wcsinx

..before throwing codes?

I heard and felt a misfire last weekend as I was leaving the gym. Looking under the hood, I decided that my plug wires were looking pretty bad and were probably leaking. So I replaced those and the misfire disappears.

Also for quite some time now, I've noticed a fuel smell at idle and my fuel economy hasn't been as good as it once was. So I figured at 110k miles, I might as well replace the O2 sensors and see if there's any improvement. And like magic ... fuel smell goes away, idle is smoother, and economy seems to have improved (need to track it over a few tanks to confirm).

So in both cases these were things that my limited powers of human perception were able to pick up yet ... no codes .. not a single one. Why? Why allow things to go so far out of range of a nominal run condition when the sensors and programming are more than capable of detecting these things? What is the motivation on setting the tolerances so wide on some of these conditions?

Gordy M

The engine has to live and run smoothly from Desert Valley to Pikes Peak so the Parameters for your Long Term fuel Trims (LTFT) are +/- 25 without throwing a code. While this is an over-simplification, as you engine runs every so many cycles it updates the information and makes adjustment. In your case the heart of these changes are the Oxygen sensors. When GM was developing the code for the c5--during 1992-1995 most areas had good gas with ethanol just beginning to appear on the market. The original O2 sensors were not made for the amount of ethanol in today's fuel. Around 2008 GM and others manufacturers updated the requirements O2 sensors and they were made more resistant to the effects of ethanol on them. The manual also says to replace them before 100k I believe. HTH

K-Spaz

Probably the flip side would be, if these cars threw codes every time the O2s or plugs got a little used people would throw a fit about that too. Tend to agree with the above too, when these cars were made pure gas was common. Now, E10/E15 is about the norm and that plays hell with the tuning and performance. I'm getting to see that first hand right now too.

wcsinx

Yeah, I get that. But at the same time ... when is a misfire ever considered normal? Maybe what I really want is to be able to put these things in engineer/developer mode where I can actually see the deviation from ideal instead of an idiot light or code popping up at some arbitrary limit.

K-Spaz

Using the misfire as an example;

The PCM watches the timing of the crank sensor pulses and determines an anomaly from the spacing between. It 'assumes' any anomaly of X magnitude is a misfire and records them accordingly. If you want to know when they happen on a car that isn't predisposed to it, you have to log it or use a scan tool to find them. They'll be there, pretty much on any vehicle from time to time. So, they're not considered abnormal enough to warrant lighting the CESL.

I do get what you're saying, but if you think of it like that and occasionally check on such things, you'd see they DO tell you, they just don't make a big deal of it. I mean, its' not like your car won't run. IT isn't even something most people would notice. And it probably has no effect on emissions or it would throw a code.

That and, considering how at the time of the C5 they were just finally getting a PCM that would 'work'. It's a miracle it does all the things it does and monitors everything it monitors.

In the end, you got it before it woulnd't 'go', so I'd call that a win. Imagine having a Dodge. SES usually means you're calling for a rollback.