Update on EFI mod -- potentiometer on coolant temp sensor circuit

[cozye]

I second Sarah's assessment of using the AFM spring for adjustment. I tried this on my car as well, and while it did make the car run very good at lower RPM's and speeds, there was so little spring pressure that at the higher RPM and throttle openings the car fell very flat and did not work well at all. I suspect this is due to the significant change needed to bring the air fuel ratio back left the spring with so little tension, the flap swings open too easily and not linear like designed. I'm sure if you are only moving 3 or 4 teeth, its fine, but the change needed was too drastic for the AFM to handle.

I tried several times to dial it in using the AFM (needed about 12 teeth), and it never worked anywhere near as well as just dialing up the water temp circuit. I played with this for a couple of months.

And Sarah, don't forget that the AFM temp sensor richens the mixture up a bit at cold temps, what was it like 62 degrees ? That may explain your slight warm weather change.

[cozye]

And Sarah, if you decide to hack your ECU, let me know how it works out and where you end up

[Jeff G 78]

Great info everybody. I might try to move the spring a few teeth and still keep the CTS mod to fine tune. If I don't like it, I will switch it back. I think that like Sarah, I would need a big AFM spring adjustment based on the amount of resistance I have added to the CTS circuit. BRAAP's write-up is great. I started reading it at midnight and was too tired to comprehend everything, but it all makes sense.

[darom]

I have also added the 5k potentiometer to the CTS circuit after chasing the lean condition of my 76 for months (it turned out to be a bad fuel pump). After experimenting with different resistance settings, it seems I can't add more vacuum to the existing 17 in Hg no matter how rich the mixture becomes. Actually adding more resistance makes the vacuum go down. The AFM is a fresh rebuilt unit to match CA EPA standards. I tried 10 BTDC, 850 rpm idle, then 14 BTDC (engine runs stronger at acceleration without pinging).

Btw, Sarah, thanks for the CTS mod guidance :-)

[FastWoman]

You're quite welcome, Darom! Yes, when you keep adding fuel, the engine will start choking, and both vacuum and RPM will fall. There's a sweet spot where the mix is just right and will maximize vacuum and RPM. I try to figure out the limits of adjustment where the engine starts dropping off on the lean and rich sides. Then I adjust to the middle, just a tiny bit towards the rich side -- maybe 2/3 into that adjustment range.

So are you saying your engine does need additional resistance in the CTS circuit to richen it enough to reach that peak of 17 in Hg, even after replacement of the fuel pump? If you're still adding resistance, how much are you using? Just curious.

I also had my fuel pump fail, but that wasn't the source of my mixture problem. I had hoped that it might be, but alas I still need about 2.6 kOhms in series with the CTS to reach my peak vacuum of 18.5 in Hg. Without the added resistance my engine runs quite poorly.

Oh, and I do find my engine runs better with a bit more advance than the stock 10 deg. I'm currently running at about 13 deg BTDC. My engine will idle down as low as 450 RPM before stalling out, but it seems to "like" about 900.

Edited June 6, 2011 by FastWoman

[Zed Head]

Tuning for vacuum level that is done at idle will have the "idle enrichment" fuel contribution from the ECU and the TPS. I'm pretty sure that I saw the same type of issue, tuning for idle properties, on the spring tension tuning procedure.

[darom]

Sarah, after I installed the rebuilt AFM with the stock CA lean settings, the best resistance setting I can get is to leave it alone at 0 to have my 17 in Hg. It is still a nice troubleshooting tool.

Re: the idle and ignition timing - I am in the same boat: the car likes 850-900 rpms with the 14 degrees. 10 BTDC makes my car sluggish.

Zed Head: you might be right about the idle enrichment. Now all of us need a vacuum gauge inside to observe it while at cruise. There was a reason why old cars used to have these gauges :-)

[Zed Head]

To complicate things even more, most cars use a ported source for vacuum advance. So as soon as you touch the throttle, you're timing advance jumps significantly. Plus mechanical advance which kicks in at 600 rpm. You can be around 30 degrees advance at low RPM, steady cruising conditions, above 2500 RPM you can be in the 40s.

And don't forget the idle air adjustment. Idle conditions are their own little world.

Not to put a damper on playing with these things, just saying that there are a lot of other considerations. What happens at idle only gets you so far.

[FastWoman]

What happens at idle only gets you so far.

Yeah, but what happens ABOVE idle requires gauges in the cabin, dynos, or somesuch. I guess I'm sometimes cheap/lazy. I'll probably end up running some vacuum line into the cabin for some on-the-road tuning someday.