FastWoman

Stanley, this is a weird little check valve with two nipples. Air/vapor passes freely in one direction, but may also pass in the other direction with a bit more force. Thinking mine was defective (for passing air in the reverse direction), I looked for a replacement and couldn't find one. I then determined/figured it really wasn't defective after all. It was simply a weird part. In a pinch, maybe you could substitute two check valves plumbed in parallel with each other and flowing in opposite directions, so that some resistance to flow exists in both directions, but air may pass either way. Preferably one check valve will require a bit more force to open.

It hurts to read this thread! What a beautiful Z! I wish you the best of luck re-locating it! If I see it here in Virginia, I'll certainly let you know. FAIW, I have a defeat mechanism on my engine. My biggest fear was that someone would take my car for a joy ride and trash it, but I don't have to worry about that now. Of course it can still be stolen, but only by a tow truck or someone who is a lot smarter than most thieves. I think a great way to create a defeat mechanism would be to wire a number of critical circuits (e.g. starter wire, fuel pump, ignition, EFI if you have it) via black wires (so there are no useful color codes) to a molex connector under the dash or perhaps in the ashtray compartment. You would then have a mating connector you would plug in to correctly connect those various critical wires. Carry it with you when you leave the car. You could also add a couple of ground wires in the car's connector -- and perhaps the horn wire for grins, so any attempt to hot-wire the connector would likely result in blown fuses.

Aw shucks! I expected to see a photo of a floating Z car!

It makes sense to me too. I had always wondered where the 4500 RPM thing came from -- sort of in the same way as the 10% of your brain thing. When people grab hold of these numbers, the original context is often lost, and the numbers seem to develop a life of their own! Anyway, the AFM pegging out at 4500 RPM at WOT makes sense. Having thought about this a bit more now, I bet the mix gets richer and richer above 4500@WOT. Air flow is probably approaching some asymptote dictated by Reynolds numbers, such that air delivery per engine rotation declines, and yet the same dose of fuel is probably delivered with each engine rotation.

So I started writing another thread and then had a change of heart. My epiphany was frought with many missing details of the circuit design -- things that I'm unable to test, because I don't really have a functional electronic test bench anymore. (One of these days...) But this is what I was thinking, and it's something you can try. It might work well, and it might not: If you add a bit of resistance in parallel between pins 8 and 9 of the AFM, you will lean the mixture. This leaning will be more at idle and moderate throttle than under heavy load. Part of what leads me to this thinking is that the altitude switch output is somehow combined in with the AFM voltage output in the '77/'78 to create the injector base pulse, which is then modified with fixed pulse width corrections thereafter. There are a number of ways to design these circuits, so it is very hard for me to draw many conclusions. However, by lowering R0 (pins 8-9), we'd be introducing a correction at a more "correct" stage of the circuitry. How much resistance should you add in parallel? I'm thinking not much. How about trying a 2 k potentiometer for starters? Pre-set it to the 2k position, and then dial it down to optimize performance. The midpoint of the potentiometer would set the AFM to approximately the ideal, stock resistance (i.e. 100 Ohms between 8 and 9), but you can adjust further. Avoid adjusting the resistance too low, as I don't know what the resulting higher output voltage of the AFM would do to the ECU. If you want to be safe, maybe wire 200 Ohms in series with the 2k potentiometer, and then wire those two parts to pins 8 and 9. That way you won't be able to adjust the 2k potentiometer dreadfully far and hurt the ECU by accident. By the way, how do other cars run up there in the ozone layer? Do all cars have a fuel economy issue up there?

Wow, you're REALLY high! OK, don't do anything just yet. I'm about to start another thread. Maybe there's a better way! Maybe, just maybe... And don't install an adjustable FPR. The FPR design in our Z's will hold pressure, which is important for prevention of hot restart problems, probably ESPECIALLY where you're situated. The adjustable FPRs (e.g. Aeromotive design) do not hold pressure after shutdown. I realize your system leaks pressure pretty fast (8 min), but if it can be tightened up to hold almost full pressure for maybe 30 min, that's half of preventing the hot restart issue. But just curious... Do you have hot restart issues? Maybe your "mile high" gasoline formulations are less volatile?

Chickenman, yes, I see we're saying more or less the same thing, at least re post 15. I can't explain why I read it differently. <shrug>

There's an ECU flow chart in the '75 through '78 FSMs. It shows the spark signal triggering a multivibrator circuit. Apparently the base width of the output is modulated by the AFM voltage and altitude switch. So the AFM output is primary to the formation of the base pulse. That pulse is then lengthened by correction signals deriving from the TPS, CTS, and ATS, as well as a correction for battery voltage. At no point is there any sort of frequency-to-voltage conversion. I feel certain that would be reflected in the flow chart, as other details of lesser significance are indicated (e.g. ignition wave shaping).

