Hello All,

I've observed something interesting with my R's engine. After changing the spark plugs and reading them shortly--less than a hundred miles--after, plugs #3 and #4 are black (not wet with oil, but very sooty) but #1 and #2 are perfectly normal. Odd. Upon further investigation, I found that only the bottom of IM runners #4 and #3 are dirty--feels & smells like oil and soot. The rest of the IM is very clean. Very odd. See picture, it's not great but it's the best I could do with the IM on. The soot/oil runs all the way up to the purge inlet/IAC area. If the purge valve was leaking fumes into the IM, would that justify the oil/soot distribution? Could this be flow reversal from from the cylinder and/or leaking intake valve seals? It's my understanding that unsteady flow reversal in an IM is typical.

Enlighten me, I'm all ears. And yes, I've searched extensively. No one can call me out on searching because that's mostly what I do on forums.

Background Info:
-105k Miles, bone stock, very clean internally, very smooth, very strong
-Burns oil but I can't quantify how much yet. In the process of switching from Mobil1 to GTX, as I'm almost certain the previous owner used regular oil. Certainly drinks Mobil1.
-PCV is clean & working properly
-30 MPG with mixed driving (I drive like an old man)

Caleb

 

PCV valve is about in the center of the intake manifold. Oil vapors come out and the flow of air is towards the back of the plenum which is why those 2 runners are "sooty". At least that's my guess lol.

 

Don't think too hard about this.

PCV valve is out of the picture because the soot isn't a commonality between all runners.

You have clean air coming into the engine.

The fuel injectors are at the bottom of the runners.

Other than the PCV there is no way for oil to flow into the manifold.

So, what do you have left? Intake valves.

You proabably have a little blow by on cylinders 3 and 4 in addition to leaking intake valves. The combination leaves you with oily residue and post combustion air back flowing through the leaking valves.

A quick way to verify would be a leak down test.

 

Seems reasonable. I never doubted it was from leaking valves and/or chamber back flow, just looking for input & experience from others who may have had a similar issue.

When I have time, I'll do a leakdown, pull the IM (probably the header too), and report back.

 

You don't even need to pull the intake manifold. Just open the throttle body and listen with the air intake tube off. Save yourself a little work.