Greetings all. I searched the threads and found no thread specific to this issue, so here go's. Vehicle is a 2001 Express 3500, 5.7L with 128K on engine. It is a unique Cutaway service truck I purchased new and have taken very good care of everything on this machine since day one. I would like to get another 100K out of the truck and, since I live in the Pacific NW, rust is not an issue. I have been changing the engine oil religiously every 2K miles and decided this time to go with Mobile 1 synthetic for high mileage cars. I usually use Wix filters but could only find a K&N when I purchased the materials for this change. Drained oil, new filter. 5.5 quarts of Mobile One Synth. Now, when I start the truck there is a very low (deep) sounding knock when started. It goes away after a couple seconds, but it sounds terrible. I'm ruling out the oil filter check valve. My question is this: Has anyone experienced this using synthetic oils ? And, should I change back to the old style oil I used before. Also, The oil pressure runs 40 PSI when cold then hits 20 when at operating temp. I am seeing lower pressure now. 38 cold and 15 or so when hot. I have read about GM 5.3 knock and the "it is not a problem" service release, but this sound hurts when I hear it. Any input would be greatly appreciated. Thanks.. George.

 

Greetings, from another part of the PNW (where rust is definitely an issue).

I don't think the oil is CAUSING the knock. Synthetic is good oil. But it may be highlighting some wear that was already there. You must spend a lot of money on oil.

I would also not be so quick to rule out the filter.

 

Thank you for the response Mountainmanjoe. I'm guessing you are East of the hills where you see more snow !! I never thought about how much oil I have used. I just remember my Grandfather telling me about the importance of oil changes. That stuck with me even though I know oil is much improved from the days of non detergent brands. I do notice it takes a second or so longer for oil pressure to register on the gauge but when I looked into the K&N they state they have the check valve on the filter. The sound is so ominous it feels like its coming from the depths of the motor. I may end up changing back to the dinosaur just to see what happens. Merry Christmas !

 

I'm surrounded by coastal range mountains and can see the ocean from my window. These aren't your grandfather's cars and not your grandfather's oil. They can go much longer. I would follow the interval in the owner's manual.

If you want to chase down the sound, you can get under there with a mechanic's stethoscope. But if it's an internal problem, then you can't do much about it short of pulling the engine. If you're handy, you could drop the oil pan and check the pump and screen. Or give it to a mechanic. Or live with it. But before touching anything I would go back to your Wix filters and see if that clears it up.

Merry Christmas

 

Knocking sounds reflect worn out parts, which had got so much tolerance than oil can't fill the gap any longer. It could also be the oil itself, being to thin or too old (and got thinner). If the noise is only in the bottom then it could be the bearings in the crankshaft, or the displacement bush also in the crankshaft at the extremes of it. However a "diesel engine" kind of rhythmical continual tapping noise is typical heard when the valves or the camshaft are worn, or there is too much valve clearance. If the engine has oil plunges to move the valves, one of them may be getting "lazy"

 

Change your oil again with your regular oil and filter. I've lent my parents my vehicles and you can tell by the engine noise when they've had the oil changed at a cheap place.
I'd say the synthetic loosened up sludge but you shouldn't have any with such low oil change intervals.

 

What was your usual oil choice?

 

My usual oil has been Quaker State or similar Penn brand. So I took it out for a decent ride on the highway to get a listen and monitor the gauge. Got home and let the engine cool down completely. When I started it up I did not hear the sound at all. Nothing. I am going to monitor the situation. If it comes back I am going to switch back to my old brand which will add one more oil change to my growing list.