Mystery vacuum leak

[Maverick H1L]

I need some ideas about this stinking vac leak I have in my hybrid 3100. It sounds like it's coming from the center rear of the intake (transmission end). The valve covers have been sealed, regasketed, rethreaded (to ensure the bolts get tight), and the driver's side (RWD) valve cover has been sealed with RTV. The fuel injectors have had the o-rings checked and replaced where needed. It looks like the intake manifold gaskets are sealing just fine and I doubt the end wall RTV has been disturbed along with the sealant on the oil pump drive.

Right now, my MSII is reading around 33-35 kPa at idle, which is at 13-1500 RPM at its lowest when the engine is hot, and I know for a fact that I had originally set it at around 900 with the idle kPa at around 28-29. I can hear the leak, but I can't find the thing no matter what I do. The idle intake pressure doesn't concern me anywhere near as much as the fact that the engine is idling 50% higher than I would prefer...

Anyone have any clues? I'd like to get this taken care of sometime in the next week before the inspection time. I have a 10-day sticker on it at current...

[1988GTU]

The PCV working correctly? Check the valley and make sure its sealed there? EGR port good? Brake booster line not sliced or threads bad?

[TGP37]

Ideas, check around the TB/Plenum seal, IAC seal, cracked vac line inches from the plenum....

Or maybe the leak is somewhere else and you are just hearing into something else. Have you tried isolating each vac line one at a time to rule out anything down stream?

I doubt it but even the throttle bar running through the TB, could those seals leak?

There is a possibility it is a cracked plenum, it happens far too often. Even with stock torque on the bolts, they still crack easily.


Maybe apply pressure to the plenum and spray a mild soapy water around the suspected area. Won't take much as it will bubble up real fast.

[Maverick H1L]

Well, so far I have the following:

1. If I connect up my charcoal canister, BIG leak. It's currently disconnected, BOTH LINES. When I do hook it up to pass visual, I'm going to plug the vacuum fittings...

2. No problem with brake booster, check valve, vac hose, or plenum fitting. Already checked that. Disconnected the hose at the booster end and plugged it with a screwdriver, no change.

3. When I rethreaded the valve cover with the PCV valve in it, I pulled up too hard on the valve to get it out of the cover and snapped the nylon line. It has been fixed with hose. The valve has been wrapped with electrical tape to seal to the grommet and to the line's elbow, and the line has been wrapped with tape to seal to the elbow as well. This wasn't a problem before I wrapped everything with the tape, just in case.

4. I went and got the nylon pipe that runs from the intake bellows to the other valve cover. The pipe itself ended up not fitting right with my EGR setup, so I cut the ends off of it and spliced them together with hose, and the splices have also been wrapped with tape to prevent leaks. I'm having a slight issue keeping the valve cover nipple in the port on the cover.

5. After I went and got gas today (getting home was a pain since the engine stalled twice and tried a couple more times), I waited until it cooled off and disconnected my FPR to move out of the way, since my FPR is mounted to the firewall right behind the back of the upper plenum. With that out of the way, I put a pair of gloves on and smeared RTV silicone down on the end of the intake where it meets the heads and block. After reassembling everything and also tightening the nuts on my headers (where they meet the heads), the weird leak seems to be gone, but that stupid breather pipe fitting wasn't completely in the valve cover. Even with the breather pipe loose in the valve cover, the intake pressure stayed about where it has been since the mystery leak started creeping up.
6. EGR has new gaskets. I'm eventually going to plug the hole leading into the plenum behind the TB since the MSII can't run it and it's just there for "show me".

I'm going to find a way to get the breather pipe to stay in the valve cover and see what happens. I was originally going to use just a breather filter in the hole (they stick in there just nice without any hardware or anything), but the ignition module assembly and bracket are in the way . I could use a Camaro 3.4 bracket, but I really can't see spending more money for something really not necessary...

[1988GTU]

Well, so far I have the following:
I put a pair of gloves on and smeared RTV silicone down on the end of the intake where it meets the heads and block. After reassembling everything and also tightening the nuts on my headers (where they meet the heads), the weird leak seems to be gone, but that stupid breather pipe fitting wasn't completely in the valve cover. Even with the breather pipe loose in the valve cover, the intake pressure stayed about where it has been since the mystery leak started creeping up.

You modify the lower intake to clear the tb area? Take too much or not enough material away if you did mod that area?

[TGP37]

How about the fresh air feed for the PCV system? It draws metered air if you use a MAF. Any pics or video? Could help.

[1988GTU]

What vehicle is this in by the way?

