'76 280Z Question on dist/manifold vacuum, dist timing, fuel pressure, brake booster

[Jennys280Z]

14 different model numbers of AFMs across nine model years '75-'83

http://www.thezstore.com/page/TZS/PROD/11-3040

[cozye]

At some point this weekend I'll do a resistance test again on my AFM and give you the numbers for reference.

Just FYI, the AFM needs to match the ECU, that's why there is a bunch of different part numbers.

[FastWoman]

Hey Jenny, I'm back. Sorry for my long absences. We're randomly leaving town on real estate matters (looking). It's not a permanent sort of thing -- or BETTER NOT BE! This will all get a bit better for me when the roads are cleaner, and I can drive my Z. (Well, or we can take the little Miata. Either is a pleasure to drive, but I do prefer my Z.)

The EFI is somewhat of a black box system, so it's hard to know EXACTLY what info the ECU uses from the AFM. However, if I were designing the system, using a potentiometer (e.g. the AFM) for input, I'd send a fixed voltage through both legs of the potentiometer and tap the voltage off of the wiper, sending it to a high-impedance input circuit. The objective would be to avoid passing any sort of real current through the delicate wiper contact. That would minimize oxidation of the contact and would maintain accuracy despite wear. If I could have input into the potentiometer design, I would also have a high precision resistance ladder alongside the main carbon trace to stabilize the relative voltages. Anyway, the AFM seems to be constructed with all these design attributes, and I suspect the ECU does pull off the voltage from the wiper (not a current) via a high impedance circuit. It would only make sense.

With this sort of design, absolute resistance values aren't nearly so important as relative resistances. (It's a design that can age rather gracefully.) The most important thing is that the carbon trace and the wiper contact are both clean and protected from oxidation (via a thin lubricant film).

Anyway, could your AFM still be out of spec? Well, maybe. Maybe mine is too, although I certainly verified all the resistance readings and made certain I had a smooth voltage gradation from full-closed to full-open. (Note here: You have to use a needle-movement multimeter to pick up on any jitteriness in the voltage output -- preferably one that isn't nicely damped. Oddly, a cheaper meter is going to do a better job.) That said, even if your AFM is out of spec, you could still tune your system reasonably well with that potentiometer in series with the temp sensor, assuming the error is to the lean side (but possibly even to the rich side, using a method I haven't had occasion to try). Would it be perfect? Well, maybe not PERFECT, but it would still be close, and it would be a whole lot cheaper than a rebuilt AFM. In fact a manually tuned system with a slightly-out-of-spec AFM is probably going to deliver a better mix than an untuned system with a rebuilt AFM.

FAIW, I also have to wonder what these rebuilding companies do when they rebuild an AFM. Do they replace the potentiometer assembly? Or do they simply clean the unit up, verify proper functioning, make it look pretty, put it in a nice box, and ship it out? I strongly suspect it's the latter. I also see items such as rebuilt ignition modules and ECUs, and in that case I know it can only be the latter. In fact I suspect some rebuilds even skip the step of verifying proper functioning and simply hope that the part won't come back. I could be wrong about this, but I'd be surprised.

Oh, you asked whether the AFM could suddenly develop bad resistance values. IMO, the answer would be "no," unless one of the resistors went bad and opened up (not likely, especially since you're getting close to the right numbers). OTOH, I would think a new vacuum leak could be a sudden thing. You found a few of those along the way.

Edited January 28, 2011 by FastWoman

[Zed Head]

FAIW, I also have to wonder what these rebuilding companies do when they rebuild an AFM. Do they replace the potentiometer assembly? Or do they simply clean the unit up, verify proper functioning, make it look pretty, put it in a nice box, and ship it out? I strongly suspect it's the latter. I also see items such as rebuilt ignition modules and ECUs, and in that case I know it can only be the latter. In fact I suspect some rebuilds even skip the step of verifying proper functioning and simply hope that the part won't come back. I could be wrong about this, but I'd be surprised.

I asked MSA about this before I bought mine. They said that they use an improved design over factory, with a thicker carbon trace. Of course, when I got it, it said rebuilt by "Fuel Injection" corporation or something similar (not Python, another rebuild wholesaler) so MSA does not actually rebuild themselves. But maybe MSA sets the specs. I think that they replace the electronics but keep the mechanical components that are still good. I've had the cover off of mine and the internals looked brand new.

On the other hand, I think that many companies that rebuild alternators, for example, just test components and put them right back in if they test in-spec. They even say so in the packaging that comes with a rebuilt.

