Originally Posted by Chalt

I would like to add that you can rent a leak-down tester from an auto parts store. Since you have 65 psi compression on #1 and #3, you need to find out why the readings are low. Replacing the speed and reference sensors is a separate issue from low compression readings. A leak-down test on each cylinder will help you identify what exactly is leaking. The test will pinpoint if you have:

1. Leaking intake valve
2. Leaking exhaust valve
3. Worn rings (leaking past rings)
4. Leaking head gasket (blown)

Sure, you could Not do a leak-down test and pull everything apart and just start replacing everything. But knowing precisely where failure has occurred prior to disassembly is valuable information.
If you choose not to systematically and methodically test things, you will always be in guessing mode and that's expensive and unnecessary because you are spending money on parts you didn't need and your time on repairing and/or replacing parts that were good.
When you get lost, slow down and test, test test. Also, document, document document your tests. Write down dates, times, what test readings were. Make notes about what you worked on and tested. You can also use your phone camera or video to help you document test readings and such. That way you will know for sure what you have done or not done. Troubleshooting without documenting is a scattered/ shotgun approach. When you test methodically and record results, you will be able to communicate to yourself and others what is going on.

Here is a pretty short generic leak-down test video (with annoying music). It explains the how and why for a leak-down:

https://youtu.be/hGv70RyiGGc?si=OxrBQ0yoX2wNrHh2

Here is a bloke doing a leak down test on a 944:

https://youtu.be/djqh_86lGfQ?si=6fjRThMWvyzDl4g1

If you want to learn more about leak down testing, search it on youtube and/or a search engine.