Quote:
Originally Posted by Nerys
dcb for the other guys solution. is it pegging fuel trim because of the vac leak or is the vac leak not letting it peg enough fuel trim? IE we don't WANT to use more fuel :-) (just clarifying)
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The ethanol needs a different air fuel ratio to properly work. Even a the amount you get from running E10 changes the desired ratio from 14.7:1 to something like 14.1:1 I am just guessing on the numbers but that seems close to me. The OBD1 computers only had a limited range they could deviate from the fuel table so they seem to be adjusting to the limit and that is where E10 is more or less happy with. It is close enough that there will be no extra room if there is the slightest problem with the car. So a very small vacuum leak would normally make it run slightly richer to compensate and the car would run just fine at 14.7:1 with normal gas. But with the table already at it's limit there is no more room left to compensate and you get terrible mileage from running lean. On light throttle highway cruising you can get away with lean running. On acceleration or medium throttle that will get you knock and that is really bad for mileage and power.
Newer computers have more ability to self tune so should not be affected as much. But a OBD1 system is really not much different than a carb system when it comes down to it other than the 10% or whatever feedback adjustment it can make on the fly from the O2 sensor depending on the car's computer setup.