Where did all of my oil life go??

[Benjamin Simon]

I have some engine computer replaced today (ECM?). I just happened to looked at my oil life, it was at 58% this morning. I got my car back this afternoon and the life was down to 29%. WTF? I could see if it said 0% or 100% but 29%????

No, it did not appear have been driving 1000s of miles this afternoon.
Yes, I am positively certain it said 58% this morning and now say 29%.

It is nothing serious, but I am curious. Should I replace it according to this new number or calculate it based on the old?

[dkozloski]

I'd just go with what it says now. That's the conservative approach.

[Caroutisine]

That doesn't make sense. Maybe something was wrong with the calculation the old computer had. I agree that going with the new reading is a good idea.

[Benjamin Simon]

How are the calculations made? Do the ECM do the oil life calculations? Just really weird.

[pjbizjak]

I would say to monitor your oil manually by checking the dip stick. If the oil is still clear on the dip stick you don't need a change, however if it is dark then I would get it changed and reset the oil life index.

[WillySTS]

It counts engine RPM and measures ambient temps and engine temps. Maybe you did something to it that you don't want to tell us about.

[C&C]

If I assume that the ECM was new then I'm baffled, but if it was a used ECM, well maybe that's the oil life the previous vehicle had on it. And to an earlier post about the color/clarity of the oil, that is not a criteria to evaluate the condition of your oil.

[CadillacSTS2003]

did your car overheat?

[Benjamin Simon]

Originally Posted by WillySTS

Maybe you did something to it that you don't want to tell us about.

No Sir.

[Benjamin Simon]

Originally Posted by C&C

If I assume that the ECM was new then I'm baffled, but if it was a used ECM, well maybe that's the oil life the previous vehicle had on it.

Very good idea! I am sure GM does not trash the bad computers, and I just get a refurbished one.

Damn it, my car is under warranty. I don't want refurbished parts? How can I find out if this is in fact a refurbed ECM?

No, it did not get hot to my knowledge. Maybe the raced me car when it was in the shop.

[Superjim]

Originally Posted by Benjamin Simon

Maybe the raced me car when it was in the shop.

That thought crossed my mind also.

Jim from Texas

[Aurora5000]

The GM engine oil life monitor counts engine revolutions and accumulates the number for the basis of the oil life calculation. It adds deterioration factors for operating temperature, start up temperature, soak times, ambient, coolant temperature, etc... There are a LOT of factors that "adjust" or affect the slope of the deterioration but the fundamental deterioration is traced back to the ZDP depletion that is inescapable with engine revolutions. The specific rate of ZDP depletion is readily measurable for any given engine so that is the fundamental item that is first calibrated for the oil life algorithm to tailor it specifically to that engine.

The other things that determine oil life such a acid build up, oxidation, petane insuluables such as silicon from dust/dirt, carbon or soot build up from the EGR in blowby, water contamination, fuel contamination, etc.... are all modeled by the multipliers or deterioration factors that "adjust" the immediate slope of the line defined by the engine revolution counter as those items can be modeled in other ways and accounted for in the immediate slope of the ZDP depletion line.

The algorithm was developed over the course of many years by several lubrication experts at GM Fuels and Lubes, spearheaded by Doctor Shirley Schwartz who holds the patents (with GM) for the algorithm and the oil life montitor. When the idea of the oil life monitor first progressed from the theoretical/lab stage to real world testing/development/validation. There were fleets of cars operated under all conditions that deteriorate the oil life for any and every reason and , thru oil sampling and detailed analysis of the oil condition, the algorithm was developed, fine tuned and validated to be the most accurate way invented yet to recommend an oil change interval by.

The oil life monitor is so effective because: it is customized for that specific vehicle/engine, it takes everything into account that deteriorates the oil, it is ALWAYS working so as to take into account THAT INDIVIDUALS driving schedule, and it tailors the oil change to that schedule and predicts, on an ongoing basis, the oil life remaining so that that specific individual can plan an oil change accordingly. No other system can do this that effectively.