P0301 misfire at idle (puffing noise at muffler)

racecar825

racecar825

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As the title says, I have a misfire on cylinder 1 at idle. It's a 1998 4.0 with 165k miles. I can feel slight vibrations while in the car and it is idling. I had this code a few months ago and changed the plugs, coil wires, distributor cap, and the distributor rotor and the problem went away until now. I tried clearing the code but it comes back within a few minutes. I did the screwdriver test on injector 1 and injector 2 and they sound pretty similar so at least the signal to the injector is okay, still need to test the actual injector. I do notice that it is harder to start now than before.

Today while diagnosing it, i noticed a weird puffing air noise coming from the exhaust I have not heard before. Any idea what this could be? It is only audible near the muffler and not near the engine see the attached video.

 
Sounds like a cylinder not firing. Pull the plug, check the condition of the plug / gap and make sure the wire is seated well. Reset the code and try again.
 
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Once you have ruled out spark and the fuel injectors; do a compression check on that cylinder to see if the problem is a valve or rings. You may consider removing the valve cover to see if there is a rocker arm or push rod problem. Hope I am wrong, but that sounds like partially collapsed lifter....
 
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I'll work on ruling out spark and fuel tomorrow. My plan is to pull the plug and check the condition/electrical contact with the coils. If thats not bad, then I'll move to swapping injector 1 and 2. I have a spark test light and fuel pressure gauges coming in soon so I'll use that too.

What are the symtoms of a collapsed lifter? I was planning on changing out my valve cover gasket anyways since it's leaking. What should I check for besides any obvious wear/looseness?
 
Bosch plugs seems to have problem in a lot of vehicles, the 4.0 included. I would take a bet that if you confirmed that the rest of the ignition system is good that the issue is with the Bosch plugs. Swap them with some Autolites.
 
Update:

It seems to misfire more when the engine is cold.

Today i swapped the plug in cylinder 1 with an Autolite Irridium plug with no luck. I checked that the coil was good with an inline spark light and that lit up. The bosch plug that came out was pretty good (a little gunk on the tip, but that might have been from me hitting dirty block with the tip of the plug)

Also tested Cyl 1 injector signal with a noid light and it looks good. Swapped Cyl 1 with Cyl 3 injector, no change. Fuel pressure reads ~45 psi at prime and ~48 psi when running, so that looks good. Injectors did look a bit dirty near the o-ring, so i wiped that off with a towel.

Tomorrow I will check on possible vacuum/intake leaks and see if the IAC/throttle body is clean. If I can't find anything there I will have to open up the valve cover and take a look there. Compression tester is coming on Thursday via Amazon Prime.
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