How About This One, AFR/Fuel Experts .. ?

[Norm K]

Since installing my new K27 7200, RarlyL8 headers, Borla muffler, Leask WUR and AFR gauge I've been just short of obsessed with getting my AFRs set.

Today though, I ran into something I've not seen discussed (or even suggested - perhaps because it sounds so silly) on this or any other forum.

When I ran my car ('80 930) last Saturday and Sunday the warm idle AFR fluctuated between 13.4 and 13.6. Was out of town until today when I checked them again. Same readings; 13.4-13.6.

Now my strange AFR issue: My seven year old battery was shot, so I bought a new one this afternoon (after the aforementioned AFR readings), brought it home and installed it. Started the car, warmed it up and the AFRs were running 14.4 to 14.6, a full point higher. I never opened the engine lid, just installed the new battery. I'm completely flummoxed as to how a battery can affect AFRs. It can't, can it? Yet it did. From a very consistent mid-thirteens to mid-fourteens in about 45 minutes - without ever opening the bonnet.

I wondered if perhaps the power interruption had some affect on the AFR gauge, but I'm fairly certain the power to it is completely cut off every time the key is turned off. In addition, not only did the AFR readings indicate a more lean condition, the idle speed dropped as well (supporting the theory that the car was indeed actually running leaner), by a little over a hundred RPM. Richened the mixture a little at a time and once the AFRs were back in the mid-thirteens the idle returned right back to where it had been.

Anyway, sorry for the length of the post, but I'm sure curious about this. Total coincidence ... or not?

[s5uewf]

just a wild guess, but perhaps with now higher voltage you are getting a hotter spark at your spark plugs and burning more of the fuel??

[Norm K]

I wondered the same thing, sort of; wondered if somehow the dead cells in the battery were limiting the entire electrical system and affecting the WUR's ability to function properly.

[mark houghton]

Your AFR electronics/computer has within it a voltage regulator...but of course. If your input voltage falls above or below whatever the circuitry is able to 'regulate', you may see some readings off-calibration.

I say that because I was fighting the opposite when my alternator voltage regulator took a dump. It was fine running down the road, but if I shut her down for a few minutes (such as to stop for some cigs or booze, or maybe to pick up a loaf of bread....you know, stuff to live on), upon starting back up my AFR's would be fine until I hit around 2500 rpms...then they would take 1-1.5 point dive. Scratched my head 'till it was bleeding on why the hell my AFR's were doing the funky chicken initially. The tell-tale sign that something was amiss was the smell of rotten eggs (i.e., battery boiling) whenever this weird AFR thing would occur. So, in my typical Fred Flinstone fasion, I hooked up a voltmeter and started monitoring things. Lo and behold. when the 'heat soaked' voltage regulator started acting up, so did my AFR's. Process of deduction, my dear Wattson. I was seeing upwards of 18 charging volts and it was reeking havoc with my AFR readings...with no signs of changed running conditions such as idle, etc. New voltage regulator = fixed.

The opposite may be true in your case. Your toasted battery was not putting out a high enough voltage, thus focking up your AFR readings.

REVISED
Well I re-read your post...hmmm, you had symptoms (idle speed dropping). You were undoubtedly seeing low battery voltage that may affect things like the strength of the coil charge and thus spark to the plugs, maybe stuff like your lambda frequency valve and computer, fuel pump speed, stuff like that. Voltage changes can do weird things. Sounds like you got her adjusted back to normal with the new battery. I bet the symptoms don't return, whatever their ultimate cause.

[mooney265]

14.6 is too high for warm idle. IMHO EDIT: Oh, now i see that you did adjust the mixture back to the mid-13's... missed that on the first reading...

Sent from my MB855 using Tapatalk 2

[JFairman]

The fuel pumps may be running faster with the new battery and making higher system and control pressure if the old battery was tired and down around 11.5 volts or less and the alternator charge while running wasn't keeping the cars regular running voltage up around 13.5 volts where it should be and probably is now with the new battery.

With the new battery and higher system voltage running the pumps faster it might lean out CIS when no adjustments are done to compensate.
Sounds like you adjusted the idle CO adjustment to where it should be now and got it running good again. Hopefully it will stay where it is now so you can enjoy it.

Just a guess and to check that possiblity you need a voltage gauge to see what the pumps were getting before and after the battery change and a CIS pressure gauge to check system and control pressures to see if they changed.

[heliolps2]

+1 on the fuel pump and the correct voltage to the WUR will also raise the warm pressure, leaning out the AFR's