?Misfire? - only under heavy throttle

This one is a bit of a head-scratcher for me.

2013 Prius plug-in - this is the car we bought last fall that had been overheated. The overheating destroyed the head gasket and warped the head. We had the head professionally rebuilt, put it back together with new Toyota parts, and put 5000+ miles on it with no problems apart from a CEL for an oxygen sensor.

This past week it started doing something odd, but only under heavy throttle. Keeping it under maybe 75% throttle avoids the problem. But going beyond that 75% mark, it acted like it didn't have any more oomph. It would rev, then drop, then rev, then drop. One time I let it keep doing that a few times, which yielded the triangle of death. But disconnecting the 12v battery for a few minutes brought it back to where it had been.

Putting a code reader on it yields the oxygen sensor CEL, plus usually a message about a coil misfire. Pretty sure it's on the original coils. The time I got the triangle of death I got a wild array of codes (33 of them, I think) - everything from hybrid system problems to a "small fuel leak" (there's a code for that?) to low engine performance. But keeping the code reader on after that, we're back to the oxygen sensor and the coils.

Is this odd behavior really consistent with a coil going bad, though? It only quits at high RPM?

I have a set of junkyard coils I can try swapping in (from a wrecked Prius cab, presumably running OK prior to the wreck) - but there's a lot going on here these days, so I wanted to check here whether that was likely to help. Thanks in advance!

 

Thanks for the feedback, each of you! To answer the questions:

Yes, we cleaned the EGR exhaustively. We tried many different ways, but ultimately what worked for us was a combination of strong chemicals, physically loosening up the crud with a few different tools, and also using a pressure washer.

We also replaced the plugs (with NGK iridium) when putting the car back together. My attitude was that I wanted to replace anything likely to fail while I had the car apart -- also all gaskets and the water pump (which had over-revved when the car had run out of coolant, which is how the previous owner overheated it). Incidentally, it hadn't run out of coolant because of the head gasket failure. It ran out of coolant, which caused the overheating and the head gasket failure. We know this because the previous owner was honest about having accidentally left a coolant line disconnected after changing the right headlight bulb, and also because there were signs of coolant having splashed everywhere under the hood afterwards.

I didn't fix the existing CEL yet because it came and went, and because it didn't seem to impact drivability or mileage at all (and, honestly, because life has been too busy to take apart the car for a problem that didn't seem to affect using it). I had taken a glance to see whether there was an obvious bad wire to cause an intermittent fault like that, but didn't see anything that jumped out at me.

Car has 110k miles. MPG has been wonderful, averaging 59 despite a long (up to 60 miles round-trip) commute.

Coolant level hasn't budged and the problem doesn't seem to have gotten worse. No smoke or odd smells with the exhaust.

It feels like it's easier to provoke it when the car has been running for awhile and is up to temperature.

Okay, other symptoms. This is all going to seem a bit vague, apologies in advance.

- It sometimes feels like the misfire comes right when the car would kickdown to low gear.
- Sometimes (not always) the car seems to 'buck' a little more when the ICE comes on, compared to what it had done.

Ah, one other question - I remembered last night that my junkyard coils are off a gen4. Visually they are the same as the gen3 ones, although the part number is different. Anyone tried swapping those before? If you have, does it work?

Thanks again for the help.

 

The oxygen sensors arrived via Amayama, $163.21 delivered for the pair. Not bad for genuine Toyota. Installed them the day before yesterday... lower (downstream) one was pretty straightforward, upper (upstream) one was not! Take off the heat shield first (three bolts, 12mm as I recall) and you can get it with a plain old 22mm wrench. Easier than trying to sneak behind the heat shield IMHO.

Took it out for a spin - no misfires! Hurray! We used full throttle over and over again: on-ramps, merging, passing, just enjoying being able to use full throttle. Not a single misfire.

Satisfied that I had that problem behind me, I went to bed. The next day, I took the Prius for a drive, and ...

Misfiring. And more misfiring. And more misfiring. Oddly I still get O2 sensor codes, even though the parts I installed were genuine Toyota (and Amayama has a good reputation, I haven't seen any concerns about whether their parts are real). I also get a bunch of codes for coils, usually all four of A, B, C, and D. It's almost always when asking more of the engine, and I can largely avoid provoking it just by staying out of the throttle.

Tried the gen4 coils, the car didn't like that at all.

Pulled the spark plugs, nothing very interesting to report there.

Borescope inspection likewise didn't reveal anything very interesting.

Ordered a set of coils from Amayama. Anything else to try in the meantime? I had been so tempted to buy a set of Duralast coils from the local Autozone, but the Amayama ones ended up being only a little more (like $20 difference across four coils, for original Toyota vs some unknown manufacturer) that I decided I'd just accept the wait.

But why on earth did the car run so well for one evening? What's up with that?

 

Ack, somehow I didn't see the notification that I had new replies. Arg.

Sparks are NGK 93501 Iridium IX from Rockauto. Completely agree about parts from places like fleabay often being questionable.

I did resist the temptation to use the Duralast coils. Parts from the local dealer are not in the budget, so we ordered from Sharjah via Amayama. (4x 90919-02258 for $260 delivered, to give you an idea of the savings...) Since it's Sharjah they are presumably ordering them through Partsouq, but giving a somewhat better price than Partsouq does. I have used both Amayama and Partsouq in the past, and while you'll wait for the parts, they will only charge for what they actually send.

The coils should get here in another week or two, I'll try to update at that point.

 

This turned out to be very close to the right answer!

Amayama canceled the order, but Partsouq took care of us. An hour after they arrived, I was installing them while charging the traction battery. I know, not advisable, but Providentially this actually led me to the answer: while I was working on the windshield wiper housing, I glanced over at the valve cover area at the exact moment that a fat spark came from the one ground wire attachment...

It turned out that both ground wires from the wiring harness to the valve cover (not the big one going from the engine to the chassis) were only finger-tight. Looking back, I reattached them while reassembling the engine after replacing the head gasket, but I must not have cinched them down enough right then. The car made it 6000 miles before this became an issue.

Replacing the oxygen sensors helped for an evening. Looking back, I am guessing this was because reaching the upstream sensor involved me climbing onto the engine, such that my knee probably bumped the bad connection and restored it temporarily.

Moral of the story: if you have a misfire after reassembling your Prius -- do check those grounds!