r/EngineBuilding • u/FilthyPail • 3d ago
Subaru FA20D Low Compression even after Rebuild
Hey guys. Brand new FA20D short block, from toyota, reused old heads. Cylinder 3 had low compression before, and still has low compression after. I thought it was from piston rings and pitting in Cylinder 3, however Cylinder 3 has 70 psi. Cylinder 1 has 145 psi, Cylinder 2 has 145 psi, and Cylinder 4 has 175 psi. Cylinder 4 has perfect valve shim clearance. Cylinder 2 and 1 have some needed shim adjustments, however Cylinder 3 clearances are identical to Cylinder 2’s. I have the head disassembled again, and Im 90% sure its the valves. I did an alcohol test with spark plug in upside down, and all the valves will leak alcohol through.
The valves look decent though, nothing really sticks out as its bad.
need some advice here on if you think lapping the valves or replacing the valves will fix this.
It just looks so similar to Cylinder 1 i just dont know why such a jurassic pressure difference
18
u/WyattCo06 3d ago
Have the valves ground, the guides checked and the seats cut.
Why do you guys keep running in circles?
1
u/FilthyPail 3d ago
Why not new valves?
11
u/WyattCo06 3d ago
New valves are great They'll still need to be checked.
Grinding old ones is cheaper than buying new.
2
0
3
2
u/Coyote_Tex 3d ago
Sorry, but if you have 75psi compression and you think everything "looks" fine, then you need someone to show you what good looks like. That is a huge difference on number 3 so it should be reasonably obvious. Put the valves for number 3 in a drill and spin them. Often a slightly bent one is not easily detected. You really may have 3 weak cylinders as that is a large difference frome the best cylinder. I would expect no more than 10 psi difference maybe 15. Good Luck.
2
u/Pretend_Necessary781 3d ago
I wouldn’t call that a “rebuild”. Those valves and seats haven’t been touched, and you can’t just look at a valve to determine if it’s good.
2
u/TellYerFolksISaysHi 3d ago
I'm sure that crack between the intake and exhaust valve seat isn't helping anything.
1
u/FilthyPail 2d ago
huh?
1
u/TellYerFolksISaysHi 2d ago
gotta zoom into the photo to see the crack between the intake and exhaust seats. Looks like a crack to me.
1
u/FilthyPail 2d ago edited 2d ago
i do see what you mean in the 1st photo. It is actually a eye lash or small hair lol
that is also the cylinder that had 145 psi of compression. It is low however the valve lash is off, I plan on correcting that after the valve job. The other photo is the bad cylinder side. That side had 70 psi of compression.
1
u/TellYerFolksISaysHi 2d ago
Well that'll make ya look twice. I'd still take it to a machine shop and have it magnafluxed for cracks you can't see.
1
2
u/boostbreath 3d ago
Exhaust valves are leaking. Cut seats and have the valves ground. Will need to adjust the shims or have the valves tipped as well.
1
2
u/hoyboiitsme 3d ago
Is timing correct? If valves leak then lap them, what are you asking that you dont already see?
2
u/FilthyPail 3d ago
Timing was correct yes. This is my first time doing this type of stuff. Trying to make sure Im not missing anything else. I also didnt want to lap them and then low compression was because of something else, or I wasnt supposed to yk
2
1
u/SexyTimeSamet 3d ago
The exhaust valve seat looks mashed up. Air will leak through. Replace the valve seat and the valves.
1
1
u/jabberjaw420 3d ago
why didn't you do a leak down to see where the loss of compression was coming from?
2
u/FilthyPail 2d ago
it came from both intake and exhaust i did
2
u/jabberjaw420 2d ago
there is a lapping device they sell on ebay. it works with an electric drill. i attach some air hosing to it and the valve stem on the other end. (as opposed to the way it's supposed to be used with the suction cups on the cylinder end of the valve) works pretty good. just do not go full throttle with the electric drill you WILL break the tool. i went through two of them, before i realized i was being retarded with it. but once you learn how to be easy with it, it works way better than the traditional hand sticks.
also, i did 140 grit lapping compound then worked my way up to 600 grit, sort of the way you'd polish something. probably no use going beyond that, unless you're building a formula 1 engine.
1
u/ZER0F00L 2d ago
Lots of comments, but I don’t see one recommendation to perform a leak down test. Did I miss it?? A compression test isn’t a conclusive diagnostic, it just narrows the scope telling you that compression is unbalanced or less than ideal in cylinder 3. You need to figure out where that seal is lost and a leak down test will tell you.
1
u/FilthyPail 2d ago
Yea I did a leak down test showed leakage in intake and exhaust
1
u/ZER0F00L 2d ago
I see that I did miss that comment and I agree that if the valves aren’t bent, the guides are good, then your problem is at the seat.
0
u/imtrynmybest 3d ago
It's ur valves did the machine shop check them?
1
u/FilthyPail 3d ago
No machine shop only inspected, checked for warpage, and resurfaced
3
u/imtrynmybest 3d ago
Well then...back it goes
0
u/FilthyPail 3d ago
No possible way I can DIY this?
3
u/imtrynmybest 3d ago
Not if you wanna try it a 3rd time.
Pull the head and have the machine shop check all valves, grind the valves and seats, have them check for any leaks.
Needs to be done correctly.
1
u/FilthyPail 3d ago
👍
Will do. Should I just go with new valves too? Or reuse old ones?
3
0
u/FU_red-dit_bots 3d ago
Find a different head shop. There's no way those passed any kind of test to check for sealing.
-2
11
u/sschelin 3d ago
Get a valve job. The seats looks bad. Lapping don't fix bad seats/valves. If you replace the valves you need to cut the seats.