A little background. I purchased a 2003 SE about 2 months ago. 60k miles. Seems to be in great shape. The engine idled a little rough at times, but overall it ran very good.

Over the weekend I did an engine flush with ATF, ran a can of seafoam through the engine, changed the oil (went with Mobil 1 0W-40 and M1-301 filter), and added a can of seafoam to the gas tank (with 91 octane premium fuel). After all of this, the engine ran much smoother and also had less top-end (valves, lifters, rockers, tappets, etc.) noise. However, now I have noticed that after running the engine, when I put the vehicle in park, I get a fairly loud knocking sound that seems to be coming from the driver’s side top of the engine. It can only be heard at idle. As soon as the rpm is brought up to anything above idle, the knock goes away. It almost sounds like a diesel engine. Any ideas on what could be causing this and anything I could do to make it better? I don’t think the noise was present prior to flushing the engine and changing the oil. The previous owner claimed to be running Mobile 1 oil, but I have no way of knowing if this was the case or what viscosity he was using.

 

Check you oil pressure. Have you checked your VIN to see if you are in the oil pump range?

I would never suggest you put seafoam in your oil, if that is what you did. Sounds like you might have a little rod knock. Need to get your pressure checked.

 

My VIN is out of the range (not sure that makes me feel much better). No, I did NOT put seafoam in the oil. Don't think it's necessary with synthetics. I DID use the seafoam treatment through a vacuum line on the upper intake and also DID put seafoam in the gas tank.

 

Only does it with a hot engine and not cold?

 

Yes. Only when hot. There are some noises when cold, but not this noticeable knock. Since I'm running a 0W-40 oil, it ought to stay reasonably viscous when hot, right?

 

No the oil will thin way out when hot.
I think it is in oil getting to thin when hot.
Try a 10w-40.
Where do you live?
Flat tappet engines need a thicker motor oil, you could probably get away with the 0w-40 in the dead of winter in Siberia, but no way in hell you can use it in Texas.

 

I'm in KC, and believe it or not, Mobil 1 recommended the 0W-40 grade since their 5W-40 (which is what the Owner's Manual suggests for my climate) is for diesel engines.

But back to your suggestion....wouldn't a 10W-40 have the same viscosity as a 0W-40 at hot operating temps?

 

You would think so but no.
The top number is the "amount" of protection that the oil offers and the bottom number is how thick it is.
So a "0" motor oil is as thin as water when cold and even thinner when hot.
So a 0w-40 is a "0" motor oil that protects like a straight "40" weight motor oil.
It offers that amount of protection because of additives. The bigger the gap in numbers the more additives they need to add.
You can use diesel oil in a gas engine but cannot use gas oil in a diesel as it does not offer the soot and particulate protection that a diesel engine needs.
Drain and refill with 10w-40 and the noise should go away.
I use Mobil 1 5w-30 in the winter and Mobil 1 15w-50 in the summer.

 

Spike, thanks for the info.

I switched to Mobil-1 10W-40 over the weekend, and unfortunately that did not fix the knock. I've noticed that it isn't always there. For example, I pulled into my driveway yesterday and put it in park. The knock was there. I raised the the hood, go to get a stethoscope tool I have to try and find the noise, and the noise basically went away before I could find it. It was still there, but so faint it was drowned out by other engine noises. But just this morning, when I got to work and parked, it was back. Any ideas?

 

You need to stop driving it till you can have the oil pressure tested at idle when cold, at 2000 RPM's when cold, at idle when at operating tem and also at 2000 RPM's when warm.
The fact you are not in the vin range means little, so don't get to comfortable with the notion that you don't have a problem, cause you may do to the fact that others have been found outside the vin range.