P0303 Code

Swap the ignition coil from #3 and see if the problem moves. That would verify an ignition coil issue.

ie. say you swap #1 & #3 coils and the code changes to P0301; that would tell you that coil is bad - since #3 cylinder is now firing. You can do the same with the injector and spark plugs; though that would be a lot more work. ONLY move ONE thing at a time while testing; so you don't have to guess which part actually failed.

 

Have you recently replaced the spark plugs? Did you get them off amazon or ebay???
If so, they are very likely cheap chinese copies.

Coils last a long time. Mine have 382,000 + miles. Swoping the #3 and 4 coils is a good way to
see if the coil is bad. If the miss fire stays at #3, it's the plug.
Bosch plugs from Autozone have worked well for me.

If the coil is bad, do not get cheap ones! Unless you like removing the windshield wiper assembly a lot.
They likely but a few hundred miles. $12.50 for Prius coils, is extremely cheap!

Check the coolant tank, is it at the full mark? If it is, the head gasket is likely okay, at least for now.

Usually, the gasket fails between cylinders 1 and 2. But sometimes it's 3 and 4, but it's rare.
You likely will never see water in the oil in a Gen 3 Prius. By the time that happens, you'll need a motor.

 

Did you clear the codes? I guess I had forgotten to tell you that.
Why did you change 1 and 3? Why not 3 and 2 or 4?

It's confusing....

Perhaps you should remove the spark plugs and look at them. And look down the spark plug holes
and see if any of the pistons look cleaner than the others. And you can inspect the plugs as well.

You can also use an inexpensive bore scope. And if you put pressure on the coolant system, you
could see coolant leaking past the head gasket, if it has failed.

Are you certain the engine is actually starting? The hybrid battery will turn the engine over for 15-20
seconds before disengaging. Do you have fuel in the tank?
Perhaps check all the electrical connections, and check the fuel lines, you could have a leak. You did
say you smells fuel.

It does take some still to replace the head gasket. Have you ever changed a head gasket on any engine?
It's the same.

 

Good Luck!

 

Sounds like its not starting at all. A hybrid motor spins the engine so fast people often think the dngine is running.

No

 

#1 is always the one closest to the harmonic balancer. Never use left or right references on the car, since most people don't know their left from their right. Depending on if they're sitting in the driver's seat or staring at the engine from the front of the car. Driver's side vs passenger side sometimes works too; unless your talking about a UK car vs US car. See what I mean????

 

as you stand at front of car, facing engine bay, yes: cylinder one is at your left (aka "front") end of the engine.

Last link in my signature is the full engine section of the repair manual, should have that info. (On a phone turn it landscape to see signatures.)

How long have you had the car?

Addendum: just browsed the aforementioned "full engine section" myself, and could not find a diagram corroborating this cylinder ordering. But it's lore...

 

EGR (Exhaust Gas Recirculation) system, including intake manifold, ever cleaned? If not, it'll likely be fully clogged, and has caused head gasket failure.

 

Sounds like you have a problem with the coil currently in #4 position. At close to 300K miles; IMHO - I'd go ahead and replace all 4 coils. Toss out the know bad one, keep the other 3 for future troubleshooting. Are those Autolite spark plugs iridium? Iridium plugs will go 100K+ miles w/o issues.
re: spark plugs. The brown one is fine, the darker blacker one is burning some oil or running a little rich in that cylinder. Should probably run some injection cleaner with your next tank of gas; that might clean-out a slightly leaky injector issue. If the slightly blacker plug is from #3, that would explain the darker color - unburned, rich fuel mixture from intermittent misfires.

Hope this helps....

 

A value of zero on a cylinder misfire counter means no misfires, while a higher number represents misfires per 200 revolutions.

 

I would just get any random quality set of coils; since they're easy to get to and your going to have 3 relatively good coils to do a quick swap out to test - if you have future issues. Don't dismiss the local auto parts stores; some of them will give you a "lifetime warranty". IMHO - Worth the slightly elevated cost, if your going to drive the wheels off of that car. Ditto with spark plugs; just keep the receipts in the glove box and toss the spare coils in the spare tire wheel well; so you don't lose them in your garage mess.
I had an intermittent coil on my GMC @ 210K miles. It didn't completely die for another 6 months and anytime I pulled over to try to troubleshoot it - it would start working again. My code reader stated P0300, so no help there. I even pulled onto the shoulder of the highway a few times to try to troubleshoot it; only for the problem to disappear again. It eventually failed long enough for me to troubleshoot it, so I replaced it - just the one. Fast-forward 9 months later, the same thing started happening again; so I shot-gunned all the coils and haven't had an issue since. That was 60K miles ago. I only held onto the newest coil, figuring that the rest of them wasn't too far behind intermittently failing too. The set was way cheaper than the individual coil. The single coil, I purchased at a local chain auto parts store, w/lifetime warranty.

Good Luck......

 

The first rule of diagnoses is to make it simple. By moving plugs and coils, you made it unsimple.
If you have a misfire on ONE cylinder, you move THAT coil with the one next to it. Clear the codes.
Run the engine. If the misfire moves, the coil is the issue. If it doesn't move, it's the spark plug.

Those plugs do not look like iridium plugs. Either get the ND plugs from the Toyota Dealer, or try
autozone for the Bosch plugs. https://www.autozone.com/p/bosch-spark-plug-96339/1043174?searchText=spark+plugs&productPartGroupId=azpg4514&productBrandId=BBHK&productUniqueId=5935644815472

Same for the coils. You don't really want to go cheap with these parts. How often do you replace them???

Since you like to do a lot of work, just buy GOOD spark plugs, install them, and drive.
You may not need coils at all. But if you still get misfires, then buy the coils.

 

Don't go cheap on plugs or coils; they may fail quickly and you would have been better off just replacing the one failed coil with oem.