Guys,

Hope you guys can point me in the right direction. My 04 2500 5.7 is having an intermittent misfire at idle (stopped) and is sometimes sluggish to take off (feels like its misfireing). At highway speed seems to run normal, mpg sucks but seems normal... 10-12 mpg range.

Changed the plugs over the weekend because they were way past due, but no change. I am left with ignition coil, injector or something mechanical in the head valves/rocker seats etc. Ecu has thrown several codes, cyl misfire (didn't show a #) and o2 sensor bank 1 unit 1 reads 1.27v and stay open loop.

Before I pull the cover off, I would really like to isolate the cylinder(s) of concern. What diagnostics are you guys using that indicate misfire? I have a elm327 unit but the software I have dosent tell me much for this problem. Is the ecu capable of deticting misfires and identifying cyl? Any tests that have worked in the past? I dont think temp readings would be too telling as it's intermittent and of course doesn't do it when sitting in the driveway. Anything else I should be checking?

 

The code itself should tell you which cylinder it is whining about.

Your O2 sensor is likely toast. That *could* lead to a random misfire problem. (P0300)

 

The only code it is throwing is the P0300, while I was at the parts store buying the o2 sensor I used their diagnostic tool. It indicated the same P0300 and said to replace the camshaft sensor. Swapped that in last night, same issue. I am going to do crank sensor hopefully tonight and clean the throttle body. I dont think it is a faulty 02 sensor causing the problem since it starts running rough before the sensors are warmed up. I will get that changed out this weekend when I dont have to work on a hot exhaust system.

Seems like most tools struggle to read misfires on dodge/Chrysler vehicles in this age range from the reasurch i have done.. this would be helpful info to have.

 

Unplug your front O2 sensors, see if the engine runs any better.

 

Unplugged 02 sensor, no change. Need to pick up a crows foot to get the sensor out, its stuck good but will do that over the weekend.

Also changed out the crank sensor no change there either.

When I got home from work i put a temp gun on the headers and found out the passenger side is running extremely hot.. i was getting ~700 deg initially then dropped to about 400 at idle. Driver side was ~400 initially then dropped to about 200 after a few minutes at idle. idle.probobly is what fried my o2 sensor.

 

For removing the sensor, just snap the wire off and use a normal deep well socket. Use a wrench to install the new one. Or if you have a fancy split socket, use it ONLY for install. Otherwise it'll likely split wide open and you'll be out a tool.

I've had dodge obd2 tell me which cyl when there was one, that code means you have multiples. I'd replace that o2 and retest. Is the same side that is getting hot the sensor that is getting flagged as an issue? Lean will cause more heat. Excessive heat can cause misfire conditions. You may be overthinking it.

 

Problem seems to be solved. Changed o2 sensor to little change but after a pcm reset I think it got back in line. From the looks of the 02 sensor that bank was running really rich. Both sensors went closed loop, had good voltages and there were no misfires. I guess I will see after the learn cycle.

Last item that seems odd, manifold pressure on the diagnostic readings was arround 10-11 in-hg however hooking a vacuum gauge up i was 18-20 in-hg. Anything I should be concerned with here?

 

MAP sensor is Manifold Absolute Pressure, so, your readings are actually correct.

 

You should always pull power after you replace sensors so any poor learning is completely forgotten... If only that would work with people, right? :-) That ELM bluetooth interface combined with the torque app will allow you to monitor your fuel trims. Once it is running well, take a couple snapshots so you can see what good looks like. Then you can do comparisons down the road when you notice issues. MPG on older, well used hemis in 1/2 tons tend to be about 12-14 mixed and can be as high as 20 on nice level highway for speeds under 60mph. I had a 04 1500 QC 4x4 with 20 inch wheels and 3.92 gears that I bought new and had for 11 years / 140k miles or so.