5.9l Misfiring

[Nysm1]

Ok, first things first, I'm a mechanic by trade and have literally ran through every test and wiring diagram imaginable. So my 1997 Ram 1500 has been having a low idle misfire for quite a while. If my foot is on the accelerator the misfire is gone, but as soon as I'm at idle it starts up again. Thats nothing weird for these trucks, but what is weird is that its never the same cylinder that is misfiring. I have run through all the wiring, changed the ENTIRE fuel system, changed pickup coil, ignition coil, run leakdown tests, compression tests. I've done EVERYTHING that I could imagine EXCEPT change the heads, motor, or PCM. I'm here asking for help because this truck is worth the effort, but I'm stuck. What could I be overlooking?

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[pacofortacos]

On my 2001 5.9, I had several exhaust seats that were sunk into the head. It would give an intermittent idle miss - the exhaust valve wouldn't seat every time. Still had good normal throttle power.

Was very hard to track down, I just knew it wasn't anything else at that point and figured it had to be in the heads - didn't even spot the problem when I first took the heads off. Didn't actually see it until I pulled the valves out.

 

[dapepper9]

Some Amsoil power foam or Mopar Combustion Chamber Cleaner could do some good or even just drive it a little bit and when it's warmed up, get on it. Do some high throttle runs or take it on 10 mile highway run with overdrive off. Good way to burn out any carbon.

The other things to check are the wire crossing tsb and coolant temp sensor can be a finicky ***** particularly on those 96/97 trucks.

What kind of plugs are being used?

 

[Nysm1]

Some Amsoil power foam or Mopar Combustion Chamber Cleaner could do some good or even just drive it a little bit and when it's warmed up, get on it. Do some high throttle runs or take it on 10 mile highway run with overdrive off. Good way to burn out any carbon.

The other things to check are the wire crossing tsb and coolant temp sensor can be a finicky ***** particularly on those 96/97 trucks.

What kind of plugs are being used?

Champion plugs. I even tried both hotter and colder plugs to see if either helped with no change. I have a Snap-On Solus and have checked every single signal available and they all are spot on.

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[turkeybird56]

NYXM!: First, welcome from Central Texas. 2nd: U have done everything I could imagine to do and a lot I do not know how to do.....I was thinking low spark output, but U said U changed the coil etc.,. So to be honest, U got this backyard guy stumped:

Weird thought: Fuel pump not pushing enough fuel at low idle??? Not sure if U have a rail, or TBI or what in yer truck: Just a thought.....

 

[dapepper9]

Champion plugs. I even tried both hotter and colder plugs to see if either helped with no change. I have a Snap-On Solus and have checked every single signal available and they all are spot on.

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Are the plugs copper? These things can be picky about that. How is the wire routing?

Regarding the temp sensors, 97 has 2. 2 wire is what the pcm uses, single wire is for the cluster. If the scanner is saying the reading is good you're probably fine there but i still wouldn't rule it out. Diagnostic system on these is fairly basic.

I would still try and run it hard or do a chamber clean. If it's had a pretty easy life it's not uncommon

 

[Nysm1]

Are the plugs copper? That year actually has 2 temp sensors, 1 that the pcm reads and 1 for the cluster gauge.

Are the plugs copper? These things can be picky about that. How is the wire routing?

Regarding the temp sensors, 97 has 2. 2 wire is what the pcm uses, single wire is for the cluster. If the scanner is saying the reading is good you're probably fine there but i still wouldn't rule it out. Diagnostic system on these is fairly basic.

I would still try and run it hard or do a chamber clean. If it's had a pretty easy life it's not uncommon

I was running NGK copper in the beginning because the local shops didn't have champion but yes they're champion copper plugs. Ive replaced the temp sensors and wiring almost a year ago because they were shït lol.

I also get on her all the time. I've also run a 3 step on it, replaced the plenum, cleaned everything. This is just killing me haha

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[Nysm1]

I'm also ordering a new PCM this week and may order a new harness also just to eliminate that also.

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[Nysm1]

NYXM!: First, welcome from Central Texas. 2nd: U have done everything I could imagine to do and a lot I do not know how to do.....I was thinking low spark output, but U said U changed the coil etc.,. So to be honest, U got this backyard guy stumped:

Weird thought: Fuel pump not pushing enough fuel at low idle??? Not sure if U have a rail, or TBI or what in yer truck: Just a thought.....

I thought the pump was to blame also, but I replaced it and even put an inline filter in to make sure it was clean. I installed a Schrader valve also to check the fuel pressure, still no change.

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[yoda]

You mentioned the miss changed cylinders, will it count misses on all 8 cylinders at times or just 2 or 3?
Someone mentioned exhaust seat problem, I too ran into that a couple times on the old 5.2/5.9 magnums, very hard to diag.
Before you replace the pcm, try doing a running compression test, 1 cylinder at a time, that's the only way iv'e found to diagnose valve seat problems on those engines, a regular compression and leakdown test will not always catch the problem

 

[Nysm1]

You mentioned the miss changed cylinders, will it count misses on all 8 cylinders at times or just 2 or 3?
Someone mentioned exhaust seat problem, I too ran into that a couple times on the old 5.2/5.9 magnums, very hard to diag.
Before you replace the pcm, try doing a running compression test, 1 cylinder at a time, that's the only way iv'e found to diagnose valve seat problems on those engines, a regular compression and leakdown test will not always catch the problem

Ive run the compression test and all are +/- 10 PSI. They're all about 100, which I know is lower, but at the moment I am trying to eliminate the electrical issues before I change anything engine related. The truck is 21 years old and has 259k orginial miles, has been well maintained, and has had regular maintenance.

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[Nysm1]

Also, its usually only 1 or 2 cylinders at a time. Never more than that.

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[dapepper9]

Any idea what your fuel trims look like? Real basic/simple check is unplug the upstream 02 but knowing fuel trims is better

 

[yoda]

I understand but PCMs are expensive, A running compression test is different from a regular compression test and may shed some light.
Take a plug out, screw in your gauge and actually start the truck, you need a gauge that you can release the pressure from the shrader once the truck starts so you are not measuring cranking compression. Take a reading at idle, put the plug back in, then do all the cylinders the same way

 

[Nysm1]

I understand but PCMs are expensive, A running compression test is different from a regular compression test and may shed some light.
Take a plug out, screw in your gauge and actually start the truck, you need a gauge that you can release the pressure from the shrader once the truck starts so you are not measuring cranking compression. Take a reading at idle, put the plug back in, then do all the cylinders the same way

Thats something I havent done and will do lated today. I also dont know the fuel trims off the top of my head, but I will check them later also.

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[dapepper9]

Be sure to unplug the corresponding injector if you decide to try that

 

[Nysm1]

Be sure to unplug the corresponding injector if you decide to try that

Thanks for the reminder lol. I have done that before haha

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[turkeybird56]

DELETE

 

[dapepper9]

Thanks for the reminder lol. I have done that before haha

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Like most things, easy to remember such things when you're the guy doing it lol. Ive done a lot of simple things like that in the moment

 

[Nysm1]

Ok, so today I started up the truck for work. No issue at all, the misfire was gone again and still no signs pointing me to the right direction. Also, I ran a running compression test and once again no significant issues were recorded.

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