4.6 Knocking sound at idle on cold start?
 

Seems that this issue has started in the last few weeks. Here are the details:

-24k miles
-bone stock engine, original owner
-change oil every 5k, only use Motorcraft oil filters
-only does it at idle and seems to go away after 10 minutes of running
-doesn't matter if it's cold or hot outside
-no warning lights, car runs fine

Never had this problem before. Almost sounds like a rod knock....that goes away when warmed up. Not too loud, but there. I googled this, and seems like this happens to many ford modular motors. Some link/posts say folks have even had their motors replaced under warranty for this.

I'm just wondering what to do as my warranty will expire next year. Anyone else heard of this or have this happen to them?

Thanks for any comments.

That always makes me think of an exhaust leak - because it goes away. A spun bearing doesn't go away when the engine warms up.

I haven't read this any where before - what does your google links say the cause is?

My 06 GT has the same or similar noise. When cold and loading the engine down as in accelerating lightly, it makes a similar noise. Just idling or very light load it doesn't make the noise.

On mine, it sounds more like perhaps the variable cam drives instead of the bottom end (I hope).

I've got 48k miles with regular oil changes from the Ford dealer's Fastlane ,and I don't thrash it very much, although I have lugged it down a bit too much in traffic..

Only time will tell if it gets worse.....

I found some stuff online about the modular 4.6 and 5.4 motors having "piston slap"....whatever that is. Seems to be mostly coming from F150 forums.

Found this as well:
http://www.modularfords.com/f128/col...-knock-170695/

Mine made a similar noise when cold. I switched oil brands (Motorcraft to Pennzoil Platinum) and it went away.

Motorcraft oil is very good but every engine is different.

You might try the easy things first.

Piston slap - bingo!
I just traded my 05 GTO [loved that car] because i couldn't stand it [piston slap]. Car was stock unabused with 32k.

My 08 is making that same noise. It started just when it was cold now it seems to do it even when it's warm, but only at low RPM's in a high gear. I think I just am a little crazy about sounds... always looking for something to fix. I have to remember it's a Mustang not a BMW. I actually put some "Dura Lube" in last week and it is alot less noticeable (you can all make fun of me now).

Look at the oil filter next oil change. Look for metal pieces.

The piston slap is common in almost all cars now. To increase the rpm range and thus increase horsepower numbers, the mass of the piston has to be reduced. An "ingenious" idea was to move the piston ring and shorten the skirt. This effectively fixed the mass issue and did increase the power by allowing the redlines to be moved up.

Piston slap is noticeable in my winter vehicles (subaru) and my Mustang never sees cold, so I don't ever hear it, but based on piston geometry and the fact we use a 5w-20 semi-synthetic, I can see the oil bleeding down and having some "slap" on cold starts until the oil gets pumped up top and circulates and builds up pressure in the ring glands.

By changing your oil, you allow for residual (synthetic or thicker oils) to stay in the areas that the conventional oil bleed down from and thus the noise is less noticeable (like the 2000-2004 Silverado/Sierra).

Changing your oil viscosity does one BAD thing. It doesn't allow the oil in the sump to heat up and quickly make it to the top of the motor. If you start with a higher viscosity, it takes longer for the fluid to make its way under pressure through the passages and up to the top of the motor.

You would want to put a 5w synthetic in there, but contact FORD as 5w-20 Motorcraft semi-synthetic and a 5w-full synthetic may exhibit different properties.

Ultimately if oil fixed the issue, or if the issue was more than an annoying noise, FORD would have simply specified a different oil. They make LESS money per oil change by specifying what they did, so I'm sure they have nothing to gain by telling us to use Motorcraft semi-synthetic.

Ok, I need to revisit this thread because I heard that noise for the first time tonight in our 12k mile '06 GT. We've had it for 2 months and purchased it from a friend who was very kind to it and used Mobile1 5w-20 since the first oil change. Since I use nothing but synthetic I had no problems continuing that schedule. I changed the oil last week and I heard this noise for the first time tonight after a 6hr cool down from a 60 mile drive and starting it in a heated garage.

I backed it out of the garage and had to get out of the car. That's when I heard it, otherwise I may not have heard it because I can't hear it from inside but it's obvious outside.

I wouldn't mind hearing what others are using for oil.