I have a 2002 Ram Van with ~120k mi. I'm getting two codes p0303 and p0138.

When it's in park there's not much to notice. When I shift into drive and take my foot off the brake I feel a noticable shuttering. The same thing happens as I come to a stop or take off. There's a slight hesitation at take off some times, but seems random. The exhaust isn't smooth, there's a bit of a bump to it. When I put my hand over the tail pipe it is bumping, but not to the point of getting sucked in. I'm getting a little oil up through the pcv, too. At driving speed everything seems fine.

I've replaced the coil, wires, plugs and distibutor. I've also cleaned the TB and replaced the gasket and cleaned the air control valve on back of the TB (I forget the name of it right now).

I pulled the #3 plug and it had some light black carbon on it, so I guess that confirms it's misfiring. I switch wires and plugs, but that didn't fix it. I also switch the #2 and #3 injector to see if that was the problem, but no luck. I've replaced a lot of the vacuum lines, some were cracked in half. I've changed the pcv and the much larger case vent on the opposite valve cover.

I have a scanner and those are the only codes are are coming up. When in park my LTFT is -10, but when I'm driving it varies between -7% and +4%. The pre-cat o2 sensor averages .5V and is switching. The post-cat o2 sensor is at a consistent high of around .9V.

Something that seemed odd to me was that when I turn the engine on, both o2 sensors read 1V until they get heated up. Is that a bias voltage? I thought they should both read low (~0.1-0.3V when cold)?


Thanks for reading this long winded post.

 

Sounds like you might have a burnt valve in your number 3 cylinder. A compression test should confirm or deny that. The high voltage from the O2 sensor (P0138) could mean it needs to be replaced.

 

Thanks for the reply. I've haven't been able to find much about how a post-cat o2 sensor could fail. I checked it again a few hours ago by letting it warm up and just watching for when the o2 sensors would start generating data. The pre-cat sensor started after a few minutes and looked fine, but the post-cat o2 sensor held a perfect 1V reading for about 30 minutes.

I'm going to check it again in the morning and if it's still locked at 1V I'll go buy a new one and see what happens.

 

I thought the after cat o2 sensor was just to judge the efficiency of the catcon. I did not think it affected engine function.

I am not sure on this.

 

You are correct, it's purpose is to determine catalyst efficiency.

 

That's what I've read everywhere. But I don't understand why I would be getting a high o2 sensor reading off the post-cat sensor and without high o2 sensor reading off the pre-cat sensor. The pre-cat o2 sensor is showing that it is switching between low and high (.1 to .9) voltages. I guess if I know that the post-cat o2 has flat lined at 1V it needs to be changed anyway and then go from there.

 

Because one or both of the O2 sensors might be no good and is giving a false reading.

 

If you found carbon build up in the cylinders, you may need to do a TB and IAC cleaning too. https://dodgeforum.com/forum/dodge-r...n-the-iac.html

Next, run a can of seafoam through the TB. Follow the directions on the bottle. It should help dissolve the carbon in the engine easily. Then go for a drive to blow it out the exhaust system. It's fine to use with the CAT.

 

I changed both oxygen sensors. The rear sensor has dropped from an average value of 0.9V down to about 0.5V. Still seems a little high, from what I've read, but maybe that's from running rich for quite awhile. I'm going to keep an eye on that value.

I've started looking at the Mode $06 data, which is another beast entirely. There are 8 tests that are reported. Before I changed the oxygen sensors I had four of those tests post a fail. Now, I only have two that are still labeled fail. These are non-continuous tests, and I haven't driven around that much so perhaps the tests haven't met the conditions to have been completed again. If anyone has info about Mode $06 data here are the two tests that are still a fail:

TID $11 Component ID $01 - Value = 0, Min = 48, Max = NA Result = fail
TID $13 Component ID $01 - Vaule = 0, Min = 10, Max = NA Result = fail

I'm going run some seafoam through the system tomorrow to clean it up and see what happens.

 

Ok had to look up this Mode $06 test. It is what I am familiar with as the emission system self check. Though I can not pull up the data directly my scan tool does show which component of the self tests are not complete or failed. And if you have not completed the "Drive Cycle" then yes your tests will show up as a failure even if all systems are working correctly. Complete this generic drive cycle and then redo the Mode $06 test.

RUNNING AN OBD-II DRIVE CYCLE
The purpose of completing an OBD II drive cycle is to force the vehicle to run its onboard self testing diagnostics. Some form of a drive cycle needs to be performed after DTCs have been erased from the PCM’s memory or after the battery has been disconnected. Running through a vehicle’s complete drive cycle will “set” the readiness monitors so that future faults can be detected (and potentially to pass the emissions inspection). Drive cycles vary depending on the vehicle and the monitored items that needs to be re-set. Whenever possible, follow the drive trace prescribed for the specific vehicle/monitor in question. Some vehicle-specific drive cycles can be found in the vehicle’s Owner’s Manual.
The following “universal” drive cycle can be used as a guide to assist with re-setting monitors when a vehicle specific drive cycle cannot be located. This generic OBD-II drive cycle begins with a cold start (coolant temperature below 122 degrees F and the coolant and air temperature sensors within 11 degrees of one another). This condition can be achieved by allowing the vehicle to “sit” overnight, and then by beginning the drive cycle the next day. Most drive cycles will be difficult to follow exactly under normal driving conditions, so the driver should exercise caution, road safety, and courtesy to others.
• Start the engine. Idle the engine in drive for two and a half minutes with the A/C and rear defroster on.
• Turn the A/C and rear defrost off, and accelerate to 55 mph at half throttle.
• Hold at a steady speed of 55 mph for three minutes.
• Decelerate (coast down) to 20 mph without braking or depressing the clutch.
• Accelerate back to 55 to 60 mph at ¾ throttle.
• Hold at a steady speed of 55 to 60 mph for five minutes.
• Decelerate (coast down) to a stop without braking.