Sooty #7
 

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

Re: Sooty #7
 

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!
:cheers:
Marty

Re: Sooty #7
 

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

Re: Sooty #7
 

Concur :thumbsup:

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.

Re: Sooty #7
 

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

Re: Sooty #7
 

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

Re: Sooty #7
 

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

Re: Sooty #7
 

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

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Re: Sooty #7
 

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.

Re: Sooty #7
 

Watching closely to learn!

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