Light knock at start up!

Rezid

Registered User
When i start the car up cold it makes a slight knock coming from bottom of engine. It is heard when i accelerate going to work in the morning. After 2 or so minutes it goes away and is slightly heard from time to time. I had a rebuild about two years ago. My CEL is on as well. Will a bad sensor cause a knock or noise in that area, it also runs a little rough. I'm just hoping i dont have an egg shapped rod or something like the first time i rebuilt it. I tried pulling the codes about a month ago but i got nothing. The only sensors i have replace are TPS and IAC, everything else is still stock. Anyone have any thoughts........?
 
Most likely it's a sticking piston pin. After the piston warms up it goes away. It could also be a bent rod though that is causing the piston to be cocked in the bore, but the same result, goes quiet when warm. I've had a bunch of SC motors that do this. Usually it's just a sticking pin. When it was rebuilt if they didn't put enough clearance into the piston pins then they run tight and as soon as a little varnish gets in there they stick.
 
Most likely it's a sticking piston pin. After the piston warms up it goes away. It could also be a bent rod though that is causing the piston to be cocked in the bore, but the same result, goes quiet when warm. I've had a bunch of SC motors that do this. Usually it's just a sticking pin. When it was rebuilt if they didn't put enough clearance into the piston pins then they run tight and as soon as a little varnish gets in there they stick.

Good to know!
Mine is doing that now too.
Cannot wait to get the new motor done and in!

Just gotta find time
 
Don't know the details of the rebuild, only that everything was put in new from gaskets, seals, rings, oil pump etc. One thing that leads me to think its a bent rod is that, the first time around i had two egg shaped rods and were not replaced with new ones. They had them resized. Would this put more stress on them maybe causing them to bend again. I have good oil pressure.......?
 
wow, im not a professional engine builder by any means, and someone feel free to correct me on this, but if the rods were egg shaped, even if they were re-machined, wouldnt the metal still be distorted to a certain degree, with the price of a rod vs price of another rebuild that just doesnt make sense...
 
wow, im not a professional engine builder by any means, and someone feel free to correct me on this, but if the rods were egg shaped, even if they were re-machined, wouldnt the metal still be distorted to a certain degree, with the price of a rod vs price of another rebuild that just doesnt make sense...

It is standard engine rebuilding procedure to resize rods. This has to do with the housing bore dimensions and has nothing to do with the rod being bent. SC rods are forged which means that while they are strong, they also bend rather than break. In many cases a bent rod will be indiscernible to the naked eye and must be checked on a rod checking fixture. This is not part of a normal engine rebuild process and unless the builder knows to look for this, it would not be uncommon to miss a slightly bent rod. If this does in fact happen, there can be a couple possible results. 1) the rod may continue to function more or less normally for a long time. 2) the rod may get progressively worse, 3) the piston may crack due to the stresses involved in running crooked. When the pistons crack they tend to also explode and send the rods out the bottom.

It kind of sucks because you have no way of knowing what the real problem is without disassembly. I've continued to drive the motors in this condition but if you up the HP or boost then most likely it's eventually going to come apart on you. For now I suggest being gentle with it whenever you hear noises (ie. when it's cold).
 
Yeah, I'm taking it lightly, as for now this is my daily driver. The noise isn't that bad. I was listening to it yesterday and sounds more like a rattle than a knock. Either way, have to be cautious! Gonna drive it until it starts to get worse, drives fine except for the noise. Gonna wait until tax time to maybe rebuild again and put new rods this time, if that's the case.
 
Yeah, I'm taking it lightly, as for now this is my daily driver. The noise isn't that bad. I was listening to it yesterday and sounds more like a rattle than a knock. Either way, have to be cautious! Gonna drive it until it starts to get worse, drives fine except for the noise. Gonna wait until tax time to maybe rebuild again and put new rods this time, if that's the case.

Well, a rattle and knock are two entirely different things although similar. Pistons will knock. It is unmistakable (IMO). Anyway, if you think it's a rattle, pistons don't generally rattle because they are kind of big. ;) Sounds like you need to do a little more sleuthing.
 
It is standard engine rebuilding procedure to resize rods. This has to do with the housing bore dimensions and has nothing to do with the rod being bent. SC rods are forged which means that while they are strong, they also bend rather than break. In many cases a bent rod will be indiscernible to the naked eye and must be checked on a rod checking fixture. This is not part of a normal engine rebuild process and unless the builder knows to look for this, it would not be uncommon to miss a slightly bent rod. If this does in fact happen, there can be a couple possible results. 1) the rod may continue to function more or less normally for a long time. 2) the rod may get progressively worse, 3) the piston may crack due to the stresses involved in running crooked. When the pistons crack they tend to also explode and send the rods out the bottom.

