Q: Why am I losing compression on cylinder #4 on my 2006 Mazda 6

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I bought a block that needed a piston,the block honed and resurfaced after hitting a valve in the cylinder head. I had all of that work done as well as getting all of the bearings and piston rings replaced. I used another head that was warped,had it resurfaced, a three angle valve job and had the valve stem seals replaced from the same machine shop. Replaced all gaskets including the head gasket and timing components. After reassembling the engine using torque specs the engine has a very hard time starting and shuts off after a few seconds. During start up you can hear and feel air being pushed out from the front right of the engine near the intake manifold. A compression test confirmed a leak in cylinder four. There is no oil leaking from the head gasket at all where the sound of the air/compression leak is coming from. If not the intake, could that great amount of compression still be lost between the head and block without oil and after being machined and checked as good to go?

Hi there. If you are feeling air pressure coming out of the engine in this area then you have a problem with warpage or a crack that is not seen. Once everything is put together and torqued then you should not get any air. It is possible that you got a bad head or you may have a valve issue if the pressure is coming from the intake. You need to get some assistance from a certified mechanic, like one from YourMechanic, that can come to you to [diagnose why there is a loss of compression] https://www.yourmechanic.com/services/noise-from-engine-or-exhaust-inspection

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It is possible for the engine to lose compression due to a head gasket leak without loosing any oil due to the head gasket. It is also a definate possibility that the intake manifiold is causing the compression loss. The possible causes of this issue are faulty gaskets, incorrect installation of certain parts, or that the shop that did all of the machine work made a mistake. I would recommend checking all of these things in order to determine where the compression loss is coming from.

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