Nine2Eight

The other day I accidentally pumped 8 gallons of 87 octane into my nearly empty S4 tank (I always use 93). Having done so, I decided to see how the car would react. At first I noticed nothing, but upon startup in the morning the car ran like crap on the way to work. However, upon the next startup and thereafter, the car ran like normal. Question - did the LH have to "learn" and
adjust to the lower octane? It almost had that same feeling like when the battery has been disconnected and the LH must reestablish itself.

Joe Celeste
87 S4 Diamante Blau
Tampa Bay, FL

WH928

I would suspect the computer would trim as soon as it recognizes this condition. there is no reason to my knowlege it would take any longer than that. Air temp, engine temp ,humidity will efect the preignition making you think the computer has taken corective action.The computers are pretty primative in comparission to the ones of today.

AO

The car does have a learn mode but I believe that's only after a battery disconnect and I think (dangerous) it's mostly for it to learn the proper amount of fueling to reach stoich or 14.7 AFR. If you were to put in a lower octane petrol, I don't think this would affect your air/fuel ratio, but it would affect how easily it pings/knocks. Since you have an 87, it has knock sensors and if they are working they should detect if your car pings with the 87 octane. Once it detects this situation it will retard the timing up to 9* in 3* increments until the pinging goes away. The good thing is that this protects your engine... the bad is that your car will be considerably down on power.

I wouldn't drive it like you stole it until you run this tank down and put some higher octane gas in it, but I wouldn't worry too much about it either.

mark kibort

ive tested the battery disconnect at one of my dyno runs. there was no difference in power or mixture.

I think the way the system works, is that timing is pulled out if there is a ping, not opimized for the type of gas.

mk

Ron_H

I've run 87 octane, and even 85 octane when I'm away from California, from the day I bought the car. It runs beautifully. I have plenty of power from rest to redline. I disconnect the battery if the car is going to sit for more than four hours and it starts up and runs perfectly every time. It has been doing this for about 70,000 miles. FWIW.

John Speake

Hello Joe
A start up rough running would have nothing to do with fuel octane. More likely some dampness in the igntion system ?

Assuming your knock control system is working OK (no faulty knock sensors) then with a low octane fuel, the only time you would notice any change might be when using larger amounts of throttle and rpm, when any detonation would result in the 6 deg igntion retard.

Nine2Eight

Ron,

Maybe an 86.5 is different from my 87. I always have to suffer the learning delay after reconnecting the battery if it was disconnected for more than a couple of hours.

JPC

Erling G-P

Guys, I've often wondered about cars with knock sensors that automatically retard the ignintion if you run too-low octane fuel. What happens once you return to correct octane fuel ? Does the ignition return to normal by itself, or do you have to do some kind of reset manually; perhaps at a workshop ?

Cheers,
Erling

John Speake

Hello Erling,
The system is fully automatic. When knock is detected, the EZK knock control system can determine which cylinder(s) are knocking. The igntion is then retarded on just those cylinders in 3degree steps until the knock ceases.

The system continues to monitor knocks, and if no further knocks are detected, the ignition is slowly advanced back to the stock settings.

So no intervention is required.

The only issue that does require some action is that sometimes spurious knock sensor fault codes are recorded in the EZK ECU. Then the ingtion is retarded by 6 degrees at higher engine loads/rpm, in order to prottect the engine. A battery reset will clear these spurious codes, or use a diagnostic tester to identify and clear the codes.

AO

It will return to the stock timing in seconds as long as there is no knock. I believe the stock values are for it to begin to return to stock timing 10 seconds after a knock event. It returns by adding back 1 degree of timing every second. So if you were a full 9* retarded, once the knock is gone it would wait 10 seconds and then add 1* every second after that, so a total of about 19 seconds.

So the quick answer is... it does it by itself.

dr bob

So for just moping around in congested SoCal traffic, I could save almost $4 a tankfull by running regular instead of premium. The car would protect itself.

Might be OK for winter driving here, but in summer there isn't enough ignition correction available to kill the pinging with low-quality gas. For the amount of gas I use each year in the 928, savings would be about $80 total. Maybe a decent meal for two. In the big picture here anyway, the difference in cost is down in the noise someplace.

Ron_H

Occasionally, it coughs and stutters for about.........1/3 of a second.....and then purrs along until I drop the hammer from the next light after it has warmed enough. Then it just obediently slams the machine across the intersection and slides sideways as it hits second gear. I like my 928. I just don't like the electrical gremlin that lurks therein.

John Speake

It may be hesitating when warming up before the O2 loop takes over. In that warm up phase mixture is critically dependant on MAF calibration and idle CO setting.

Have you set idle CO to 1% as per the WS manual ?

Ron_H

I haven't any idea. Devek was the last one to touch it and it has run well ever since. When I say it coughs and sputters, I mean simply that the idle wavers for about a half a second and then stabilizes. Hardly noticeable. I keep hearing how it hurts to keep disconnecting the battery, but I don't seem to notice anything adverse so far....50,000 miles of this gremlin. Lately though, I notice that I can let it sit overnight connected and the gremlin is getting weaker 'cause the it won't kill the battery like it once did.

John Speake

Hello Ron
On the 85/86 cars there is no "adaptation" and stored parameters in the LH or EZF ECUs that are lost when you do a battery disconnect.

You may find the hiccup is ambient temperature dependant.

It would be worth getting the CO set up at some point. If the CO can't be adjusted up to 1% (before the cats) then the MAF is aged and requires replacement.