1999 Honda V6 intake manifold carbon deposits


#1

My 1999 Accord with V6 and 240,000 miles pings on all gas except premium.
I removed the upper intake manifold and found a very small pool of oil in the mainifold floor next to the opening that the throttle body bolts to.
Also there is a small port in the upper manifold next to the throttle body opening that is about 80% clogged with a hard black deposit. This deposit is also in the corresponding port on the lower intake manifold.
I had a new EGR valve and decarbonization done at about 175,000 miles.

The car runs great on premium gas but it is supposed to run on regular which is a lot cheaper.


#2

80% clogged is not good. Clean out all the deposits, put it back together, and buy regular.


#3

The oil probably came from the PCV system.
Fumes and some oil mist gets drawn from the crankcase into the manifold.
At 240k miles there’s likely more gasses seeping past the rings (blowby) and more stuff going through the PCV valve and passages.

There can also be carbon deposits in the combustion chambers.
These can cause hot-spots that encourage pinging and even raise compression.

This engine could be a candidate for a ‘mild’ rebuild:
Take the heads off, send to a machine shop to resurface the valve heads and seats.
Replace the valve seals and guides if excessively worn.
Mill the head deck flat. Replace the rings, rod bearings and crank/cam seals.
Hone the cylinders.