I took my truck in for smog on Friday and it passed without any problems. I had already noticed that it isn't idling as smooth as it used to. And the smog tech noted that it had a pending 0300 and a rough idle.

It was running on 8 cylinders when I put everything back together a couple of weeks ago after replacing the intake manifold gaskets and COP boots. I was able to fix that easily since it was just corrosion on the COP terminals. So right now I'm thinking it may be a vacuum leak from removing everything.

I'm just wondering if anyone knows more about a 0300 and what exactly it means on a 6.8 besides just a "random miss". And also if anyone has any ideas of what else to look for to smooth out the idle.


Additional info that might help:

• Truck starts up instantly, it was missing badly on startup and would sometimes die after a few seconds, but a new battery fixed that.
• Idles at 800-900 RPM when warm, and at about 1100-1200 when cold.
• Drops back to idle smoothly after running at high RPMs
• MAF was cleaned with MAF cleaner during re-assmbly of the manifold and throttle body.
• PCV valve was changed during re-assembly as well
• Fuel filter has not been changed yet. ***UPDATE*** Changed fuel filter and no change
• Once again, passed smog easily and numbers indicate that it is running on all 10 cylinders and there is no unburnt fuel in the exhaust.
• Certain RPMs are smoother than others when on the road, but haven't been able to note the exact load/RPMs yet.
• Seems to have all its original power at WOT.

 

A better scanner would indicate which cylinder is misfiring. Example P0301 would be #1, etc.

Have you cleared the codes and see if it comes back?

 

I don't own a scanner, at least not yet. The P0300 was noted during the SMOG, and the tech didn't note any other codes. I wasn't surprised due to the rough idle. I don't necessarily hear a miss, it just isn't as smooth as I remember, and there is a bit more vibration than usual while sitting in the truck while idling. I guess realistically I need to get a scan tool before I expect to accomplish anything.

 

A vacuum leak would make your idle almost surge when at idle, or idling down. Have those symptoms? If not, you should be okay but never hurts to check everything.

Have you checked all your plug wires? A quick search on the internet shows that alot of pending 0300s were cleaned up with plugs, and if not, plug wires too.

 

All the plug wires (COP Boots) were replaced. I did have some corrosion that I cleaned off the connection at the COPs. I have also unplugged one COP at a time and noted how it runs and all 10 of them improve the idle when plugged back in which tells me all 10 of them are functioning.

I just replaced the fuel filter (I believe it was the original with 80k miles on it!) and it didn't change anything, though it still needed done.

I have an OBDII reader on order and I'm going to wait until I have it before I go trying anything else. The truck is back on the road at least, so this is more of annoyance than anything, plus I'm sure it won't be helping my fuel mileage at all.

 

Ok. I am going to have to ask, what brand of spark plugs do you have in there? These motors prefer Motorcraft.

 

I just replaced the original Motorcraft Platinums with new Motorcraft Platinums along with new Bosch COP boots.

 

Well the SMOG tech must have cleared all the codes. The truck has been driven 3 or 4 times since the SMOG check and according to my new BT OBDII adapter and the Torque App there are no codes present. My computer isn't detecting any miss, but of course that doesn't mean much. So now that I have full access to all the parameters my PCM can see what should I be looking for to help diagnose the rough idle? (Just keep in mind this is a '99, and although it is OBDII, if I remember right some of the parameters simply aren't available.)

 

Try to get to the misfire counter thru " live data " . If it provides that info , check all of the cylinders to see which cylinder has the most significant misfire count . If so, that is the cylinder to focus on ; it is possible that a sometime -acting coil (such as getting too hot and providing lower KW arc ) and/or sometime-acting injector (such as only partially opening ) could be to blame.

This is a tough diagnostic work. Good luck.

 

I drove the truck to town Sunday (20 miles) and to work and into town yesterday (30 miles) and I don't know what fixed it, but it is running fine now. The rough idle is gone and there is no miss. It may have just been the old fuel that I finally got rid of. I fixed the ball joints in the fall of 2011 and had a coolant leak right after that. I didn't have the time or $$$ to fix it until just a couple months ago and I'm finally able to drive my truck again. It's good to have my truck back! So in the last few hundred miles (16 months) I've done the following...
-New Tires
-Replaced Ball Joints
-Replaced Front U-Joints
-Replaced Front Brake Pads
-Serviced Front Slide Pins
-Replaced Front Brake Rotors
-Front Hub Bearings and Seals
-Front Diff Fluid Changed
-Painted Front Diff Cover
-Replaced Steering Damper
-Replaced Spark Plugs
-Replaced COP Boots
-Replaced Intake Manifold Gaskets
-Replaced Heater/Radiator Hoses
-Replaced Heater Tube (Under Intake)
-Replaced Heater Hose Quick Connects
-Installed Heater Hose Shuttoff Valve
-Replaced Thermostat and O-ring
-Coolant Flush
-Replaced PCV
-Cleaned MAF
-Replaced Fuel Filter
-Replaced Battery
-Replaced Serpentine Belt
-Installed Driving Lights in Grill
-Installed New Head Unit
...and most importantly...installed brackets to hold my LED Maglite!!!

 

Just an update...

Although the truck is idling and seems to be running fine I did notice some surging while accellerating hard, especially while I was towing my Nissan pickup home yesterday from work (long story involving a new radiator). Couldn't even tell it was back there since it only weights about 3,000lbs. Anyhow I was showing my new BT OBDII reader to a friend at work and it is finally showing a 0310 so I have a misfire on cylinder 10. I'm hoping that after addressing this it will finally smooth the idle out the rest of the way and smooth out the accelleration. I'll update this once I look into it.