It kind of sucks because you have no way of knowing what the real problem is without disassembly. I've continued to drive the motors in this condition but if you up the HP or boost then most likely it's eventually going to come apart on you. For now I suggest being gentle with it whenever you hear noises (ie. when it's cold).

I see said the blind man...It makes sense, I guess my logic is just to buy new, but then again, someone may not want to do a complete bottom end, just what is not working properly.I guess with every different engine there are minor differences with wear,tear,etc. Some one like you Dave who works on these motors days in and days out knows exactly what to look for and have seen it all, all the "normal" car guys (I.E not us) wouldnt think twice to look at something like that. you learn something new everyday!
 
I see said the blind man...It makes sense, I guess my logic is just to buy new, but then again, someone may not want to do a complete bottom end, just what is not working properly.I guess with every different engine there are minor differences with wear,tear,etc. Some one like you Dave who works on these motors days in and days out knows exactly what to look for and have seen it all, all the "normal" car guys (I.E not us) wouldnt think twice to look at something like that. you learn something new everyday!

I agree that buying new these days is just the way to go. We work on all sorts of old V8 stuff at our shop and I lament daily whenever we have to recondition old parts. It used to be that reconditioning was the only way, but now when you can buy new set of 4340 rods for $250 for most motors, it just doesn't make sense to fix stuff. But with the SC that is a bit of a problem since there are no new replacements. You have to go up significantly in cost to get an aftermarket rod not so much due to the rod but the custom pistons you have to order to use them. :(
 
I agree that buying new these days is just the way to go. We work on all sorts of old V8 stuff at our shop and I lament daily whenever we have to recondition old parts. It used to be that reconditioning was the only way, but now when you can buy new set of 4340 rods for $250 for most motors, it just doesn't make sense to fix stuff. But with the SC that is a bit of a problem since there are no new replacements. You have to go up significantly in cost to get an aftermarket rod not so much due to the rod but the custom pistons you have to order to use them. :(

One off car, One off parts, One off price, or maybe one-hundred on price, still workin that out for myself. hah
 
Did some more checking and noticed engine makes a rattling noise at the very end when engine is turned off. Sounds like somethings clunking around in there or loose in there somewhere, but really wont know unless its disassembled. It kills me that i don't have the time and funds to fully address this issue.:mad:
 
Just for the fun of it pull the oil dip stick out and start the car cold. Bought 2 Sc`s that way and owner`s thought the engines were bad when it was the dip stick hitting the crank. I have your problem right now with a NA 3.8 but it had a head gasket failure and they kept on driving it till it got real bad. Changed headgskets and found this little knock mostly when cold. Before doing the gaskets it had no cold start knock but the plug was fouled in that cylender. Think I will let it go til it blow:rolleyes:
 
Did the dip stick thing and knock is still there. I notice when i turn my car off the car still runs for about a second or more and tack bounces up high then drops when car finally cuts off. Could it be my timing thats off and would this make my engine knock??
 
Did the dip stick thing and knock is still there. I notice when i turn my car off the car still runs for about a second or more and tack bounces up high then drops when car finally cuts off. Could it be my timing thats off and would this make my engine knock??

Sounds like you have a fan that's wired incorrectly causing residual voltage after you shut it off.

As for the noise, try removing the supercharger belt and see if the noise is still there.
 
I do... My fan was not turning on at normal temp and a shop was charging me an arm and leg to fix, so they wired it up to where its on at all times. Is this bad? As for the supercharger belt. Do i remove it and then turn the car on?
 
Yep, Remove belt and start her up. I forgot about the blower makeing noise. Had a few of them do this as well. It can make a noise that sounds like a rod or bearing knock. Fan will put a bad load on alt and burn out both early. Its best controlled by temp. They do make some aftermarket temp sensors with a relay.
 
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Got around to it again today and supercharger makes no noise. Took the belt off and sound was still there. I google'd around and read on some forum about bad pcv valve can possibly make engine knock. Which i replaced months ago, i replaced it with another one i had and the noise went away. I cant hear anything anymore...., well for now. Gonna give her a run later and check it out!
 
mine does the same thing seems to thump when its cold goes away in a bout 3 minutes of driving i am very careful not to beat on it when its cold
once it goes away its all good my motor has about 3500 miles on it now i use lucas 10-30 oil ,lucas oil stabilizer and a good filter at every change
 
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