Vacuum leak test, valley pan

[Shorthorse]

I've been trying to help isolate a suspected vacuum leak. No signs of any leak externally using all the usual methods. I'm wondering about the leak possibly coming from under the valley pan, inside the engine. Has anybody ever experienced that? Is it even possible? Is there a way to test for that with a running engine?
Thanks

 

[lewtot184]

if the angles on the intake are off then it is possible to have a vacuum leak.

 

[Ski 61701]

Take an unlit propane torch and let the gas flow around the valley pan while motor is running. Go over the entire manifold not just gasket area. Rule out cracks in the manifold. Also if you have power brakes unplug hose from booster and plug it. This way you can test the booster if it has an internal leak

 

[beanhead]

You bet its possible...vacuum gauge can give you a clue, lower than normal and/or unsteady needle...oily looking plugs is also a sign. Those leaks will drive you crazy the only sure way is to pull the intake & pan and see if any oil around your ports or in the chambers..sucks! but at least then you know for sure

 

[Shorthorse]

Thanks for the responses guys. Yes it's very frustrating. The engine will run above two grand but any less and the vacuum gauge falls rapidly to near 0 and it dies. As I said, every test on the outside indicated no leaks. I was hoping there was some way to test for a leak from inside but I could not imagine one. It's time to stop wishing and get to work pulling it apart.
Does anyone know the angles & dimensions of the intake so I could check to see if it's been cut? The car is a race car with a low deck 400 block. The intake is an old Weiand TM6. Visually it looks Ok, but.....

 

[Ski 61701]

Have you tried the propane torch? If you have a leak the gas is going to find it and the engine will smooth out. Going to save you a bunch of time and $$$. Is your carb working properly?

 

[oldbee]

I don't think you need a propane torch, if it won't run below 2000 you almost definitely have a vacuum leak; unless you have a really screwed up carb. Power brakes? Maybe it's leaking?

 

[pwDave]

Thanks for the responses guys. Yes it's very frustrating. The engine will run above two grand but any less and the vacuum gauge falls rapidly to near 0 and it dies. As I said, every test on the outside indicated no leaks. I was hoping there was some way to test for a leak from inside but I could not imagine one. It's time to stop wishing and get to work pulling it apart.
Does anyone know the angles & dimensions of the intake so I could check to see if it's been cut? The car is a race car with a low deck 400 block. The intake is an old Weiand TM6. Visually it looks Ok, but.....

Shorthorse, been there and done that, plug all the holes in the valve covers, pcv, oil fill breathers, do not allow the crankcase to get air any where, then start the car, if it runs better, you have found the leak, if it doesn't, keep looking. Dave.

 

[pwDave]

Shorthorse, take the intake and gasket off and use some of that stuff that is used to check rod and main clearance. sit the manifold on without the gasket, mine was .000 at the top of the port and .006 at the bottom of the port, was never going to seal in any shape or form, my heads had been angle milled and ruined. Dave

 

[69Bee]

Shorthorse, take the intake and gasket off and use some of that stuff that is used to check rod and main clearance. sit the manifold on without the gasket, mine was .000 at the top of the port and .006 at the bottom of the port, was never going to seal in any shape or form, my heads had been angle milled and ruined. Dave

Dave, the heads weren't angle milled, and aren't ruined. All that happened is that some shop did not level them properly before milling them. Any shop with the correct Mopar intake alignment fixture can cut the intake side of the head at the correct angle, and the intake will seal properly.

 

[miller]

It's time to stop wishing and get to work pulling it apart.

If your going that far, how about setting the bare intake onto the head surfaces, without the gasket. If there is an angle difference, it will sure show then. Also will show any interference in the fit.
Same thing pwDave says, but just using a feeler gauge.

 

[Shorthorse]

The intake appeared to fit perfectly without gaskets but once the valley pan & gaskets were in place, the bolt hole alignment on the intake was slightly off. Enough to have to take a file to some holes. It was slight but the fact that it didn't line up after adding the thickness of the gaskets bothers me.

 

[69Bee]

The intake appeared to fit perfectly without gaskets but once the valley pan & gaskets were in place, the bolt hole alignment on the intake was slightly off. Enough to have to take a file to some holes. It was slight but the fact that it didn't line up after adding the thickness of the gaskets bothers me.

This is what happens when the block and/or heads are milled. The heads move closer to the engine centerline, and forces the intake to sit higher. When the intake side of the heads are milled, the manifold will sit lower as it should. Ports and bolt holes will match up. If you got it to fit with filing the bolt holes, you should be okay, but if it is off too much, a vacuum leak could be created at the bottom of the ports. If you don't want to remove the heads, you could have the intake milled (maybe 0.010" should work). The main reason I like to mill the heads instead of the intake is that once done, it is married to that motor.

You also mention gaskets and valley pan. The gaskets are not usually needed, just the valley pan. This could also be causing your mismatch problem. I only use the valley pan with RTV on the end rails and corners of the pan. I use spray copper coat on the ports.