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!
:cheers:
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]

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

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.

[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

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[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.

[jss06c6]

Watching closely to learn!

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[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.

[Ccmano]

Dom,

Didn't you have an issue when the porting was done with a pinhole leak in the injector housing? I wonder if this might be the same issue.
H

[ssrszz4]

I know that '90's didn't have exhaust valve seals and was designed that way on purpose.



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[XfireZ51]

Dom,

Didn't you have an issue when the porting was done with a pinhole leak in the injector housing? I wonder if this might be the same issue.
H

When I first ported the injector housing, I went thru the bolt boss at the very front of the IH. Vacuum leak into #2 primary runner. Welded that.
There have been other cases of oil seepage through a very porous casting of the cylinder head runner.
#7 is close to the PCV. Wonder if its related to crankcase venting and possible lack of restriction between IH and cylinder head.
Question for Mr. Haibeck.

[XfireZ51]

Interesting post I ran across while doing research on another topic. This appears to describe what I observed after doing the compression check.Same bank as well.

http://www.zr1.net/forum/showpost.php?p=71453&postcount=1

[jss06c6]

Dominic,

Any updates?? Very interested to learn what is causing the fouling of #7 plug.. sure hope it's something "reasonably" easy to fix..

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[XfireZ51]

Dominic,

Any updates?? Very interested to learn what is causing the fouling of #7 plug.. sure hope it's something "reasonably" easy to fix..

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Right now there isn't a further update. At this point, we'll begin looking at it after the 4th holiday or sometime after that. Given the results of the compression test and checking of injector and plug wire resistance, there's nothing obvious. It could easily be a leaky injector. But right now that's speculation. It could be something more involved. I'll have Dr. Polatsidistein have a look and I will update.

[jss06c6]

Thanks. Watching with interest..

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[Paul Workman]

A fuel injector leaking would show up if you pressurized the fuel rail and observed the time it takes to bleed off pressure.

As I recall, it took over 15 minutes to bleed off the first pound of pressure when I did mine the last time, once I found the leak (a bad check-valve in a fuel pump).

One other thought: isn't the vacuum line for the pressure regulator just about directly over the #7 plenum runner? You know where I'm going...!:icon_scra

Just some things to ponder before pulling the motor...:cheers:

[XfireZ51]

A fuel injector leaking would show up if you pressurized the fuel rail and observed the time it takes to bleed off pressure.

As I recall, it took over 15 minutes to bleed off the first pound of pressure when I did mine the last time, once I found the leak (a bad check-valve in a fuel pump).

One other thought: isn't the vacuum line for the pressure regulator just about directly over the #7 plenum runner? You know where I'm going...!:icon_scra

Just some things to ponder before pulling the motor...:cheers:

Well yes the PR vacuum line is by #7 but plenum volume is supplying vacuum so its hard for me to gather how it discriminates on #7.

[Dynomite]

One other thought: isn't the vacuum line for the pressure regulator just about directly over the #7 plenum runner? You know where I'm going...!:icon_scra



I have no clue........please describe for us less familiar :handshak:

[secondchance]

One other thought: isn't the vacuum line for the pressure regulator just about directly over the #7 plenum runner? You know where I'm going...!

Are you thinking fuel pressure regulator leaking internally dumping excess fuel into #7 cylinder?

[XfireZ51]

Are you thinking fuel pressure regulator leaking internally dumping excess fuel into #7 cylinder?

I would suspect that FP would be affected whcih would show up across the board for the plugs.

[mike100]

Poor man's leakdown test: What is the compression on one crank compared to other cylinders?

[Dynomite]

Well yes the PR vacuum line is by #7 but plenum volume is supplying vacuum so its hard for me to gather how it discriminates on #7.

As I recall the Fuel Pressure Regulator last time I had one fail (someone else ZR-1) there was raw gas all the way forward coming out the Air Intake in front of Throttle Body which.........raw gas got there through the FPR vacuum line.

Poor man's leakdown test: What is the compression on one crank compared to other cylinders?

Yes.....I would be more suspect of the uneven compression. Blue or Black smoke for example.....burning oil or gas??

You say gray/blackish exhaust??

[XfireZ51]

...
Yes.....I would be more suspect of the uneven compression. Blue or Black smoke for example.....burning oil or gas??

You say gray/blackish exhaust??

I do get a gray/blackish exhaust coming out when I jump on the throttle that appears to clear out. However, no such indication at startup or steady cruise.
I'm leaning a bit more towards out of phase cams on the driver's side. That side of the motor exhibits noticeably lower compression #s than the passenger side although all with the exception of #3 appear to be within the 10% tolerance level.