I'm About Done With The %**&ing Efi

[Zed Head]

Thought of this a few days ago - maybe you have the wrong speedo gear in your transmission.  Are you really getting a calculated 10 mpg, or just saying "wow it's on E already"?  No offense, but 10 is a nice round number.  The gauge hits E after about 12 gallons from Full.

 

Also, I don't think you've mentioned your driving style on these bad mpg trips.  If you're climbing a lot of hills and goosing it often, you could easily burn some extra fuel.  It's a performance engine, designed to burn fuel if asked.  Your plugs don't look like 10 mpg plugs, as noted.  I took another look at your post 44 numbers and they look just like my old setup (except for the TPS NG) with stock injectors, that typically got 18-20 mpg.  That's mostly flat land, but with goosing.  

[mjr45]

Zed, I'd be happy with a good solid all around driving 16MPG.

[S30Driver]

One thing no one has mentioned is the BCCD.  I made the mistake a few years ago adjusting it trying to "improve" things.

Made it about 3 miles, started running pig rich, fouled the plugs, and died at a intersection.  Fortunately, I brought the 12mm wrench with me just in case.

Anyway, if you have not touched it,  maybe unplug the line to it and see if you smell any gas.

[FastWoman]

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.

Edited March 24, 2015 by FastWoman

[Chickenman]

Zed, I'd be happy with a good solid all around driving 16MPG.

 

I get 28 MPG ( Imperial gallons ) on my 1976 with 3.90 gears and a ZX 5 speed. So you should be able to get a lot better than that.

 

Personally I would send the stock injectors to a company like http://injector-rehab.com/shop/home.php and have them cleaned and balanced. If your " new " injectors are rated higher than the originals, then no amount of futzing about is going to correct things. That will throw the whole fuel curve out of whack.

 

I'd also invest in a WB AFR meter such as an AEM. These are reasonably cheap and you get real time AFR mixtures with no guessing. Invaluable tuning tools.

[mjr45]

I may opt for a new set of injectors some FJ707T, the $ amount to get new ones is about the same as getting the old stock ones cleaned and rebuilt. The driving I do is pretty much up and down hills on twisty roads so I expected the MPG to not be great, but it should be better than it is.

 

Sarah,  my house is almost twice as high up as Denver(a horrible place) and visitors from the flatlands often get short of breath walking 20' up to the door, "there's no air up here." I will pick up a pot next time I'm in town probabaly a day or two depending on weather and since it snowed AGAIN last nite, it won't be today, and wire it in just to see what it does per your recommendations.

 

I don't think the BCDD has ever been messed with, but I'll see what happens when unplugged, who knows at this point.

 

As I've said, the car runs well just has crappy MPG, I'm also thinking I'll replace the TPS because its NG @ WOT. I'm still on the fence about new injectors and maybe an adjustable FPR but I've read they are not all that great. Here are all 6 #1 on the right. #1 looks pretty good, #2 not as good, #3 a little worse, #4 worse about like #2, #5 bad, #6 a lot worse than the others. In person 4,5,6 look much worse, pic makes them look better.

 

Thanks again to everybody.

Mike

Edited March 24, 2015 by mjr45

[Captain Obvious]

BCDD should have no effect on mixture at all. All it does is provide a path around the throttle plate. Think of it as an idle speed control. I'm not going to rule anything out at this point, but the BCDD shouldn't have anything to do with this.

 

As for an altitude switch, I looked through the manuals as well and I don't see any mention of the altitude switch prior to 77. I'm thinking they added that for the 77 year and beyond. I agree with FW in that I wouldn't go shorting pins on the ECU. Probably wouldn't cause any problem, but not worth the risk.

 

So unless you're triple positive sure you've got the right injectors in there, you need to address that. Where (and when) did you get your replacement injectors? Did they come with the car or did you put them in?

 

 

 

 

[FastWoman]

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?

[mjr45]

Haven't had any hot restarts so far this year, had an occasional problem in the past in the summer, it doesn't get real hot temp wise in the summer, just feels hotter cause we're closer to the sun, got to use SPF 5000.

 

Captain, there is no altituce switch but there are 2 wires hanging by the ECU that I suspect is for this.

 

After looking at the pic of my plugs, it looks like there is no consistency to the burn on all 6, #1 looks the best almost right on, but the rest look worse in terms of burn with #6 being the worst, so maybe its faulty or wrong injectors, don't know.

Mike

Edited March 24, 2015 by mjr45

[FastWoman]

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?

Edited March 24, 2015 by FastWoman

[240260280z]

There is an old post about the EGR rotting through and polluting the intake with too much exhaust. It is worth a check.

[mjr45]

Fastwoman, that might be worth a try. No, my wif'e Kia(blah) gets about 38MPG and my truck around 20.

 

Blue, don't have an EGR.

 

Mike

Edited March 24, 2015 by mjr45