Intake leak

Reply Subscribe

Thread Tools

Search this Thread

Aug 24, 2002 | 11:29 PM

mchild

Thread Starter

Senior User

Joined: Aug 2000

Posts: 407

Likes: 0

Intake leak

I have an `85 460 that has had a rough idle. I checked the intake manifold for leaks by spraying carb cleaner along the top edge of the manifold where it meets the head. Each time I sprayed the engine vacuum went from 17 to 20 inches for a moment, the rpm’s would increase, and the motor would also smooth out.

So I tighten the intake manifold bolts (finding all a little loose) and the motor ran smoother and the vacuum went up to 18 inches. But, since the vacuum went to 20 inches when I sprayed the carb cleaner before I tightened the bolts, I tried it again afterwards. Once again it went up to 20 inches which I am assuming means I still have a leak but, apparently, not quite as bad.

Is there anything else I can do other then pulling the intake and replacing the gaskets? If not then is there a better gasket to use? What about using an RTV sealant around each intake port?

Thanks,

Mark

Aug 25, 2002 | 03:44 PM

TorqueKing

Posting Guru

Joined: Feb 2002

Posts: 1,712

Likes: 1

From: Atlanta, Georgia, USA

Intake leak

get a Fel-Pro blue stripe gasket, most any parts store sells these. Use RTV black ONLY at junctions where the end seals meet the head. Some prefer to use RTV in place of the cork end seals, this is fine, but there's no reason for the cork seals to be inadequete unless your heads have been milled. In this case, unless your block was also redecked to accomodate for the lower sitting heads, you should use RTV in place of the end seals. DON'T use RTV on the head side of the intake gasket, this can turn out disasterous if done improperly. It won't take but an hour or so to do the whole thing, and the gasket is cheap, maybe $8.99 for the whole set. TK

'77 F100, 302 (the aftermarket Prodigy), C4
Cadet Second Lieutenant John F. Daly III
South Carolina Corps of Cadets, The Citadel
The TorqueKing

Aug 25, 2002 | 05:56 PM

Vampiro

Cargo Master

Joined: Sep 2001

Posts: 3,135

Likes: 0

From: Canada

Intake leak

what about that black insta gasket stuff? it repels water, gas, oil, everything. i used it and it seems to work well. always remains flexible like a glue

Aug 25, 2002 | 10:13 PM

TorqueKing

Posting Guru

Joined: Feb 2002

Posts: 1,712

Likes: 1

From: Atlanta, Georgia, USA

Intake leak

No "gasket in a tube" will ever work as well as a real material gasket. The only time you should ever use RTV is to aid in sealing corners, if you use it as a primary material, it has less shear resistance, and is very prone to bulging into water passages, preventing full flow. Besides, if you do use Silicone instead of the correct gasket, you will be in a world of hurt to remove the intake should you ever need to again. You'll have to scrape all the old stuff off, otherwise it will never seal correctly. Modern Fel-Pro gaskets, especially the Blue-Stripe line have a no-stick coating which keeps it from getting sheared by bi-metal engines (iron block, aluminum intake or heads, etc.). Trust me, it's much more difficult to remove a part when it was siliconed in place. Maybe if it was a Toyota, or another car that didn't deserve love, I'd say go ahead and do whatever, but if you love it, do the correct repair!

'77 F100, 302 (the aftermarket Prodigy), C4
Cadet Second Lieutenant John F. Daly III
South Carolina Corps of Cadets, The Citadel
The TorqueKing

Aug 25, 2002 | 11:36 PM

Vampiro

Cargo Master

Joined: Sep 2001

Posts: 3,135

Likes: 0

From: Canada

Intake leak

true true, but in emergancies it will work, and i have had to do it

Sep 1, 2002 | 09:40 AM

stilllearnin

New User

Joined: Sep 2002

Posts: 1

Likes: 0

Intake leak

Mark

I had the same problem last summer on my stock intake (83 460). I took the manifold to my local engine machine shop, which verified that it was warped. I opted to replace it with an Edlebrock Performer EGR manifold, instead of having it planed. Worked great.

Gary

Sep 1, 2002 | 11:35 PM

mchild

Thread Starter

Senior User

Joined: Aug 2000

Posts: 407

Likes: 0

Intake leak

Thanks for the comment.

After posting I went back and re-torqued one more time but this time went to the top of the torque spec listed in the Haynes manual plus I added one more lbs. for a total of 31 lbs. ft. After doing so the intake vacuum went up some more and was nearly what it was when I spayed the carb cleaner at the seam. I was getting between 19.5 and 20 inches of vacuum with only about a half of an inch fluctuation. After I did this last re-torque I sprayed carb cleaner again and I really could not tell whether it was increasing vacuum because of the cleaner or it was simply the normal fluctuation. At that point I decided to let well enough alone. Motor runs much better now. It I do pull it I will check for it being true and if not then I will go with an aluminum manifold.

Regards,

Mark