Missing vacuum line.

[captcorvette]

Have been chasing vacuum leaks, inept (me) mechanic, bad advice from local agencies, missing issues, VATs issues and much, more but slowly making progress thanks to the help, explanations and experiences of many of you. Seems like this is an adventure in "what comes next?".

Chasing down possible leaks one by one when at least one item came to the forefront. Have had the ECM off and on adding piggy back chips, checking pens, looking for any possible damage etc.. Then like the old "forest for the trees" I notice the vacuum line on the bottom of the ECM is gone. Yep completely gone. The "L" fitting is there on the bottom of the ECM housing but no hose in site----any place. Will now have to wait until I get home this evening to break out the FSM to try and backtrack from the ECM to where the hose pipe should be but there are no obvious hoses or pipes that are not connected to any area I can find.

Good thing I like puzzles. But perhaps this is a further sign of progress. One of the reasons I don't like to let anyone else ever work on my toys but sometimes we are forced to let other get their hands in the mess.

Any ideas out there as to where the pipe/hose has gone? How it could be gone without even a remnant remaining?

[sammy]

there is a plastic line that will come from under the plenum to the rubber fitting . jjust take a carefull look ,bet its there somewhere .

[captcorvette]

Thought there should have been one from under the plenum but there is only one on the driver's side and it goes to the one that I had just used to check the vacuum pump which is new and is working correctly. Put my finger over the pipe and the pump shuts off holding vacuum. Finger off and reconnected and the pump runs all the time.

[tomtom72]

Just for reference. That vacuum line that appears to be among the missing is from the left side actuator for the port throttles ( secondary blades in the head ports ). It tells the ECM via the MAP sensor under the ECM bracket that the secondary actuation system has enough vacuum to function properly and if needs be the vacuum pump to activate, thus augmenting vacuum to keep the secondaries "ON".

I am fairly sure that any parts store sells that hard plastic line in small spools.

just another point of reference. It seems with our cars one has to be wary of just using the soft rubber vacuum hose as a quick fix. Our motors develop a good deal of vacuum and tend to suck shut standard vacuum hose. I think that is one of the reasons why our motors have that hard plastic line everywhere.


Tom

[tomtom72]

This link will give you some background on the secondary vacuum system.

http://www.corvetteactioncenter.com/...diagnosis.html

In one of the picts it shows the hose assembly from under the plenum. In that pict, in the upper portion you can see the hose extension from the left side actuator to the MAP under the ECM, which is the hose you are seeking.


Tom

[QB93Z]

The vacuum line you are looking for is tucked into a wire bundle under the plenum and crosses the back of the engine to get to the vacuum sensor under the ECM.

Note: on later cars the vacuum sensor is under the plenum.

Search all of the wire bundles near the brake fluid resevoirs and around the brake booster, That is the direction that the tubing comes from.

If you still can't find it, I will go out to the garage and get an exact location.

Good luck,

Jim

[captcorvette]

Thank you everyone. Still can't find the bloody tube/pipe. Did stop by a few parts houses this evening on the way home and so far no one has the hard plastic piping. Will try the agencies tomorrow but first need to find the missing pipe source. I have a suspicion that one of the other set of hands that have been in this engine might have just pulled it out not knowing were it was supposed to go so the easy fix was bin it. But finding this problem is progress. Fingers crossed that I don't have to take the plenum off-----again.

[BlackbirdZ07]

I just got through replacing the secondary vacuum harness in my '90, which included this hose. A couple of the posts above are correct; this hose comes from a T connection at the driver side secondary actuator.

If you can't find anything that will work, this hose was just fine in my car and I could send it to you. I would probably check to make sure nothing else is missing first. If anything else is missing I'll send it if I have it, or it may be best just to order the secondary vacuum harness if enough is missing or damaged. Unfortunately, yes, you'll have to take the plenum back off.

[captcorvette]

Jim, Tomtom, blackbird, sammy and all the others we have success! Spent about an hour this morning tracking the plastic pipes down and had managed to find a roll of the hard plastic pipe but it is yellow. No worries yellow would do if it had to!

Opened the bundles and checked every wire/pipe. All went where they were supposed to go at least from what the FSM, sage advise givers and logic would dictate. Checked the rear of the plenum time and time again. No pipe.

Just before biting the bullet and pulling the plenum I decided to use my mini flex camera and look under the plenum. AH HA! There the little digger was and it was doing exactly what everyone said----going to the rear of the plenum. Looked again from the back of the engine and couldn't see the pipe. Flex mini cam to the rear and trace the pipe.

Found it!!! It was neatly cello taped to the bottom side of the big wire bundle at the back of the engine so it could not be seen from above. Would have NEVER seen it without the camera or pulling the plenum.

It is reconnected now and will give it a test this evening when finished with this afternoon duty/work.

But again this is progress and I am pleased about that part. Will have to determine how many issues this does or doesn't solve. Then if enough issues sorted out will install Marc's chip tomorrow on my day off. If The Boss doesn't have too many "honey do's" scheduled.

Much obliged to all.

[tomtom72]

Alright! Don't ya just love it when a plan comes together! Nice find.