Ricardo

So over the past week my GT has developed a slight lope and a definite misfire at idle, part throttle, and cruise, also in between easy shifts it will misfire when dropping RPMs . But there is absolutely no loss of power and the misfire disappears at full throttle. It pulls hard with no hesitation at WOT.
Did a few searches here and found a few posts explaining this is common with o2 sensors going bad. Took it to the dealership and they found a few misfires but said o2 sensors are working perfectly. They changed my fuel filter and said the misfire didn't return. It came back after a few miles!
Cleaned throttle body, sprayed the MAF with MAF cleaner, checked for any vacuum leaks.
Ran Scantool software and STFT is within normal operating range and o2 voltage seems to be too.
But then I see the o2 sensor on one side do this during a misfire for a fraction of a second. The other side doesn't drop off like this and this only happens during a misfire.


Now it's down to COP, spark plugs (4k on current OEM replacement set-followed TSB to the letter), and o2 sensors.
Is this enough information to make an educated guess on my next step or do I need more data? I can screenshot any data needed. Forgive me for being such a n00b to this kind of problem, I've trying hard to research all I can. And thanks for any help, it's frustrating when a dealership says they fixed an issue and it reappears.


EDIT: Just to be absolutely sure, I ran SCT, Scantool, and Torque for any codes and all of them detected no codes this entire time.

jpplaw

Plugs usually go bad under load.
COP's usually go bad under load.
Fueling issues are usually under load.
However, your problem is just the opposite of the above.
If it disappears at WOT, then it is a closed loop problem. Try changing out that O2 and use the Motorcraft brand only.

dlazrael

My situation wasn't quite like yours, but what ended up happening is I had some water get down past the COP boot and got to the top of the spark, the end result was the spring piece inside the boot corroded and caused me the problem, I replaced that part ($7 at autozone) and worked like a champ - so you MIGHT want to check the boots/connectors before running off spending over $200.... Though with the data you have it does sound more like an O2 sensor

Mr.Ski

An exhaust leak will cause a misfire like this too.

Ricardo

thanks guys! Im changing my front o2 sensors right now, the rears were turned off in the tune anyways.


I do have an exhaust leak from the manifold to midpipe that just will not go away. been there since day one but I'll throw some copper gasket maker on it and report back.

Ricardo

and by 'change' I meant find out a manual transmission holds heat a lot more efficiently than a 4.6dohc engine can and that getting that DS one makes one curse life as a man with big hands and forearms

Ricardo

o2 sensor change did nothing...moving on to the plugs, if that doesn't work then it has to be the coils. I cant believe a DEALERSHIP could't find the cause with all their fancy computer diagnostics and knowledge.

drewsky

Try the exhaust leak problem .. Hopefully will solve it !

moosestang

It doesn't have to be the plugs or coils, it could be a compression problem.

Ricardo

this is good to know as a possible issue. This is a great lesson in how complex our engines really are! Wish I could just soak up all you guys's knowledge about our cars.


edit: if it means anything, my car has been babied its entire life with just one recent dyno run and three stoplight encounters. no racing or beating on it.