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It runs strange below 500 rpm at idle but runs fine at high idle. My dad is a mechanic and he says the second piston isn't getting enough compression to fire, we've check the spark and it has spark but we havnt checked the compression. Need help!!

I would recheck the idle rpm. Are you sure your not looking at the 750rpm line. 500 is nearly the same as the 0 mark on the tach. Also what car do you have year and sub model.

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1990 subaru legacy awd ls

I'd suggest checking compression as your dad suggests. See what it turns up. I'd also suggest a leak down test since you'll already have the plugs out. It's most importantly imo that compression be balanced rather than the reading, since all compression testers seem to read slightly different. Let us know what you find out.

Good luck.

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In the older scoobys my dads ran into crack intake gaskits and he's heard of revised metal gaskits????

I've heard of leaky intake gaskets on these engines. Easy test is to spray carb cleaner around the intake while the engine is running to see if it straightens out. Are you getting a check engine light?

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There isn't one

engine light on? if so, what codes is it giving you?

There is no check engine light

Well below 500rpms pretty much any car is going to idle like shit. I would start off by figuring out why your idle is lower than the 750-800 that it should be. Check for vacuum leaks, intake leaks, clean the MAF sensor, replace air filter (if dirty). How long has it been since your plugs have been changed?

There is a lot of general maintenance items that if ignored could cause the issues you are having.

A service history of your car you be very helpful for trying to figure out what your issue is.

As mentioned, the tachometer can be a little misleading. The "bold" hashes between the numbers are the 500 hash marks, and the smaller less bold hashes are the 250 hash marks. The bold hash below the 1,000 rpm hash mark is 500 rpm. The lighter hash above that is 750, so if you're idling at that lighter hash mark below 1,000 rpm, then it's idling around the proper rpm.

Here's a picture for reference.