Can someone tell me if you have a low vacuum reading, needle steady 10-12, and you have checked everything thoroughly and suspect an internal vacuum leak around the manifold gasket, if it is an internal vacuum leak around the manifold gasket, would you get accurate readings with a static compression test and a running compression test to help you make sure before you pull the manifold? Or is there another way to confirm an internal vacuum leak besides checking for external vacuum leaks very thoroughly?

Thanks
mjac

 

Take out the PCV valve and remove the breather cap. Then with the engine running block both of those openings by hand so they are air tight. Allow the engine to run with these blocked for a little while and then remove a hand from one or the other. Is there pressure or vacuum inside of the engine?

 

I want to make sure of what you are saying. You cover both openings with your hands air tight, let the engine run awhile, remove your hand from one opening and then see if there is pressure or vacuum on the hand still on the other opening? Do you repeat this procedure in the opposite direction?

I did block off the breather hole with tape and took a vacuum reading at the PVC grommet using a piece of rubber hose to reduce the grommet opening to fit the vacuum gauge hose and it was still a low, steady vacuum reading of 10-12.

Thanks
mjac

 

With both of the holes blocked by hand it should be easy to tell if the engine is pulling the case into a vacuum(not normal) or building a little bit of pressure from the cylinder leakage(normal). Your idea of using the vacuum gauge should work too. If it is pulling it down to 10 or 12 inches in the crankcase you must have a pretty big leak because normally the seals won't allow it to go much lower.

 

I understand with both hands making an air tight seal over the PVC and Breather openings you should easily be able to tell if there is slight pressure or vacuum, but you said to take one hand off, I thought that had something to do with telling

Okay, there should be no vacuum at the PVC grommet with the vacuum gauge rigged up there and the breather blocked off. If I had a cracked ring, or a valve sticking or not seating, or a rounded cam lobe or lobes causing a valve or valves not to seat properly, from what I understand the reading on the vacuum gauge be wildly bouncing or swinging. If I have vacuum at the PVC and/or Breather and taking a vacuum reading at the manifold port on the carburetor shows a steady (no bouncing or swinging) low vacuum of 10-12 inches, everything points to a bad manifold seal and an internal vacuum leak?

If it is an internal vacuum leak from a bad manifold seal, taking a static and a running compression test would be useless?

Thanks
mjac

 

Where ya at?

Thanks
mjac

 

What I meant was block both holes air tight using both hands with the engine running. After it has run for a bit just take one had off to see if the case has pressure or vacuum. If it is bad enough you might be able to just feel it without removing one hand. Dodge pickups with the Magnum 360 used to be bad about sucking in the gasket on the bottom of the intake manifold. Blocking both holes was a good test. Usually they would really start using oil all of a sudden when that would happen. Does your engine use a lot of oil?

A year or two ago my 460 did this exact thing on a trip out to Kansas. It started using a quart of oil about every 60 miles running on the interstate. I never saw it smoke either. But the blocking both holes test worked.

 

If I am understanding you right you pull one hand off and then feel with the other hand if there is vacuum or slight pressure. Okay, you say there should be no vacuum at the PVC or Breather. I taped over the breather hole sealing it, rigged up the PVC grommet to receive the vacuum gauge hose and got a reading of 10-12 inches which I suppose is doing the same thing as the hand test. Does that tell you that you have an internal vacuum leak from a bad manifold seal?

The truck burns no oil.

Can the rockers be so out of torque they can cause problems. They have not been torqued down for 60-70 thousand miles.

Thanks
mjac

 

I think that your test is telling you the same thing as the "hand" test would but you're a bit more scientific. It should never pull the crankcase into a vacuum with the engine running. It should always generate a little bit of pressure from the leakage past the rings. It sure sounds like a leak from the manifold gasket. It is weird that it isn't burning oil but anything is possible. Rocker arms won't cause this condition.

 

I wasn’t more scientific than your hand test, that is just the one I already did and was wondering if it was the same thing.

My two main questions are these. If I had a broken ring or a valve stuck or not seating or a rounded cam lobe, the vacuum gauge would be wildly bouncing or swinging. So everything is pointing to a bad manifold seal? If it is a bad manifold seal a static and running compression test would serve no purpose?

If I have to change the manifold gasket, I have an Edelbrock gasket set this time, the last one was a Mr. Gasket I think, this will be the third time I have had to change the manifold gasket, I want to get it right. The first time was when I changed the camshaft because of rounded lobes. I was hard headed and used the end gaskets despite everyone saying not to us them. The rear right one blew blowing water out of the manifold and I had to change the manifold gasket again. That time I used ACDelco 10-2006 RTV Engine Sealant to seal the manifold ends but I think I used a Mr. Gasket gasket. When you do your manifold gaskets, do you put sealant on the gaskets, one or both sides? It seems I had to put a little on the head side to keep the thing in place when I dropped the manifold on it.

Thanks
mjac

 

Most of the time I like to glue the gasket to the head using a glue like weatherstrip adhesive. On the Ford Windsor style engines the stock style intake gaskets are made so that they can hook onto the ends of the head gaskets to help keep them in place. This can be helpful but I have seen one where for some reason that little tab on the end of the head gasket got bent upward and was caught between the manifold and the gasket. This caused a leak..

I usually put a smear of RTV around each of the water ports and the exhaust crossover but I don't put anything around the intake ports. Sometimes RTV on both sides of a gasket can be bad and act sort of like grease which can make the gasket squish out.

On any engine that has water going through the intake I try to always use a gasket that's metal reinforced. Hot rod gaskets that are soft and don't have that tend to fall apart after a couple of years if they are run with coolant.

When you have the intake off it wouldn't be much more work to remove the valvetrain and then inspect each lifter and lobe one at a time just to make sure that your cam is ok.

 

Sometimes I do not realize what you are trying to say until later after reading it.

when you say glue the gasket to the head with weatherstrip, you mean the hanging tabs don’t you, you don’t put weatherstrip on any mating surfaces do you?

I suspected too much RTV was not a good thing, you just smear some around the water ports, what are you calling the exhaust crossover, I thought that was a tube on the back of the heads on later model 302s for emissions.

Then you said Hot Rod Gaskets, I didn’t realize you were calling the Mr. Gasket and Edelbrock Manifold Gaskets Hot Rod Gaskets. That might be just what happened, the Mr. Gasket manifold gasket may have deteriorated. Then after all of this I did not ask you the most important question. What is the BEST manifold gasket I can use, this is my third go round.

When I have the manifold off, I will take a look at what you are talking about inspecting the valve train, might give me piece of mind and tell me know how good the oil is working. But right now we are hoping manifold seal and that is it. If you were doing this would you pull the valve covers and torque the rockers?

Thanks
mjac

 

I called up Cometic and the guy is going to make me a High Temperature Steel Intake Gasket, IR-149060-HTS, for $26.48 plus $8 shipping.
Supposed to be built for high compression, blown applications, with alcohol and nitrous in competition.

Thanks
mjac