[T3] gas mileage, voltage, etc.

[Daniel Nohejl]

The sticker fell off, so I’m not sure how old it is, but it’s less than 5 years. The older battery (4/12 on the sticker) is in our other car (battery reads 12.53V despite the car not having been driven in 2 months), so the one in question is probably between 2 and 4 years old. I’ll have to look for a receipt. Based on what I’ve read, I suspect that the battery could be at fault regardless of age. I guess I’ll see what the charger tells us. 

I know there’s a correlation between low voltage and running rich on D-Jet, but I can’t quite reason out how a bad battery can contribute to rich running and bad gas mileage if the charging voltage is otherwise correct since the 14.1V I found at the regulator should be what’s reaching the brain and other components. Of course I neglected to check power to the brain during all of this as I’ve been trying to focus on components not related to FI. I should probably check power into and out of the FI power relay as well as at the 2 wires to the brain in the engine compartment just to be sure.


Daniel


> On Jun 20, 2017, at 7:18 PM, Sean Bartnik <sjbartnik at mac.com> wrote:
> 
> How old is the battery?
> If it's 5-7 years old then it's on borrowed time. 
> 
> I think charging it overnight on the charger will tell you what you need to know. If after that it still isn't fully charged then it's likely on the way out. 
> 
> -Sean
> 
>> On Jun 20, 2017, at 7:03 PM, Daniel Nohejl <d.nohejl at gmail.com> wrote:
>> 
>> The in-town gas mileage on my ’69 FI Squareback has taken a nosedive recently. Normally at this time of year in NYC, I get around 20mpg when doing my normal commuting and putting around. It’s 18mpg in the winter. My last few tanks have averaged a miserable 16.25mpg!!
>> 
>> Over the past two weeks, I've checked all the obvious things I could remember to check: tire pressure is fine, no gas leaks, all temp sensors ohm out okay warm and cold, nothing is disconnected in the engine bay, dwell fine, timing fine, spark plug wires fine, idle speed mostly fine but more on that in a moment. Generator brushes are fine, voltage at D+ on the generator is about 15V at idle and the same at D+ on the regulator. B+ on the regulator is between 13.8V and 14.1V at idle and it measures the same at the battery. 
>> 
>> The only things I’ve found are:
>> 
>> —occasionally at idle, the speed will drop as the motor bogs down a bit and gets rough, as though a cylinder was half dropping out or something. I had the DVOM hooked up to the battery and when this happens, the system voltage tends to drop after a few seconds, often as low as 12.8V though I can’t tell if this is a cause or an effect. 
>> 
>> —despite the idle voltage mostly being at 13.8V and cruising voltage anywhere from 13.8 to 14.2, my battery won’t fully charge. In the last week or so, I’ve found it to be anywhere from 12.4V to 12.3V to 12.55V in the morning before work or in the afternoon post-work, but it never gets back to 12.6. This evening, it dropped down to 12.4V only 45 minutes after driving around for more than an hour. So it’ll charge up a bit, but not fully.
>> 
>> So is my battery giving up? I brought it inside to trickle charge it and see if it’ll come all the way up, but I’m not optimistic. I suppose I could also test total generator output, but I was satisfied by the 15V at D+ and the correct charging voltage at cruise and most of the time above idle. Should I not be? Could a bum battery even with an adequate charging system contribute to very poor gas mileage? Am I chasing ghosts here or perhaps merging some unrelated issues?
>> 
>> 
>> 
>> 
>> 
>> 
>> 
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