The Honda K24 motor (2.4L DOHC I-VTEC) is used in quite a few of their vehicles. I have two of 'em. One in a fairly high state of tune in our premium fuel '06 Acura TSX and a more workhorse regular fuel version in our '12 CR-V. Apparently since around '08 and until recently in 2016 there has been a reoccurring and basically unresolved issue of what Honda refers to as "Engine Rattles at Cold Start-Up" in their Service Bulletin 09-010. The problem is well documented in various forums and You-Tube videos. Basically, on cold starts the engine makes a loud chatter for a couple of seconds after start up. Specifically, the source of the problem is the VTC Actuator on the intake camshaft (item 7 below) and the Service Bulletin 09-010 describes replacing the defective VTC Actuator. The VTC Actuator is part of the I-VTEC system and it's function is to advance intake cam timing on demand.
My out-of-warranty '12 CR-V with 45K miles had gotten to the point that it rattled on nearly every cold start. Some say this is just a harmless nuisance but it bothered me and I can't help but think that valve train life (specifically cam chain and tensioner life) is compromised by the kind of violent impacts necessary to make a noise that loud. I wanted mine fixed. FYI this is an expensive repair when you're paying a dealership ($600-$800) because it takes several hours of shop time and common internet "wisdom" is that the fix was only temporary because Honda could only replace the old part with a new instance of the same part and the problem would likely return at some point. However, early in 2016 Honda revised the VTC Actuator design and supposedly the newer part resolves the problem permanently. Let's hope so!
As you can see in the parts fiche above there are two part numbers listed for the VTC Actuator.
14310-R44-A01 is the old part and specifically referenced in the Service Bulletin
14310-R5A-305 is the new design part according to the parts manager with whom I spoke.
Heads Up! There may still be the older version in dealers parts bin - be sure you buy the newer one:
Service Bulletin 09-010 is very thorough so I'm not going to reproduce that here (see link above). Bottom line, it took me about 4 hours and was actually easier than I expected. Of course, YMMV. And yay!, so far, the problem has been resolved. The Service Bulletin recommends discarding the old VTC Actuator on removal but being a curious sort of guy, I disassembled my old one to try to understand how this thing works. For other curious types here's what I found.
When you pull the camshaft cover off, here's what you see.
The VTC Actuator advances and retards intake cam timing depending on instructions from the ECU and works with the more conventional VTEC technology of using multiple cam profiles depending on power demands. Note the mild and more aggressive intake cam profiles. The cam followers transition to the more aggressive profile when instructed by the ECU. Older VTEC motors had only this function. With I-VTEC Honda added variable cam timing.
Removed from the camshaft the VTC Actuator looks like this:
Remove the cover and here's what's inside:
There's a spring loaded piston that 1) locks the "spider" by dropping into a recess or 2) allows it to rotate depending on oil pressure supplied to he unit. It is this rotation in the "unlocked" position that advances intake cam timing.
A different view of the extended piston
When the piston is extended (i.e. the VTC is locked), it fits into this recess in the backing plate
Piston removed and spider fully rotated clockwise (i.e. fully advanced)
The spider is free to rotate when the piston is removed or when it is compressed by oil pressure. The entire unit is pressurized with oil and the spider runs on hard plastic "bearings"
The camshaft is hollow and carries pressurized oil to its bearings and to the VCT Actuator
So, there it is. This thing is really pretty simple but is obviously a highly precise machined component. I have no idea what Honda did in revising the part and wasn't about to take my new one apart to figure it out; but, I suspect they have included some kind of return mechanism to force the unit back into locked position at shutdown. At any rate, mine appears to be fixed for now. Got my fingers crossed!