On a 98 Legacy, I get a code telling me cylinders 3 and 4 are misfiring, car runs fine, what and where is the sensor detecting these misfires, just curious

Perhaps you need new spark plugs and plug wires, which trips the sensor.

By taking the second derivative of the crank angle sensor input, the ECU can calculate the angular acceleration of the crankshaft. If there is a misfire the piston won't be pushed down with enough force, and the crank won't accelerate like it should.

By taking the second derivative of the crank angle sensor input, the ECU can calculate the angular acceleration of the crankshaft. If there is a misfire the piston won't be pushed down with enough force, and the crank won't accelerate like it should.

nice!  that's awesome.

 

as mentioned misfires on that particular engine are usually due to needing plugs and wires.

cheap aftermarket wires and non-OEM plugs don't do well in those engines either....

 

though you don't specify which engine yo'ure talking about, but that is true for them all in 1998. 

 

if it's a 2.5 liter engine then oil leaking into the spark plug tubes can also cause this issue.

that can't happen on a 2.2 though.

How long have you been getting these codes? Have you cleared them and both came back? Any other codes besides those?

 

Is it an auto or manual trans?

I got these codes on my 98 Outback with the 2.5 in it 5 years ago.  They just poped up after a 35 mile drive and while I was sitting at a stop light.  The engine stumbled a little when it happened but then it ran fine.  I checked the codes and cleared them and they haven't come back in the 5 years that I have been driving the car.  The plugs, wires, and coil had less than 20,000 miles on them when it happened. 

 

Sometimes things just happen.

presslab, excellent, I wouldn't had ever figured out how it knew what cylinders were the culprits, I didn't figure that the pin on the ECU sending the pulse to fire could sense anything because the igniter would act as a buffer and voltages and resistances would pretty much all be the same (igniter has been replaced so I know it's good). All plugs are firing and if I pull any one of the four off the coil the engine nearly stalls, but my idle now is not as smooth as it was and there is a slight "shake" to the vehicle at idle (one or two cylinders low compression or something). The plug wires are brand new Denso and my son put new Pulstar plugs in, the engine is the DOHC 2.5 and the tranny is automatic.. Maybe possible lazy injector?  The codes 303 and 304 have been for months now, inspection is due by Nov 1st. Anyone know what ECU pin goes to the CEL? The codes stay off for the first fifteen miles after I clear them.

The engine should not be nearly stalling just from removing one plug wire.

Pulstar plugs are probably the problem.

Certain spark plugs do not play well with wasted spark ignition systems.

Switch back to regular NGK V-Power copper and the misfires will probably go away.

 

Removing the bulb or the wire for the CEL bulb will probably get you an automatic fail. The bulb must work to pass emissions testing in VA, I assume that is true for other states as well.

Fairtax, I take it in today for inspection, you're right about removing the bulb as they look for it when turning on ignition, beside they do readings from the diagnostic plug anyway, I get about fifteen miles before the CEL goes on after a code clear so they shouldn't detect this minor fault, in that it passes and gives me some more time I just don't want to look at a CEL staring at me. The engine won't stall when removing one plug wires but idles so rough and poor that it does confirm the plug wire just pulled was a good firing cylinder