Am I running too hot?

[drober23]

I posted to the AFM adjustment thread, but didn't want to turn that into an ongoing discussion on my learning curve with AFM settings, so I started a new thread.

I had installed a Dakota Digital CHT gauge, and as long as the weather was cold (or even chilly) my CHTs were nice and low. As ambient temperatures started to warm this spring, so did my peak CHT readings. As they approached, then exceeded 400 F, I began to get a bit fretful. The first think I checked was a spark plug. Here is the #3 plug, with the crush ring removed as that is where the CHT sender is.



I decided it looked a little to whitish-grey for me, and that I must be running lean. After asking a few folks, I decided to get an AFR gauge. The PLX wideband gauge looked flexible and affordable, so I bought one and welded a bung into my exhaust. I am running a '72-'74 exhaust with a thunderbird collector and muffler. The bung for the sensor is located just before the muffler, after the collector.

Since I do most of my driving on the highway (65 - 70) I paid careful attention to the readings in that range. I was averaging about 13.8. If I floored it while already at top speed, it might get to 13.2, but usually not below 13. Idle AFM was also around 13.8 (hmm, a little rich?), and I would hang in the 14 range while puttering around at low load. My average fuel economy to this point had been about 16.8 MPG (or so).

After reading a lot, especially Colin's AFM Adjustment post, I decided my full throttle AFR readings were too lean, and started to tinker with the AFM.

One iteration of AFM settings left me good at idle, but very rich at load (11.5). I remembered Colin's results near this setting, so I ran it for a while. My CHTs were a good 20 F lower than with the lean settings, but my mileage suffered a lot (14.5 mpg for 200 miles). I decided to shoot for 13.1 AFR at load, and 14.5 at idle.

I managed to get pretty close to this. My current settings have me at 12.9 - 13.0 at 65 - 70 mph. And about 14.7 at idle. Colin suggested that I take some readings a bit lower than 70 mph, so I did that on the way home tonight. It was about 70 F, and no real wind to speak of. 55mph averaged about 13.5 and CHT about 335. 60 mph averaged about 13.2 and 350. 65 mph put me to 13.0 and 358, and 70 mph stayed at 13.0 and CHT rose to 370.

Fuel tank needs filling on the way to work tomorrow. Then I will be able to report the last 400 miles or so of readings at these settings (2 fillings worth). Here are my #4 and #3 spark plugs now (#4 on left).



A second view


Looks less whitish than before to me, but I am open to opinions.

Thank you.

[Bleyseng]

Looks less lean and ok to me. I am partial to running the triple tips.

Whats the mpg now?

[drober23]

Filled the tank this morning. Average for fuel economy for the last 350 miles is 16.5 mpg.

[Amskeptic]

Colin suggested that I take some readings a bit lower than 70 mph,
55mph averaged about 13.5 and CHT about 335.
60 mph averaged about 13.2 and 350.
65 mph put me to 13.0 and 358,
70 mph stayed at 13.0 and CHT rose to 370.

I don't remember anything but asking you to post full throttle readings at 55-60-65-70 because that is the critical period for engine temps. Those full throttle readings tell us whether or not to adjust the wiper or the spring or whatever combination gives us a nice lean part throttle and idle, versus a power-circuit richness.
Your numbers above look FINE.
Colin

[drober23]

Bleysing, thanks for the comment on the plugs. I know that "white is bad", but I don't have a ton of experience reading the plugs. The second picture looks better to me too.

Colin, Ahh, I misunderstood your other post. But thanks for pointing out that my current readings are fine. It takes me a while of reading, living, and experiencing to get some things wedged into my brain sometimes. On my way with this now!

Did some testing today after work. Warmed the bus up and did 4 runs, each from a stop. Shifted to 4th around 47 mph and accelerated at full throttle to 75 mph. Ambient temp 71 F, light to no wind.

The 4 runs were very repeatable. Only the 65 mph reading varied at all between the runs, and it only varied 0.1 back and forth. Here are the readings:

MPH AFR
50 12.0
55 11.7
60 12.0
65 12.35 (averaged)
70 12.7
75 13.0

Also, as I re-read my posts in the other thread, I see I hadn't thought everything through. I mentioned that I get a difference of over 25 F in CHT when there is a 25 F difference in ambient temps. It occurs to me that most of my warm temperature readings have come on the trip home from work. In general, I fight a bit more wind on the way home than on the way to work. So at least part of that difference reflects a real difference in how hard the bus is working. I will try to benchmark differences in CHT when factors other than the temp are the same as I can.

