Leaking intake manifold?

[rgtrough]

Hey guys, How do I tell if I have a leaking intake manifold gasket? There are traces of oil in the valleys to the right rear of the carb. Timing is an issue. I sometimes have a backfire through the carb. I only have 15" of vacuum at idle off the main vac. port. The headlights won't go down. All in all the car does not run as strong as it should or did when I first fired it up.

Any help as always is greatly appreciated. peace Bud :cool

 

[JohnGrawcock]

I assume you've already done a thorough visual inspection looking for loose or cracked vacuum hoses. Don't forget the big one going to the brake booster.

Next I would correct the ignition timing for two reasons. First, an engine that is running poorly may not draw much of a vacuum. Second, a backfire through the carb while you’re under the hood can be dangerous.ad

If you’re sure that everything looks OK it is time to put your ears to work. With a major vacuum leak you should hear the engine sucking air. Use a stethoscope or simply hold a piece of rubber hose up to your ear and trace the sound back to its source. Remember to check around the base of the carb and between the manifold and heads.

When you think you've found the leak you can verify it by spraying some starting fluid on the area. If the starting fluid gets sucked in through a vacuum leak the rpm will increase momentarily. Again, I don't recommend this step on a backfiring engine.



Good luck,

John G.

 

[AKRAY4PLAY]

Bud,
John has a solid method of checking for vac leaks. i would use WD-40 instead of starting fluid in your case. basically any flam-fluid or spray will work, even hair spray. the theory is the leak sucks in the extra fuel and increases the idle speed. your timing may be off from having a difference in vacuum. 15" is not bad. i noticed you are running a 383. what cam do you have??? cam can make or kill idle vacuum. my 406 is about 8.5", pretty lumpy idle. is you distributor recurved for your combo?? are you checking timing with vac advance disconnected and capped?? a little more info would help. if you think it is an intake gasket, i would pull all vac lines off the carb and cap the ports. this will eliminate the possibility of leaky lines. once the intake is tested, reconnect the lines one at a time and test each one completely. it can take some time to find the leak(s), but you'll get it. check the vac cans on the headlights, the stem seals are notorious for drying up and failing. good luck, Brian

 

[rgtrough]

Hey John and Brian, thanks for the feedback. Tonight I did all that was suggested. Sprayed the fluid. No change. Set timing from the beginning. 10* initial/35* all in. Runs ok but not all there. Then I traced and inspected every vacuum hose I could find. I replaced all that were even remotely suspect. That did it! Starts and runs great.
Brain, everything in the engine is new this winter but the pistons and rods. Money kept me from replacing those. They only have 6000 miles on them. The cam is a comp 280h magnum with 1.6 roller rockers. This combo makes for a total lift of .522. I have not had the dist. recurved yet. I just got the motor back together last Wed. and have been fighting this timing/vac problem ever since the cam break in. How do you run your brakes and lights with such little vac? Mine are much better now but I still would like some more. Also what vac advance port to you use on the carb. I seem to have better results with the portal or no vacuum port over the vacuum at idle port.
Again thanks to all because I know I could never have done any of this without the help of the members of the CAC.

 

[JohnZ]

Use a full manifold vacuum port (not the "ported" vacuum port) for the distributor vacuum advance; will give you the pre-smog vacuum advance system, with full vacuum advance at idle (better idle cooling and idle stability).

 

[RMS73]

On my '73 L-82 I found the intake manifold was leaking into the engine. Checked the oil dip stick tube and found 8" Hg vacuum there. Removed the intake and replaced the gaskets and my vacuum went up. Up, that is from 8" Hg to 15" Hg.

Roger

 

[AKRAY4PLAY]

Bud,
glad you found the problem. old vac lines are great for causing headaches. sounds like a nice package you have. very streetable and fun. have you tried bumping your timing up to 12*??? most mod small blocks like 36*-38* total timing. i am running 16* initial and 36* final in my 406. it is all in around 2500rpm, but i don't run the vac advance. it would help me, but with the cam i have, jetting is hard enough. the lights and brakes work good, if the rpm is over 1200. the combo peaks around 22 in/vac at 3200rpm. luckily my vac booster check valve works well, i only have one stab at power brakes when idling. but i will live the gremlins to keep my 600 hp. take care and glad you got it lined out, Brian

 

[joe1975]

Hey Bud, do you think this solved your problem, Because I'm having simalar issues with my L-82 oil right rear manifold plus I'm getting detonation @ 12deg. fac.spec
Vaccum?

