Misfire Detection. What sensors?
 

Anyone know what sensors are involved in detecting cylinder misfires in the LNF. I always assumed it was the O2 sensors, I was under the impression that if they caught an extremely rich spot in the exhaust stream, that it was assumed that the cylinder must not have ignited, causing the surge of extreme rich in the exhaust. So the ECU sees this spike, and sees where it happens and calculates which cyl should have been exhausting at that time, and throws a misfire code on that cylinder.

I am wondering if i am wrong... Because recently my car has been misfiring all over the place under no-load (according to the ecu). It is perfectly fine under load.. but under deceleration, coasting, or idle... It flashes my check engine light at me, and throws codes for cylinders 1,2,3 and random cyl misfire. (never 0 or 4, always 1 2 3)

At first i noticed most of the time it would scream about misfires, i would see a dip in the readings of the downstream O2 sensor... (oh yeah i am using HP tuners to log and monitor and tune) it would dip almost off the gauge for a second, and then the engine light would start flashing.

BUT i never felt anything... on a 4cylinder at idle/coast/decel you should notice if 1 cylinder chokes... let alone 3/4 of your engine.... i never noticed a thing. was running and idling fine. Even the upstream O2 (wideband) kept a steady reading... Nothing else i can see indicates any sort of misfire, apart from the dip in the downstream O2.....

I do have a hahn catless DP... and i am using the spark plug anti-fowler to remove the O2 from the direct line of the exhaust stream.... I just thought that was the issue... the rear O2 was just acting up from the spacer... (i guess it is common for the spacer to only help for a while then start acting up)

But recently I did the stage one upgrade... and after i flashed the new tune, the misfires all stopped. for about a week... and now they are back again. But this time i dont even notice the dip in the downstream sensor anymore... it is just random misfires with no good reason.

So i am just wondering if anyone knows what could be going on here.... What else might simulate a misfire? shouldnt i feel it sputter or buck or surge or see something in the logs if the engine was actually misfiring? I know my friend had a car with a misfiring cylinder... and that thing would hardly even run with a single cylinder misfire.... the engine was jumping all over, like it was trying to break out of its mounts. I know you get the same reaction if you pull a spark plug wire on most 4cyl... so i really doubt it is a real misfire... but it still bothers me. hoped you guys had some ideas.

Welcome to my world :lol:

Same exact story as you described besides mine is p300,p301,and p302. I recently installed an aluminum flywheel at the same time the misfire codes started showing up. I've swapped coils and plugs around to see if the misfire follows the cylinders and got nowhere. The codes only pop up at idle and got to the point where everytime I came to a stop the check engine light was flashing.

You'll never guess what has fixed the problem so far??? :)

Are you getting misfires on all cylinders or just one ?

You don't have spark plug wires, You have coil packs that sit on top of them ..

It's either one of those coil packs, a shitty tune, a spark plug .

OR

You have a broken valve train. OR your intake has got a hole in it and you are taking in un metered air VIA the crank case vent connection .. OR you gotta bad MAF sensor or dirty one , Or map sensors doubt that one ....

First thing you need to do is figure out what cylinder is misfiring ..

If its all of them I wouldn't even know where to start, compression test ....

I've had an issue where the fuel pump wires frayed closest to the wiring harness and caused it to do weird stuff but never threw codes...


Figure out if its just one cylinder , move that coil pack that sits on top of the spark plug to another cylinder , keep track of them , see if the misfire goes along with the move to that cylinder.. If it does you gotta bad coil pack. If it doesn't you got deeper issues , You can try swapping plugs but 90% of the time that doesn't do shit..

Not to scare you off but I got P0300 P0301 P0304 before... I Changed spark plugs, and even changed the order of the coils to see if the misfire changed form cyl 1 to cyl 2....I finally did a cylinder pressure test...Cyl 1 25psi cyl 4 0.....2 cracked pistons luckily it was covered under warranty..... Good Luck

Alot of ppl that went to a light weight f/w get misfires at idle. Turn on the ac and i bet they go away. Or increase the idle rpms

If I'm not mistaken the crankshaft position sensor plays a role in determining a misfire or not If it occurs infrequently at high rpm it can be a mis-detect due to accuracy at higher rpm but in your case id say that frequent is well beyond that.

