Hello! I have a 2013 prius with around 150k miles. In the last week or so, it has had a very faint rattle intermittently at start up (sort of a flapping sound-- the first time it happened, I actually thought I had a flat tire). The check engine light came on and it is giving p0301 which I know can indicate head gasket failure. The coolant and oil are at normal levels and look normal. Just now I replaced the spark plugs and swapped the first and second ignition coil without replacing them. My understanding is that this way I can tell if it's just a bad coil (because the code would change positions). I tried to clear the p0301 code but can't clear it as it's permanent. Now here is where I'm confused-- does this mean that p0301 is definitively still an issue that the car is detecting? Or do I need to drive it around some and read the codes again to see? Any suggestions for next steps? I'm not particularly skilled at working on my car myself so I'll probably take it to a mechanic for anything more complex than spark plugs but helpful to know what this crew thinks!
it's probably the very beginning of a head gasket leak, but if you don't get the symptoms again, maybe it is the coils
Where did you swap the plugs? Since it's the #1 cylinder is where the code is, you could swap the #1 and #2 plugs. Then swap the #1 and #3 coils. Check the #1 plug and see if it's wet. An inexpensive bore scope that works with an iPad is good enough. If the cylinder is semi clean, you know the gasket is leaking. And with a pressure test, you likely will see the coolant leaking, if it's a head gasket failure. Seems like it might be just starting to fail, so you might not see a huge lose of coolant. Watch it EVERYTIME you drive the car. Check the levels everytime also.
Sorry, I meant to say I replaced the plugs! They needed replaced anyway. Then I swapped the #1 and #2 coils. I didn't notice any oil or moisture on the plugs but I can look into getting the scope. I don't drive all that much, probably have put under 15 miles on it since the first time I noticed the noise. Unless I'm likely to do damage, I'll probably drive it this week and confirm that the codes are still present and then take it to a mechanic for testing on the headgasket.
I meant to ask if you replaced them, but I got distracted while typing... Kinda surprised you can't clear that code. Perhaps you need to different reader? But if you get a misfire on #2, you know it's the coil pack. But that is not very likely. How many miles are on it?
My experience with noise related to misfire is that it is very loud and very metallic sounding. It does not take very much coolant leaking into a cylinder on shutdown to cause misfire on startup. If you don't drive far enough to get the engine up to operating temperature the cooling system will not heat enough to build pressure and it's that pressure that pushes the coolant into the cylinder after shutdown. A coolant system pressure test would be a good idea because if you are at the onset of head gasket failure it will only get worse with time/use.
Thanks everyone. Mileage is 150k. The code was listed a few times, once as "permanent" and once as "stored". It did clear the stored p0301 code and turn off the check engine light. I would not describe the sound on start up to be very loud generally, but it was loud enough to make me think something was very wrong the first time I heard it. Less loud since. It's possible I'm just not driving it far enough for much coolant to be leaking in. I'll take her for a longer trip this week and see what happens.
Doesn't really matter how much you drive. Just that the engine gets to operating temp. Then the car sits, and as the engine cools, the coolant leaks into it. MAYBE, it's just condensation. If you stopped the car when it was cooler, you'll get some condensation in the manifold or a cylinder. Thus the knock. If it's knocking pretty much every time you start the engine, you have an issue. Just keep a close eye on the coolant level. Try to check it at the same settings each time so you can get a more accurate reading on the coolant level.
You've got a gen 3, with 150k miles. It's making odd noise at start-up. It's got code p0301 (misfire cylinder one). You replaced all plugs and swapped cylinder one and two coils. You tried to clear codes, but P0301 persists. I'd borescope inspect the cylinders, preferably with engine coolant system pressurized, in the morning after previous day driving the car. If it's the head gasket, the overnight sit will give any coolant leakage a good chance to accumulate, and the pressurizing will accelerate the leakage at the head gasket seam. Head gasket failure is very common at those miles, and it usually starts with cylinder one. EGR system ever been cleaned?
