Engine Cylinder Balance Test: What It Indicates If It Passes?

DuderMcMerican

Active Member
Mar 7, 2016
148
9
28
Woodbridge, VA
Hey Gang,

Total newb question (again), but what does it mean if your car passes the cylinder balance test?:shrug: All I've been able to find so far is what a fail means, e.g., weak power output from a cylinder, but not so much on the "good news" end. In other words, which potential problems might a "pass" help rule out? For instance, would a passing CB test indicate good piston rings?

I got to thinking about this because I've been having some wicked blow-by, which pops out my PCV valve and grommet on a regular basis. I've read that excessive blow-by can indicate bad piston rings, so I'm wondering if (and hoping that) my positive test results can rule that out.

Note: I have tried two new PCV/grommet setups--first from Advance and second from LMR--so I don't think that's the problem. The system blows either setup out in short order (BTW, the Advance PCV/grommet looks (is?) exactly the same as the one that comes in the LMR kit--obvi didn't realize that beforehand :doh:). Would a Motorcraft unit make a difference? I'm thinking probably not because the blow-by seems pretty fierce. One time I noticed white/gray smoke coming out of my air cleaner box, which I think I read is an indicator or super blow-by, yes? I was able to rig up a piece of rigid plastic tube--actually the one that normally goes from the oil fill tube to the intake--to keep the PCV/grommet pinned down in its hole, but I'm guessing that's not an officially-sanctioned "fix." :nono: Haha. I just had the tube lying around because I replaced it with rubber tubing so I could pop an oil catch can in the line.

Addendum: I understand that a compression/leakdown test can indicate bad rings, but I haven't gotten around to that yet.

DM:flag:
 
Last edited:
  • Sponsors (?)


Passing the cylinder balance test posts a 9 or 90 when it finishes. That means all cylinders are putting out equal power. If one or more cylinders have less RPM drop when the fuel injectors are turned off, those cylinders are not contributing the proper power.

You could have all 8 cylinders with bad rings and if the RPM drop was equal for all 8 cylinders, you could still get a 9 or 90.

How to do a compression test:
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent/loan. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the throttle wide open with a plastic screwdriver handle between the throttle butterfly and the throttle housing. Crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 PSI. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

I generally use a big screwdriver handle stuck in the TB between the butterfly and the TB to prop the throttle open. The plastic is soft enough that it won't damage anything and won't get sucked down the intake either.

A battery charger (not the trickle type) is a good thing to have if you haven't driven the car lately or if you have any doubts about the battery's health. Connect it up while you are cranking the engine and it will help keep the starter cranking at a consistent speed from the first cylinder tested to the last cylinder.
 
  • Like
Reactions: 1 users
Passing the cylinder balance test posts a 9 or 90 when it finishes. That means all cylinders are putting out equal power. If one or more cylinders have less RPM drop when the fuel injectors are turned off, those cylinders are not contributing the proper power.

You could have all 8 cylinders with bad rings and if the RPM drop was equal for all 8 cylinders, you could still get a 9 or 90.

Makes sense; thanks!:nice: I had wishful thinking. Haha

I have a Harbor Freight compression tester; just haven't used it yet. I'll be sure to follow up with leak down if cylinders are suspect. Boy, I sure hope they're all squared away! My wife will NOT be at all happy about the idea of droppin a couple $k on a rebuild..:nonono:

Thanks again!:flag: