Q: Loud knocking sound when engine is running

asked by on December 12, 2016

I have a loud knocking sound coming from the engine when the car is running. There are no OBD codes pending or present. The sound is not present if I try to start the car with the fuel pump disconnected. I took the cylinder head off to find the cause and only found some carbon build up with a broken knock sensor. I replaced the knock sensor, cleaned the pistons, and the cylinder head. The valves looked fine they werent broken or bent that I could see. Then I put the head back on and I get the same sound. It's only when the engine is running and under combustion that it's present. I checked the timing and it's fine.

It started with with fluttering sound while driving at freeway speeds over the period of a week and then the engine just shut down. No bang, no loud noise just turned off like you would do with the key normally. No codes at the time either.

I have no idea what it could be except a bad rod or bearing. Naturally I hope it isn't, that will be expensive.

My car has 116000 miles.
My car has an automatic transmission.

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Internal engine components each have their own signature noise. An experienced technician can often get a good idea of what is causing a knock by listening to the sound. A connecting rod bearing usually sounds off when you rev the engine up to about 3000 RPM then back off of the throttle. A main bearing has a low pitched knock that is stronger when under a heavy load. Bad pistons will also respond to load, but have a higher pitched sound. There could be any number of other causes of your engine noise and the best thing to do is have an experienced ear on the case. Contact Your Mechanic to have a technician come to your home or office to listen to your engine knock and advise you on what to do about it.

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