Help Vibe Gods

[litefoot]

I have an AWD Vibe had my PCM and cat replaced. Before the replacement, I could get about 23 mpg city. After the replace I started at 19 mpg city, it raised to 21 mpg city. Then I had a error code reset, it fell back to 19 mpg city.500 miles later it jumped to 23 mpg city. I have a few questions; 1) Is the PCM in common terms, the car’s computer? 2) Does the vibe’s computer have a learning period, to adjust itself, like other GMs ? Would it be safe to say it is about 500 miles? 3) Would resetting an error code, reset all the data and restart the self adjustment process? 4) In the manual it states you should run 86 octane, I get better mpg with Mid grade 89 octane, Is the normal with the new PCM?Thanks

[joatmon]

the PCM, or powertrain control module, sometimes called the ECU or engine control unit is the car's computer, it's behind and below the glove compartmentThese do have a learning mode. Don't know if the learning period is 500 miles, or maybe some amount of running time. It is possible that resetting the error code could restart the learning process, or perhaps the learning process is constantly on-going. I'd think it would always be self adjusting, maybe resetting it causes it to start from default, so changes may be more noticeable.4. There is a lot of debate about the effect of different octane on power and gas mileage in the 1ZZ engine. Around me the lowest they sell is 87. seems to me that regardless of the learning of the PCM, or using 86 vs 89 octane, mileage of 20 - 23 is still lower than it ought to be. car is rated for what, 28/29 city?

[ragingfish]

Quote, originally posted by litefoot » 4) In the manual it states you should run 86 octane, I get better mpg with Mid grade 89 octane, Is the normal with the new PCM?Actually, the manual I have states a minimum of 87 octane.Are you putting in 86? If so, that's not helping...and it would explain why you get better mileage. If you're putting 87 in, there should not be a considerable difference in efficiency between the 87 and 89.Could a 1 digit difference in octane rating make that massive a difference? Who knows. But it's something to consider...

[kostby]

As I understand it in layman's terms, there's potentially 'more energy' in gasoline with higher octane rating. 'Racing Gas' as used in NASCAR and 'Aviation Gas' are something like 110 octane, as I recall.The key is whether or not YOUR engine computer is 'smart enough' to DETECT and MAKE USE OF the extra potential energy and translate that into A) better driveability=no pinging on hard acceleration and hills (not usually a problem with engines designed to run on 87 octane anyway), B) additional miles travelled per gallon used (the 'real world' result we're hoping for) , and/or C) more horsepower/torque as measured on a chassis dynomometer (the 'lab result' that engineeers and product testers look for).If you notice a distinct mpg difference between 87 and 89 octane, AND the mpg difference is significant enough to justify the (assumed) additional cost of the additional octane, then by all means USE IT!

[litefoot]

You're right it is 87. I made a typo, which is what I have been using. I have the AWD model, it is rated for 26 city and 31 highway. Has anyone ever reached 26 MPG city. I have been to the dealer many times. Basically I am being told, it is AWD, if I get 20 -23 that is normal. I was hoping for 24-25 city. But the egg smell is gone. So I was just going to settle for running 89octane and getting 20 – 23 city. Maybe in the warmer weather it would jump up to 25mpg city.

[ragingfish]

It is UNREALISTIC to ever expect to attain those EPA ratings. They are done in a lab, on a dynonometer, and do NOT mimic real life driving.If you get with 2-3 mpgs of those numbers, you're doing well in my book.

[Flip-Side]

Quote, originally posted by kostby »As I understand it in layman's terms, there's potentially 'more energy' in gasoline with higher octane rating. 'Racing Gas' as used in NASCAR and 'Aviation Gas' are something like 110 octane, as I recall.I use to think the same thing, but it is a myth. Octane is added to gasoline to prevent it from spontaneously igniting from compression rather than spark. The higher the compression of an engine, the higher the octane you need. Then again, you will notice that some cars made for 87 octane run smoother on premium gas because it ignites exactly when spark is given instead of knocking, but you will notice no difference in cars today. My old 1990 Oldsmobile with the 3.8 v6 would gain smoother running by using premium, but that was it. Octane has nothing to due with the potetial energy of gasoline, just its resistance to pre-detonation.http://auto.howstuffworks.com/question90.htm

[goodvibe]

Just to be clear. I assume that there are no mods. Did you get an error code that again had to be reset after the service was completed? If so, there's still more going on here.

[litefoot]

I had the PCM and the cat replaced, do to the rotten eggs smell.The error code was a loose gas cap, this happened only once after the service was done. There are no mods.

[remusrm]

if u think u will get 26 mpg, i think u need to drive ur car on the moon with less drag. sorry. if you get 20city and about 25 hightway you should b happy on a 4 wheel drive. my FWD auto does about 27 city and about 30 max highway. also, if you drive maybe 30 street and 55 highway you migh get about what the sticker says and not rev past 2200 rpm. also, the test in labs is not to have the car run, but how much carbon monoxide puts out on different levels of accelerations. ever wonder why a honda civic never gets good gas milage? if u go past 3.3k rpm on a civic will burn like a 3 litter engine.