Tuning With An Air/Fuel Gauge

[240260280]

On 9/12/2021 at 10:59 PM, chaseincats said:

That's what we're trying to figure out.  It happens every few weeks and is fixed by kicking the pedal a few times.

Only TVS is connected to the pedal.... look there

[chaseincats]

On 9/14/2021 at 6:45 AM, 240260280 said:

Only TVS is connected to the pedal.... look there

Is the TVS another name for the TPS?

Also, regarding the "sticking afm" would it be the resistance wiper that would be sticking or the air door inside the afm?

Edited September 27, 2021 by chaseincats

[240260280]

tvs is often referred to as tps.

Not sure on afm sticking.  It would certainly  reduce air flow if the flap stuck.

[TomoHawk]

The only way(s) to see if it's sticking and why is to remover the AFM and look inside, or take the cover off the side.

[chaseincats]

Final update: the afm wasn't sticking per-say but there was a fair bit of play in the flap/wiper-arm, so I'm guessing one of the bushings inside the device has had it.  It's possible that 'lean boot' was the car starting with the arm not in the right spot because of the flap/wiper-arm play (you could move it forward/back a good quarter inch).  I completely made that up but sounds feasible to me.

THAT SAID, I got a different afm without that issue and retuned it with the gauge and no lean-boot issues so far (knock on wood).  This AFM was unmolested and even still had the cover factory-glued shut and the plug in the idle enrichment hole.  Knowing that AFM was tuned to factory spec allows me to do this:

 

Steps to tune a 280z without an air fuel gauge:

- Be sure your AFM is sitting at factory configuration - if the cover is glued on that's a good indication its factory (if this isn't the case, get one that is or an air fuel gauge)

- Be sure your engine does not have any vacuum leaks (smoke test it)

- Disconnect your altitude sensor under the dash (if your car came with one)

- Disconnect your EGR vacuum hose and cap it (the stock afm configuration is based on that being connected I think).  Cap your carbon canister line too if you'd like although the amount of air coming in from that compared to EGR is trivial and barely changes the readings at all.

- Be sure your coolant temp sensor's (efi one not the temp gauge one) resistance tests within factory spec in the FSM (the temp sensor changes mixture richness so that needs to be operating properly for this to work)

- Check that your TPS is in spec by following this guide

- Take the cap off your throttle position sensor (TPS) and GENTLY bend the WOT arm away from the middle arm by the base NOT THE TIP (like in the pictures in previous pages) BE CAREFUL.  Datsun built the arms very close together so the car will default to running richer because they couldn't configure the engines like this at the time.  In my case, anything over 1/4 pedal pressure had the TPS tell the car computer to use WOT enrichment.

- With the engine running and using a multimeter (or good eyes), have someone hold the engine at highway speed rpm (4k?) and be sure that the TPS arm is not touching either of the other arm contacts (this is highway cruise mode)

- Now, mark your AFM's black gear's current position then turn the black gear forward (clockwise) 4 teeth (the idle enrichment screw does not need to be touched)

- If you did all of those (especially the TPS adjustment and have no vacuum leaks) then your car SHOULD idle between 13.8 and 14.2 and have a highway cruise of 14.7 (5th gear ~3800 rpm at 65mph)

 

NOTE: this is with an engine with no vacuum leaks and tuned to factory spec (valves, timing, etc) according to the "ET" chapter of the FSM.  All efi connectors have been cleaned according to this list.  These are the steps I followed over the past year and should work on all 280z engines within factory spec but I am not responsible if it does not work for you and something negative happens.  At the end of the day, these cars were tuned to run rich out of the factory because running too rich won't destroy the engine, running too lean will but getting it just right like this is a pleasure. Datsun didn't have the ability to test the engine running as a unit this precisely - they could only test each sensor before it went in and smell the tail pipe as there were no wideband o2 sensors in the 70s for this use if at all.  Getting an o2 sensor to do this or even confirm your work is highly recommended, but if you don't want to, then follow these steps.

Edited February 15, 2022 by chaseincats

[Captain Obvious]

Excellent. Thanks for the closure write-up, and hoping the replacement AFM gets rid of the lean boot.

At least you found something tangible! Let's hope that was it!    

[chaseincats]

Bringing this back for a special "happy new year" edition of this thread with one new observation.

Much like the 240z's big orange carb filter's "winter/summer" switch, I've found that adjusting the AFM requires a 1 tooth adjustment depending on the season (in order to stay within those air/fuel numbers).  When it gets cold, richen it one tooth (clockwise) and when it warms up in the summer, lean it one tooth (counter-clockwise).

I'm assuming this is due to cold air being denser and thus leans out the car, but I'm no scientist it's just what I noticed on the gauge after all this time.

For me this means that the summer tuning is 3 teeth to the left of the stock tooth (that you should have marked) and winter decreases that by 1 tooth meaning that the winter setting is 2 teeth to the left of the stock configuration.

 

Happy tuning my friends.  Let 2024 be the year of 14.7

Edited January 8 by chaseincats

[Zed Head]

There is an air temperature sensor in the AFM that is supposed to handle that.  You might ohm it out and see if it's working.

There is also the issue of winter mix gasoline.  They make the mix more volatile so that it will light off and burn cleaner in cold weather.  Modern cars with powerful computers and O2 sensors handle it well.  Older cars not so much. 

The potentiometer on the coolant temperature sensor circuit is a simpler way to get a similar effect.

[chaseincats]

9 hours ago, Zed Head said:

There is an air temperature sensor in the AFM that is supposed to handle that.  You might ohm it out and see if it's working.

There is also the issue of winter mix gasoline.  They make the mix more volatile so that it will light off and burn cleaner in cold weather.  Modern cars with powerful computers and O2 sensors handle it well.  Older cars not so much. 

The potentiometer on the coolant temperature sensor circuit is a simpler way to get a similar effect.

The air temp sensor is showing readings to spec on both the old and newer afm per the fsm which is good.  I never found that sensor did much if anything since starting this process which I always found odd.

Regarding adding a potentiometer on the coolant sensor, doesn't that only have ability to add resistance to the coolant sensor thus increasing richness?  I don't believe you can lean the car out with a potentiometer on the temp sensor but I could be wrong.

Edited January 9 by chaseincats

[chaseincats]

Spark plug question.

Plug image below.  NOTE: cylinder 3's plug's ceramic was cracked so I pulled the valve cover and noticed that cylinder's exhaust valve was loose by at least .004 and has now been fixed.  All other valves were sitting at factory spec.

I pulled the plugs (they've been in there for a few years) and they're white on the top.  I have been using NGK Iridium BPR6EIX-11 plugs.  Since the car is running leaner than stock 280zs (14.7), should I change the plugs' heat to BP5, keep them at BP6, or change to BP7?

Edited January 21 by chaseincats

[jonbill]

I'd just go with cheap bpr6es.  Does your engine burn some oil? It looks like there are deposits on the spark plugs.

[madkaw]

Those plugs show lean and or hot - I would be concerned if my plugs looked like that . I haven’t read your complete thread , but those do not look good