Engine running very rough after cold night...

What do you remember about the last time you used the car before it sat in the cold?

Any chance that could have been a brief use (move it around in the driveway or something, or anyway short enough the engine didn't really warm up)?

 

maybe some unburned hydrocarbons settled back into the intake manifold after the warm up cycle yesterday?

 

Atkinsonized engines push some unburned fuel into the air intake. That area may not of warmed up and some fuel pooled instead reaching the carbon trap.

 

it might be better to take it for a spin than just let it go through the warm up, idk.

maybe something you'll have to experiment with.

 

MIght be a factor. Very short run-times (say just a few seconds) then shut down have been found to cause knocking at next start up, on previous gens, gen 3 in particular.

FWIW, our '10 can be sitting for a week sometimes (on a 12 volt charger). Never any start up problems.

 

The car didn't throw any codes?

Yeah, I would turn the heat on and let the car run longer until it got to normal operational temperature.

 

You did the equivalent of a very short drive in which not everything got roasty toasty.

 

I only drive on the weekends and my car starts right up on Fridays. I would skip the warm up runs-

 

Toyota's analysis of the rattle after cold soak did not involve unburned fuel in the manifold. It involved condensed clear water in the manifold's EGR passages, which are small-bore with outlets lined right up with the cylinder head's intake ports.

Exhaust is 13 percent water, and the analysis seems to imply that after a non-warmed-up brief run, in cold weather, an excessive amount of that water condenses and remains in those small passages.

On the next start, the EGR valve should be closed, as it is when starting, so there isn't flow through that system. But the small passages contain some water, and air at atmospheric pressure, and starting the engine pulls the intake down to the typical vacuum for idle, so the air that's behind the trapped water expands, and the water gets squirted into the cylinders.

It has only happened to me *knocks on nearby wooden object* one time in the last eight years, and sure enough it was on a cold morning when the night before I had just started the car to move from one parking place to another. But omigosh did the engine rock'n'roll and shake the car and sound awful, until it cleared up.

The TSB that Toyota released for the issue included a redesigned manifold with the EGR passages a funnier shape, apparently to make the water-snorting less likely. But whichever manifold you've got, it seems you can avoid the issue fairly well by not doing that very short drive before a cold soak thing.

 

EGR issue is possibly correct. 2010 Prius here and I had an EGR failure a few years ago. Engine sounded so scary missing and clunking (barely running). I panicked and shut it down. Tried two more times same thing. Apparently engine was warming during the attempts and on 4th try engine cleaned up so I drove it to the dealer where they confirmed EGR fail.

 

I wouldn't think it's the EGR in a 2023, it would surprise me since they resolved that issue in the Gen 4's.

If it were me, I'd get the oil changed if it's been longer than 6-months no matter how many miles are on it. Also, get it all the way warmed up and do an Italian tuneup on her...helps push out a lot of gunk and junk in the ICE.

 

Keep in mind that the condensed-water snort should not be confused with an EGR "issue".

The condensed water ends up in the EGR passages just because, well, exhaust is 13% water, and if the conditions and temperature are right, the water will condense there.

The next time the engine starts, the water gets blown out simply because it's there and air at atmospheric pressure will expand into manifold vacuum and shoot the water out, not because anything has gone wrong with the EGR.

A car could have an EGR issue also, but that'd be its own thing.

 

. Sounds scary but it doesn’t take much these days for me to be worried. Hope your baby just had a temporary hiccup!

 

In my experience, that avoidance method works roughly 99.5% of the time, so yeah, only "fairly well." Maybe only 98% when the ambient temperature is in the 40-60°F range, combined with high humidity. This is in a 2011 with original intake manifold.