This has been a on and off problem for a few months now. The first time it happened I was driving at about 30mph and everything up to this point had been fine. But while slowing down at a red I heard a little sound. And when I came to a full stop it idled rougher than usual and there was a very noticeable thumping sound coming from the engine bay. When it turned green and I pushed on the gas there was significantly less power and the thumping became more violent. luckily my destination was only a block away and I made it there. I then shut it off and left it for about 3 hours. Then I called a friend who knows more about cars than me and he came to look at it with towing equipment in hand. He looked at it and said to drive it home so I drove with him and it kept thumping the entire time. It was worse when stopped in drive and would get better in neutral but wouldn’t go away. At lower rpms the thumping was most violent. And at higher rpms it was almost un-noticable. He seemed to think it was ok though even though it had way less power and was just kinda freaky. He said to just keep driving and see how it went. So I did, I tried to stay at or below 35 because it wasn’t as bad at low speed. But strangely it slowly got better until after a few weeks it was completely back to normal! I was amazed and thought my car was magical. It stayed fine for a few more months then just yesterday out of the blue it came back exactly like before! The day before I hadn’t done anything different but the next morning when I started it, the idle was more rough. At first I didn’t notice but as soon as I put it in drive I heard the thumping again.
I don’t know if this is related but sometimes it also just won’t start. In the summer it was fine but just this last winter whenever it was really cold it would just refuse to start. Like it wouldn’t even turn over. The first few times I thought it was the battery but jumping didn’t help. Eventually I figured out it just needed to sit for a bit and sometimes slam the door or wiggle the gear shift etc. And it will eventually start. Usually within 10 minutes, it has done that this entire winter and now that it’s warming up it’s started happening less.
Also when I first got the car it had some engine knock but after replacing the bearings between the Piston and crank shaft it stopped. But the bearings where in absolutely terrible shape. One was broken in half and they all had chunks missing… But after cleaning up the metal shards and giving it a few oil changes everything seemed fine. Except something didn’t do back in quite right. I had to take the engine mount off to get the oil pan off (terrible design) and while I was working I think the transmission sank a little even though it was on a jack stand but when everything was back together it would make a similar thumping sound when shifting into reverse. Everything else was completely normal but it felt like the engine dropped a bit when shifting into reverse. That never went away but again car friend didn’t think it was a big deal. Also when it started thumping the mpg went down by about 8.
Sorry this is so long, what do you think it could be? FYI the car is a Nissan Sentra xe 1.6L 1994.
What is your milage? what is the maintenance history?
It has 123,000 miles. I’ve only had it for about 10 months but when I first got it I changed the oil to full synthetic and changed the filter, replaced the distributor cap / rotor and the spark plugs, new air and fuel filters, replaced rear brakes, did a seafoam treatment, and replaced the Piston to crank bearings. Note that after replacing them it started making a thumping sound when shifting into reverse.
You now need to replace the crankshaft to block bearings and maybe the piston to crank (rod bearings) bearings again. What you described for the old rod bearings convinces me the crank bearings also need replacement and the crank needs to be re-ground and all new undersize bearings installed.
I am sure the noise you are hearing is bearing knock from the crank moving up and down and back and forth.