P0300
P0303
P0304

Hello there,

I replace spark plugs 3 and 4, to no change in misfire. Pulled cap and rotor and cleaned contacts, no change to misfire. This misfire started suddenly Wednesday, but the misfire smooth out at highway speed. I have a crack at the lower collector on my manifold, have had it for several years. I did notice that the exhaust leak sound sounds more distinct now. I have no oil in the water or water in the oil, or missing water. It does hesitate in acceleration with medium throttle, under heavy throttle no hesitation, stumbling. At highway speeds in overdrive if I let the RPMs drop below 2000 I do feel a engine vibration, but goes away as I accelerate. I have done a lot of reading through the forums, and trying to inch my way to find the problem. I am digging up old cap and rotor and cables to check if the newer ones have gone bad. All plugs will be replaced, as I go down the checklist of what I've read.

What are the many rabbit holes I can jump into here. As of this posting I am still working on her.

Thank you guys for being here for me in the past and letting me pick your guys's brains.

1999 automatic federal emissions
Middle-aged 366,028 miles

Ismael

 

Sorry I forgot some things, IAC recently replaced, map and temp sensors have been replaced in the years.CPS was replaced a few months ago. Didn't need the CPS replaced for a no start, was a broken coil signal wire, but kept it and have a spare.

 

If that engine actually has 366k on it it would seem appropriate to check the compression when you develop a sudden misfire in adjacent cylinders.

 

Plug wires and cap and rotor?

 

All new plugs, replaced cap,rotor, and cables with the was working good back ups ( the last set). Pulled valve cover and nothing broken on top, all moving as it should. Going to go get compression tester. Got a short reading from the codes scanner on the O2 sensor was jumping up and down on voltage, believe I was for heating the sensor, the other tick box was for fuel trim. Putting valve cover back on and then to the AutoZone for a tester

 

Long day, keeps trying to rain on me. Did a vacuum test, used vacuum connection off manifold to valve cover.
got a bouncing reading between 16 and 17 at my rough idle. Blip the throttle to wot and let go, reading drops down low then to about 25ish then back to fluctuating 16-17ish. Working on compression now, well after changing to dry shirt, stupid sprinkles

 

Forgot one thing, I did disconnect injector #3 while idling, didn't seem to make a change. Did it a few times

 

Compression test done. Two tests each #1, #2, #5, #6. Several tests on #3 and #4, dry and wet.

#1: 140 -145
#2: 140-140
#3: 40-45 dry - 50 wet#4: 40-40 dry - 50 wet#5: 150-150
#6: 160-150

3-4 Valves? Or head gasket? As before no oil in water, no water in oil. Not losing water. This sucks she's my daily.

 

Need to check and see if there's any exhaust gasses in the cooling system. Could be a head gasket.

 

You won't know until you get in there but I doubt , based on compression numbers, it's valves. Seems very unlikely both cylinders would have such similar numbers if it was valves.
Suspect it's more likely that after a zillion miles the head gasket is leaking across the top web between the two cylinders (see post #3above ;-) )
new head gasket would fix it if that's the problem unless the head is warped.
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She lives again, running good as ever. Blown between 3 and 4. Good for another 300,000.