What fixed the problem???? do tell... we are all in suspense!

thought i had mentioned this.... but it only happens at idle, coast, and cruise... never high rpm, never acceleration, never under load. It is only when the engine is winding down or at rest.

It is on 3 of the 4 cylinders.... i get codes for cyl 1 2 and 3... never 4 or 0 (not sure if they count 1234 or 0123) but only ever those 3.... never seen one for the 4th. also get a random misfire code too. but never all 4... rarely just one.

I have a stock flywheel.... so not an issue of the aluminum ones....

I had the same thing dealer replaced my head then stayed that way . Then replaced the pistons just saying

So far...and im saying so far. A new battery has kept all the check engine lights off.

If you have a catless down pipe, remove the rear O2 sensor and remove the DTC for the rear O2 sensor with your HPT. I have a catless dp and no rear O2 and disabled it in HPT, never any blinky misfire codes.
Just puting that out there.

interesting.... i tried this out... and sure wnough... AC on... no blinky engine light... as soon as i turn ac off... suddenly i get blinking at idle, coast, cruise... hit ac again.. light stops blinking... but i have a stock flywheel. (i think, unless original owner changed it... but i doubt it with only 6k miles when i bought it.)

this is also an interesting tidbit.... because when i bought the car it had no batery, so someone donated me an older battery.. It worked just fine... but it is older, and it has died on me a few times here and there... So i have been suspecting a battery issue for a while now... but figured as long as the car starts for me, why bother with it... but if im getting misfire codes due to battery weakness... (which makes sense at low rpm conditions due to alternator not spinning as fast and not putting out as much juice.)

Hmmmm might be workth looking into... i will go do a load test here soon... see how she looks.

I thought about that... but I live in an emisions state... and just fear visual inspections. So i would prefer to keep it... But maybe i can disconnect it... and just leave it in there.... see what that does. (i already have the DTC disabled due to catless DP)

im also getting misfires while cruising and low rpm acceleration but not throwing any codes. much more noticeable when the engine is cold. still working on finding the issue, doing a vacuum leak test this week and if there isnt a leak im getting new spark plugs. doesnt really bother me much but i dont want to run lean with unmetered air coming into the engine...

I was actually doing that for who knows how long... Thank goodness for the GMS1 kit... when i removed my TMAP from the lower charge pipe, it was busted in 2... the nipple that goes into the charge pip was busted clean off... only thing holding the TMAP body itself on was the bolt. Huge vac/boost leak... i couldnt figure out for the life of me why the car idles like total crap. When i found that i was like DANG! that explains it...
im sure running for months or what not with a vac leak was no good for my plugs...

So as soon as i put the new sensors in, she idled perfect. but the misfires came back later... (but as i mentioned, you dont feel a misfire... you dont see any signs of misfire in HPT, but you get the CEL and DTC codes for misfires, and blinking CEL in given situations)

But i may just replace all of the above items... battery, plugs, (TMAP done)... Might even pull out the MAF and spritz her down with some maf cleaner.... (my turbo has a slight oil leak... and everything in my intake stream is lightly lubricated...) so im sure a oily maf doesnt read too accurately.

OBDII (not just the LNF) detects misfires through the crankshaft position sensor. The PCM expects to see the crankshaft accelerate with each piston firing, which is measurable in degrees of rotation.

When the PCM sees a graph where there is an inconsistancy in the acceleration of the crank (one cylinder not contributing enough power) it can determine which cylinder should be firing at that time and throw a misfire code for that cylinder.

^^^ I said this on another forum where it was asked.

Cleveland and its surrounding counties are the ONLY ones that check emissions in Ohio. Solution: Move away from Cleveland. :lol:

But yes, just unplug the sensor. Zip tie the connections together (side-by-side or something) so they aren't dragging on the ground.

And if they do a visual check... you'd be screwed anyway, since you don't have a cat. :lol:

Any update on the OP's issue?

The new battery has still kept all misfire CEL's off for me.