Engine slightly "misses" at cold start-up?

Clogged injectors tend to stay clogged after warmup. Modern fuels and filters are much better so it's rarer to find injector problems.

To me this all sounds like class water in the system. The fact it misfires slightly and it's cold. Once the engine is up to temp the water evapourates and the engine runs fine. Once it cools down the water condenses back into liquid form read to cause problems on the next startup.

Simply pop the engine cover off and pull the spark plugs out. They may need attacking with a wire brush to clean them up. Put the plug back in and spray some water dispersant like WD40 and reconnect up.

I don't know the exact setup on the prius but on other cars once the leads/coils have been connected a bit of Vaseline is useful to add some form of seal to discourage water getting back in.

It's dead easy a good thing to eliminate first

Ive had this on some cars and this has always fixed it.

 

----could moderators repost this in the care,maint & troubleshooting--

may need attacking with a wire brush to clean them up.
I liked most of this answer but the wire brush....I wouldn't use on these fine tipped iridium plugs. I know spark plugs used to be routinely cleaned, is there a preferred way to clean these, with some solvent? I might try an intake cleaner like sea foam if you see deposits (directly on the removed plugs themselves) . My old plugs a 100+k certainly had some buildup but they were replaced. Hobbit also covers some of the possible water splash problem here 100K maint - 2 along with some nice closeups of used spark plugs. Checking them will help you narrow down the problem.

One web page with iridium cleaning tips http://www.globaldenso.com/en/products/aftermarket/plug/qa/tech/qa_23.html

 

It doesn't sound all that unusual, really. Check the oil level, if it was just changed it may be too high. If you have been buying gas with 10% ethanol you've been getting all the injector cleaner it would ever need.

And for pity's sake, don't scrub the spark plugs! This isn't a Massey-Ferguson tractor or a fine Lucas product :_>

 

Can you explain what you mean by "misses"? I have a 2009 as well and maybe I will pay more attention next time I start cold.

 

Oh brother! It's a 2009! There is -no way- an injector is clogged or a sparkplug is fouled! It's new. What you are noticing is probably normal. A cold engine will not run perfectly smoothly. It -may- learn after a few months and run smoother during warmup, but at any rate, just about any car will run a -little- rough at cold startup.

Oh, and water will not cause this only when cold. There is no distributor cap. There are no spark wires. The -only- water that could be there would be in the fuel tank. It would miss warm or cold.

 

I know what he is talking about, I have a 2007 and it feels like it is misfiring when its cold outside, but everytime it "misses" if you listen a bit closer you can actually hear a difference in the sound of the engine each time afterwards it sounds like it is slowly adjusting the idle speed downward as it warms up. I believe this is normal. If it is really misfiring it will throw a code no matter how small of a miss it is. I literally have to clear codes on a VW of ours once a month, and everytime a misfire is involved it logs it even if it didn't throw the MIL / CEL. I would assume the Toyota would log these as well so you could always go to autozone and have them read you the engine codes if you don't have your own reader. I have always gotten the "no error codes found" whenever I have connected mine.

 

I have exactly the same problem with my 04 gen 2. Did anyone figure it out? I drove a rental one once, and it didnt make this problem!

 

When the Prius gasoline engine is first started (cold start), the ignition timing is retarded, and idle speed is a bit higher (1,300 RPM or so) than it will be once the engine is warmed up. After a few seconds the ignition timing will be advanced, and idle speed will slowly come down to normal, which is 1K RPM.

This is all normal.

Regarding cylinder misfire, a DTC may not be logged until the misfire rate on a cylinder is 25% or more, so the absence of a DTC does not prove that there's no problem. However in the OP's case with a new vehicle, it is unlikely that a substantial misfire is present unless water got into a spark plug well.

 

I'm wondering if there's any new information on this.
When I first start the car and the ICE cycles it is really rough, and sounds like someone dumped a bag of marbles in the engine. If I bring the revs up to about 1400 it settles down, also at end of the warm up cycle the idle drops and it smooths out as well. It only seems to be at the initial ICE start after turning car off.

I've checked plugs, gapped to 1mm. Di-electric grease on coil boots.
Cleaned MAF, and throttle body.
Cleaned all connectors for coils and injectors.
I have Marvel mystery oil in the fuel.
I have a new PCV valve, and the hoses were clear.

Any ideas?

 

Bag of marbles on a G2 may be a bad cop. There is a few youtubes on it. Sounds like the motor bottom end is blown. Sounds really bad.
But just needs a new coil on plug. I would have bet money it lost a main bearing.

After the marbles stops shut the car off and then put it in inspection mode and let it fully warm up and get under the front of the car under the engine and listen for blown bottom end sounds thumping and banging or thudding.

 

The sound disappears when the revs are just above and just below the rev range that the marbles sound happens, so I don't think it's the engine.