weak battery - fuel consumption

[jamesmilsome 0]

I read on the SNS website that a weak battery causes the ecu to dump 25% more fuel? My battery is not the best so is it worth getting a new one? It was flat so I charged it and it has been fine but does seem to struggle.

 

cheers

[dr_mat 0]

Interesting theory. The only way it can be true is if the battery is so weak that the ECU goes into limp home mode. But then your alternator would have to be non-functional aswell..

It may be true, I don't know for sure, but it sounds a little odd to me.

Weak battery is irrelevant once the engine is turning..

[Kevin Bacon 5]

The ECU would need to see 10V or less to go into panic mode, but the other systems probably wouldn't even run if that was the running voltage.

[marcus 0]

I would say this is an odd theory too, BUT i replaced my battery last week and have to admit getting a slightly better mpg reading.....maybe this is one possible reason??? or coincidence?? who knows, who cares :-).

[Henny 0]

IIRC, the ECU only needs to see 10V for a very small amount of time to switch into limp mode, after which it's in Limp mode until you restart the system (ooh, it's like rebooting a car! :lol: )

 

If you have a problem with your battery, IMHO, it's entirely possible that it's dropping enough voltage for the alternator to struggle when first started and idling to drop the voltage to under 10v for a second or so, which will cause limp mode to be instated... :|

[jamesmilsome 0]

I may just replace it to be on the safe side as I know its poor anyway. cheers for that lads. Would performance not be down in limp mode as it is still running well (just crap mpg - may be my driving style though).

[Henny 0]

it'll be running richer than normal, so there will be a slight performance downturn, but if the rest of the system's OK, it'll still run reasonably well... 8)

[bcstudent 0]

The alternator on my 1.8 16v is very weak. Once the battery starts to get a little tired it affects the way the car runs drastically....for about a day until it's too flat to start the car. I really should sort that.

[CaiosG60 PWR 0]

Here's a quote from a page on engine managment that G-Man give me the link too..................tooo edatacate mooo! :scratch:

 

"Battery voltage

 

If the voltage of the vehicles battery varies then it is likely that the time taken to open the injectors will vary. Since the EMS times the overall injector pulse if the injector takes longer to open then the time it remains open will be that much shorter and therefore the fuel introduced to the engine will be correspondingly less. Some EMSs have a correction applied to the base map of injector times for variations in voltage; the corrections are usually small but during shorter injector times (idle and cruise) they can be very significant to the efficient running of the engine."

 

Think its saying that if the voltage is low and the injectors take longer to open then the ECU will increase the fueling to compensate!.

[Kevin Bacon 5]

Nope, it's saying that the injector pulses are a fixed timeframe and if the injector pintle takes longer to open than usual, hard luck, you get less fuel!

 

However, it also said some ECUs have base level compensation but I doubt any VW ones do.

[CaiosG60 PWR 0]

Isn’t that what I said? except I didn’t say some.

 

"Think its saying that if the voltage is low and the injectors take longer to open BUT then SOME ECU's will increase the fueling to compensate."

 

That better! :-) :-)