I have a 08 ram 1500 automatic with 155000 miles on it. It had a head gasket go out after replacing the head gasket I get a rough idle with vehicle shaking. A code appears as random misfire (p0300)The code flashes at idle only. After 2000 rpm the code clears itself the shaking stops and vehicle runs fine. I can't really afford to throw parts at it was wondering how I narrow down what could be wrong.
I have checked compression all seems fine.
i have replaced
coils
Spark plugs
Egr valve
Valve cover gaskets
Head gaskets
Rods
Rocker arms

 

What spark plugs ya got in there? How is your fuel pressure?

 

Ngk spark plugs I haven't tested the actual psi of the fuel hose but took the hose off primed the pump and it shot out.

 

What type? Are the rare-earth plugs? These engines tend to not like those much. Standard copper plugs are best, and least problematic.

 

So I did go with copper it had iridium in it before that caused corrosion in coils. But coils were also replaced.

 

Try taking the coil from the suspect cylinder and moving it to another non related cylinder. See if the fault follows the coil.

** also, move the spark plug (s) in that suspect cylinder to completely different and unrelated cylinders. If it is one of those components, the fault will follow the part. Ignition wire too??
Take notes to keep track, (easy to get confused and forgetful)(for me at least)

I attended an Champion spark plug seminar (aviation recurrent training) and the speaker espoused about how if you ever drop a spark plug, to 'drop it a second time'...
... right into the garbage bin.
They're contention is that from impact, that a spark plug could get a micro fracture in the ceramic/porcelain that while it may look great and may even test under pressure and high voltage, that while operating at temperature that it could 'leak' or ground out and that it should not be considered a good spark plug anymore.
Did a sparkplug fall to the ground by any chance?

Let us know


I hope this helps you.

Please return and tell how this information worked for you, what you find, and how you eventually fix your vehicle (even if it is that you brought it to a mechanic / dealership for repair)
It is your feedback and final repair report that will make these forums useful to others in the future.
Many times I search and find the same exact fault that I am having, someone suggests how to fix it, the patron follows the suggestion and then never returns to say YEA! or NAY...


The forum thread just goes dead because the person fixed their fault and has not a care to offer the answer anymore. Please don't be that guy.

Cheers!

 

What engine??

Consider putting this information into your signature like many others do so we never have to ask.
including aprox mileage

I do it because I am always speaking about my engine and I forget that not everyone reading know what engine, vehicle, trim, and or mileage I am referring to...

Cheers!

 

Computer more or less controls everything. So its good to start looking at what the computer is seeing and doing. Bluetooth OBD2 with an android app like Torque is probably about the most cost effective way to get started. I see these things are now half what the used to be... like only $7 now. The torque app is free, or get the pro version for like $5.

I'd check for vacuum leaks, if you use propane to sweat copper plumbing joints - likely you don't have to spend anything to check for vac leaks. At idle, a small leak can have a effect - where at higher rpm it has much less effect.

 

Another vintage trick is to spray WD-40 or starting fluid (ether, USE EXTREME CAUTION) and recently I see quite a few technicians using BrakeClean by the case...
Spraying it around a running engine and listening for a change in RPM (like fj5gtx suggested)

Many many times I see our mechanics using the BrakeClean to 'clean' off oil residue, and everywhere... it is a cost effective, quick evaporating, convenient blaster
AND , there is always and 'and'
... IT IS BAD PRACTICE.
So many parts are plastic, Electrical connectors, vacuum tees, intake plenums, radiator tanks... plastic is used everywhere under the hood and especially EVAP components, that pesky EVAP system that the genius' in Congress and the EPA mandated that out fuel tanks be sealed and onboard tested so gas fumes don't gas off into our Earth atmosphere.

Brake Kleen (and other volatile chemicals) can cause chemical stress in plastic parts. wash away plasticizers, make these parts brittle. Use them with caution.
Yet another reason why I MUST be the only one who works on my vehicle.

I hope this helps you. Please hit the [ LIKE ] button… 😊

Please return and tell how this information worked for you, what you find, and how you eventually fix your vehicle (even if it is that you brought it to a mechanic / dealership for repair)
It is your feedback and final repair report that will make these forums useful to others in the future.
Many times I search and find the same exact fault that I am having, someone suggests how to fix it, the patron follows the suggestion and then never returns to say YEA! or NAY...


The forum thread just goes dead because the person fixed their fault and has not a care to offer the answer anymore. Please don't be that guy.

Cheers!

 

So I have tried to find a vaccume leak before with starting fluid. I. Not sure if that's a good idea or not I was told it works but had no results. So I sprayed into the throttle boddy while running and it killed the motor. I have done this before on older vehicles and it didn't kill them. Should I try again with propane? Thanks for all the help I will be swapping plugs and coils tomorrow to see if I can test that idea