In-situ Intake Gasket Test

[Tony H.]

My engine rebuilder proposed a test to determine whether my recently rebuilt 1970 LS-5's intake gasket is leaking. The test he proposed calls for the removal of the carburetor and the rocker arms. He will then attempt to apply a vacuum to the intake. I suppose this will be done with a sealing plate placed where the carburetor goes. If vacuum can not be established, the intake to head gasket(s) is the likely culpret. If intake manifold gaskets are again the problem, then he will check the intake's and heads' geometry for possible warpage. Hopefully he can machine these components as necessary and still preserve the parts for NCRS fun (preserve matching numbers of my car).

When the engine work was done at the latter part of last year, all the valves were replace, teflon positive-seal valve seals and cast iron type valve guides were also installed so I have confidence that the oil is not escaping passed the seals. My engine builder said that he has seen this problem a 100 times on big blocks and 99 of 100 times, it's the intake gasket that is not sealing properly. He also asked me if I used a sealer for the intake ports. As you may know, I followed the service manual which only calls for a sealant to be used around the coolant ports. My rebuilder says that he uses a brown sealer that comes in a can that is brushed on and apparently GM uses the same product (maybe it IS a GM product). Any idea what this sealer might be? He also told me that the front and rear rubber seals (supplied with the intake gasket set) are not used anymore by GM (since about the mid 70s). Instead, silcon sealer is used.

I have scheduled my mechanic to prepare my car for the intake manifold gasket test which will take in the week of June 12. I'll let you know how it turns out then.

By the way, my gas mileage last night (300 highway miles) was about 16+ mpg.

[Robert C.]

Re: Another Test

Tony, your rebuilder's advice sounds good to me. There is another test many mechanics use to find vacuum leaks at the intake. You start the car and let it idle. You then use a bottle of propane gas, open a little ,and point the gas jet opening, and slowly move it all around the intake gasket area. When you hit a leak the engine smooths out very quickly. Hopefully someone who knows the exact way this test is done will chime in here, as I'm not 100% sure I've got it all correct.


Texas Chapter NCRS

[Tony H.]

Re: Another Test

Bob,

I hear you on your test. I did not confirm external leakage (air rather than oil) but that would be a good way to check. I'll go try it after dinner and see if there is improvement in idle (due to the propane getting sucked into an external intake leak). However, the intake gasket was leaking internally. In other words, the leak was on the bottom side of the intake and inboard side of the head and the oil that is sprayed up in the engine (above the cam shaft and below the intake) gets sucked into the intake/head interface and slurped passed the intake valve into the combustion chamber. That's why the oil coats the intake passages of the heads but not the intake passages of the intake manifold.

External as well as internal leakage would point to a warped head or intake. Hopefully the latter and hopefully correctable without loosing my matching numbers intake. Here's where the preservation part of NCRS comes into play!!

Thanks for your comment.

[Joe L.]

Re: In-situ Intake Gasket Test

Tony-----

I like it! This is one test that I hadn't thought of. But, conducted under the conditions which you described (carburetor off and sealed; rocker arms off and valves closed), it ought to work well. I'll be interested in the results.

As far as the sealer goes, don't waste your time. The sealer which you are describing sounds like Indian Head. It's fairly dark brown material which comes in a can or a bottle with a swab-type applicator affixed to the lid. Very good stuff, too; I use it myself. But, it won't cure this problem. I tried it and about 7 or 8 other different sealers. None had any effect whatsoever.

As far as the end seals go, ALWAYS use silicone RTV in place of the rubber or cork end seals that come with the kits. Throw the rubber/cork seals away. You've got a much better shot at a good seal using the RTV. Use Permatex Ultra Copper or Ultra Black. Both are very high temperature resistant and they expand when they come in contact with oil, improving the seal.

Also, it sounds like your rebuilder did all the right things when he rebuilt the engine. All new valves and cast iron replacement guides are THE way to go. I don't like the Teflon positive seals too much, but lots of folks have used them successfully. Like I mentioned, I prefer the Viton-type positive seals or the 92-96 LT-1/LT-4 positive seals. The 92-96 type won't work on a big block, though.

[Jerry Clark]

Re: In-situ Intake Gasket Test

Tony:

It sounds like your engine rebuilder knows his stuff, then again , why did this situation get by him in the first place. I have followed your posts from the start, would like to see you rewarded for all your diligent efforts soon.

Best of Luck

jer

[Dale Pearman]

Re: Another Test

You got it right Bob; that's how I test for leakage.

Varooom

[Tony H.]

Re: In-situ Intake Gasket Test

Well, I asked the same question: Why would this leaky intake get by the shop in the first place? His response was that sometimes, the parts go in straight but warpage occurs with the heat generated by the engine. Sometimes the warpage is just enough to cause this situation.

My latest thought I had late last night was that perhaps the heads were in their opposite positions when originally installed in the car (the left head was on the right side and the right head on the left side). There may have beeen slight warpage of heads and intake originally but not enough to cause any problems. However, when reassembly occurred after the rebuild, the heads were swapped right to left and vise versa and the compounded irregulatity did not conform to the intake or allowed additional warpage. ???? Just a thought. perhaps there is a way to determine by the position/measurements of the geometry of the heads (while installed) and the intake (once removed) whether that is a possible solution.... Perhaps not, but sure will be nice to find a nice solution to this frustating problem...

I'll keep you posted!

[Paul Y.]

Re: In-situ Intake Gasket Test

Tony,

I have been dealing with the same problem and have replaced the intake gaskets four times. The last time I doubled up two and glued them together with gasketcinch. This worked well until I took it out on the highway for 8 hours at about 80mph. My machine shop friend told me that I need to pull the heads off and get the proper angle surfaced on them. I bought the car a couple of years ago with the problem so I don't know if the heads were shaved or what. It looks like they might have been a little. The lack of vacuum sure reaks havoc with the headlights and wiper door. I don't want to do any surfacing to my intake manifold as that seems to be a patch instead of a fix. I am trying to get the time to pull my heads off and will keep you posted as to my progress.

Regards,

Paul Young