[Rocketdog]

I love Seafoam. I have used it on every used car I have owned.
I have used it in the spray form and the liquid form to clean parts and as an engine treatment. It can be added to the fuel or oil, but I have never put it in the oil.

The key I found after a couple unsatisfactory treatments (little smoke) was heat sinking the intake manifold.
Use it as directed, but make sure the engine if heat soaked...like up to full operating temp for 30 minutes.
Then, have somebody run the throttle to keep it running, and put the entire can into the throttle body. This will take several minutes. When the can is empty, immediately shut it down and let it cool completely...like for an hour at least.

Remember, it took 20+ years to get that dirty, it will not come out immediately or all at once.
However, if you do as stated above you will have a smoke show like no other, I promise.

I ended up removing the intake and cleaned it manually, then ported it for kicks. Upon removal, I found a lot of carbon on both the intake and exhaust sides of the heads.

Before installing the Mark engine, I pulled the intake and exhaust manifolds and cleaned everything manually. Even that engine, with only 60k miles on it had a lot of carbon in the intake and exhaust portions of the heads. There was not much inside the intake manifold itself, and using Seafoam, I was able to get everything clean like new. The Seafoam really softens the carbon deposits.

I even made a Seafoam drip system to run in the PCV tract to constantly clean the top end if needed. A simple lubricator or Filter, Lubricator for an air compressor tool works pretty well. I used to fill it up and let it drip about every 2 seconds, and it kept my 2v NPI engine really clean, and that one had almost no valve stem seals left and smoked hard.

Good luck

 

[Rocketdog]

At 12.99 a can I would try the wait for an hour. Far more satisfying results.

 

[Rocketdog]

I don't think you can hydrolock a V8 with a spray can of Seafoam.
The long heat soak has worked for me for many moons, that's why I recommended it. I don't recommend sucking Seafoam in through a vacuum line without a way to meter it.
Good luck!

 

[Rocketdog]

I guess I just used the seafoam wrong then. Does the car you are taking about have a metal intake @Rocketdog? Is heat soak significant for a plastic intake?

Like I said, I use it on all my cars. To answer your question about metal vs plastic intake manifolds...it does not matter because the engine temps remain the same. The plastic obviously has different properties than aluminum but the specific gravity or mass is irrelevant as the temperature envelope is constant.
Get it hot, spray the whole can in there and shut it down for an hour.
Your next start up will be dramatic, I promise you.
My Corolla has a plastic manifold and 275k on it. I did that one in February and got the locals all stirred up with the show.
That thing smoked as much or more and for longer than the 170k NPI 2 valve did. It was spectacular on a crisp -7f NH morning.

 

[Rocketdog]

They all look pretty clean from that view.

I would be very cautious of putting any substance in your oil. Any at all.

The gunk you are after lives in the components downstream of the throttle body, and inside the combustion chamber, intake and exhaust ports of the head. Not internal to the rotating assembly.

If you use Motorcraft synthetic blend oil, you should have no issues of crud building up, it is very good oil.

Frankly, your best bet is to pull the intake and exhaust manifolds after doing the Seafoam routine. The Seafoam will soften the crud nicely.

Then you can pretty much spoon out the bulk crud, and further clean with a brass brush and Seafoam spray until spotless.

My SOHC had about a Dixie cup
of crap in both the exhaust and Intake ports and manifolds. About 12 Dixie cups half full came out
My DOHC had about half as much. Still, it was enough to actually spoon it out.

 

[Rocketdog]

My point was not real clear.
Pulling the manifolds is a job, and best done when upgrading.
I pulled my SOHC intake after looking at it with a scope before and after the Seafoam
I pulled my DOHC manifolds prior to installing the engine.

The Seafoam did not really do much on its own beyond a spectacular smoke show. It did soften the gunk.

Both cases revealed a surprising amount of gunk, and I doubt 1 can of anything would remove it. Maybe if it was repeated many times, it would remove more.

Beyond working well as a parts cleaner, I can't say that it does a lot. I do add it to fuel on occasion and it is my go to upper engine parts cleaner.
Put stuff in your oil at your own peril, I won't condone it.
To truly clean a 20 plus year old intake and or exhaust manifold, there is no substitute for manually cleaning them.
The mechanic in a can BS espoused on here is amateurish at best.
These cars can vote and buy a drink, and there ain't no way a can of stuff will remove 20 years of gunk.
The Seafoam smoke show is cheap entertainment, that's about all.

Good luck!