Sooty #7

[XfireZ51]

Recently, I started experiencing a case of gray/blackish exhaust exiting the rear. Motor no breaking up. And at first I chalked it up to car sitting around during the winter. Blowing out some carbon. But it has continued. But only when I go WOT.
I pulled some plugs for a compression check and #7 was not in good shape. It was pretty black and sooty. I replaced the plugs and did about 700miles to MI this weekend. Car ran great, ~ 24-25mpg. Pulled new #7 plug and again sooty.
Checking plug wire and injectors w engine hot:

Injectors (primary/secondary) both check out at ~15ohms HoT.
Also checked plug wire resistance thru coil HOT, 14Kohms end to end. Same as other
cylinder pairs.
#7 also tested out at 220psi which put it a bit below the 228psi average for the 8 cylinders.
Compression was:
Lft Rgt
235 235
210 230
215 245
220 240

[DRM500RUBYZR-1]

While the injector tested good electrically, I am very suspicious of it's performance.
Classic symptoms of an overly rich condition, and specific to one cylinder.
If spark is sound, which your observations suggest, then an injector pouring fuel would be a logical suspect.
Good luck and keep us posted!

Marty

[Ccmano]

There are two kinds of soot. Soot from running too rich, aka a leaking injector. And then there's soot from excess oil in the cylinder, which could be a ring, valve guide, gasket or porous head issue. Highest probability is the injector. Resistance, as was pointed out, is irrelevant when leaking.

Do you have a photo of the plug and the soot?
H

[Dynomite]

Originally Posted by DRM500RUBYZR-1

While the injector tested good electrically, I am very suspicious of it's performance.
Classic symptoms of an overly rich condition, and specific to one cylinder.
If spark is sound, which your observations suggest, then an injector pouring fuel would be a logical suspect.
Good luck and keep us posted!

Marty

Concur

I get a bit better fuel mileage (26-28) sixth gear on highway....and have always had compression around 220 + or - 5 lbs. Your compression seems to be from 210 to 245 or 35 lbs difference. I run Haibeck 91 Octane Chips in ALL ZR-1s. Anything specific to one cylinder suggests an issue albeit your #3 is the lowest compression and your range of compressions is not what I would expect.

[XfireZ51]

Valve timing could also be a source of lower numbers on the Driver Side v Passenger Side.

[XfireZ51]

So here's the #7 plug. BTW, rear exhaust tips were also sooty.

[Ccmano]

Hmm... crusty might be a better description. That could be oil. My first impression was actually coolant in the combustion chamber with that white crusty stuff but oil can look like that too in early stages. It's probably not fuel.
H

[jss06c6]

Dom,

Are all other plugs normal? Have you checked the PCV? Valve guide leaking? Hard to think that you have piston ring issues in only one cylinder. Anyone ever tried to get a bore scope inside an LT-5 to check piston & cylinder wall?

Hope this is something simple!

Steve

Sent from my QTAIR7 using ZR-1 Net Registry mobile app

[XfireZ51]

There is an oil consumption issue going on. It accelerates when running at higher rpms. Marc and I discussed it at BG. Marc has a thought that it may be due to a mismatch between cylinder heads and injector housing regarding the restriction of crankcase pressure. The car will be going to Dr. Politsidistein shortly where he'll be looking up the exhaust, checking cylinder heads, and injector housing matchup.

[Paul Workman]

Is it oil, or is it fuel...is the question.

What we have observed/know:

-compression, bank-to-bank is clearly a little higher on average on the right bank than the left, and #7 is second to highest on the left (lowest average) bank
-Loop resistance thru the coils comparable with non-affected cylinders
-gray/black noticed at exhausts
-all HOT(?) injectors resistance within tolerance
-plug porcelain has a black soot layer and inside steel surfaces have crusty material evident and unique to that one cylinder
-motor (you say) seems to run strong - no issues noted
-headers with SW "X" pipe, Dynamax 17288 twin outlet mufflers w/ an NPP on each of the two mufflers

Black sooty plug and some crusty deposits as well, and observed black/gray exhaust emissions at WOT - points to OIL (most(?) likley) or excessive fuel

Supposing, for the moment, it is oil affecting ONE cylinder. Where can it come from?

A broken ring? Compression test = inconclusive. What would a leak-down test reveal?

Bad or missing valve guide seal? Possibly. If so, one valve in the lot of them showing signs of coking wouldn't be a surprise, considering the soot issue. (Now if we had a coolant leak in the IH, the valve(s) would be cleaner than all the rest (ask me how I know...) But, if it IS a guide seal, nothing short of pulling the cams is required, and that usually means pulling the motor to facilitate re-phasing the cams. Then the question is whether it is worth it? But, of course if you did pull the cams to install new guide seals, when the cams are reinstalled their phasing can be checked and/or matched with those of the other side at that time as well...

Your motor is ported. Could there be oil seeping through one of the runners? This has happened to more than one ported head! Any black streaks initiating from a spot of the sides of runner walls closest to the valve springs? (Yours wouldn't be the first, I can tell you! You might need a probe to look down there, unless you remove the IH too.)

Assuming it is a fuel issue:

Injector leaking fuel? A fuel pressure leak-down test would indicate leakage, but just where isn't so easy to say. However, if the plenum is off that would be a good time to do a double-elimination and swap injectors from a good cylinder to #7, but then too install different injectors to take the place in the donor cylinder. (Or, you could just swap between two cylinders, but then you'd have to pull the plenum again should what was originally #7's injector(s) prove bad.)

Hmmm.... That's all of the testing I can think of that you may not have done. But, often a test results in new evidence that can lead in an unexpected direction. Would be interested in what you find.