Naively, I would expect top CHT to increase somewhat as ambient temps increase. I haven't gotten a good feel for other people's experiences with it from what I have read.

[SlowLane]

...full throttle readings...

Colin, do all of our L-Jet vehicles have WOT switches? I thought those were added in the late 70's. My AFR goes very rich when the WOT switch is engaged. I'm pretty sure the ECU is overriding the data it's getting from the AFM and temp sensors to give as much fuel as possible, so I don't see how you would glean any useful AFR adjustment information out of that.

I think I've figured out what gauge to get next: a fuel-injector duty-cycle indicator.

[Amskeptic]

...full throttle readings...

Colin, do all of our L-Jet vehicles have WOT switches? I thought those were added in the late 70's. My AFR goes very rich when the WOT switch is engaged. I'm pretty sure the ECU is overriding the data it's getting from the AFM and temp sensors to give as much fuel as possible, so I don't see how you would glean any useful AFR adjustment information out of that.

I think I've figured out what gauge to get next: a fuel-injector duty-cycle indicator.

External switch was deleted in '77. Now it is wiper reaching end of its travel.
There is a logic gate in my request, a sort of IF this > THEN that, whereas "that" does not have any causal bearing on "this".

IF wide open is lean > THEN you must advance wiper rich, whereas IF wide open is rich, you can choose to use the wiper or the spring to lean out any partial throttle richness. The ECU does not override anything, it merely adds or subtracts to the pulse width as determined by program + ts 1 and 2 + afm. If you are set to 14.1 at partial high speed throttle and floor it, the ratio may drop .5 let's say. If you are 12.7 at partial high speed throttle and floor it, it may drop .5 again. I hate to say it, but that damn spring has plagued my readings enough to tell me that they are all related at all positions.

The only counter-intuitive information that I'd like to add, is that you should not be alarmed with leaning above 70 mph. The engine loses enough volumetric efficiency above 4,200 rpm, that it is not indicative of blow-torch valve killing leanness to watch your mixture move from 12.5 to 13. In my world, however, I am not going to allow the mixture, at full throttle, to go above 13-13.5 at full throttle ever.

Please, SlowLane, if you have experimental questions and the means to test them, please offer your results here.
Colin

[SlowLane]

if you have experimental questions and the means to test them, please offer your results here.

Gladly. Hope you don't think that I'm simply sniping from the sidelines.

My Canadian-spec '81, with a working full-throttle enrichment switch, drops to an AFR of about 10.0 under pretty much any condition when I floor it, which is why made the assumption that the ECU was overriding all or most of the other sensor data.

At speed, AFR tends to be about 12.0 - 12.5 with part-throttle input. It can easily fluctuate by about as much as 1.0 with just minor throttle adjustments.

I have an analog AFR gauge, so perhaps the observing experience is different than with a digital gauge. With analog, I can't get precise readings to 0.1 precision, but I can watch the effects of various actions. With a digital gauge, you can get precise measurements, but they may be either averaged over a sample period, or simpy sampled at a particuar instant, at which point they are displayed to the user at a rate of change that is reasonable for the human mind to process.

[drober23]

As far as gauges are concerned, I'm pretty sure mine (the PLX) is not doing any averaging. The number changes several times per second though, so the time between readings is small.

It also has the ability to make a "strip chart" graphing out the last 30 seconds of data. This is nice if you are looking for trends or changes. It can also output a voltage reading ( 0 - 5v), but I don't have anything other than a multi-meter to hook that up to.

I think my current plan is to keep my AFM settings as they are for now at least. I don't drive over 70 much, and only rarely pop over 75. So the rising full throttle AFMs don't scare me too much. I'm open to opinions on this though.

Before I started, my idle AFM readings were in around 13.6, and cruising at 70 was 13.8 (and sometimes up to 14). My spark plugs were the white you see in the top picture in my first post of the thread. When I floored it, it rarely went under 13 when I was at speed. During the course of adjustment, I discovered my mixture screw had been turned all the way in ("a gleefully irresponsible" way of masking a past vacuum leak I fear).

Now the mixture screw is 4.0 turns out from fully in. The static was moved about 1.5 teeth toward rich, and the spring was turned 6 teeth toward lean. Warm idle is now in the mid 14s, and cruising at 70 is right around 13.0. The spark plugs are as you see in the second picture of the first post in the thread. Also, the CHTs are a bit lower than they were previously.

Now I can relax on this a bit and dive into the million other things the bus needs. Looking into the rear brakes is next on the list!