Tapping/hissing sound
Tapping/hissing sound Hey all,
I have been chasing a tapping/air sound in my motor since January. To me it sounds like air escaping during the tap. My friends just think I am crazy. With a stethoscope I can hear the noise loudest between 5&7. I did a compression test and all cylinders were between 170-185 except 1&5 which were both just under 155.
Since I first heard the noise, I have upgraded from LS7 lifters to Johnson 2110 and rebuilt the trunnion, new headers and X Pipe/true duals. I finally said screw it and took it to a shop. They pulled the head and found a small amount of carbon build up on cyl 5 and they were "100% certain" the valve was hitting the piston. Cleaned it up and put it back together only to still hear the same noise. Ran a borescope in the cylinder and no marks on the piston. I am running out of options. No codes and fuel trims are fine on that side, +/- 3%. There seems to be a miss now when there was no before. (Probably getting worse?)
Will a leak-down test help at all or should I just start tearing down the motor? Is there anything else I can do before tearing it down to try an diagnose this issue.
Any help is very appreciated.
I have been chasing a tapping/air sound in my motor since January. To me it sounds like air escaping during the tap. My friends just think I am crazy. With a stethoscope I can hear the noise loudest between 5&7. I did a compression test and all cylinders were between 170-185 except 1&5 which were both just under 155.
Since I first heard the noise, I have upgraded from LS7 lifters to Johnson 2110 and rebuilt the trunnion, new headers and X Pipe/true duals. I finally said screw it and took it to a shop. They pulled the head and found a small amount of carbon build up on cyl 5 and they were "100% certain" the valve was hitting the piston. Cleaned it up and put it back together only to still hear the same noise. Ran a borescope in the cylinder and no marks on the piston. I am running out of options. No codes and fuel trims are fine on that side, +/- 3%. There seems to be a miss now when there was no before. (Probably getting worse?)
Will a leak-down test help at all or should I just start tearing down the motor? Is there anything else I can do before tearing it down to try an diagnose this issue.
Any help is very appreciated.
Last edited by nine8ss; Oct 1, 2018 at 03:32 PM.
How many miles are on the engine? The compression test seems just a little low to me, but if has had a lot of miles it could just be worn in a bit. First thing that comes to mind when I hear hissing would be a vacuum leak. Look over the upper engine and make sure all your vacuum lines are connected. There are ways to test with starter fluid but if the hissing is loud you may be able to find it by visual inspection.
A vacuum leak leak will cause a misfire and won’t show up on a compression test or leak down test.
A vacuum leak leak will cause a misfire and won’t show up on a compression test or leak down test.
Speed Engineering 1 7/8 headers. 130k on the motor. Previously owned by an older lady that was meticulous about maintenance. All cylinders still have crosshatch. The air sound is just for a 1/2 second as the tap occurs. Not constant like a vacuum leak. To me it sounds like compression leaking past something...rings/valves.. not sure.
