1991 325ix Low compression

[ghatid]

So I've been trying to figure out this problem I have where my car idles perfectly, but when I tap on the gas, it has issues getting back to idle and the rpm drops really low before coming back up. Also, when I'm driving the car, if I let go of the gas suddenly when going at a constant speed, the car feels like it's engine braking.

After a bit of work, I figured out that I have a cylinder with low pressure. I did a compression test and got:

170, 155, 155, 115, 150, 160.

Then I dropped in some oil into the 4th cylinder and the pressure went up to 170.

All signs seem to point to worn rings.

Is there any downside to trying some restore 6 treatment or something along those lines in order to get the compression back up before ripping the whole engine apart?

When I bought the car (less than 150 miles ago) there wasn't this problem, but I thought it would be a good idea to run seafoam through it, and though it cleaned up my idle, I think it might have also cleaned out the gunk sealing the piston.

I'm guessing my next step is a leak test. Is there anything it could be other than bad rings? Everyone seems to say that the rings on the m20 are unstoppable. Also, since I know one cylinder can't keep pressure as well as the others, can't I just connect a pressure line into the cylinder and run pressured air to listen for the leak? Would a leak down tester tell me anything else?

[ghatid]

Hmm, now I'm even more confused.

I tried doing a "leak test" (leak test without measuring pressure) by pressurizing each cylinder at about 20psi at each of their TDC.

When I did it for cylinder 4, I found that I could hear very clearly that the air was leaking from the hole that is to the bottom right of each exhaust valve. I thought that meant that it might be a valve seal, but when I did the same for the other cylinders, I could hear the same noise (just more clear at the hole that's closer to the cylinder I was checking).

Where does that hole lead to and what does that mean for my car?

http://imgur.com/4xQVrYa

[chriswin3]

Need to do the wet test on all the cylinders and compare. Comparing wet to dry isn't really too informative.

Sent from my Nexus 7 using Tapatalk

[ForcedFirebird]

That means you are pressurizing the crank case. Those holes are oil drain back to the pan. Normally on a leak down, everything would be sealed up, throttle open, dipstick out. If the intake valve was leaking, you would hear it in the intake manifold, exhaust in the exhaust and bad rings and you would hear it at the dip stick. Sounds like that cylinder has a bad ring(s).

[ghatid]

Thanks for the input. I figured that's what it meant, but I'm pretty new to working on engines.

It seems that the correct way to fix this would be to pull apart the engine and do a full rebuild. What are the chances that there is something really bad with the engine that will require me to do more than just switch out some rings/gaskets? My understanding is that I should also take the head and the block to some machinists to get them measured or something something something.

Is there anything else I should try?

[ghatid]

Okay, so I thought I'd update because I'm pretty relieved. I put in engine restore 8 crap (I don't like using "magical additives" but I don't have the space to do any decent engine work) and drove it around for a few hundred miles.

My compression numbers are now:

165, 160, 165, 160, 162, 165

Somehow cylinder #4 seems to have regained the ability to hold pressure.

Now I just have to figure out why my engine struggles if I tap on the gas really quickly and the lights dim and the economy dial goes up to 50 and down again.