Vacuum Leak 97 MKVIII

[eglade]

My Mark has recently developed a major vacuum leak in the intake valley area. It appears to be the manifold itself as a spray test and visual inspection reveals no leaks in the upper area of manifold or connections.
The leak is a solid, loud hiss but only affects driveability intermittently. It starts and runs fine and only occasionally exhibits a fluctuating idle. This condition does not last long and no trouble code is displayed. What puzzles me is that the leak would seem to be in the intake. If a large quantity of unmetered air were to enter, would'nt the computer sense the lean condition and go richer to compensate, thus throwing a check engine light ?
The leak is very loud, but not easily accessible. I'm getting ready to start pulling components for more access to leak area. I have only been able to do a quick driveway diagnosis and plan on diving in deeper when time permits. Any vacuum controlled solenoid or connections under the throttle body, runners or plenum area I should pay attention to ?
Anyone else have this problem?
Any assistance would be appreciated

 

[Frat-man-du]

I had a rip in the gasket for my IAC - was not that loud, but never threw a code but got rough idle at the stop light, acceleration was fine.

Also the rubber hoses next to it were all cracked so I replaced those at the same time.

 

[eglade]

Thanks for the info. The idle at stoplight sounds similar. Will check at IAC during troubleshooting.

 

[Jibit]

exactly where did you spray? try spraying between the connection of the upper and lower manifold, near the injectors, around the throttle body. There's no telling where it could be coming from. make sure the intake tube is attached properly to the throttle body. I've had a couple of times where I would tighten it too much and squeeze it slightly off the throttle body causing a leak. I don't think the engine knew any bette because the same amount of air was still passing through the MAF.

 

[southernbrothersgarage]

I had a similar problem on my '93. It turned out to be the flex tube from the exhaust to the egr valve. It sounded like a vacuum leak at first. I don't know if the gen 2 cars have the same setup but it's worth a look.
Dave.

 

[eglade]

Thanks for the info. The lower intake is broken directly under the alternator. I am looking for an aftermarket replacement. Dealer says they had a redesign, but no longer stock the intake. I prefer to upgrade while I'm in there. Any suggestions ?