My sisters truck transmission crapped the bed shortly after I got my new (to me) car so I gave her my old chevy which was very close to being sent to the junk yard. She desperately needed a vehicle and it was still running at the time. (2004 chevy classic, 280k)
Several months later (now) it’s developed a misfire. It started out as a p0303. I pulled the coil pack assembly, removed the boots then laid it sideways on the valve cover to where there was a small gap between the coils and the valve cover. All 4 coils were firing although it seems like my first attempt #3 was reluctant. (it now works every time) I pulled the #3 plug and it was fouled with hydrocarbons. This car has had but doesn’t currently have a system too rich code. I also noticed both fuel trims go negative -15 to -20 when you rev the engine, but is near 0 at idle.
I swapped the plug and it’s oil stained boot (spark tube seals?) with the #2 cylinder hoping the misfire would follow it only to now be greeted with a p0300 when I put it back together. I also noticed it began to feel like a two cylinder misfire. Cleaning the plugs didn’t do anything, I put a fuel pressure tester on the fuel rail and fuel pressure was steady at almost 60PSI, revving the engine it stayed the same. I applied vacuum to the fuel pressure regulator and it dropped so I’m guessing that’s functioning properly. I’m not sure exactly how to test for a leaking injector. Kind of scratching my head I went back to the ignition system. I used a test light to verify there is power and control coming out of the ignition control module. I’m second guessing myself about the coil thinking my test may have been flawed. Should I have tested the coils with the boots on using a test light? I didn’t even think of testing the boots themselves.
I also cleaned up the pins from the ignition module that plugs into the coil because I noticed there was a little bit of oil in there that got by the gasket somehow. I then cleaned the plugs again and noticed somewhat of a difference.
Now when you first start it cold it feels like a single cylinder constant misfire with an additional cylinder missing every once in a while and when it idles down and warms up the two cylinder misfire becomes constant (it wants to die). Before I cleaned it it felt like a constant two cylinder misfire either way. Me messing with the ignition system and noticing a difference albeit small afterwards makes me think this is a spark problem and maybe not fuel related like I originally thought but tbh I don’t know what to think.
Our plan a few days ago was to junk the car when she had a potential lead on borrowing another one but that didn’t pan out so I’m back here trying to do what I can with this.
As mentioned, above, checking for injector leakdown and compression are both worthwhile.
But I’m also wondering why you keep throwing those old plugs back in. A new set will be cheap. The coil pack is likely a different story in terms of $$, so more questionable to just try on a “hunch.” But if it’s all original then it’s all suspect.
Personally, my next step would be checking compression as @Nevada_545 suggested. It will give you a definitive answer to whether or not to continue chasing your tail on spark and fuel.
You guys make great points. I actually replaced the plugs and the coils about five years ago due to a misfire on #2. That time it was just a bad coil, may not be so lucky this time. I don’t want to spend $20 on a set of plugs only to find out it’s a mechanical issue, but it is counter intuitive you’re right.
I’m kind of kicking myself for not checking compression earlier given the age and condition of this vehicle.
I just came across something pretty neat. I still have my notebook I wrote the dry/wet compression numbers down in 5 years ago. #3 is the weak link. Hell of an early warning.
Ya. Def. check compression before spending any more time on anything else. It wouldn’t explain why the misfires didn’t stick with cyl 3, but it’s still best to just check for the mechanical issues before spending more time than needed on other stuff.
When the bad plug/coil was moved to another cylinder, both the original #3 and the new cylinder home for the bad plug/coil was also misfiring. So the code went from a single, specific cylinder to a general misfire code. Seen that happen before so that’s my explanation for it…
BTW, #1 in the historical test looks like a valve might be slightly leaking. It didn’t come up like the others in the wet portion of the test.
OP, do make sure the throttle blade is at WOT while doing the cranking compression test for all 4 cylinders both dry and wet… So all 8 cranking sessions…
Well, I got hold of a compression tester. I don’t know what to make of this. #1#3 and #4 were all over 180psi dry. #2 was completely dead and read 0. There was barely a burp when I pressed the release valve. I tried it at least 3 times and it registered 0 every time. My guess would be a broken valve spring who knows. I really can’t explain how the other 3 cylinders test better now than they did five years ago lol. I really wish I would have done this from the beginning.
With the ignition and fuel system taken out of the picture it’s going to be something I’m not screwing with if it requires opening the engine or pulling the manifold. I like this car because it’s a guinea pig and the one I learned how to wrench on but in all reality the next step is probably the junk yard. Now I’m gonna have to find somewhere else to store all the precious junk I had shoved in that trunk.
A broken valve spring will make a LOT of noise, you would not miss that. More likely you have burned an exhaust valve. At zero compression, a good share of the valve face would be missing. You could also check the valve lash at the #2 exhaust valve, it could be really tight and not letting the valve close completely but that would also indicate that the valve is seriously burned and most if not all the dace is burned off.
sounds like it’d be a blast for a driveway owner. Maybe some day. Perhaps it’s gunked up, the bit of piston I could see through the spark plug hole was pretty nasty looking so maybe its like that. This car was ran from 132k to it’s current mileage on probably 3 oil changes. When I was looking at it a few days ago I noticed some exhaust or oil smoke was coming from around the throttle/intake area on a cold start. I couldn’t tell exactly where though. Never noticed that prior to this misfire developing.
