Random thumping after rebuild
09-14-2021, 12:01 AM
#1
Random thumping after rebuild So just got my truck back together after a rebuild. New rods and pistons, Stage 2 TSP truck cam, valve job on the heads. Pretty sure the heads were milled a sizeable amount. On first startup, after messing with it a bit and getting it to idle, it started doing this random thumping noise. It seems like it's on random cylinders at random times, some thumps more concerning than others. I have oil pressure and these thumps aren't showing up on my knock sensors. My only guess is maybe detonation in the headers due to excess back pressure, but that is a complete stab in the dark and I don't even know if that's a thing. Thought about disconnecting the headers from the exhaust piping and seeing if anything changed. Attached a video, any advice would be greatly appreciated!
09-14-2021, 07:04 AM
#2
Can’t hear the video. Is there black smoke and a strong smell of fuel. Are your headers stainless if so what color are they turning. Is the exhaust hooked up? Open headers can cause a fake lean condition.(sending extra fuel) Oxygen sensor needs some exhaust pipe behind it to prevent it from getting clean air. How big are your injectors?
Last edited by jasons69chevelle; 09-14-2021 at 07:12 AM.
09-14-2021, 07:54 AM
#3
Can’t hear the video. Is there black smoke and a strong smell of fuel. Are your headers stainless if so what color are they turning. Is the exhaust hooked up? Open headers can cause a fake lean condition.(sending extra fuel) Oxygen sensor needs some exhaust pipe behind it to prevent it from getting clean air. How big are your injectors?
I do smell fuel, haven't had it running in the daylight so not sure about smoke. Stainless headers, they don't appear to be getting abnormally hot but I haven't ran it much either. Stock injectors. Exhaust is connected out the back, headers collect into a single muffler, then run to another muffler, then dual pipes out. One of the tail pipes has a bit of a kink in it... Ran fine before I did the work to it with the pipe like that.... But thinking that may have something to do with it. The setup ive always thought was wierd tho but never questioned it.
09-14-2021, 09:34 AM
#5
It almost sounds external like something clunking in the trans tunnel. Do you have anyway to monitor the engine data.
09-14-2021, 10:08 AM
#6
09-14-2021, 04:19 PM
#7
ModSquad
iTrader: (6)
Sounds like it’s backfiring through the intake ever so slightly. Did you measure pushrods?
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09-14-2021, 09:03 PM
#8
That's exactly what I was thinking, You could hook up a vacuum gauge and see if it bounces according to the noise, If it's backfiring into the manifold it would see a slight pressure spike when it happens. If it's an exhaust backfire noise it would also be obvious at the tailpipe.
09-14-2021, 11:11 PM
#9
Well I was thinking it was the pushrods. I did go quite a bit shorter than stock, 7.275. I even posted this thread because I was coming up with such short rods: https://ls1tech.com/forums/generatio...-pushrods.html
So I double checked it this evening... Lifter calls for a 0.04 to 0.06 preload, and getting 8.5ish turns on the length checker. So 8.5x.05=.425.....0.425 +6.8 puts me at 7.225, plus preload of 0.05 puts me at 7.275. Also tried the turns to 22ft lbs from zero lash method just to see what I got... Got 1 full turn so that puts me right in that 0.04-.06 preload range. The machine shop did tell me a 0.100-0.120 shorter lifter wasn't out of the realm of possibility.
Looked over my grounds and wiring... Not seeing anything amiss there. Read somewhere getting the coil packs in the wrong side could cause intake backfire but that just doesn't sound right. Will go ahead and swap them since I've got them.
Vaccum lines I just replaced... They seem like they're in better shape than they've been since I've had the truck.
So I double checked it this evening... Lifter calls for a 0.04 to 0.06 preload, and getting 8.5ish turns on the length checker. So 8.5x.05=.425.....0.425 +6.8 puts me at 7.225, plus preload of 0.05 puts me at 7.275. Also tried the turns to 22ft lbs from zero lash method just to see what I got... Got 1 full turn so that puts me right in that 0.04-.06 preload range. The machine shop did tell me a 0.100-0.120 shorter lifter wasn't out of the realm of possibility.
Looked over my grounds and wiring... Not seeing anything amiss there. Read somewhere getting the coil packs in the wrong side could cause intake backfire but that just doesn't sound right. Will go ahead and swap them since I've got them.
Vaccum lines I just replaced... They seem like they're in better shape than they've been since I've had the truck.
Last edited by Benny82; 09-15-2021 at 02:49 AM.
09-15-2021, 08:22 AM
#10
Do you happen to have a caliper that can measure the pushrods? I ask because when I called around for pushrods only Manton claimed the could get the pushrods to within .007 length. All the rest said within .025. Not acceptable when you have a preload range .035+/-.005 and .093 total travel.
When I ordered my pushrods I had a few different sizes. None of them were exactly the size I ordered. The largest out of spec was .005. Because we grouped the sizes that were close together splitting the difference on some rods. I ended up ordering 11 more rods. I didn’t like the preload I was getting and wanted it closer to spec. I would have had different sizes on nearly every pushrod. sounds like you are using a lifter that requires a low preload with a tight tolerance. Every pushrod will be a different length I would guess. I found with my comp cams checker it was out .017 from the 6.8+turns compared to a actual measurements.
When I ordered my pushrods I had a few different sizes. None of them were exactly the size I ordered. The largest out of spec was .005. Because we grouped the sizes that were close together splitting the difference on some rods. I ended up ordering 11 more rods. I didn’t like the preload I was getting and wanted it closer to spec. I would have had different sizes on nearly every pushrod. sounds like you are using a lifter that requires a low preload with a tight tolerance. Every pushrod will be a different length I would guess. I found with my comp cams checker it was out .017 from the 6.8+turns compared to a actual measurements.
09-15-2021, 08:42 AM
#11
Do you happen to have a caliper that can measure the pushrods? I ask because when I called around for pushrods only Manton claimed the could get the pushrods to within .007 length. All the rest said within .025. Not acceptable when you have a preload range .035+/-.005 and .093 total travel.
When I ordered my pushrods I had a few different sizes. None of them were exactly the size I ordered. The largest out of spec was .005. Because we grouped the sizes that were close together splitting the difference on some rods. I ended up ordering 11 more rods. I didn’t like the preload I was getting and wanted it closer to spec. I would have had different sizes on nearly every pushrod. sounds like you are using a lifter that requires a low preload with a tight tolerance. Every pushrod will be a different length I would guess. I found with my comp cams checker it was out .017 from the 6.8+turns compared to a actual measurements.
When I ordered my pushrods I had a few different sizes. None of them were exactly the size I ordered. The largest out of spec was .005. Because we grouped the sizes that were close together splitting the difference on some rods. I ended up ordering 11 more rods. I didn’t like the preload I was getting and wanted it closer to spec. I would have had different sizes on nearly every pushrod. sounds like you are using a lifter that requires a low preload with a tight tolerance. Every pushrod will be a different length I would guess. I found with my comp cams checker it was out .017 from the 6.8+turns compared to a actual measurements.
