OK, guys. Here's my issue:

My '95 coupe recently developed a stumble on acceleration, and what felt like an occasional miss while cruising at around 1800-2000 rpm. After some late night searching on the Forum, and pulling the codes, I decided to check the vacuum connection from the MAF intake. I pulled the hose from the fitting and blew into it. Sure enough I could hear the air coming out somewhere under the water pump where the hose disappears. I got under the car and tried to trace the vacuum line, but could not locate it coming out under the car. Granted, between the frame, springs, etc., there isn't exactly great visibility there.

Since everything I was finding on-line and in the codes pointed towards a MAF vacuum issue, I decided the easiest thing to do would be to Tee off one of the vacuum fittings on the passenger side intake manifold and connect forward to the MAF fitting. Short, sweet and clean. Started the car, checked vacuum at the MAF fitting and went for a drive.

Throttle response was immediate, no stumble at speed; It felt like a whole different car!

Problem is...I still have an open vacuum connection somewhere on the front of the engine that I cannot find. The old vacuum line runs down the driver's side of the water pump and disappears into the voids between the water pump and the accessory drives. As far as I can tell, it does not come out beneath or beside the block, but I cannot be sure. Several of the vacuum diagrams I have seen on-line appear to connect it to an EGR valve. Is this correct? If so, where is this valve located?

Any help would be appreciated. While I love the new attitude the 'Vette has, I am still getting a code 72, which I believe still indicates an open vacuum port. Thanks in advance!

 

From your description, it sounds like the vent lines for the OptiSpark. There is a vacuum line that runs down from the driver side intake manifold, down to a fitting on the bottom of the Optispark(distributor) to pull moisture and ozone out through the bottom of the unit. There is also a fresh air tube that goes down there too. That fresh air line comes up to a little fitting on the air intake pipe between the throttle body and the mass air sensor, also driver side. Sounds like one of those lines has come off down below. Unfortunately, they are buried down behind a mess of accessories on the front of the engine. You might have to pull the belt and hoses off to get in there. Here is a photo of the Opti that shows the lines right at the bottom.

 

Hear is another view. The vacuum line from the driver side is just visible above the accessory bracket and snakes down the driver side of the water pump and disappears behind the plug wires. the fresh air inlet hose comes up near there too (although I don't see it here in this photo) and the fitting goes into the air cleaner pipe.

 

Yeah, I remember those lines from when I did the Opti replacement last summer, and remember thinking what a B**** it would be if they ever came loose! The vacuum line from the manifold seems to be OK. I guess its the fresh air inlet line that has come loose. I

I assume that there is vacuum pull through this line drawing air into the opti unit(?) The car DEFINITELY runs better with the new vacuum line attached.

Thanks for confirming my suspicion.

 

The line from the bottom of the opti goes to the fitting on the front of the intake manifold. That is the vacuum line that draws air through the distributor and into the intake manfold. This line is supposed to have a check valve and a flow restrictor/filter inline, if I am not mistaken.

The line from the top of the opti goes to the intake tract between the MAF and the TB. This line provides fresh air into the distributor.

 

Yes. That line to the distributor doesn't really pull a vacuum, it only provides a small flow of air through the distributor to pull moisture out the bottom and fresh filtered air in through the other port. If your fresh air hose is off of the distributor, you could pull in dirt or moisture, especially if you go through any water that might splash up on the hot engine.

 

That certainly makes sense since if you have vacuum out, there has to be airflow in. Hopefully I can get at this before too long and replace the bad line. The car is not a DD, but I don't need to risk a premature Opti failure!