Chasing a pesky cylinder 3 mis-fire (P0303) issue on 1999 5.2L.

I want to explore the possibility of bad valve or cracked head.

My question is: if it _were_ a cracked head, or carbon build-up on a valve, would that trigger the P0303? I would think that the only way the PCM could tell if a cylinder was not working would be Ignition or fuel flow? If those two items are good (I have already found that spark is good on #3 and I've swapped Injectors and reset the code-store), then the PCM could only "know" from the exhaust / O2 sensor side, and by then, it would not have data to know "which" cylinder was mis-firing.

Next step is the compression test.

One more question -- I did not put the Combustion Chamber cleaner in the engine after the plenum gasket replace -- If I had carbon build-up on a valve -- would I stand a good chance of getting it clean with CC Cleaner ?

(I was afraid it might gunk up the cat is why I didn't do the CC Cleaner -- is that a mis-guided fear?)

Thanks ,
Lee

 

Is the top of the injector clear? If you look in my sigline I have a thread devoted to my engine rebuilding. Three of my injectors were plugged, and I think #3 was one of them. That could be part of your misfire problem.

 

I believe the injector is clean -- I had them all out and cleaned with carb cleaner. They all looked the same, no buildup or trash and I swapped them around. (Fuel rail looked okay, too). It's not the greatest cleaning, but in spite of the swap-outs, it's a consistent P0303 code. Takes about fifty miles to display the code after I reset with a battery disconnect and switch-on.

Thanks for the info,
Lee

PS: Thanks to the poster who mentioned the "noid" light -- I wish you could see the look on the faces of the guys at the parts places. I guess I haven't found a good place -- but maybe I could just buy a setup.

 

When i first bought my truck i was chasing down a misfire for a few months. When i first bought it the plenum blew, because of the lemon law, the dealership fixed it. After the plenum was fixed the "check engine light" was still on for a misfire in cylinder 7 i think? i know it was one of the back ones though (where most of the oil would drain into because of the blown plenum). Well, i brought it back to the dealer multiple times, after they told me that nothing was wrong, well i finally brought it back and told them i was not picking it back up until the problem was fixed. Well i got a call a while after and he said that they took the head off and one of the valves was cracked (the plenum was probably blown for a while before it got fixed to cause this). So if your plenum was ever blown, i would definitely check your valves.

 

the misfire problem will show up if your head is cracked and you cant trace it down to anything... generally, it cracks between the valve seats and you can see a carbon trail between the valves. If you yank the head and that is the problem, i'd recomend yanking the other one and swapping them both for a set of the HD's, the castings are pretty affordable at under $300 bux each, last time I looked. (hughs has them).

do a compression check and leakdown test on the cylinder to make sure it's sealing, if it is, then i'll bet it's the head being cracked. I did a comp test on my heads before pulling it apart for a r/t head install and found that the readings were all good, just idled crappy. when I yanked the heads, 2 cylinders on 1 head, & 1 on the other head were cracked. New heads and it was running great.

 

i dont havea misfire but my truck idles like garbage,, compression/leak down all test good.. i know for a fact i got cracked heads.. ive heard cracked heads can also give you a spark nock after all the normal remedies are already been tried.