 

[JohnGrawcock]

I don't think any of us have addessed the oil leak

Hey Bud, do you think this solved your problem, Because I'm having simalar issues with my L-82 oil right rear manifold plus I'm getting detonation @ 12deg. fac.spec
Vaccum?

The oil on the intake that Bud mentioned appears to be completely unrelated to his vacuum and tuning issues. The oil is most likely leaking past the valve cover gasket. This leak is way too common on GM engines because they only have those four little bolts to hold the flimsy valve covers down.

What usually happens is the bolts get over torqued at some point in their life forming a dimple in the gasket flange. Once the flange has become uneven the covers will not seal even with new gaskets.

The problem can be corrected by flattening out the gasket flange. To do this:

1. Remove the valve covers.
2. Hold the cover upside-down with the topside of the flange against a hard flat surface.
3. Beat the flange flat with a hammer.
4. Reinstall the covers using new gaskets and torque spreaders under each of the four bolts.
5. Torque the bolts to factory spec.

The torque spreaders act to spread the force of each bolt over about a 3 inch area rather than just under the head of the bolt. Much better seal.

 

[JohnGrawcock]

I wanta play too!!!

AKRAY4PLAY,
Have you got a good source for a 400 short-block? Any heat trouble with the extra 6?

The reason I ask if that I've been thinking of building a 400 small-block. I want to do something more like a road race motor. Quick revs and a fat mid-range would make for lots of fun in a street driven shark. I'd settle for a messily 400 to 500 peak hp.

In fact, think I'll shop around for some dyno simulator software this afternoon so that I can work on that torque curve.

See Ya,

 

[JohnZ]

Oil stains/spots/pools on the intake, especially adjacent to the intake bolts, are a result of hot oil from the lifter valley migrating up the bolt threads and out from under the bolt heads (all intake bolts except the extreme four corners extend into the lifter valley). Remove the eight intake bolts one at a time, apply thread sealer, and re-install.

 

[rgtrough]

Hey JohnGrawcock, it is very possible that the oil is coming from the valve covers. I will give your method a try. Thank You.

Hey Joe1975, I am still experiencing the small amount of oil in the valleys of the manifold. The car is running much better now that I have close to the proper vacuum. Check your total timing with a advanced setting timing light so your total timing is in the 34 to 36 degree range. Anything higher than this may be the cause of your detonation.

Hey JohnZ, Your advice is always greatly appreciated. I used thread sealer when I installed the manifold. When setting the timing using the full manifold port on the carb, should the vacuum advance tube be connected to the distributor? I can feel the improvement at idle, but it seems off in the high end. Maybe it's time for a professional to look at my distributor. peace Bud

 

[JohnGrawcock]

Hey JohnGrawcock, it is very possible that the oil is coming from the valve covers. I will give your method a try. Thank You.

I forgot to mention that you'll want to use some threadlock (i.e. "LockTight") on those bolts.:duh

 

[JohnZ]

Hey JohnGrawcock, it is very possible that the oil is coming from the valve covers. I will give your method a try. Thank You.

Hey Joe1975, I am still experiencing the small amount of oil in the valleys of the manifold. The car is running much better now that I have close to the proper vacuum. Check your total timing with a advanced setting timing light so your total timing is in the 34 to 36 degree range. Anything higher than this may be the cause of your detonation.

Hey JohnZ, Your advice is always greatly appreciated. I used thread sealer when I installed the manifold. When setting the timing using the full manifold port on the carb, should the vacuum advance tube be connected to the distributor? I can feel the improvement at idle, but it seems off in the high end. Maybe it's time for a professional to look at my distributor. peace Bud

When setting base timing, the vacuum advance line must be disconnected and plugged so the advance diaphragm doesn't move; after setting the base timing at the lowest possible idle rpm, re-connect the vacuum advance, re-adjust the idle speed and idle mixture screws for the highest steady vacuum reading at your desired idle speed, using a vacuum gauge connected to a full manifold vacuum source.