according to the atlantic tech tips pages, this is normal. Furthermore, the 12v output test in the FSM seems to work smoothly.. however.. follow my train of thought for a second.

The wiper on the potentiometer gets a lot more wear in the first 30% of travel. It gets a lot of wear around idle, and at cruising because that's where it spends the majority of it's life. I performed the 12v output test while moving the flap slowly and it seemed to lower the output progressively. I wonder though if this is misleading. When we loosen up the spring on the AFM and make the mixture richer, we are effectively reducing the voltage output of the AFM to the ECU. Also the physical position of the wiper is moved significantly, possibly out of the more "worn area" of the carbon trace.

I just wonder if the test procedure in the FSM, FI Bible, Tech tips pages, etc.. is the end all be all final answer.

Is it possible for there to be enough wear on the potentiometer at the idle and cruising areas to have incorrect or intermittent resistance and affect idle/part throttle operation?

low vacuum could be just the symptom.

These are some reasonable assumptions that can be made:

Adding enough fuel seems to correct the problem

Any significant source of vacuum leaks has been ruled out

When adding more fuel to the idle mixture, higher throttle performance suffers (presumably because the AFM flap would be open a lot more)

We aren't the only one's who have suffered this problem and it could be wear related item.

Consistent compression across all cylinders seems to rule out a valve or internal engine issue. Even blues tech tips pages has an old worn out 280z motor pulling 18.5hg.

The issue does not seem to occur at all when giving the car more than 30% throttle. This could be due to TPS "full enrichment" or possibly the AFM flap opening past the worn area.

I know some speculation and suggestions have been made that internal problems could cause this, like worn rings, leaking valve guides, etc.. but two facts seem to contradict this. One is that the vacuum gauge hand is steady, and in a valve leaking scenario this should not be. Second, worn rings would cause engine smoke of some sort. I have none. Even if you make the argument that the rings are seated and working on compression stroke, but leaking or allowing blow by on intake stroke, oil would go by as well and create that puff of smoke on deceleration. I just cant make sense of any internal engine scenario.

I've spent countless hours on the internet, google, forums, etc.. researching "280z low vacuum" "280z hesitation", "280z stumble", etc.. and I've found lots of descriptions that seem similar, quite a few discussions, but never a solution. Every discussion I've found abruptly ends. Some discussions are not exact, they might have had fuel pressure issues etc.. But I have found several cases that appeared to be similar, and never a posted resolution. I have found a couple threads where they thought they had a resolution "like bending the TPS full enrichment contact to engage all the time" etc.. But these are just hacks to work around the problem, not real solutions.

I think timing should be ruled out as well. I've done the valve timing, and ignition timing until I'm blue in the face. Sure, advancing ignition timing helps. It doesn't solve the problem. The reason it helps seems to be the fact that advanced spark helps a lean mixture by allowing it a longer burn time. But again, aren't we just working around the problem ? The only thing I have a hard time with on ignition timing is this "ported vs. manifold" vacuum. Obviously manifold vacuum on the vacuum advance will advance timing a bunch at idle and part throttle conditions and would be ideal. However It's not the factory configuration, and I'm quite sure this car didn't run like this from the factory. Also, the vacuum advance mechanism wasn't designed for manifold vacuum souce so I'm not sure how that would affect the overall operation of the vacuum advance with a much higher vacuum source.

All of this comes full circle and keeps me thinking that there has to be a metering issue of some sort. It's the only thing that has a solid explanation (if you throw out the FSM test proceedure).

[FastWoman]

Hi Cozye, I feel certain it's not an engine wear issue, because given the right fuel/air mix, my engine DOES have a lot of pep and does pull a normal vacuum. I'm also pretty certain it's not a vacuum leak issue because there's nothing left to leak -- unless I did an awful job of mounting up the intake manifold (which I didn't). It's not valve timing, because I'm verified within specs (although I'll soon need to switch to the #2 hole to remain within specs). I think it's definitely a metering or fuel delivery issue.

