How to dignose vacuum leak underneath manifold

[BJM]

I have swapped many many parts now and even changed injectors to find a rough miss at idle. I really think its a vacuum leak on the under side of the intake.

I changed the intake gasket last winter and everything seemed fine but I slowly developed this miss over the summer. I have swapped everything and blocked off all vacuum lines to no avail.

The miss is bad enough that I can no longer idle at 700 RPM without stalling randomly and the engine droops severely other times.

I am left with yanking the intake off just to take a look. I must admit I do not like the FelPro gasket (96033). I am used to big squishy gaskets like used on Chevs.

My spark plugs all pretty similar looking. How can I figure it out without actually removing the whole intake?

 

[Turbo6inKY]

Have you grabbed a piece of rubber hose, put one end on your ear and gone hunting for leaks with the other end?

 

[MistaScott]

What do the scan #s look like at idle?
If you have a leak, BLMs should tell you.

 

[BJM]

I have not actually tried that, I will.

 

[BJM]

Scan values are all good looking by way. Direct Scan never shows anything useful when the speed dips. I am thinking its one cylinder only, unlike a vacuum hose leaking which affects all the cylinders.

Do manifolds ever crack?

 

[odell4o8]

intake gasket leak,

i had a intake gasket on my stage headed motor to leak and it pulled oil from lifter valley and smoked at idle real bad, it did not smoke when boosted. it also had a whistle sound coming from the intake it still idled ok ,

 

[CHANCE]

if you have a propane bottle like the small torch kind what you can do is pull the pcv valve out of the manifold place a piece of rubber line on the bottle and stick it in the pcv hole start you car and then slowly turn the bottle on if you have a vacume leak the rpms will go up but do it slowly you dont want to get the crank case full of propane

 

[GNBUICK87]

The best Way

Buy a can of carb cleaner and squrit along the intake between the valve cover and the intake if the car idles up it is leaking sucking air we use this method at the dealer

 

[BJM]

I have tried everything I can from the outside of the engine. I tried both propane from one of those little plumber's torches as well as carb cleaner spray. I got nothing around the edges of the manifold. I tried some propane into the valve cover vent and the engine did rev up but I did not pull the PCV. It probably went up the PCV valve. I will try it with the PCV hole instead.

You guys are referring to those hand held propane cylinders with a small torch on the end right?

 

[CHANCE]

yes the small propane torches

 

[tc86gn]

Try using a squirt bottle filled w/water.

Much safer and if there is a leak, you will DEFINITELY hear the difference.

There are more complicated ways of finding a vacuum leak but this is the easiest and safest.

 

[ThikStik]

I had an internal vac leak on a cylinder and it did not hurt blm's. Engine ran sporadically but with a good idle. K was worst problem.

 

[BJM]

Originally posted by tc86gn
Try using a squirt bottle filled w/water.

Much safer and if there is a leak, you will DEFINITELY hear the difference.

There are more complicated ways of finding a vacuum leak but this is the easiest and safest.

I tried the water thing, I tried propane, I blocked vacuum lines, etc. I can't really squirt water into the PCV hole, that's why I am trying propane.

BTW what more complicated ways are there, I am pretty well at the end of rope with this.

 

[HellOnWheels]

If it is a "timed" miss try this....get away from all lights turn off all car lights open hood let engine idle and look for spark jump just make sure you let your eyes adjust to the darkness.

Good luck,
HOW

 

[DR BOOST]

its probably the throttle shaft. ive seen this quite a few times.

 

[Pronto]

Maybe your hyperfocusing on a vacuum leak. Maybe it's an electrical or injector issue.

 

[tc86gn]

I agree w/the above post. If you haven't found a vacuum leak by now and have checked everything possible for a vacuum leak, then it is posssible that you have another problem.

 

[Rickracer]

Find someone who has a good ignition scope

If you get it scoped by someone who really knows how to use it, they will be able to pinpoint any cylinder that has a vacuum leak. A cylinder balance test should show it. Even pulling the plugs and examining them closely may give some indications. If your intake is leaking at the gasket, you will likely be pulling some oil in as well, it should be evident on that plug.

 

[ThikStik]

BJM, do you have any idle smoke? Also, is your plenum gasket good?
PS :Not hijacking thread i hope , but contemplate this: I have all new stuff on my motor and have idle smoke. An oil film covers inside of plenum area and intake surfaces all the way to back of throttle body. Its not turbo, or pcv, and heads are redone. But this intake is common to old motor that also idle smoked. Why under any reason would the oil film cover the back of throttle body on down to the intake valves?!??!?

 

[BJM]

Re: Find someone who has a good ignition scope

Originally posted by Rickracer
If you get it scoped by someone who really knows how to use it, they will be able to pinpoint any cylinder that has a vacuum leak. A cylinder balance test should show it. Even pulling the plugs and examining them closely may give some indications. If your intake is leaking at the gasket, you will likely be pulling some oil in as well, it should be evident on that plug.

Could you tell me more about the ignition scope? What does it do?

I did change the plugs as part of this mess and one of them was little darker than the others. Check out this photo. I always photograph plug when I take them out. Last years did not look like this.

Recent Plugs Changed

Do these plugs look okay?