I have a 1997 2500 4x4 5.9 auto with 187,000

I have been getting the Misfire in 5 mostly, 3 often and random others as well as RANDOM misfire code.

I changed the usual stuff after checking about this issue.

Plugs, wires, cap, rotor, did the wire re-route, coil. Re checked all my work and swapped pack to original parts to see if it was a certain part.

The wire re-route made the code take longer to come up and it seems 5 is the first and most common one to come up. I have replaced the #5 fuel injector as well, just to eliminate that.

The idle is sometimes rough but other than that the truck seems to run fine.

I have pulled spark plug wires and injector wires individually to see if that changes anything and it does not make a significant difference between cylinders.

Not sure where to look next?

Any suggestions?

 

Intermittent misfires can be pretty tricky to diagnose. You can do a power balance test, a spark test, and a compression test to help pinpoint the problem. If you dont have a OBD II diagnostic scan tool you can do the power balance test by individually pulling your ignition system's secondary wires at the plugs and watch what the rpms are doing. each time you pull a wire the rpms should drop. no rpm drop or very little drop compared to other cylinders indicates a weak cylinder or misfiring cylinder... You can test individual secondary wire with a spark tester from a parts store. Low compression can contribute to a misfire. To do a compression test disable the fuel system and start the engine and let it stall. disable the ignition system and put a compression gauge in a spark plug hole and crank the engine till the needle bounces 4 times then record the reading. do this for each cylinder. all the reading should be within 20% from the highest reading. On average good compression is in the 120-130psi range. Low compression can indicate leaking valves, leaking head gasket, cracked head, bad rings. A wet compression test can pinpoint it to bad compression rings. its the same procedure as a cranking compression test except you add a teaspoon of oil into the cylinder. if compression improves you need new compression rings. If you have the equipment you can also check the fuel injectors but since you replace one to no avail, it doesn't seem to be likely that they have a problem.

 

I have done the spark test. Some cylinders drop RPM more than others. Is this normal?

The truck sounds tight and runs good with average 12+/- MPG and does not use oil.

 

Durring the power balance test the cylinders that drop the most are the most efficient (contributing the most to turning the crank) it is normal for there to be a slight difference between the rpm drops. Its the cylinders that don't effect the rpm that are the problem. Were all the plugs firing properly when you did the spark test. I common cause of intermittent misfire is a lean burn misfire. Can you check your fuel pressure? or have a scan tool you can hook up to look at long term fuel trim?

 

I do not have a way to check the fuel trim unless this Ultra gauge my neighbor let me borrow will do it. I'm pretty sure it will.

What am I looking for?

I was looking at O2 sensors and saw a big variance in the readings but I don't throw a code. Also it took 200 miles for the Cylinder 5 code to come up.

 

long term fuel trim tells you what the pcm is trying to do... such as if you have a very positive long term fuel trim the truck is trying to add fuel to fix a lean condition. very negative and the truck is trying to take away fuel to fix a rich condition. the front o2 sensor switches from hi to low when the truck is running correctly as the engine is cycling from rich to lean. the downwind post cat o2 monitors the operation of the cat and should not cycle like the front. dont check an 02 sensor as they are not quick enough to monitor the cycling of the 02 and some will just average the readings.

 

A clogged catalytic will do this. Happened to me several times before I "replaced" it

 

Just curious if you have found anything that helps your issue. I have a 97, 5.9 and I am throwing a P0300, 303 and 305 like you. I also notice I have a little rpm surge at idle (200-300rpms) when it's up to operating temperature, and my rpms at idle seem low, both in gear and in park. The truck runs great above 1,000 rpms, no misses that I can feel, no surging and it almost always throws the code when it's idling only. I've tried the TPS and IAC already, so I'll be changing plug wires and plugs and routing them correctly soon to see if that helps.

 

Take your throttle body off and clean all the sensors with break cleaner ( less harsh on plastics) then run a can of sea foam in the tank. This should help.. If not loom into replacing o2 sensors, catalytic, and maybe even a plenum kit.

 

5 and 3 are next to each other.... not really a good sign. Losing any coolant? What do the plugs look like?