Also, one more data point - I tested the AFM off of the 1978 parts car that I had for a while and I got 100 and 180. The engine ran great, but again it was with no load, just garage tuning.

And Jenny, to your question, the car would drive at a steady throttle setting, but it would stall and buck if I tried to give it throttle,with some intake popping. It seemed like it wanted to go, but there was no way to feather the throttle or do anything to get it to drive right. I had just bought the car and got it running, so had no points of reference to work from.

Luckily it came with an FSM so I went through the testing procedures and found the AFM problems. When your first post came up I was second-guessing myself, but the fact was, in my case, that the rebuilt AFM made all the difference. If someone chimes in with a great-running engine with 126 and 226 AFM readings, I'll be confused again.

Edited January 28, 2011 by Zed Head

[Jennys280Z]

Little update: So now my car has new plugs, plug wires, cap and rotor. I'm timed 12 degrees BTDC. I haven't driven the car yet but will tomorrow I hope. But I moved my AFM wheel two teeth leaner than it was before I adjusted it in the first place. I'm doing this so I can get a more likely feedback tomorrow during my test drive what these changes will do to my car.

Until tonight I only adjusted my AFM wheel once, moving it about 3 teeth CCW, and then I lost a half inch of vacuum, and my car ran no better than before, even with new plugs and air filter.

The case for running rich: Despite the high smooth idle with my TPS enriched, I didn't run any better on the road and probably ran worse (less pickup, more helicopter, more smoke and gasoline smell) AND, after adjusting my AFM those three teeth richer, combined with moving static advance back to 10 BTDC, I had some light popping out my tailpipe while driving. These observations, combined with cozye's opinion about my old spark plug photos is causing me to believe that I have a rich-running issue.

A rich condition, that my former mechanic may well have been trying to deal with at my last tuneup. When he leaned out my AFM and put in hotter plugs.

Anyhoo, I'm looking forward to my "extra lean" road test tomorrow. I'm about two teeth leaner than ever and I am so hoping my car responds positively to this change. And I really hope a few of you lovely 280Z owners will check your AFMs this weekend. Then by Sunday I'll have a much better bearing on what is wrong with my car.

Something is telling me that a bad AFM might be causing these "lean like" or "rich like" symptoms. And even frustrating me and making me chase after them the wrong way. But I'm still hoping for a 280Z owner to post a 126 and 226 and tell me I'm okay. I've been reading a lot about how some of these Z's seem to wander into a state of leanness, whether by ECU-fade or whatever else, but maybe my car's gone the other way?

Edited January 29, 2011 by Jennys280Z

[cozye]

Can you put the AFM back to the stock setting, or do you not know what the stock setting is ?

[cozye]

Also, I'm not saying you were running rich. Just looking at your plugs they didn't look like you were running overly lean. The one plug did look a little rich, or like it was misfiring because it was more black while the others were tan.

[Jennys280Z]

Can you put the AFM back to the stock setting, or do you not know what the stock setting is ?

If the notch I've identified isn't the marker for stock, and if that notch at 12 o'clock with a pristine glue blob isn't nothing but a coincidence, then I was at the stock setting yesterday and now I'm about 4-5 teeth lean.

Nobody's paid much attention to that to either confirm or deny it for their own AFM, but I'm convinced of it myself.

Edited January 29, 2011 by Jennys280Z

[Jennys280Z]

Also, I'm not saying you were running rich. Just looking at your plugs they didn't look like you were running overly lean. The one plug did look a little rich, or like it was misfiring because it was more black while the others were tan.

Okay I hear you but...Generally, they were all dirtier than in the photos before I cleaned them up the first time. And we're not really talking about enough miles put on them after that--- 25 -30 miles if that. You can see on the metal in the photos where I sanded them. I think in the case of the #5 cylinder's plug - the dark one- I didn't clean the insulator but only the electrodes.

[Jennys280Z]

Little update: Just got back from a long test drive after leaning out the AFM 5-teeth from where I was before (two teeth leaner than ever).

Outside temp here is over 60degF.

Idle was awful warming up. 550RPM though a steady putt-putt otherwise. When the temp gauge was almost fully warmed up I got in to drive...idle was probably 650-700 at this time. I put it in reverse and released the clutch pedal, it shot backwards with no threat of stalling despite no throttle.

After driving it pretty hard for a half an hour, my idle speed was 800RPM, if that.

Never heard a backfire, from a few full throttle runs, a few full throttle hill climbs in 2nd and 3rd, to revving to 5000RPM and back to idle (with no load).

