I have removed my y7 intake manifold and installed a Y8 intake manifold (98) on this car while keeping the same same y7 throttle body. But now I am having a high idle problem, could this possibly a vaccum leak?

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Do a smoke test, see if u can locate the leak. Mine was idling weird when I did the y7 to y8 manifold swap. Turns out I had too much tension on the throttle body clamshell spring. Causing a very tiny air leak directly behind the coil spring. Couldn't find the leak til I did the smoke test.. Then it became clear. After that, it idle perfect. Still idle perfect.

Also, check your throttle body gasket. Make sure I use the correct one.

 

This happened as a result, I am convinced its probably one of the gaskets? either bad head gasket or intake manifold gasket. I did forget to mention I replaced two bent valves. I will have to check.