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My new to me 2001 Corvette with 144k miles has a P0300 misfire code. As I literally got in the car for the first time after purchase, the CEL came on and showed the P0300. The CEL turned off after about 30 minutes of driving but the code remained stored. The DIC code reader only shows the P0300 (for codes related to the engine anyway). The engine at idle does have a noticeable skip/shake to it every couple seconds and is a little rough. It idles around 600-630 RPM. Does that seem a little low? It feels strong when driving however, only issue is seemingly at idle. Also should mention the exhaust has a carbon/gunpowder smell.
Maintenance that I have done so far is cleaning the throttle body (covered in oil), cleaned the MAF, and replaced the PCV valve.
The PCV valve has been bypassed it seems for some time. I noticed it after purchase that it was just a straight hose going in behind the throttle body where the PCV valve should be, and the original PCV housing was tucked up underneath it with just the the grounding cable holding it in place. I fabricated a new housing for the PCV and installed that hoping it was just acting like a vacuum leak causing the rough idle but it runs the same.
I would be looking at the ign. coil(s) , spark plugs and ign. wires. The spark plugs should not be black or sooty looking. If you find a black sooty plug, the next thing is to determine why it is black and sooty. Is it a weak coil? bad plug? excessive blow-by? leaky injector? There are a multitude of possibilities here. Report back the plug condition a picture will help. If it's not a sparkplug then you'll have to do some more troubleshooting. My guess is excessive blow-by but that's just a guess. Good Luck!
I pulled one plug from each bank. That said, plugs and wires may be in order since half of the wire boot connectors are barely or not even connected at the plug. I barely pulled on a few and it was just leaving the connector behind on the plug. One just ripped the bare wire out from the boot. I have a new set already. One plug was only half seated.
I just had this DTC... found one plug wire burning (arcing...) on one of the header tubes. Replaced wires (and plugs..) the DTC and rough idle are gone.
No black soot on those plugs. They're not the problem your looking for but since you have them out might as well rule them out completely and replace.
I'm in the process of doing that right now. I got them all replaced (all were ACDelco except for the hard to get one on the back next to the brake booster, that was an Autolite)
One on the passenger side that I me ruined wasn't in all the way was because it can't go any more than about 4 threads. I have a theory that it isn't cross threaded but there is just a bunch of sludge in the rest of the thread from running like that for who knows how long. Any tips on what to do here? Run a tap through it?
Are you saying you can't get the plug to seat in the Cyl. head? If so inspect the threads first for damage if ok, then go ahead and run a tap thru it but gently and put grease on the tap to catch the metal shavings. What did the rest of the plugs look like? the first two? I hope.
I’d say about 75% of misfires are ignition related...you really have to give those plug boots a good push to seat them fully...you should hear a good click !!
Are you saying you can't get the plug to seat in the Cyl. head? If so inspect the threads first for damage if ok, then go ahead and run a tap thru it but gently and put grease on the tap to catch the metal shavings. What did the rest of the plugs look like? the first two? I hope.
Forgot to mentions, but all of the plugs were clean, just white residue on them like the ones pictured. What size thread tap would this take?
It is tough to get my camera in there to see but this is why I suspect it is just gunked up. You can how far I can thread it in and the rest is just black.
Originally Posted by C5 Diag
I’d say about 75% of misfires are ignition related...you really have to give those plug boots a good push to seat them fully...you should hear a good click !!
Yes, I'm making sure they click solidly on to each plug and coil.
Forgot to mentions, but all of the plugs were clean, just white residue on them like the ones pictured. What size thread tap would this take?
It is tough to get my camera in there to see but this is why I suspect it is just gunked up. You can how far I can thread it in and the rest is just black.
Yes, I'm making sure they click solidly on to each plug and coil.
From this picture the threads look bad. But its just a phone pic. not sure on tap size. use the sparkplug threads for the tap size. 14mm something ??
I ran a tap through it. A couple small flakes came out but the new plug threaded right in and it is holding tight. I took it for a good 25 mile drive and it runs great. But the rough idle is still there. I forgot to mention that I cleared the P0300 code about 100 miles miles ago and it still hasn't come back. Before my test drive I had given the car a wash including the muffler tips. After that 20 miles there was a very small amount of soot on the bottom of the tips. Not sure if that means anything.
I ran a tap through it. A couple small flakes came out but the new plug threaded right in and it is holding tight. I took it for a good 25 mile drive and it runs great. But the rough idle is still there. I forgot to mention that I cleared the P0300 code about 100 miles miles ago and it still hasn't come back. Before my test drive I had given the car a wash including the muffler tips. After that 20 miles there was a very small amount of soot on the bottom of the tips. Not sure if that means anything.
Rough idle could be a leaking intake manifold, or possibly dirty injectors. It's hard to tell. Maybe you could find the cylinder(s) that have the miss by pulling sparkplug wires one at a time, the one that has the least RPM drop is the offending cylinder. Do not pull them off with your hands, it will knock you on your ***. Get plastic wire puller and use it to pull the boots. Let me know.
Rough idle could be a leaking intake manifold, or possibly dirty injectors. It's hard to tell. Maybe you could find the cylinder(s) that have the miss by pulling sparkplug wires one at a time, the one that has the least RPM drop is the offending cylinder. Do not pull them off with your hands, it will knock you on your ***. Get plastic wire puller and use it to pull the boots. Let me know.
I use a 12 volt test light to do a cylinder drop test...NEVER got shocked !!... you just have to keep the tip of the test light closer to the end of the coil pack than your fingers when removing and installing the plug wire !!
Food for thought. Removing plug wires on a running engine causes the coil to produce maximum voltage, then collapse back through the coil creating excessive heat and can damage the coil. I wouldn't advise pulling plug wires to do a cylinder drop test and just letting it run with a wire disconnected - even for a short amount of time.
Food for thought. Removing plug wires on a running engine causes the coil to produce maximum voltage, then collapse back through the coil creating excessive heat and can damage the coil. I wouldn't advise pulling plug wires to do a cylinder drop test and just letting it run with a wire disconnected - even for a short amount of time.
As long as you give the spark somewhere to go (i.e. using a test light connected to ground)...you’re fine !!
As long as you give the spark somewhere to go (i.e. using a test light connected to ground)...you’re fine !!
Yes - as long as the spark can get to ground and return to the coil properly through the ground path all is good. My comment was meant to prevent people from pulling a wire and letting it hang (post 14). I see people give that advice all the time "Just pull a plug wire to check for..." That and the one where people tell others to disconnect a battery cable on a running engine to check an alternator make me cringe.
Last edited by Ed Ramberger; Apr 23, 2020 at 03:38 PM.
As long as you give the spark somewhere to go (i.e. using a test light connected to ground)...you’re fine !!
Sorry I haven't responed for a while, I've been pretty busy. I'll go grab a test light and test the coil packs the way he does in the video. Am I looking for the same kind of thing, the distance the spark travels (less travel compared to others would be a weaker spark)? If it's a weak spark would that cause the soot on the tail pipes which seems to be caused by unburned fuel?
Sorry I haven't responed for a while, I've been pretty busy. I'll go grab a test light and test the coil packs the way he does in the video. Am I looking for the same kind of thing, the distance the spark travels (less travel compared to others would be a weaker spark)? If it's a weak spark would that cause the soot on the tail pipes which seems to be caused by unburned fuel?
C5Diag was describing a cylinder drop test for a comparison of how much the engine drops RPM when each cylinder is deprived of spark and doesn't fire.
Engine Misfire P0300