The car has better much pickup now! (much better sorry ) It still has a gargle at part throttle/low RPM but this might be attributed to having zero vacuum advance from my distirbutor.

I think the moral to the story is becoming this: I had a bad thermostat. I ran the car several times in the cold without it warming up...and not driving it either. Not addressing this problem immediately made me focus on how bad the idle was prior to replacing the thermostat. After doing that, I still didn't have an opportunity to rightly "drive" it in warm weather. And now that I think of it, the last time this car ran this good was last summer/fall when the weather was this warm.

There is a divergence here, between how the car idles and how the car drives. I've sacrificed my idle speed for a much healthier road performance. For whatever reason this is the case.

Still know my vac advance is shot on my dist. Am still wondering whether my AFM has a chance of being alright (and if this leaning out process is actually just a way of dealing with the failed/failing AFM without replacing it).

Thoughts?

Edited January 29, 2011 by Jennys280Z

[Zed Head]

That's good news Jenny. Maybe I was close, thinking that the 126 and 226 resistance readings would give a shifted fuel curve from the AFM. If I had known more when I got my car, I might have got in there and tried adjusting it but I barely knew where these forums were at the time, and was just working from the FSM.

Your vacuum advance should not be active at idle. If your vac advance line is connected to ported vacuum, which is the stock configuration, then you will have no vacuum advance at idle when the throttle is closed. So I would not look there to fix your idle problem.

You didn't say how long you took to warm up. For at least the first minute or three, your Auxiliary Air Regulator should keep your idle up (around 1200 to 1500 rpm, slowly dropping as it warms up). If it is not, then it is either stuck closed or open, or is not getting power. The test to see if is open is to pinch shut one of the hoses connected to it, and see if the idle goes down. It should be open when cold and closed when warm.

Your car is driveable with no really bad problems. Looks like you're winning.

Edited January 29, 2011 by Zed Head

[Jennys280Z]

That's good news Jenny. Maybe I was close, thinking that the 126 and 226 resistance readings would give a shifted fuel curve from the AFM. If I had known more when I got my car, I might have got in there and tried adjusting it but I barely knew where these forums were at the time, and was just working from the FSM.

Your vacuum advance should not be active at idle. If your vac advance line is connected to ported vacuum, which is the stock configuration, then you will have no vacuum advance at idle when the throttle is closed. So I would not look there to fix your idle problem.

You didn't say how long you took to warm up. For at least the first minute or three, your Auxiliary Air Regulator should keep your idle up (around 1200 to 1500 rpm, slowly dropping as it warms up). If it is not, then it is either stuck closed or open, or is not getting power. The test to see if is open is to pinch shut one of the hoses connected to it, and see if the idle goes down. It should be open when cold and closed when warm.

Your car is driveable with no really bad problems. Looks like you're winning.

Wow thanks Zed. Sure I'm not blaming my idle problem on the lack of vac advance, just the noticeable gargle/miss I still hear from 1000-2500 RPM on the road. Is this correct thinking?

The idle problem is something else. And my car was likely running too rich (but why?). In fact I remember experiences driving the car in the past when plugs would foul resulting in dead cylinnders and a bad running car. We put over 50,000 miles on the car ourselves (well, me and my ex ) and I really started exploring the repair bills and trying my best to remember the history I've had with it myself.

OT: The air regulator is where I heard a whistle (vacuum leak!) right after I changed those lines/clamps. So that popped my "vacuum leak" cherry when I accidentally made one myself hehehe The surface of the connector(s) on the Air Regulator are slightly conical, making the standard style hose clamp I replaced necessary to be tightened very tightly to stop the leak.

And now this drive today had the benefit of a new cap and rotor for what that was worth. I'm definitely firing on all cylinders now at least! It feels like a straight six now and not a straight five.

I've never seen a used dist cap up close before last night. The rotorhead was blackened and had a tiny notch in the metal where I presume the "spark" crossed. The six contacts on the cap were all blackened on their faces too.

I don't have any experience to know better, but I suppose it's possible that the new cap and rotor singlehandedly made that much of a difference in the way it drives, and my adjustment direction on the AFM is incorrect. I just don't know how much difference 4-5 teeth of the AFM makes having no experience with them at all.

For at least the first minute or three, your Auxiliary Air Regulator should keep your idle up (around 1200 to 1500 rpm, slowly dropping as it warms up).

OMG Zed I'm going to have to do my homework on those now. ty