New plenum, huge vacuum leak

[vetteboy86]

I purchased a plenum which I ported. I put it back on the car, and now I have one huge vacuum leak. The car doesn't want to run. Reason I think I have a vacuum leak, is because I can hear the hissing sound over the motor. To me that is odd.

It almost sounds like when you run the car with an open throttle body, and no hose to the air filter. Only I have everything hooked up.

There are only so many places it could leak. I used new gaskets around the runners. I put the mat sensor, back in. I have the two nipples on the passenger side connected. I have the line for the brake booster back in.Where else could it be. It doesn't seem to have sealed good around the runners. What is the proper sequence to tighten the bolts. Also does anyone know the torque spec. I dont want to go crazy on the aluminum parts.

Thanks

 

[Ken]

I purchased a plenum which I ported.

Hmmm... Makes me think, how good are your porting skills?

Are you sure the gaskets didn't slip on you when you reinstalled the intake?

 

[vetteboy86]

Okay, it wasn't a full port. I just opened up the entry to the throttle body.

What do you mean by the gaskets slipping. I do not think they moved hardly any. I had to move them slightly to slide the torx bolts through.


If I put a thin soapy film over the areas where the plenum meet the runners, could that help me find the leak. I thought soap would bubble or something. Or is that an air leak? Either way I cannot find the leak. I am thinking I have to pull that plenum off, and put the old one back on.

 

[Vettefan87]

Hmmm... Makes me think, how good are your porting skills?

Are you sure the gaskets didn't slip on you when you reinstalled the intake?

I am no expert, but wouldn't the gaskets have to move a good distance for there to be a vacuum leak? The worst I thought could happen is it could block one of the passges on the plenum. Something is definetly not right here. My brother is not a imcompetant person, so I highly doubt it is mechanic error.

 

[jrbartholomew]

to find a vacuum leak ive used either/starting fluid.with the engine running give it a quick shot spray at joints and gaskets. the engine will rev when the either finds a leak.you have to be very careful and just use small sprays.hope this helps.

jeff

 

[vetteboy86]

Jeff,

Thank you for responding to my post. However I might have more going on then just a vacuum leak. What size vacuum leak would cause the car to not want to run at all.

I start it, and it revs to around 1200 RPM's or so, then just dies.

 

[jrbartholomew]

sounds like maybe 2 surfaces may not be flat or maybe warped .you could also have someone crank the engine as you spray the either. i would think a leak that big you could almost see. good luck.

jeff

 

[vetteboy86]

Yeah, I keep checking all the lines. I dont think a line unplugged would cause such a bad running condition. I did change the plenum, so that is really the only other place to look. I have checked the gap between the runners, and the plenum, and it is fairly consistant.

Does anyone know a order to tighten the bolts, that would secure the fit. Also is there a torque spec. Maybe I have them to tight, is there such a thing?


Craig

 

[fredk]

Intake must be torqued to 35 ft lbs and according to the Haynes tighten front to back drivers side first then same for the passenger side. RTV sealant should be used on front and back of intake and around coolant passage holes in the gaskets.

 

[vetteboy86]

I didn't take the intake manifold off. Just the plenum. Are the plenum torx bolts that same torque spec. I was wondering what order the plenum to runner bolts should be tightened. Also do I need to use sealant on the tube runners to plenum gaskets.

 

[vetteboy86]

Problem fixed!


I put the original plenum back on the car, and no vacuum leak. I knew it wasn't my mechanical skills

Well I am going to button everything up tonight or tomorrow after work. Thanks to all who replied.


Craig

 

[G Winter]

Now I think we all want to know why that plenum leaked. If you can find the reason it may help all of us avoid a similar problem.

 

[vetteboy86]

Well that is exactly my thought. I wasn't sure if the planes on the plenum is a different degree. The one I put on is dated to be an 89. I am not sure why it didn't work. I do know that the vacuum leak I was experiencing was from around where the tube runners attatch to the plenum.

When I put the old plenum back on, all I attached were the necessary lines to the plenum, and it works fine. So that rules out a leak somewhere else. I almost feel like putting it on again, just to see if I can get it to work. This might be another day though, because it has been like four since I have driven it, and I really want to get it out.

I would be curious to know if anybody has sucessfully put an 89 plenum on an 86. I used the same new gaskets on both installations. I was relieved, when she started on the first turn of the key. No stumble, or bad idle. I did clean the TB and the IAC when I had everything off.

Well if anyone has had the same experience let me know. Also if anyone has done this let me know too.

Craig

 

[Edmond]

That's a mystery to me as well. I would think that the plenums from all L98's would fit without any problems.

 

[vetteboy86]

I cant understand it myself. I am not going to get rid of it. I think I might maybe in the winter, put it back on, and try it again. I am getting close to the cruise fest, and driving weather these days is hard to come by. So for now I am going to put it on the shelf.

Edmond,
I am just happy I got a good deal on the plenum, and not out any serious money

 

[Edmond]

You should sell it. There's nothing wrong with the plenum. I never put it on my car because I managed to salvage that little nut for the booster hose and that was stripped on my plenum.

 

[vetteboy86]

Selling it is an idea. I really would like to know why it didn't work. If I do figure out the deal, and get it working on my car, I might try doing some polishing or something cosmetic to it. I really would like a polished look on top of the engine.

 

[Moonunit 451]

If you've got the time, you should try it one more time. I can't imagine why it wouldn't work, and you really don't want to send something on to someone else if there's a problem with it for some reason that you suspect. Happy to see you're back on the road though So has Justin whooped up on ya yet

Never did hear what the idle prob turned out to be

 

[Vettefan87]

So has Justin whooped up on ya yet

Nope, he is to scuured hes gonna get a ticket which is okay, yet he does a burnout in front of our house. It is an open invitation whenver he wants to go, I'm game.

 

[Moonunit 451]

Don't wanna talk about tickets :eyerole On the other hand I'm gonna hit the track this weekend and see if I can pop into the 11's. Weather's not lookin' good for a fast run though, way too humid. What about your idle Justin?