new rod bearings, now knocks at only certain rpms

[khartley]

So I put in the new rod bearings, torqued to spec, except I went a 1/4 turn over on each accidentally. Polished the crank with emery cloth very lightly, but the crank had no scarring to start with, very light light scratches. I used assembly lube on each bearing. Anyways, this is what its doing....at an idle there is no knock, all I can hear is an exhaust leak coming from I think the beginning of the downpipe. When I rev it, there is light knock from about 2000-2300 rpms, but that is all, I took it up a little past 5000. I drove it a few blocks, I was really easy on it, and it was the same way, with just knock at those rpms, but a little louder. Its a 99 n/t that I bought just to sell.
So what could it be? I'd like to figure it out without having to take the bottom end apart again, cause I'd like it sold ASAP. Is it the bearing? Maybe exhaust leak? Should I hold it at high rpms a while and hope it goes away? , not sure what to do, any advice would help

 

[khartley]

OK, I just went out and started it to see how it was today, and when I first started it, I revved it and slowly went to different rpms and heard NO knock at all...then my husband got under the hood and started revving it really high, really fast and it started knocking and would just do it at the high rpms.....then we let it idle and I tried to do the slow rev again, and it was doing the light knock again, but this time at different rpms....I only went as high as about 3000 rpms and I just shut it off.

So now I don't know what to think...is the bearing just not sure if it wants to spin or not? Or is it something totally different?

 

[fallenauthority]

Did u replace the bearings due to a spun rod bearing? If so, your crank is done, no matter what, it will have to be removed and turned(not sure if these cranks are non serviceable like the 4g63t cranks) and polished and new bearings put in again. I had the same problem, and the damage was one such a small level that i couldnt see any but it sure enough spun the new bearings i put in mine too. Looks like you took on a bigger task that you thought by picking this thing up just to turn around and sell it.

 

[khartley]

Yeah, sucks cause the engine has to be pulled for the crank to come out. But what I don't get is why it wouldn't be doing it at an idle....any time I've ever dealt with a spun bearing, it always knocks at idle too, which this one doesn't.
Well, regardless, even if I resell it broken I'll make money on it. This guy already offered me $1800 for it with the spun bearing....so if I didn't fix it I'll get at least that.

 

[khartley]

Oh but another question for you....was it your #4 bearing? And did it spin right away after you replaced them, or did it take a little while?

 

[fallenauthority]

it took 3 days but im sure thats cause i never got it high in the rpms and went 2 miles each way to a from work. then the 3rd day i decided to get on it and see if the problem returned and bam ass soon as i hit it and went past 2300 rpms it knocked and stopped. Yea it was my number 4 bearing. Mine didnt do it at idle either, that is until i figured i would just replace the whole block so i drove it to and from school and work for a week and by the time i parked her for the swap it was ticking at idle, revving, driving, all the time. sounded horrible.

 

[dr1665]

The crank on a 420A is serviceable to a point. From what you're describing, I'd say you're looking at a mandatory bottom end rebuild. Internals should be alright aside from needing the crank and journals turned for oversized bearings, but I wouldn't try driving that car very far. I would be willing to bet that symptoms continue to get worse the longer that car is driven up to and including a new window in the block, if you push it to hard.

No new information to you, I'm sure, but figured I'd let you know you're probably going to have to cut your losses on this one.

 

[khartley]

Yeah, I knew that if just new rod bearings didn't work that meant the crank was too scratched up. I don't consider it a loss though, cause it was basically a free car, so anything I get out of it is profit.

 

[Strife]

I had 2 rod bearings die on my 99nt and I rebuilt the entire longblock my self in my fathers garage. I did not have the crank turned. The engine has run well if not a little loud for over 16,500 miles now. Maybe I got lucky? Not trying to argue just stating my experience.

Here's why I posted. I too had the exact same problem you are describing, and believe it or not the problem turned out to be the bolts that hold the crank to the plate for the clutch module. It sounded just like a cross between an exhaust leak and a loud knocking. I know I torqued the bolts to the spec in the shop manual but that's what the problem was. I retightened the bolts and the noise went away. (I found out it was the clutch because I took it to the dealer for analysis)

Its a long shot but I hope it helps.

 

[fallenauthority]

I had 2 rod bearings die on my 99nt and I rebuilt the entire longblock my self in my fathers garage. I did not have the crank turned. The engine has run well if not a little loud for over 16,500 miles now. Maybe I got lucky? Not trying to argue just stating my experience.

Here's why I posted. I too had the exact same problem you are describing, and believe it or not the problem turned out to be the bolts that hold the crank to the plate for the clutch module. It sounded just like a cross between an exhaust leak and a loud knocking. I know I torqued the bolts to the spec in the shop manual but that’s what the problem was. I retightened the bolts and the noise went away. (I found out it was the clutch because I took it to the dealer for analysis)

Its a long shot but I hope it helps.

It would be nice but i would bet 100 bucks that its still the rod bearings, just for shits and giggles drop the pan and pull the bearing that spun before and see what it looks like.

 

[VelocitàPaola]

Per your other thread, perhaps your rod caps bolts have failed? TTY bolts cannot be re-used.