[Maverick H1L]

You modify the lower intake to clear the tb area? Take too much or not enough material away if you did mod that area?

Not sure what it is you're asking here... Only thing modified to put in the intake was the top of my timing cover. Block, heads and intake are untouched.

Breather pipe is in roughly the factory position just in front of the throttle body. I made a hole in the silicone coupler for it (no, it doesn't leak, I've checked that already).

It's a 1988 Z28 that was bought as a body shell and got the V6 and transmission out of my other car, since the body on that was shot.

[1988GTU]

Not sure what it is you're asking here... Only thing modified to put in the intake was the top of my timing cover. Block, heads and intake are untouched.

Breather pipe is in roughly the factory position just in front of the throttle body. I made a hole in the silicone coupler for it (no, it doesn't leak, I've checked that already).

It's a 1988 Z28 that was bought as a body shell and got the V6 and transmission out of my other car, since the body on that was shot.

Is it pushing up against that area still? is what I am asking.

[Maverick H1L]

Is it pushing up against that area still? is what I am asking.

Um, first, that's the FRONT of the intake (RWD), and second, there is a good deal of clearance between the thermostat housing and the timing cover. The problem was at the transmission end of the intake.

[1988GTU]

From looking at another camaro 35oo swap I witnessed something similar but was on the front cover hitting the TB area. I guess since this is a production swap I should know everything.

[caffeine]

I'm going to find a way to get the breather pipe to stay in the valve cover and see what happens. I was originally going to use just a breather filter in the hole (they stick in there just nice without any hardware or anything), but the ignition module assembly and bracket are in the way . I could use a Camaro 3.4 bracket, but I really can't see spending more money for something really not necessary...

You can drill/tap that valve cover to 1/2" NPT if you want to run a breather filter just use a 90* fitting and it will clear the coil bracket.

[Maverick H1L]

Well, I've been working on this issue and I'm still stumped.

So far I have:
1. Swapped the gasket on the driver's side (2-6) valve cover
2. Rethreaded and used a stainless threaded insert on every hole in both valve covers EXCEPT the lower bolt over #6 which currently has an M8 bolt in it (removed the metal insert from the bolt seal and it fits fine)
3. Used RTV on the gap between the head and intake edge by the #2 cylinder and also on the valve cover gasket itself
4. Replaced the o-ring on the M8 bolt AND added a fuel injector o-ring on the cylinder head end of it (where the original seal had the lip on the inner edge of the bore)

I'm still getting the sucking noise from the area of the M8 bolt. I can't get it to stop no matter what I do. I've put an additional o-ring UNDER the valve cover, another between the head of the bolt and the factory bushing, another between the bushing and the bore in the valve cover, and it STILL leaks from that area. And I also seem to have the issue that if I hook up the charcoal canister to the engine vacuum supply, there's a leak there too (need it connected for emissions visual inspection). I can get the breather pipe to stay in the hole if I pin the end of the PCV valve pipe (that I guess originally ran to the canister purge valve on the 3100? and yes, it's plugged as well) between it and the ignition module bracket. The valve cover is TIGHT. The bolts are TIGHT. I don't understand where the vacuum leak keeps coming from and it's driving me bonkers.

Right now, I have the throttle stop screw backed out to close the throttle plate completely. It's running at about 32 kPa (it HAD been down to 29 at idle and would go down a LOT under a cruise condition), but I still have the high idle problem and it's not really controllable by the IAC. The idle speed is around 1000 cold and around 1500 hot. I have the IAC to the point that if I add more steps, the MSII starts OPENING it and INCREASING the throttle (it also opens it and increases the throttle if I drop steps... IDK why the IAC is working like it is but the wiring is correct as I checked the harness before installation and the MS to GM ECM adapter is the same one used in 1227165/MS swaps and was bought from DIYAutotune the same time as the MSII kit).

[1988GTU]

Is the TB that you have on there the one that came with the engine? If not, check the gasket, flange, and TB mating surface. If the seal/gasket is not the right seal/gasket meant for that plenum and TB combo, it will cause issues.

[Maverick H1L]

Is the TB that you have on there the one that came with the engine? If not, check the gasket, flange, and TB mating surface. If the seal/gasket is not the right seal/gasket meant for that plenum and TB combo, it will cause issues.

93-95 3100 upper plenum and throttle body, bought together. Gasket is correct as I used a 3100 full engine gasket set (except I had to buy a 3.1 RWD oil pan gasket and make my own water pump gasket for it).

Again, the problem is with the (what would be the REAR) valve cover over the #6 cylinder.