Low compression #6

[Jnikic]

During an open track event two weeks ago, I noticed that my motor was running hotter than normal, though the gauge never went past the half way point. A little while into the next session, and the motor suddenly refused to rev past 5.5k (in 4th gear only, lower gears were ok up until that point) - it was almost as if I hit a rev limiter, it simply wouldn't accelerate further and hit a wall.

I came in and checked my oil level (it was okay), and fuel pressure (3.5psi on the PMO regulator). AFR's were also normal, though rich as usual. During the next session, everything was okay until 15-20 minutes in when the same problem arose - again, I noticed the motor was running warmer than usual - and so I pit in. After it did the same thing the next time out (an hour or so later), I decided to pack it up and made the 240 mile drive home without any issue - but I did notice that the motor appeared to be sprayed with oil from the catch can before I left.

My mechanic looked through the car this week, but couldn't replicate the problem. During normal driving, the motor pulls hard and appears strong. But compression test results show the following:

1. 148
2. 125
3. 146
4. 166
5. 140
6. 100

So there's obviously something going on with cylinder 6 - from the leak down test, my mechanic thinks its a broken ring.

The motor was "rebuilt" by the previous owner 25k miles or so ago with larger cylinders (98mm on a 3.2 case = 3.4), but I don't have too many details beyond that. It seems as if we could just address cylinder 6, or proceed with a complete top end - BUT my question is whether or not I'm okay putting that repair off until the end of this season. I caught the track bug bad and have an event next weekend.

Will I do any damage to the motor by running? Do you think the problem I encountered could be caused by a broken ring/low compression on cylinder 6? Or is there something else at play concurrently? Thanks!

[HawgRyder]

If it is a broken ring...you will be OK up to the point where a piece of that ring decides to migrate down past the piston and damage everything in the crankcase!
A broken ring will probably damage the cylinder wall...the piston...and maybe the head.
The quickest check I can think of to find out if it's a ring problem...do a compression test...first dry...then wet (with oil) to see if the numbers go up.
If there is a BIG difference...it's probably rings...not much difference...probably valves (seating).
Bob

[JFairman]

If there's an uneven puffing from the crankcase breather at idle speed or above and when blipping the throttle that may be from excessive blow by around broken compression piston ring(s) on number 6.
How does the number 6 spark plug look?

If piston blow by gas seems smooth from the breather even when reving the motor then maybe it's one of the valves leaking from an overrev contact with the piston or maybe a worn exhaust valve guide on that cylinder.
Good luck with it.

[Flat6pac]

Thats a lot of variation in compression readings. 9.5 JE should show in the 180#s new and fresh with a valve job. You should look at all of the pistons.

Bruce

[Jnikic]

Here's a look at the #6 plug:



I'm noticing some roughness in the idle, a low spit from the exhaust every so often.

[docrodg]

Agree that is a lot of variation. Cylinder 2 has something as well. The oil issue says rings to me, but could be valves too. Only choice to be sure is teardown the top end (including cylinders) and look at EVERYTHING. A leakdown test may help narrow suspicion prior to teardown, but if you are taking off the heads may as well take off cylinders and check those (plus a rod bearing or two). I am tho by nature one of those "check everything while in there" kind of people if it doesn't take a lot of parts that are $$$$. The idle roughness and spit may be related to this issue or just a mixture problem.

Plug looks like it is eroded a little, running hot maybe. Could be a combination of problems.

[gab997]

I had the same issue with my motor, I had an excessive blow by issue and decided to finally get to the bottom of the issue. I completed a leak down and had issues on #4 and #6 had each cyl on tdc and the air pressure was was coming through the breather. If it was a valve issue it would be coming through either the intake or the exhaust. I ended up dropping the motor off to Mike Bruns and found two cracked rings and most f the other ring needed new ones and there was piston damage more than likely to detonation.

So sounds to me you have a piston and ring issue with 6 and more than likely a ring issue on 2. Need to tear down the top end to see exactly what your dealing with.

Good luck.

Greg

[Porshaah]

Originally Posted by Jnikic

I noticed that my motor was running hotter than normal

A hot running engine is typically an indication of a lean running condition and the plug appears to agree with this.

Originally Posted by Jnikic

AFR's were also normal, though rich as usual.

This statement seems to contradict both itself and the condition indicated above .

Originally Posted by Jnikic

from the leak down test, my mechanic thinks its a broken ring.

What were the leak down results?

Originally Posted by Jnikic

Will I do any damage to the motor by running?

Depends, do you feel lucky?

Originally Posted by Jnikic

Do you think the problem I encountered could be caused by a broken ring/low compression on cylinder 6?

Yes, it's likely caused by one of the compression rings being broken, although I would have expected a bit more oil on the plug. You might want to do another compression / leak down test to see if the the results repeat.

Originally Posted by Jnikic

Or is there something else at play concurrently? Thanks!

You really won't know for sure until you see things physically by either a bore scope or tear down. And even then, it's often difficult to determine, without measurements.