Q: Engine makes weird noise

asked by on July 31, 2016

I start my 02 Bonneville and the engine makes weird knocking noise and gets a little louder under acceleration somtimes the car wont even start at all but when it does it makes the that noise agian i could probably take it on a quick 5 minute drive before the engine cuts off and the car wont start up till its been sitting there for a couple of hours i also have a vid if need to i can show it please help me with this issue thanks.

My car has 197590 miles.
My car has an automatic transmission.

The knocking noise and the dying are most likely two different problems. Diagnosing sounds is inherently not possible without being present. Even with a video. There are some very specific noises a car can make that with experience a technician learns what they are. In your case, remember I haven’t heard the noise, a knock on start up and on acceleration is usually a crank shaft issue. The crank shaft has two sets of bearings. The main bearings and the rod bearings and both make distinctly different sounds. Only experience has taught me their differences. I could not have heard the difference if someone had tried to explain them to me in a classroom or in a conversation. Either way, it will at the very least require the oil pan to be removed and the cranks bearing to be inspected.

But don’t jump to this conclusion without an experienced technician has listened to your noise. As for your dying while driving, the 3.8l has an ignition module that commonly displays this problem. As the motor warms up, the internals of the module expand and the motor uses spark. When diagnosing the GM 3.8l, I look for spark and injector pulse. If it has injector pulse an no spark, I suspect the ignition module. If it is missing spark and injector pulse, I suspect the crank sensor.

If I suspect the cranks sensor, I connect a scan tool and observe the crank sensor data and waveform. From this I will know if the crank sensor is the issue or if further diagnostic time is necessary.

Keep in mind, these are the pattern failures I have seen on the GM 3.8l motor. I recommend you book an appointment with a certified technician, such as one from YourMechanic, to diagnose your starting issue and the noise you’re hearing firsthand.

Good luck!

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