Vacuum Leak/Sucking Sound - '88 MJ (Video Included)

[boxyjeep]

I've been trying to get my '88 MJ running right, but continue to experience a rough idle and what sounds like an intake leak somewhere around the #4 cylinder.  So far, I've replaced:

1. Exhaust manifold & gasket (replaced gasket twice)
2. Throttle body cleaned, gasket replaced, MAP connector tapped & replaced
3. Valve cover gasket
4. Ensured all manifold bolts are properly tightened in correct order
5. Checked head surface for approximate flatness using straightedge

 

I'm about to pull my hair out on this one.  I didn't see any obvious issues or cracks when I took the manifolds off the last time.  I just pulled the heat shield off in order to get a better look, but still don't see any visible cracks or other issues.  I ordered a mechanics stethoscope and will be using that to try to figure it out.

 

Any ideas or suggestions would be super appreciated.  Really hoping that the head isn't warped or cracked.

 

 

[Dzimm]

Maybe try a little soap and water to see if you get bubbles at the seal.  Would at least narrow it down to an exact spot if that's where it's at.  

[boxyjeep]

1 minute ago, Dzimm said:

Maybe try a little soap and water to see if you get bubbles at the seal.  Would at least narrow it down to an exact spot if that's where it's at.  

 

Forgot to mention, tried that as well but didn't see anything noticeable (will get a video on this).  The exhaust/intake gasket was my first thought since I used the cheaper one that was included with the new exhaust manifold, so I replaced it with a good Fel-Pro unit and sprayed both sides with Permatex High-Temp Copper spray (but still had the same issue).

[Dzimm]

1 minute ago, Chargum85 said:

 

Forgot to mention, tried that as well but didn't see anything noticeable (will get a video on this).  The exhaust/intake gasket was my first thought since I used the cheaper one that was included with the new exhaust manifold, so I replaced it with a good Fel-Pro unit and sprayed both sides with Permatex High-Temp Copper spray (but still had the same issue).

It's possible it's on the bottom side of the manifold, did you look from underneath? 

[boxyjeep]

Just now, Dzimm said:

It's possible it's on the bottom side of the manifold, did you look from underneath? 

Will be checking that once I get the stethoscope.  I just removed the intake manifold heat shield to make things easier to see, but nothing notable.  Also checked from underneath using mirror and from the ground, but couldn't see anything significant.

[Jeep Driver]

Injector Orings?

[Dzimm]

6 minutes ago, Chargum85 said:

Will be checking that once I get the stethoscope.  I just removed the intake manifold heat shield to make things easier to see, but nothing notable.  Also checked from underneath using mirror and from the ground, but couldn't see anything significant.

Hmm.  What's the air movement like in that area from the fan?  Would you be able to keep a match lit to see if it will draw the smoke in? 

[boxyjeep]

44 minutes ago, Jeep Driver said:

Injector Orings?

 

Forgot to mention--I replaced these when I replaced the exhaust manifold.  No leaks.  They are definitely a bit noisy, but there is no fuel leaking and the vacuum leak doesn't appear to be coming from them.

 

43 minutes ago, Dzimm said:

Hmm.  What's the air movement like in that area from the fan?  Would you be able to keep a match lit to see if it will draw the smoke in? 

 

There is a bit of air movement from the cooling fan.  May try with the match (wish me luck--don't want to burn the garage down).

[HOrnbrod]

Try a cigar:

 

 

[Dzimm]

22 minutes ago, Chargum85 said:

There is a bit of air movement from the cooling fan.  May try with the match (wish me luck--don't want to burn the garage down).

Yeah keep a fire extinguisher handy. 

 

7 minutes ago, HOrnbrod said:

Try a cigar:

 

 

This is a better and safer method, might be hard getting the smoke to your leak if it's in a manifold tho. 

[HOrnbrod]

Make an adapter for the throttle body throat and wedge the butterfly open. Used to do this with carbs back in the day...

[Motorhead X]

You might try the cigar method with the other end of the tube near the manifold...

[boxyjeep]

Re-torqued the bolts and it appears to not be leaking.  Will keep an eye on it.

[Jeep Driver]

2 hours ago, Chargum85 said:

Re-torqued the bolts and it appears to not be leaking.  Will keep an eye on it.

Do you have an aftermarket header/exhaust manifold?

 

Is the flange on the exhaust thicker than that of the intake?????

 

Take a close look at it. 

[boxyjeep]

On 10/9/2018 at 6:24 PM, Jeep Driver said:

Do you have an aftermarket header/exhaust manifold?

 

Is the flange on the exhaust thicker than that of the intake?????

 

Take a close look at it. 

The exhaust manifold is aftermarket but built to OE specs.  I was also concerned about thickness differences between the intake and exhaust, so I measured them and found they were the same.

 

This is still holding steady after re-torquing the manifold bolts to 25 ft/lbs in proper order.