2001 Malibu - slight knock in engine

[pricer88]

My 2001 Malibu has a slight knock in the engine when it is started cold. After running for a few minutes, the knock goes away. The engine doesn't seem to have any other problems. Gas mileage is between 25-30. The car has 75,000 miles on it. I have only had the car for about 3 months. I really like it (only paid $5K). I hope it doesn't have a major engine problem.

Thanks in advance for any help you can offer.

[teler86]

raise the octane fuel you putting in and change brands if your using crap like speedway

[jdevarie]

Does it only happen when it is cold? If so, see what weight oil was put in last. It could be that a heavier than required oil weight was used and while cold it takes longer to be pulled up by the oil pump and fill the oiling system thus the knocking.
I had a similar situation with my F250 Super Duty V-10 truck with 22K miles at the time. I had no time to bring it to the dealer for an oil change and had taken it to one of those jiffy lube places to have it changed. They put in the wrong weight oil. They used 10W-30 when the truck required 5w-30. The next morning I went to start it and it almost sounded like a diesel engine with all this knocking noises coming from the engine. They went away after about a minute of the engine idling. I looked at the oil place receipt and confirmed that they had used the incorrect weight oil. I had it replaced and the truck came back to its normal self.
Hope this helps.

regards,
Jim

[DaYtOnAt2]

The slight knock is actually a combination of bad tolerances between the piston and wristpin and the low quality aluminum used in the cast pistons. Basically if you let your engine "warm up" before driving and don't beat on it when it's cold you'll be okay. Once the aluminum expands with the heat increase the clearance closes up and the clicking/knocking goes away. Every 3100, 3400 I've seen since '99 has made the same noises.

[gapboi210]

There is a bulletin on this. My '01 did the same thing. I took it in to the dealership. It was a design flaw in these engines. When the engine is cold, the pistons are contracting much more than the cylinders are. The rings cannot compensate for howmuch they are shrinking. Newer motors have shorter wider pistons for less friction and thus beter fuel economy. So when the pistons are cold, they are rocking slightly, this is called piston slap. Dealership said it ws a flaw but just let the motor warm up and dont place a laod on the engine till it does. I went to another dealership and they said the same exact thing but they pulled the motor put and replaced all the pistons with high performance polymer coated pistons. runs great and no more knocking or tapping sounds.