Do you have any Y pipe on the truck or are you running open headers?

 

Any chance that coolant or something got on the R/S O2 sensor? That will give you both the lean code and misfire....I'd start with swapping the O2's side to side and seeing if the lean code follows....

 

I agree with Billy, check them O2's they will cause hell. Mine were bad and ended up fouling out my plugs also.

On a side note what code reader did you get?

 

Both O2s are brand new, and I think I've figured it out.

It runs fine if I keep the RPM up, but once they drop (and exhaust velocity slows accordingly), I get a "pending" lean code. Once the lean code stops being "pending" and shows up as just a lean code, I get the code for "random/multiple misfire".

So my thinking is this:
When the exhaust velocity is low, since the O2 is so close to the end of the exhaust, it is registering some ambient oxygen, which it interprets as running too lean, so it adds more fuel, and thus, the misfire.

Does anyone see any flaws in my logic?

 

that's why I asked you if you are running open headers. The o2 sensor would be exposed to open air and dump all kinds of fuel in.

 

I also had this problem when i installed the cam in my 5.3 .

I used my EFI LIVE scanner and found misfire on cylinder #4 .

I had a loose rocker arm that was trippingn the knock sensor setting the P0300and flashing light.

I bet you just have something loose. But a good scan tool will prob pinpoint it.

Also \make sure your Tune has the misfire count raised due to increased valvetrain noise to prevent false misfire readings.

Good Luck!

 

if it's flashing like that, it's a cracked plug or bad wire usually. i've never seen a bad coil.

 

Sounds like we thought of the same thing at the same time.

When you did your swap, did you have to do the CASE relearn?

If it were a cracked plug/bad wire, wouldn't it throw a code for an individual cylinder, not the "random/multiple" code?

And also, the code only comes up when exhaust velocity is low.

I actually just re-torqued the rockers today just to be cautious, and they were all right at 22 ft. lbs. No knock sensor codes.

 

Nope, fired right up perfectly.

I'd say you're problems are due to the open headers. How come you haven't bolted up the OBX y pipe?

 

No it will flash p0300. mines done it multiple times