PorKen

I took off my intake again to install (8mm spacers/isolators), and found black goop growing again on the intake legs, and half of the head ports. It has been less than a year since I did an intake R&R, where I cleaned everything. (The original 120K mile goop was 1-2mm thick!)

There was an odorless gel like substance coating the lower intake legs, which I think is the precursor to the goop. Below the injectors is clean from the direct gas spray.

I don't have any oil in my intake, as the crankcase is not vented to the intake at all. So the only thing it can be is gas! What I've thought was oil coking up the last 2-3" of the intake legs is actually gasoline fogging/blown back up into the intake, and turning to black goo over time.

Another interesting find is that in the head, only the ports with long straight legs feeding them had goop collecting. The legs which curve back on themselves did not have much if any goop. Is it turbulence which cleans the walls?

PorKen

It will be interesting to see if, after a few months, it returns now that the intake is cooler.


You see it on other cars too, like this W12 Phaeton.

dr bob

Ken--

I'm thinking that oil from intake guides is drawn back through the manifold as air flows towards valves that are open. Just a guess, though.

That W12 is sure a nice compact piece!

PorKen

I thought about it being from valve guides, but the jelly I found above each fuel injector makes me pretty sure it's old gas. Come to think of it, the air flowing towards other cylinders gives the 928's batch fired gas a reason to be backing up the intake.

Click on the pic for the thread (re thermostats) - the intake manifold is a $10K hand ported piece!

leperboy

Ken, the black goop I found in my intake when I first opened it had the odor of stale gas.

Matt

the flyin' scotsman

Had the same on my intake..........solvent took it all off.

In before MK...........the valve timings is good?

Bill Ball

Interestingly, when I removed the intake manifold and supercharger from my car after a couple of years and 20K miles, the inside was coated with a sticky goo. I thought it was oil, but there are no breather lines going into the supercharger or manifold. It even got all the way out to the plenum feeding the SC, but that is probably from the blow-off valve that recirculates compressed manifold air back into it. I couldn't say if it was gas residue or oil from the valve stems.

heinrich

If you have or had a leaky fuel pressure regulator or damper (yes, damper ) then this can also happen...

PorKen

The twin screw manifolds are all open inside, and the injectors spray straight down into (and around) the intake ports. Gas probably sprays all over the place most of the time.

heinrich

Bill Ball

Whatever gas is spraying allover seems to be minimal as gas mileage is not affected by the blower under street driving. So far, no nuclear explosions either.

Lizard928

Ken, I think that you are correct with it being gas,

However the reason that I believe it is on some ports and not others is not 100% because of the intake design. But instead due to the timing of the batch firing,

Say at idle you get 4 squirts per revolution and those squirts are in time with certain valves opening. The other atomized fuel is sitting in the intake longer and has more time to settle and build up on the walls. This may also explain as to why sometimes the 85/86 intake manifolds backfire abit!

Some it could be the plenum design, I just dont think it all is.

BTW I may have a set of intake spacers out of phenolic you may be able to persude me out of, shoot me off an email.

PorKen

I ran across this pic from my old '86.5.

I was wrong, it's not the straight legged ports that have more buildup, it's the ports fed by the 5/8 side plenum...

PorKen

It does seem to be connected to the firing order.

The dirtiest ports are 2, 3, 5, 8, which are the even ports in the firing order.

1-3-7-2-6-5-4-8

(These cyls are all fired by the 5/8 distributor.)

PorKen

Maybe it's a ram effect difference between the two sides? I have a spare '85 intake (unfortunately not a very dirty one), and looking in the different legs, it could be the amount of filth is the same, but it's at different levels. As if one side's plenum is likely to draw or push farther than the other.

I measured the horns inside the plenums (out to the hose) a while back, and determined that the intake length they were shooting for was roughly 400mm from the horn to the head. Maybe the slighly different shapes inside each plenum is enough to change the ram effect?

1/4
7 - 29
4 - 98
T - 75
1 - 98
6 - 76

5/8
8 - 84
3 - 29
T - 75
2 - 29
5 - 111


Both plenums have the same volume. (I filled one with water, then poured that water into the other.)