How are misfires detected in OBD-II ?

[Stu]

Had a crazy idea today: adding OBD-II to an old vehicle. I know that a OBD-I panther was converted using the newer harness, but what about for a carbureted vehicle? I am a senior in computer engineering so I technically could do something like this if I thought it out well enough, but my main question is this: how does the OBD-II computer determine misfires? Does it monitor spark? Monitor the opening of the injectors?

[texasvic]

It watches the CPS.

[Grand Marquis GT]

A OBD-II Carb car?

Now i've heard it all

[Stu]

It watches the CPS.

Crank or cam?

What does it do with it? Coordinate data between where the crank/cam should be and where spark is?

[85MercPark]

Crack, crack I say!!!

[texasvic]

Crankshaft position sensor. The OBD-II looks for a deacceleration.

Here's a quick link

Linky...

Another feature of OBD II is its measurement of engine misfire. For this the system uses a crankshaft-position sensor, which can observe minute fluctuations in crankshaft acceleration and rotational speed that occur with a misfire. The problem here comes in when shifting or driving on rough roads: these and other forms of driveline shock also influence crankshaft fluctuations.

So some automakers use such devices as accelerometers, the ABS system, or an internal engine algorithm to determine if the vehicle is riding on a rough road. If it is, the system can unlock the torque-converter clutch to dampen some of the shock so it won't be transferred to the engine. Now here's the really bad part: since it is impossible to do this clutch-disengagement on manual transmission cars, some of those models with manual are no longer produced due to their engine-misfire false alarm problems.

Also, detecting misfire is difficult in some engines at high rpm, where the ignition pulse rate is almost submerged by the engine's inertia, making it hard to decipher signals from the sensors. Plus, inline 5-cylinder engines, V-10s, and certain V-6s can create erratic crankshaft-speed readings throughout their speed range. Inline-6s can be strange also, so the signals from a crankshaft-position sensor need to be carefully filtered and processed to be dependable.

Some systems are really high-tech. They measure misfire by measuring the electrical values of the ionized gas between the spark plug electrodes. Needless to say, this system is expensive.

[85crownHPP]

[Stu]

I say the same thing about your rediculous LED's. At least an OBD-II system would be of some use.

Anyways--I would have thought the computer would be monitoring spark, not the cam. Sounds pretty complicationed; maybe something for me to tackle after a few years in industry.

[DuceAnAHalf]

what would you use the OBD system for on a carburated engine?

[Stu]

what would you use the OBD system for on a carburated engine?

No idea--I was just thinking out loud.

[DuceAnAHalf]

i would not think that there would be much for it to do. Maybe monitor everything but it could not really change anything.

[Southern_Pride]

I don't exactly know why youd want to do but you could. It be one of a kind that way. The only one in the world.

[phayzer5]

-10 points for non factory-correct computerized engine controls.

[boXman]

[85crownHPP]

I say the same thing about your rediculous LED's. At least an OBD-II system would be of some use.

at least my LEDs are pheasable, and doable, and functional.