cranking PSI

[imean340]

Chasing around a misfire i've had since I put the car back together, I found a spark plug with a flattened ground. Thinking it could be something serious I did a compression test. It turns out I think the spark plug was just dropped (two of my buddies were helping me by putting the plugs in while I was finishing up some stuff under the car...not blaming them but maybe it was dented already and nobody noticed it because I told them not to check the plug gap as I was in a hurrry).

Anyway, I tested the cyl in question and I got 120-122psi vs another cylinder for reference that got around 150-155psi

Oiled it up and sure enough compression went up. Im not about to fix a ring on this motor as it is still pretty young and I really dont have the time right now. Figure that one cylinder is at most 20% down from the other. I need to check all the cyls now and make sure it's only one but if it is, what kind of power do you suspect thats costing me on my motor (more info on motor in sig)?.  Should I be worried about trying to run a 100 shot of nitrous like that? It's a fairly new motor (5 years, under 10k miles) , speed pro forged pistons, bored .30 over ect... think I may have had this problem since it was new. Have had a slight issue with the dipstick getting blown out just a little at when beating on the car but figured it was just because it's a 39 year old dipstick tube lol


Any advice/suggestions/opinions are appreciated. Thanks!

[6packCuda]

120 is not terrible, but it's not too great either. If the rest of the cylinders are around 150, a cylinder leakage test on the low cylinder might be a good idea. Do you have a pcv valve and breather in your valve covers? If it's blowing the dip stick out, it sounds like you're building up too much pressure in the block. Could be missing pcv, plugged pcv or excessive blowby.

[Chryco Psycho]

you have a problem for sure , 15% max difference & you are above that + you have confirmed a ring problem so you need to pull it down & find the problem , possibly the ring gaps are just lined up & need to be rotated 

[imean340]

6packcuda, I do have a PCV. Im pretty sure the leaky compression ring is causing the excessive crank case pressure.

Cryco, unfortunately it'll be a while till I can get the time to pull the motor out and fix the ring...not to mention I just painted everything in the engine bay/spent all that time and money on paint and sandpaper ect... only to have to pull the motor again. Fixing the ring is just something that I wont be able to do for a while and as a money issue too, if im going to pull the motor out again im going to want to upgrade some things so the cost is going to snowball. I wanted to try to put it off for as long as I can. Im wondering what kind of power do you think it would be costing me if it's just the one cylinder? The car drives great, runs strong ect... I don't have a track anymore so maybe the e/t's aren't where they should be but seat of the pants it certainly doesn't feel like the motor is hurting. Im thinking of just spray it with a 100 shot, see if I get the times I want when I do go to the track and hopefully run that till I can afford/get the time to pull the motor. Aside from making sure the crank case is being vented well on spray, anything I should worry about with doing this?

thanks!

[Supercuda]

I couldn't advise spraying an engine with a confirmed ring leak. That is just a disaster waiting to happen. At least pull your pan, the appropriate cylinder head, and pull the offending piston/rod assembly. It really could be as simple as ring gaps lined up, or it could be a broken ring, that is slowly scoring your cylinder wall beyond repair. It's a bummer of an option list, but at least it's realistic.

[imean340]

Thanks, money got tight so I think spraying it is out of the question anyway. I was thinking about a broken ring the other day actually, but what made me think it's not the case is wouldn't pouring oil into the cylinder do little to seal a broken ring (or quite honestly, i'd figure it would do little to help a lined up ring gap as well)? I could be wrong but I figured that the oil would only work when a ring is worn down and the oil just helps to fill in the little bit of looseness between the ring edge and cyl bore. It seems like if there was a gap or a broken area, the oil wouldn't do as good of a job as sealing it and you'd only see maybe a little improvement in cranking PSI. When I put oil down into the cyl, I saw a full improvement, it was right back to where the other cyls were. Of course this is just my guess on what would happen, im sure you guys have more experience than me....am I right or wrong?

Since I probably don't plan on fixing it soon (going to have to work some more overtime to pay some bills, classes will be having finals soon ect...) yet I still want to drive it, any way of telling if I am hurting something in doing so? I keep a close eye on metal shavings in the oil...don't see much. I can see the piston with a flashlight and it doesn't look like there are any oil deposits on it. Plug comes out clean every time.

[Chryco Psycho]

oil will temporarily seal a broken ring or lined up gaps
 no way I knwo of to know if you are hurting it by driving it , just a chance you take

[Supercuda]

A borescope will give you the wanted information without teardown, as long as the piston is about 1/3 of the way down the bore. If the ring is broken, there will be a line down the cylinder wall right where the break is scratching it, and lined-up ring gaps will show no unusual wear at all. It only requires that you unscrew the sparkplug, and use a tool that costs at least $150-200 for a low-line, full-optical scope. My video scope cost me $350 from my Matco tool dealer. A good friend with a borescope is cheaper than owning a scope, but harder to achieve, very often. If your car were in Colorado, you could call me to use my borescope, but we all know how that works. NEVER is the tool we need, in the place that we need it.

[moper]

GO drive the car a bit now that the hole is firing. It's most likely a fuel washed ring in that hole. It may come back a little once there's a few heat cycles and miles on it. I've seen dead holes driven for a thousand miles come back most of the way after driving under normal use. The NOS will simply put a strain on the PCV system and gaskets... But you can still run it if you want.

[imean340]

Thanks for all the replies guys! This problem has really been wearing at me being that I was so happy to get the car finally running again before winter and it runs  better than it ever did with the new tranny from Terry and looks better than ever too with all the time I spent painting/cleaning.

Supercuda, I think that's what im going to do. I could probably use a cyl bore camera in my arsenal of tools...I may not be able to afford it but it's something I wouldn't mind paying off on the credit card as it could save me a lot of time/trouble/money now and in the future.

Moper, man that's what I like to hear....I really hope it's the case! I was driving the car around with a dead plug for a couple of miles so im sure that cyl was getting its share of fuel.