Regarding wear of the potentiometer: You'd be surprised just how robust a carbon trace can be. As long as the wiper successfully makes contact with the trace, all shoud be fine. Wear issues would not affect resistance or voltage, for the most part. In fact what the ECU reads is voltage, not resistance, and there is almost no current conducted through the wiper contact. I've worked with electronics long enough to STRONGLY doubt there would be a potentiometer wear problem, so long as there is good electrical contact between the wiper and the carbon trace. I've verified on my AFM that I have excellent electrical contact and that the voltage readings are correct throughout the movement range, as near as I can tell. (Note here, I did re-center the wiper on the trace, using the center glue-globbed adjustment. Mine was off less than 1/16" because of a slight bend in the flap from intake backfire. This tiny adjustment didn't really have much of an effect, though.)

I always wondered about the ignition timing. Your explanation that lean mixes burn slowly explains a lot. It explains the hard-blowing exhaust, where the mix might still be burning even during the exhaust stroke. It also explains how intake backfire might occur, so at least some of this makes sense to me. I suppose advancing the timing would make the backfire less likely, so I'm more open to considering a bit more advance.

Anyway, I've been a scientist and programmer in various stages of my life, and all of this involves fault tracing and problem solving. Right now, this is my thinking:

(1) The air flow meter has a certain shape that is constant and not subject to wear or degradation. The flap in mine has been lightly bent, but only lightly. I might try tapping it back into shape, but I doubt it will make any difference. If anything, the direction of the bend, combined with my repositioning of the wiper, would result in a richening of the mixture -- which obviously hasn't occurred.

(2) Ignoring the bend, as long as the spring tension is to specs, then the flap deflection will also be to specs. A given air flow will result in given flap deflection, which will result in a given voltage output at the wiper. Although we don't KNOW exactly what the factory specs are, we have a reasonable approximation from the Atlantic Z folks, and the adjustments that made my engine run correctly resulted in less than 1/2 of the tension at half deflection than what the Atlantic Z folks describe. I'm confident there's NO WAY this is within the margin of error. Something else is WAY OUT of specs in the system.

(3) I've gone through my entire system and have verified that every sensor is within specs. Of course there could be cumulative error at play; however, Bosch/Nissan would have designed the system such that the engine would still run reasonably well even with a conspiracy of cumulative error.

(4) There's a possibility that a gremlin made a solder joint go cold in the ECU. It certainly wouldn't hurt to re-flow the connections. (Ztrain is the guy who found that problem, BTW.) As I said, I've already tested my spare ECU on the prior incarnation of my engine and found it not to make any difference in the way the engine runs, so I'm doubtful I have an ECU problem. Still possible, though.

(4) The only thing I have not verified is the fuel delivery -- specifically the fuel pressure (which I've checked with a rather cheap meter) and the injector flow rates (which I haven't measured at all). This is an easy thing to measure. I'm guessing that the pulse width is correct when the AFM is adjusted to specs (provided the ECU is good -- which I think it is). However, if the fuel pressure or flow rates are wrong, then all of that would be meaningless.

So I'm really narrowed down to three systems to check -- ECU, injectors, fuel pressure regulator. That's not so bad. Everything else is verified good/within-specs.

Unfortunately this project will need to wait for several days, as it's raining today/tomorrow, and I'll be busy with life during the weekend. So tune in later!

BTW, here's a quick pic, by popular demand, that I snapped of my engine before the rain started coming down. In fact you can numerous sprinkles that hit my engine in the couple of minutes it took for me to grab my camera. :mad:

I did it in Duplicolor's MetalCast blue. The "NISSAN OHC" lettering on the valve cover was done by filing flat, painting black, filing off the paint, masking, painting the blue, and then clear-coating the whole thing. More pics later, but first I want to get the thing running right!

Edited November 4, 2010 by FastWoman

[cozye]

Nice looking engine job! Looks like you insulated your fuel rail, I'd be interested in your method.

I moved my afm contacts just a hair up so that they would be on new carbon on the trace. It made no difference.

I too have a new intake/exhaust manifold gasket. And I've been questioning the install of that plenty too because it does feel like an intake leak, although I've done smoke tests, boost test, carb spray, etc.. and not found anything.

I'll be keeping a close eye on your progress. I'm very frustrated at this point.