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Engine Knocking on cold Start

Hello, first thank you for your time. I have a 2002 Pontiac Grand AM GT with 151,000 miles (I have had the car for 6,000). I recently just changed the oil with 5w-30 which is the 3rd oil change I have done and have used the same brand and viscosity. It seems the oil is a little overfilled this time (maybe 1/4-1/3 of an extra quart). I noticed a knocking coming from the engine (closer to bottom of engine than top) on a cold start. It sounds like a rock being knocked against a hard surface. I can only hear the knocking if I am outside of the car, if I am in the car I can’t hear it at all. The knocking lasts for maybe 20-30 seconds. If I press the accelerator (when in park) the knocking goes away, and if I let off the accelerator then it comes back. I can’t really test to see if the knocking persists while driving since I can’t hear it from the cab.

I know I should do all the routine maintenance when getting a used car (spark plugs, etc), but I bought from a friend who put spark plugs on maybe 25,000 miles ago.

I read that maybe I should be using different viscosity of oil in the summer (even though the manual says 5w-30 all year, I live in North Central Nebraska)? Is the knocking maybe due to the slightly overfilled oil? Should I try to drain that excess out? Should I just go ahead and replace the spark plugs and wires as well?

I drive ~750 highway miles a month and hope I can get this car to last a few years. So far it seems pretty good, have only had to change the ECT sensor and thermostat on it, suspension bushings still look good. I just never heard this noise before and it kind of freaked me out. I probably would have never heard it but I just happen to start the car and needed to grab something from the garage.

When an engine knock is most pronounced when releasing the throttle a “rod knock” as in Rod Knox, is indicated. And when the dipstick indicates that the engine is overfilled there is a possibility that gasoline or coolant is leaking into the crankcase. Smell the dipstick. If it has the odor of gasoline it would be advisable to not start the engine until it has been drained and refilled with the proper oil and the source of the contamination found and repaired. If coolant is leaking in the oil will look like a milk shake and as with the gasoline, the oil should be changed immediately and the problem investigated.

There is no odor of gasoline and coolant is not is not getting into the engine. The overfilling is just my mistake of putting a little too much oil in the last change (again it is 1/4-1/3 extra quart). The rod knock you speak of, would it just do it only on a cold start and only for 20-30 seconds? I thought rod knock was continual and gets more progressive as you speed up?

Piston slap is also a possibility.
Can you post a sound file so that we can hear it?

I will try, won’t be until at least tomorrow at the earliest. Might just try and drain some oil out and see if the noise goes away. That is really the only thing different about the car, but I know things can happen for any reason especially with that many miles.

If the knock totally disappears when the engine is fully warmed up piston slap is a likely cause and rod knock is considerably less likely. And that is usually a much better situation, @Samsus.

I drained some of the oil. Knocking is still present but much quieter. It is possible this has been going on longer than the recent oil change, as I can’t hear it in the car. Lasts maybe 30 seconds after cold start (haven’t tried warm start to know for sure). It is louder on the bottom of the engine compared to the top. Attached audio (from under engine) as requested.

I would suggest that you start using 10w30 in the future. Your engine will like it better. Use it year round, a synthetic 10w30 will help with cold winter starts.

Sound to me like valve clatter caused by weak lifters. The sound goes away when the lifters pump up.
Is this the 3.4L OHV V6?

I agree with @insightful because it sure sounds like valve clatter to me. My Dakota (3.7 V6) does this for about a minute then quietens down completely. It’s done this for years.

Yes, this is the 3.4L V6. I will switch the oil weight and see how it likes this. What is the time frame I should repair this?

Don’t use 10W-30 oil. This could make matters worse.

The 5W and the 10W is the weight of the oil when it’s cold. So the 10W oil will take longer to get pumped to critical engine components.

Listening to the audio it sounds like there’s stuck lifters. Stuck lifters will cause valve clatter in the valve train until the oil reaches them and pumps them up.

To free up stuck lifters you want to add this to the engine oil.

Follow the directions on the can and see if the lifter noise goes away.


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ok thanks for all the advice. will go and pic some of that stuff up and try it out.

I’m having the same problem with my 02 dodge neon right this minute any thoughts on what it might be ???

I had the same problem, it ended up being the tensioner going bad! It sounded terrible like a valve noise for a bit then would go quiet.

@lbstefanski … diagnosing engine noises is what pro mechanics are especially good at. They do it all the time, and learn the subtle sounds of each problem by listening to and diagnosing hundreds or thousands of examples over the years; i.e. they learn by the school of hard knocks … no pun intended … lol …

This is an excellent problem to take you your local inde mechanic for an opinion. Posting an audio file here might generate some ideas too.

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you are having the knock for 20-30 seconds on cold start?

My car still has the knock. I think it is lifters like one of the replies suggested. The sound basically goes away or becomes really quiet when I add sea foam on a regular basis (It says what adding to the gas tank and adding to engine oil does I can’t remember off my head which is for lifters, guessing adding straight to engine oil, but I was basically doing both for awhile). I now put Heat into my gas tank when filling in the winter, and sea foam in my oil when it is getting close to oil change (because the sea foam really brings out some dirt in the engine and after a couple hundred miles with it in it needs to be changed). I haven’t had any engine issue or power train problems since owning this car (had to replace strut assemblies and hit a deer but those are unrelated). I don’t drive it much (usually down to the creek and back for the dog, and 25 miles for a trip each month). Sorry I can’t really offer to much more than what I have said: I believe my issues were lifters and sea foam really helped that specific issue, but just be wary using it in oil makes it dirtier faster.

I’m inclined to agree with the recommendation to let another shop, a reputable independent mechanic that’s been around a while, listen to it.

Rod bearing knock will, when you rev it by hand under the hood, become a hard rapping sound. It can even be detected by the antiknock sensor and trip a “misfire” code, the sensor not being able to differentiate between the pressure wave from a misfire and the pressure wave from a rod bearing knocking. The engine will react by faltering, as the ECU cuts back on the injector pulsewidth and retards the ignition timing. An experienced mechanic should be able to tell its signature sound immediately. Rod bearing knock can come & go… but it’ll get worse over time until the bearing fails completely.

In short, I’d suggest having an old timer look at it.
It might help too if you could record the sound as the engine is revved and post it here.

i understand that it is always best to have a pro look at it, but is it really worth it for a $500 car? At least in my parts there are always neons for $500.

I once took my car to 2 places at they both said it was the serpentine belt and ended up being the timing belt. Both looks cost me $70. It just seems weird that there are replies here for a car worth $500 to have a pro that is going to cost $50-$100 for a diagnosis.

What would you suggest then?