Rod knock

I took my son’s car (2001 82K miles) to have oil change and coolant service done at a national tire store. Took it the night before and left key with instructions in exhaust vent in service bay door. Manager called next morning and said there was severe knock and recommended not doing the service and said engine was on last legs and should be replaced or traded as soon as posible. It was 7 degrees over night and was knocking when I picked it up. However when I drove it today (low 20s) it wasn’t knocking as much and when I got to work it was running fine. How much of this is temperature related and how much should I worry

What grade of oil was in the car and how long has it been since it was changed? Was the oil level full?

5w 30 synthetic blend. Oil was full Changed about 4500 Miles 5 months. Oil has been regularly at 3000 to 5000 miles

Several questions.
Was there any knocking when you took it in and is/was the knocking present at idle speeds?

If it’s a rod knock then temperature should have little or nothing to do with it and it should be worse when the engine is warmed up due to the oil thinning out.

I don’t know how much it has been knocking, kid is at college and I only see this car about 6 weeks a year. (Just enough to pay for the work done on it). It was knocking at cold idle.

I’m wondering if what you are hearing is piston slap rather than a rod knock. Piston slap is most audible when the engine is cold, but goes away as the car warms up. Piston slap is caused when the skirt of the piston slaps the cylinder wall. It ususally isn’t a cause for concern.
The old Pontiac straight eight engines engines that were produced through 1954 were known for piston slap. Maybe these old grand daddy Pontiacs passed the piston slap gene down to your Grand Prix.

One other thing that makes me suspicious about whether you have a rod knock is that when the oil is thicker from the cold weather, the knock should be less pronounced and become louder as the car warms up. When I was in college, I was on an out of town trip with some other students. The driver had a 1950 Pontiac and it developed a really loud rod knock on our way back late at night. We stopped at an all night service station and the mechanic drained the oil and put in 90 weight hypoid gear oil. We were able to make the rest of the 25 mile trip back to campus at 20-25 miles per hour. The next day, a wrecker hauled the car to the scrap yard. The thick gear oil did get us in.

GM had the piston slap problem that Triedaq mentioned on many post-1998 engines. I’ve attached a link that should provide some insight. I too suspect this is what you’re hearing.

As far as that national tire store is concerned, I’d avoid them. They should have been able to give you much more definitive information than simply saying the engine is shot. And recommending that you not bother getting the oil changed is definitely not what I’d expect from a shop. If you truely had bearing knock, I’d expect a shop to recommend a different base weight of oil. Not changing the oil at all is never a solution or even a bandaid.

I guess some things never go away. When I was a kid, our neighbor had an almost new 1947 Pontiac. He complained about the piston slap in the engine. Sixty-two years later it seems as though GM hasn’t completely solved the problem.

I think the tire store is on the same page as you MB, they want to avoid the OP also. These guys get faulted if they do the job and they get faulted when they call people up and say “come and get it, I don’t want to work on it”.

What rings loud and clear here is a car owner that leaves maintiance for someone else, it doesn’t sound like junior has kept up on things.

Interesting point about the shop. You may be right.

Thanks for the input.
Tire store just does oil changes and other basic maintenance. They did suggest high milage oil if I hung on to the engine.
I have another shop do the real work on the car and they did the oil change this trip. Car was warmed up when left at shop so the knock was gone.

New question; is there any merit on high milage oils or does this just make loss leader oil changes profitable for the shop?

High mileage oils have special additives in them. One thing these do is help condition seals and gaskets to reduce leaking. That’s actually why I use it, though there may be other benefits as well.

If the knock goes away once it is warmed up then it isn’t likely rod knock. This kind of thing also suggests using a thinner rather than thicker oil - no thinner than what the manufacturer recommends though!