Low compression on cylinder 4
 

Hey guys, here is my compression test results (na8 stock motor (unknown milleage) in an NA6 chassis, na6 stock ecu):
#1: 170psi
#2: 180psi
#3: 180psi
#4:160psi

its running well, just feels low on power. My plan was to do all bolts on and ms3 this winter, but now the low compression reading on #4 kinda bugs me... Wondering if seafoam could help it? Should I just rebuild before doing the bolts on?
All opinions are welcome! Car is used mainly for autox.

Either just run it and itll probably be fine...
or 1.8 swap and then get your bolt-ons for greater dividends

It already has a 1.8 swap.

I'd run it as is.

Thanks for your feedbacks! Additionnal info, putting a little bit of oil in cyl 4 bumped up the compression to about 180... wondering if carbon build up could cause a ring to not seal properly... Car as been run on rotella t6 or amsoil full synthetic 5w40 for the past 10 years, and runs on premium gas.

I've certainly heard of seafoam cleaning up rings and increasing compression, but I honestly doubt it. Might be worth a shot though.

I'd just send it, then plan on building a motor down the line. Those compression #s aren't that bad, the low cylinder is just slightly out of the 10% variation window.

The seafoam might be worth a shot, as Fireindc said. Sounds like you've been running good oils so it may not be an issue. If it does nothing you are out less then ~$10.

Last time I did this I followed this process, it might be helpful:

1. Get engine up to temp and shut down
2. Remove spark plug wires
3. Insure spark plug wells are clean from debris, clean as necessary
4. Remove plugs
5. Add 2+oz of seafoam per cylinder
6. Before it gets a chance to drain out, turn crank 8-10 full rotations with a ratchet on the crank bolt with the plugs out
7. Reinstall plugs
8. Let sit overnight (Skip this step and you risk hydro locking the motor. This is bad. Do not skip.)
9. Start and bring to temp, go drive
10. Change oil & filter

You'll end up with some seafoam puddled in the valve reliefs in the pistons but it shouldn't be enough to cause havoc, unless maybe you have incredibly dished aftermarket pistons. There might be some smoking but this is pretty normal for seafoam. Dumping anything in your engine has risks so proceed with caution and do so at your own risk.

Very interested to see if this helps at all :bigtu: Good luck!

This looks definetly doable... i’ll keep you posted for sure!

Try the seafoam and otherwise send it. I had #3 a little low on my 1.6 (163ish? Compared to 180ish on the others) and just sent it. After a few years of ragging on it and boost they are all around 180 now.

So I understand that if the seafoam doesn't work, I should boost it just to be safe... lol.
This forum is great!

Put forged rods and new rings with oem pistons, a cheap 3 armed hone etc.

After some time I made a compression test190on first165 on fourth
well, so what. It's not like I'm going to feel the difference with 20psi anyhow.

FWIW I enjoy pulling my engine a few years apart

Well, for internet posterity, here are the hot compression results following this exact seafoaming procedure:

#1: 180psi
#2: 180psi
#3: 185psi
#4: 160psi

Engine seems to be pulling great after the seafoam (btw this thing smokes! Lol), took about 5km before it stop smoking. Number 4 has probably suffered more from heat in the past years (stock cooling setup)... I don’t think seafoam can repair that... lol.
Think I will just send it since it runs great, what would you guys do?

Thanks!

Send it dude

Three things that might effect your results:

Battery size- miatas have tiny batteries and the cranking voltage will drop significantly if you're not helping it.
Either add battery charger or hook up a running car via jumper cables.

And the important one that no one has mentioned:
Valve adjustment. Check your valves (especially the intake). Intake valve closure has more effect on dynamic compression (what you are testing) than you might imagine.
On a engine in good shape when you adjust the intakes perfectly you will have perfect even compression on all cylinders. Exhaust adjustment will have little effect on compression readings.
In your case I would bet the intake valves on cylinder #4 are .001-.002 tighter (later closing) and the intakes on cyl#3 are a tiny bit looser (earlier closing) than on 1 and 2.
I'm a pin head regarding valve adjustment and try to match +/- .0005" That's 1/2 a thousand or "5 tenths".
Shimmed valves are a bitch to get this close; swapping shims alone you can usually get them within .001. If you want to be ---- about it you adjust by grinding the tip of the valve or the shim itself.
If you have HLAs you're doomed for an accurate adjustment. I may be wrong but I think NA8 has HLAs
And this would be why no one else mentioned it...
HLAs suck!

One other thing that could be varying your readings is the fact that the cylinders cool quickly when the plugs are out and you're pumping ambient air into and out of the cylinders.
On you next test do #4 first...

The seafoam procedure that E2OK suggested is PERFECT! He does it absolutely right.
One thing I might do differently. After letting it sit overnight I'd pull plugs and spin it to clear most of the seafoam (put a dog towel over the engine!), then light it up. This will clear 90% of the remaining seafoam.
Sea foam is GREAT for removing hard carbon but is not what you want to feed a cold catalytic converter. Does not matter if you don't have a cat.
It will reduce the god awful smoking that sea foam creates. The seafoam leaves the cylinders in the first moments of operation and is burned off in the exhaust system.

I'm also in agreement with all the other suggestions to use AS IS. The difference is not enough to take apart and fix yet.

Yeah I had to help the battery by plugging it on my pickup to make sure the starter has enough power. Yes it has hla’s... never cleaned them though, but it doesn’t « thick » at all...

Yup, send it. If you are really worried, pick up a cheap borescope and make sure there is no scoring or anything in #4.