Chickenman, you can hit 4500 RPM under little load, with the throttle between the TPS's cruising range (above off-idle and below "full"), with the engine not sucking in much air, and with the AFM somewhere in its midrange. Or you can hit 4500 under heavy load, with pedal to the metal, engine sucking in all the air it can, and AFM fully deflected. The AFM doesn't measure RPM, such that it pegs out at 4500. Otherwise the ECU would use the spark signal, and no AFM would be designed into the EFI. Think of it another way: Let's say you go down the road in 2nd gear, steady speed, with your engine running about 4500 RPM. People might look at you funny, but let's just say you do it. You'll have the accelerator pedal depressed somewhat, but not a lot, so as to maintain the same speed. Now let's say you depress the pedal a bit more -- not enough to engage the TPS's "full" position. (Heck, let's just say you don't even have your TPS plugged in.) When you depress the pedal a bit more, you accelerate. Right? How does that happen if fuel and air delivery remain the same at 4500 RPM? Wouldn't you keep going at the same speed? I'm inclined to believe the 4500 RPM peg-out occurs at WOT. Maybe airflow from that point is somewhat linear'ish, so delivering the same metered dose per revolution works well enough. Or perhaps mixture errors above 4500 at WOT are not so significant that the engine will be damaged, and efficiency past the declining leg of the power curve isn't deemed that important.

Honestly, I agree with Captain. The plugs are a bit dark, but not really all THAT bad. Your fingering test confirms that the mix is a bit (and not a whole lot) rich, at least at idle. Maybe that 9% reduction for altitude would do the trick for you. That said, I've looked at the EF section of a '75 FSM now, and I don't see any evidence of an altitude switch feature for that year. Therefore I wouldn't short those two pins together. It's likely nothing would happen (unused pins), but it's still possible you could blow something out. Just for grins, try wiring a 10k potentiometer in parallel with the CTS, and set it initially at the 10k extent. (Confirm with your multimeter.) Warm up your engine, and then try lowering the resistance a bit. Try to find the best RPM. Then go for a test drive. Try tweaking to optimize your "butt dyno" results. It doesn't hurt to try it. If it runs pretty well that way, try starting your engine cold with the same potentiometer setting to see if the car runs OK when cold. It might not be optimal during warm-up, but hopefully you can get the mix bang-on where you want it when the engine comes to temperature. FAIW, I used to have a bit of a lead foot as a kid and as I recall, my '75 Z got about 15 mpg. I also recall visiting Denver as a teenager and being quite out of breath. I didn't have my car with me, but I think I was getting about 10 mpg! It will be interesting to see all 6 plug reads.

I had forgotten the altitude thing too! According to my '78 FSM (sorry, don't have the '75 downloaded), the altitude switch (present on California models) closes above 3675 feet, signaling the ECU to cut fuel delivery by 9%. So maybe that's the problem. Maybe you're just running a non-California model at an altitude that's too high. According to my '78 manual, the altitude switch connects between pins 9 and 12 of the ECU. I'm guessing it's the same for a '75, but you should verify. You could connect between pins 9 and 12 to see if that leans you to the correct mix. If that works, you could either install a toggle switch to lean the mix for high altitude, or you could get fancier and install a proper altitude switch. (I'm guessing non-California ECUs still have the altitude correction circuitry, even if the switch isn't installed in the car.) Oh, and to clarify with the fingering-the-AFM thing, you say you fingered to the rich side. Not meaning to doubt you here, but some people do get it wrong: By that, you mean you opened the vane a bit (counterclockwise rotation of the potentiometer mechanism), right? And as soon as you moved the vane slightly more open, the RPMs dropped a bit? If so, then you're not running lean, but that doesn't necessarily mean you're running rich. Also test in the other direction. Close the vane a bit, and see if the RPMs pick up a bit. If they do, then you're running rich. When you're running at the correct mixture, the RPMs will drop when the vane is fingered in either direction. An alternative theory is that your mixture is just right, but that you have some other problem that results in partial misfiring or incomplete combustion. For instance, your ignition could be failing you somehow -- maybe a weak spark, maybe timing. Or more likely it was the dirty potentiometer, which would plague more and more as you moved towards greater airflow (where the wiper doesn't travel as much). If there were some skips in conductivity as the wiper moved over the potentiometer tracing, then voltage between 7 and 8 would have been zero, which would have been read as "maxed out" airflow through the AFM. So you might have been excessively rich under load and at a more reasonable mixture at idle. I suspect you've already fixed the problem and don't know it yet. Your first good test drive should tell.

