Matt J

Hello I am currently helping a buddy with his car. It is a 1998 Mustang gt 4.6 manual with 120k miles on it. We just recently heard a noise under the hood that sounds like a Piston hitting a spark plug or some sort of light tapping. It only does it when the car is warmed up and revved. But we are trying to figure out what is causing the noise. We did a vacuum test, but we are not sure what to look for. So we made a video hoping some one can tell us if the vacuum gauge is reading correctly or not. You can also hear the noise in the video to also. Thanks!

Here is the link to the video
http://www.youtube.com/watch?v=tFRqu...ature=youtu.be

86foxvert

your vacuum test is good, maybe a little bit of a leaky valve but ok. I think its an exhaust leak from what I can hear. Check your connections to the midpipe and look for black marks at the connections.

Matt J

Ok thanks man we are just trying to narrow it down. Will a engine compression check help with information to help narrow down what it could b for
sure?

86foxvert

not if it as exhaust leak. Do you have an aftermarket mid pipe? Cat back? If you can do it safely get under it while its running and look for a leak. I use a piece of paper and look for the paper to move if there is a leak, but there are tons of other ways to do it. Im not trying to scare you but, If no leak is found it could be a bigger problem

Matt J

Its by buddys car it has a off road h pipe with flowmaster but the noise I hear sounds like a piston tapping on a spark plus and I'm not saying that Is what I think is wrong but that what it sounds like you can hear it in the end of the video I posted

Matt J

We think it could b a valve