Chasing a P0300

[Cornelius]

Did you ever get any codes O2 related?

at one time yes, then replaced them

If a vacuum leak, (manifold) then fuel trims should go hi- over 10 up to 30+. Maf wiring also connects to TB, so a bad ground/flexing wires on Maf plug. I'm sticking with MAP, thats where the cover bolts on so check wires there too. Pull it clean it oil gasket reinstall. and add a sensor in your scanner for it and monitor.
Even a bad pcv could be enough to miss. It's the 4 unconnected cylinders thats the stumper, not side by side.

My fuel trims were that high at one point now it’s normal after vac line repair. I will look at MAP sensor like you suggested. I always thought that connection was a little soft. I should note that I did change TB gasket to eliminate one more possible point of entry unmetered air and replaced the MAF because the reading would never be more than 5 no matter what the load was. Now it reads correctly and my mpg has improved but flashing cel and misfires on 1 5 6 8 persist.

 

[Tonyrodz]

at one time yes, then replaced them


My fuel trims were that high at one point now it’s normal after vac line repair. I will look at MAP sensor like you suggested. I always thought that connection was a little soft. I should note that I did change TB gasket to eliminate one more possible point of entry unmetered air and replaced the MAF because the reading would never be more than 5 no matter what the load was. Now it reads correctly and my mpg has improved but flashing cel and misfires on 1 5 6 8 persist.

Which vacuum line did you replace? Think I missed that.

 

[exp500]

Cornelius- I'm not sure this is it, but has affected a few others.Plus it's a cheap fix if you are not afraid of it.
Disconnect battery, both terminals, touch cables a few seconds. Unplug computer with one hand grounding yourself to truck, then to computer case. Inspect pins and sockets for water and corrosion, clean both and apply dilectric grease to seal and re-install. If it is green/dirty/wet you found it.

 

[ks03]

...but flashing cel and misfires on 1 5 6 8 persist.

Flashing cel, and it fells like it’s running well while it’s flashing? Typically you’re going to feel a misfire that makes the light flash.


so, afaik misfire detection works as a drop in crank speed. if it was a bad miss I was going to say the reduction in crank speed is enough that sometimes the next cylinder in the firing order shows misses. With the firing order being 1,8,7,2,6,5,4,3 a miss on 1 can show up as misses as 1 and 8. What I’m wondering though, is that if it runs great, if there isn’t a problem with the crank reluctor wheel. I haven’t ever had a reason to even look at one, but this was a thing on 3.8s where the wheel was on the back of the crank pulley. I don’t know if it’s even possible but I’d want to try to take out the crank sensor and watch it while turning over the engine by hand... alternatively you could try changing the crank sensor, but I have no idea how much they cost, and they typically don’t present like this

 

[Cornelius]

I know you say you replaced the fuel pressure regulator, but I’m suspect of it still.

one possibility for misfires on 1 and 8 is liquids in the intake, weather it’s caused by gas, oil or something crazy.

5 and 6 are near where the vacuum line for the regulator connects.

occasionally pulling lots of fuel, then evening out, especially after a hot soak, would indicate a regulator.

im not saying replace it, but just check it a couple more times. Just pull the vacuum hose off, checking for fuel. If it’s dry, twist the regulator to point the port down and make sure everything is dry.

FPR is dry, and seems to be working. Fuel pressure holds now with the new pump

Which vacuum line did you replace? Think I missed that.

Replaced the 90 elbow of the FPR, replaced the grommet where the PCV goes, insoected and trimmed the end of the pass side breather hose from valve cover to TB

Cornelius- I'm not sure this is it, but has affected a few others.Plus it's a cheap fix if you are not afraid of it.
Disconnect battery, both terminals, touch cables a few seconds. Unplug computer with one hand grounding yourself to truck, then to computer case. Inspect pins and sockets for water and corrosion, clean both and apply dilectric grease to seal and re-install. If it is green/dirty/wet you found it.

LOL it almost sounds like I should be looking for a hidden camera for this one! What is happening with this procedure?

 

[exp500]

Glad your sense of humor is still working. Just remember everything is connected to the net! Have you found the "Candid Camera " post yet?

 

[Cornelius]

So touch pos and neg to discharge the capacitors in the ecu and inspect the plug for corrosion. Do a hokey pokey and turn myself around. I don’t quite understand how this will stop misfires. Unless there aren’t actually any and the ecu has gone rogue and is reporting fake news? I’ve never heard of that but then again I don’t know what I don’t know

 

[exp500]

What did the radio used to say? " This is a Test and Only a Test".

 

[livingez_123]

I will pop the random misfire if I let mine sit and idle for a long period of time or lots of stop and go traffic...but it's not missing. I believe it's my cam that making it think it's missing. It runs smooth, plenty of power it's just a little chuggity chuggity at an idle. I hope to get it tuned very soon to smooth the idle out. If I bring up the rpm to 700rpm it doesn't pop the code.

 

[livingez_123]

Do not disconnect battery to clear codes, use scanner. Every time you disconnect battery it goes into Re-learn mode. Needs a 50 mile drive to adjust fuel spark and trans. Don't do anything except clear codes and monitor for a few days.

I just learned something new^^^^^^^^^^^