I have a 2010 Prius with 110,000 miles. Since COVID-19 began, it hasn’t been used much. After sitting for 5-10 days in a heated garage, cold startups have become quite noisy. It’s a mechanical sound from the engine like the pistons are hitting something or something quite serious. It’s not burning oil and runs quiet after that initial start up. There is no engine light and the car runs fine. If I start it the next day, it runs normal. Can some suggest where I would begin to diagnose?
If you didn't have it before sitting, it's probably not the head gasket. The hot and cold cycles from day and night could cause condensation inside the cylinders and/or intake manifold which would cause it to knock. There are several things it could be. Look for the simplest things first. You need to drive the car for 30 minutes or so, so any condensasion can be burned off. Check the coolant level before you start it. Then check it the next morning after you've driven it and it has sat. If it's in the same spot. It's likely your head gasket is fine. It is also possible after sitting for 10+ days the fuel pressure has blead down. and it will take a few seconds to build up the pressure again. And the system thinks it needs more fuel and tries, then it gets too much. It could take several seconds for it to equal out. Thus causeing the knocking... You have had the eger circuit cleaned, correct?
My Gen 1 made that sound frequently at startup late in its life, in colder weather especially. Never did find out what it was; sold the car after it encountered an eastbound Dodge Dakota. Gen 1 didn't have EGR at all, and I never had any indication, by coolant, oil, or compression, of any problem with the head gasket. Sure was an unnerving sound though.
It's a cold start knock; search the forum and you'll find it's quite common when the weather is cold and the car sits for several days. (I had it happen for the first time this winter since I'm not driving very often now). Solution: drive it more frequently. Toyota issued a TSB and updated the intake manifold for 2012-2015, but those later cars do it too. 2013 Prius scary noise at Cold start. | PriusChat NEW!!! Service Bulletin for Engine Knocking at Startup T-SB 0012-10 | PriusChat Engine Knocking (twice) at Startup - Anyone Else Experience? | PriusChat
ALWAYS let the engine run until it shuts itself off. A good example is when you are moving the car out of the garage and parking on your driveway. Be sure to let it shut down the engine prior to you turning it off. Otherwise, you will get this nasty knocking noise at the next start. (You will eventually need to clean the intake manifold, EGR cooler and add a oil catch can for engine longevity)
I’ve become adept at shuffling the car around the driveway in that 15 second (max!) grace period, before the engine kicks in. And, a fringe benefit of a block heater: with 1~2 hour plug in, it can spoof the car into giving you EV mode.
I have read that the knocking is due to condensation in the intake manifold. I occasionally notice it, including on the first startup after being stored for six months in a non-heater garage. I asked the Toyota dealer about it and they said that "they" would have to observe it before they could do anything about it. Since it happens so rarely, I chose to do nothing about it. I do not turn the engine off before it has warmed up sufficiently to turn itself off.
before hitting the panic button, drive it everyday around town til operating temp is reached then that should take care of condensation inside the cylinder pistons. If it doesn’t, get a leak down test since it’s a 2010 so you don’t end up throwing money/parts with a blown head gasket