I can't imagine this is quite true, although we've all certainly heard it. There's a big range of throttle positions between "off idle" and not quite "WOT," per the TPS signal. Are we to assume an engine spinning 4500 or above will get the same amount of fuel, regardless, metered only in proportion to RPM? I doubt it -- although again, we've all heard it. What bothers me about monkeying with the clock spring is that this not only impacts the preload, but also impacts the airflow at which the AFM pegs out (which it does). If we loosen the spring to deliver more fuel, the vane pegs out with less airflow, and then there is no room to deliver more, except as RPM increases. This results in an engine leaning out at the very time it is worst for it to lean out -- under heavy load, revving hard.

Mike, I'm running out of ideas, except for one thing: We are taking you at your word that this is a rich running issue. However, you can get a lot of soot from incomplete combustion (also low vacuum) if your engine is running extremely lean. I know, because my lean-running engine did exactly that. There cars are far more likely to run lean than rich, given that the CTS circuit is intact. Your car might be running lean if... The exhaust blows hard at idle and spits out sooty drops of water.Fingering the AFM flap more open at idle causes the engine to even out and run a bit faster.Pulling the vacuum line off the FPR and plugging the end of the loose hose with your finger causes the engine to even out and run a bit faster.Cracking the oil cap veeeeery slightly causes the RPMs to drop. Does any of that happen?

I thought you had adjusted/confirmed your TPS. If not, that's a very easy thing to do. I'd check it (and most other things) from the point of the ECU connector. The idle contacts should break as you depress the accelerator pedal very slightly, and the WOT contacts should make when the pedal is mostly depressed (maybe 2/3 of the distance to the floor). Easy to check. You are partially correct about the "what-ifs." If the ECU sees constant idle position, it will run the engine a bit rich. Your leanest running will be off-idle and not as far as the WOT setting. Your richest condition will be after the WOT contacts engage. Although the resistance checks on your AFM are now much better (with the wiper contact resistance issue now seemingly corrected), did you ever power the thing up and confirm smooth voltage changes as you move the flap? Did you measure the resistance of the CTS and air temp sensors at the ECU connector? You'll discover/confirm more by mearusing at that connector. At that location, you will also be checking for bad connections, miswiring, shorts, wire breaks, and mystery stuff hidden inside your wiring harness (e.g. a fixed resistor that someone had wired in series with the CTS circuit in my harness). FAIW, I don't think the '75 and '76 years had the lean drift issue. I know the '75 has the Bosch ECU. At some point ('77?), a transition was made to Hitachi manufacture (and possibly circuit redesign), and that's when the lean drift problem surfaced. For all I know, there could be a rich-drift issue with the Bosch units, but I don't think so. Regardless, if you rule out everything else, you might be able to lean out your mixture by altering the CTS circuit. This would be done by wiring a resistor in parallel with the CTS. Unfortunately, this would diminish the responsivity of your system to temp changes, running it too lean when it's cold. I would use that as a last resort, but it will get you running at the right mixture if all else fails. (But check/confirm "all else" first!) Oh, and finally.... Does your engine run any better now, with the AFM potentiometer cleaned? Or is it the same?

One common problem with these Z distributors is a sticking breaker plate. Does yours move freely? Pull off the distributor cap and rotor, and try giving the thing a twist. You should feel no binding. If your breaker plate is stuck, then your mechanical advance might not work correctly, hence difficulty running at higher RPMs. That could also account for incomplete combustion, hence all the carbon you're seeing. Blue, the zig-zags in the potentiometer tracings lead to a voltage ladder that insures the linearity of the scale, probably preventing drift as the tracings wear.

Sorry about that! The description in my '78 FSM of the direction of wiper movement was a bit ambiguous. ("When the flap deflects along with a change in the intake air flow rate, the terminal 7 mounted to the flap shaft slides on the variable resistor R from R1 to R9, causing the voltage across terminals 7 and 8 to change.") It implies the opposite of what it actually seems to be, as R1 is closest to contact 8 on the potentiometer trace. Go figure! Irrespective, Blue always has great diagrams and trustworthy info! Given that the wiper movement is opposite what I thought, the resistance measurements seem to indicate at least 30 Ohms of extra resistance between the potentiometer trace and the wiper, suggesting the contact still might be dirty or oxidized. Cleaning might improve things, but it wasn't the smoking gun I thought it was. p. 50 of the Fuel Injection Guide: Leads to pins 7-8 should read: "Continuity (Small resistance)" The FSM suggests powering the thing up and looking for smooth voltage changes throughout range of vane movement. Note: This should be done with an analog meter! You won't see jitter from dirty traces very easily with a digital meter!

