What Causes Varnish in Upper Plenum

[1993_VG30E_GXE]

I notice in the Upper Intake Manifold Collector (Plenum) that there was varnish buildup before my overhaul. It was tough cleaning it out - I had to use Loctite Varnish Remover. Very harsh stuff - the harshest cleaner I've ever used - worse than laquer thinner even. It works.

Anyhoo -what caused it in the first place? Is it normal to see brown varnish stains after 16 yrs?

 

[Colt45ws]

Its from the PCV system, and it is normal. It does not even do anything. I would have left it there rather than deal with a bunch of harsh chemicals, personally. If it really bothered me I would take it to a machine shop and have them hot tank it.

 

[Merkava_4]

I'm thinking if the oil gets changed often enough,
the engine will stay clean and be varnish free.

 

[Colt45ws]

I doubt it in the intake manifold. Thats all the combustion vapors coming through the PCV. If you rig up one of those PCV catch cans, that will catch any liquid, but you will still have gasses.

 

[Kestas]

What causes it?... It's the gas vapors that condense on the surfaces after shutdown. One way to solve the varnish problem is to never shut down your engine.

 

[ARCOgraphite]

At very low rpm you also have reversion of gases from the combustion chamber due to the exhaust-intake overlap event since resonance supercharging does not come into play at near- idle rpm. You can get hot exhaust gas reversion that can coke the PCV vapour. Some cars get REALLY nasty here. Extrude honing and anodozing hardcoat of the interior will help prevent buildup on the cragy rough alumionum casting. Nylon should be smooth. This is just another byproduct of having a dry flow intake - DI is even worse. since the intake valve head does not get fuel washed.

 

[UNC017]

Originally Posted By: Merkava_4
I'm thinking if the oil gets changed often enough,
the engine will stay clean and be varnish free.


Nope, cleanliness of the crankcase has no bearing on deposits in the intake. In fact, the PCV system which helps prevent deposit in the crankcase is RESPONSIBLE for most intake deposits.

 

[punisher]

Originally Posted By: Kestas
What causes it?... It's the gas vapors that condense on the surfaces after shutdown. One way to solve the varnish problem is to never shut down your engine.


Compared to the temps in the rest of the engine, the upper intake is quite cool. The gasses that the PCV pulls from the engine constantly condensate on the inner surfaces of the intake. No real way to stop it. Just one of the "evils" we have to live with.

 

[mechtech2]

Mostly from the PCV system.
Some from the other end - from the open intake valve under certain conditions.
This is why I use a catch can.
The intake deposits are not the worst part. It is deposits on the throttle body and valves that concern me the most.