[dig_dug_mx6]

I saw this in a post in the lounge section and i was wondering what this is and what problems there are putting on performance parts???

 

[Mike 94PGT]

I saw this in a post in the lounge section and i was wondering what this is and what problems there are putting on performance parts???

OBD-II (On Board Diagnostics II) is a description of the engine control system. The first computer controlled cars had what was loosely termed "OBD-I" or the first generation of on-baord diagnostics. In this system, a sensor failure might only be caught once the sensor had completely died. For example, a TPS code might get set only if the sensor reads below 0.1V or over 4.9V.

GM had some pretty good engine control logic in the late 80s in their P4 and P5/P6 systems. The Fed in the US looked at the way GM (Delco really) did things and modelled a new standard of engine controls - OBD-II (next generation) on their methods, as well as adding a host of requirements and added systems to the checklist, including warm-up cats, additional O2 sensors etc.

OBD-II PCMs keep a very close watch on the engine, much closer than OBD-I could have hoped to. For instance, OBD-II can tell if the engine misfires (generally done by watching for minute changes in crankshaft speed) and can tell you which cylinder actually misfired. The system monitors not only for sensor failure, but also for sensor degradation. Hell, it will even tell you if the gas-cap is loose.

OBD-II is not generally liked in the aftermarket because, since it keeps such close tabs on engine performance and emissions, making performance changes that don't "upset" OBD-II might be hard. Want to put a cam in your car that results in a slightly lumpy idle? Forget it...OBD-II will mostly likely toss a code due to the rough idle. It's things like that that people shy away from.

Most bolt on mods (intake, exhaust, TB etc) won't affect the system and it will be quite happy. I even know several guys with turbo installations on 1996 PGTs who report no codes.

 

[Greg S]

Mike,
Doesn't OBD-II permantly record any error codes sot hat htey can be pulled up when you have an emissions test?

 

[Mike 94PGT]

Mike,
Doesn't OBD-II permantly record any error codes sot hat htey can be pulled up when you have an emissions test?

That's my understanding. The NVM (non-volatile memory) in the OBD-I PCM is simply a battery-backed RAM in the processor itself, kept alive by an always-hot battery feed. When the battery is disconnected, the RAM loses the stored codes. Usually, the PCM will do a checksum or other integrity check of that memory each boot-up and if the integrity is in error, it will clear and reset that memory itself. This is what a battery reset usually does.

In OBD-II, I believe they use a flash memory to store codes. Even when the battery is disconnected, the code(s) remain. There may be constructional differences among makers, but that was my understanding of the OBD-II mandate: codes should not be eraseable with a simple battery disconnect.

 

[Streezz]

damn, that was one good reply!

 

[dig_dug_mx6]

Great info. I really appreciate it. I figured it was some computer crap.

 

[BillyGnoses]

they ecu stores all the information for every reading it gets when the code appears(really handy for diagnosing) but it gets errased when you erase the code,some codes take longer to regester, if they are erratic and don't set the light you can also read them with a scanner in pending codes (with the snap on scanner anyway) i'm a little waisted so i'm not sure what the question was anymore, but i hope this helps

 

[alphaomega9186]

what were to happen if you were to install something that OBDII doesnt like any ways? what are the side effects of lets say installing the Cams that give a jumpy idle. and is there any way to swap the computer if its even worth it?