Considering all that you have done, and especially considering that your CTS measures correctly at the ECU connector, and your TPS is adjusted correctly, I think your problem would likely be a misadjusted or faulty AFM. You identified three resistance tests; however, I'm not 100% certain what you tested. I think I'm understanding this: Pins Should be You measured 6-8 180 189 7-8 low reading 218 8-9 100 112 If I understand these correctly... I wouldn't worry too much about the first and third readings. I think they're close enough and set up the voltage ladder about correctly. If you want to get persnickety, you could solder 1680 Ohms of resistance in parallel with R0 between pins 8 and 0, to give you an effective R0 of 105 Ohms, which would put your voltages in the correct proportion. However, I don't think this is your problem. The more likely problem, if I'm understanding you correctly, is your 218 Ohm reading between pins 7 and 8. This is the resistance between the "near" end of the potentiometer trace (at zero airflow) and the wiper. This value should be less than 6-8 (your reading, 189 Ohms) and should actually be maybe several Ohms. This indicates to me that your wiper and/or trace are dirty/oxidized and/or that your wiper isn't making good contact with the trace. I would try cleaning the trace and the wiper and then repeating your measurements. Once you've cleaned the wiper and the trace and confirmed a low resistance between 7 and 8, you can power the meter up by hooking +12 to terminal 9 and ground to terminal 6. Hook a voltmeter between 7 (neg) and 8 (pos). Then move the vane back and forth slowly. You should see the voltage reading vary smoothly as you do this, with no jumps or gaps. This is the acid test. If you've got good voltages, your AFM should be fine. (By "fine," I don't mean that your clock spring is necessarily calibrated to spec. That's obviously another issue.) Another issue with the '75 AFM is that it has no backfire valve in the flap (vane). A '76 AFM has this feature and might be substitute for the '75, depending on fuel pump shutoff circuitry. (I can't remember for certain whether these are compatible.) Without this valve, a bad backfire can cause the vane to bend, so that air flow turns the potentiometer a bit more than it ordinarily would. This can cause a rich-running condition. If your vane is bent, you can actually bend it back with a bit of care. I did this on my '75, long, long ago, as a kid, by inserting the end of a wooden block into the throat of the AFM to serve as an anvil, and then tapping it out carefully with a hammer. (I think I had to insert another wooden block through the other end.) That solved a rich-running issue I had. BTW, "guys and gals" is perfectly acceptable language, at least to me. And PS: Be very careful of fuel leaks! You can burn up your car and/or burn down your house that way (if your car catches fire in the garage)!

I'm not a carb person, so I don't know your options very well. It's maybe a better question for the carb forum folks. A company called Z Therapy rebuilds the old SU's. They do a very good job, but they're expensive. If shaft/bushing wear is the only problem, it might be possible to have a local machine shop fix it.

Zombi, I suspect you've deleting the heat shields for aesthetic reasons. (I admit they're ugly.) Hopefully you have headers, and hopefully that prevents the commonplace hot restart issue. However, you might want to hold on to the shields for a year or so, through changes in gasoline formulations, to make certain you don't need them. They do help with the hot restart.

I had the problem with my son's Ford Taurus. Yes, I know it's not a Z, but... It turns out the Taurus has a really crappy build of speedo, and as the mechanism wears, the spinning magnets start to contact the copper cup very lightly, causing the jumping, but only at certain speeds. The solution in his case was to replace the speedometer. However, because I'm a hopeless gearhead who hates substituting junkyard parts that are worn exactly the same as the part I'm replacing, I disassembled his speedo, ground a tiny bit off of the copper cup where it was making contact, and reassembled. Problem solved.

Just guessing, but I bet there's a lot of wear/play in the throttle shafts. When the engine is running, the vacuum pulls on one of the throttle plates, jamming it somehow against the throat of the carb. To reproduce the problem without the engine running, try applying pressure against the throttle plates with your fingers and manually opening the throttles. Vacuum at idle would be about 9 psi.

It would be very interesting to know whether that potentiometer is inline with the coolant temp sensor. If it's the same wiring as the '78, I believe that would be pin 13 on the big connector. With regard to skinned cats... Yes, unfortunately, I've seen them. Some graduate students in my lab at one university had the unpleasant job of skinning them for disection specimens in an anatomy class. I hated seeing that, because I'm quite a cat lover.

Hey Len! Welcome back! So I had suggested to you a while back that I might be selling my '78 and buying a '79-'83 T-top ZX (for the T-top and little else, as I love my '78). Well, the challenge seems to be finding a good, rust-free'ish specimen of ZX. I've not made any good progress and don't anticipate finding anything this year or next... or the next after that. So you can still consider my '78 available for beta testing.

It's only a misprint. It was supposed to say $240.00 (firm!). Love the floor pan!

Maybe your breaker plate is sticking? It's a known issue/weakness with S30 distributors. Pull off your distributor cap and rotor, and rotate the breaker plate assembly with your fingers. It should turn smoothly, under spring tension.