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3.1 MPFI LH0 constant knock or tapping sound


9705

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Hello there, totally new here. I am glad to have found this here, lots of valuable infos!

So I am the proud owner of a '90 Lumina sedan, with the 3.1 MPFI, or LH0. Since I got it, it always made this slight knocking sound, not like a rod knock, but still annoying. At first it only did it when cold, but it slowly started to do it constantly. Last night it was pretty bad, I could hear it very well even with my windows rolled up. Even at idle I could hear it. Maybe I should have worried about it earlier, but what's done is done. I know the 2nd gen 3.1s were known for knocking for a few reasons, most notably the infamous piston slap. Some people told me MPFIs were prone to that as well, some told me my engine was shot and to just let it blow up 😅 What do you guys think? I don't really believe in magical fluids to reduce the sound, since the problem by itself won't be fixed, but I could try using a different engine oil. Apart from that I don't know what to do or where to start really.

 

Thanks for your help

Edited by 9705
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I will do a video tomorrow, including clips I will film tonight as the engine is pretty well warmed up. Tomorrow it'll be a "cold" start, it's a lot more audible. 

It got better today compared to last night, so basically today is as loud as usual, so not badly loud. Yesterday night was cold and very humid, might have been why it got me worried. I'll post everything tomorrow

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I had a bad knock once appear out of nowhere.  Turns out it was a spark plug in one of the rear banks that was not properly/fully tightened had worked its way loose somehow.  Getting back there and tightening it up properly fixed it.

But seriously, a video might be helpful.  Any other symptoms?  

Once a 3.1 is warmed up and at idle, all you really should hear is injector noise.  

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Like I said, tomorrow I'll post the video of clips from tonight and the cold start I'll film tomorrow morning. Not really a cold start, more like a cooled down engine starting up, it's louder when it's cooled down. I did film a "running engine" clip while driving, windows rolled down, on an empty street serving for a future new development. Got it revving on park with the hood open and everything, should give you guys an idea. I did notice that when sitting still, the engine knocks a lot less at first, then every rev I give to it, it doesn't knock at all, not even a little, even at high revs. I usually only hear it either at cold start or when driving. Maybe it has to do with the engine having a load or something...

I never thought about a loose spark plug, honestly. Maybe I should check this out tomorrow. Did it appear and never "evolved", or did it get slowly louder after a while? Because it already did it when I bought it some time ago, but wasn't loud at all and usually only appeared when the engine was really cold. I did drive it during the end of winter. When warmer temps arrived the sound got a lot quieter, but then started to get louder with some time.

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There it is. Posted the video on my youtube channel. Don't mind the other content, unless you're interested.

 

During the cold start shot, you can hear loud noises coming from workers on the roof of my appartment building this morning. It's not from the engine. The temperature was at around 9°C/48°F.

 

Don't worry about the noise the front shocks make during the driving shot. They have been changed today lol and if you're concerned about me driving and filming, I used an uncompleted street serving for a future new development. It's unused except during the day for people who go to the nearby newly built school. It was at 11pm. 

 

 

Edited by 9705
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Update: driving it today as well. The sound nearly disappeared, for absolutely no reason. I can still hear it, but it's a lot quieter than it's been for the last few months. It's actually just like when I bought it. It has always been a bit random, but never that much. 

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1 hour ago, 9705 said:

From the manifold? What sort of problem?

No, further back.  You get a lot more noise in the car, where you're actually closer to the rear exhaust, than you do with the underhood shots.

So definitely give that exhaust system/muffler a good inspection for leaks.

 

 

Edited by pitzel
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  • 10 months later...

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