2005 Prius P0301 misfire

One guess would be oil fouling the plug. Is the plug just dark with carbon or a heavy deposit that might be burned oil? Any smoke in the exhaust that might indicate a oil leak into the cylinder?

 

Definitely check the oil level like xpcman says. Besides fouling the plugs overfilling can foul the Mass Air Flow sensor and wreak havoc with engine performance. And the MAF and plugs would go bad again if not corrected. Also check your vacuum lines and fuel injector connections since you did plugs twice.
Likely areas to check are:
Open or short in engine wire harness
Injector
Mass air flow meter
PCV hose connection
PCV hose

 

A failed cyl 1 fuel injector which is spraying too much fuel is a reasonable possibility since you've swapped spark igniters and the misfire stays with cyl. 1.

 

Have you checked the resistance of the coil pack on the #1 cylinder? If the resistance is too high, you won't get a reliable or sufficient spark to properly ignite the fuel and trigger misfires.

 

Well, you could have insufficient compression pressure in cyl. #1 or the connector from the wire harness to the coil pack or to the ECM could be faulty. Valve clearance is a potential cause.

 

Just been down this path myself. I'd take an educated guess and say replace the injector or remove them for professional check and clean. I just bought a gen 2 cheap because nobody could diagnose it. It turned out to be a dirty injector, nothing more. Using a bore scope, (camera), it too looked very oily inside the cylinder, however simply unburned fuel due to misfiring. I used to work in a Toyota franchise as a qualified tech and the 2 archilles heels of these engines appear to be the injectors fouling and oil control rings if services are neglected. I suspect it is the stop start nature of operation that makes it more prevalent in these. As I am a mechanic, and have my own shop, I tore mine down and rebuilt it just because it costs me so little and I want to keep the car. But having said that I doubt it was absolutely required. I knew it had a bad service history so elected to make sure I knew what I had. In your case, simply deal with the injector as these engines are incredibly reliable mechanically. In the case of a misfire, it must be, in simple terms, fuel, spark or compression. Yes oil can complicte the picture but the amount ti takes to cause a persistant misfire, when plugs are fresh will always show as exhaust smoke. As Patrick hinted, swap parts to different positions and see if the misfire follows the components in oreder to diagnose. Best of luck!!

 

My bet is you're correct. I've seen this exact scenario on the few occasions with these engines. Incredibly robust and have long service life however they do appear to foul injectors under certain circumstances, particularly cold weather and lots of stop start running. This is only anecdotal observational analysis, but beats the hell out of getting hoodwinked into an engine replacemnt that you don't need. Btw, I can tell you how to clean them yourself for cheap if you're confident and have the gear to do it.

 

Next step should probably be a compression check.

 

A child could do it. You need a 10mm socket and a 12mm socket. Remove the air cleaner assembly. You will look straight at the 12mm bolts that hold the fuel rail in place. Simply remove these bolts, 2 of them. Wiggle the fuel rail away from the head, not too aggressive, and unplug the wiring to the offending injector, and wiggle the injector from the rail. At this point the pressure will release but it's not too scary. Place a rag over it to stop it getting in your eyes if you like. Simply put another injector from a wrecking yard perhaps back in place,making sure the O rings are in place, use a little wd40 if it helps to re-install it and replace everything in the same order. Fire it up and bingo, I bet it's as good as new again. Maybe cost 25 bucks, if you're lucky. I can post pics if you need help. It's really a 5 minute thing. The hardest part is going to get another injector. I don't kn ow if you have Echo's over there but all 1NZ toyota engines took that same injector, so doesn't have to be a prius. Good luck. As for the head gasket etc, no that is major surgery and as yet unrelated to this procedure. You'll be amazed how simple this is. BTW, don't give yourself hell by removing the air cleaner completely, just pop it up and swing it tone side, wires still in place, just so you can get to the injectors.

Before you start, if there is dirt around the injectors, use compressed air to blow it all clean. If you don't, that dirt will go down into the intake as soon as you remove the injectors....not so god for the engine. Just a thought.

 

Nope seriously...dead easy. I can send pics if you get stuck. It's late night here now, so can do it tomoz if you like. In about 12 hours I could take step by step pics and post them. But if you can change oil, you can do this.

 

No probs Bryce, I'll see what I can do. I'm better on the spanners than the computer so maybe this will be interesting...lucky I have techno savvy kids. While I'm at it, I'll post details about how to enter it into inspection mode so you can run it to check it when done. It's just as simple.

 

You might think that it is necessary to remove the engine valve cover when you look at the fuel rail. However it seems there is enough play between the fuel rail and the valve cover "finger" over the rail, so that you actually don't have to disturb the valve cover.

 

Quite right. Yes it can be fiddly, but no, you don't have to remove anything else to do this, apart from the air-cleaner assy, even then you simply swing it aside out of the way, wiring still plugged in.