I've been trying to Google this and I've read there shouldn't be any or there should be 1-3" of vacuum.

Reason I'm asking is that my rebuilt '95, 351 is starting to leak at the rear main AND the front cover seal.... Ughhh...I've never had a front seal in the timing cover leak.
Now, I've checked the PCV valve, hose's, etc..
If I pull the fresh air line off the air box and cap it, with my Vacuum gauge in the dipstick tube, it starts climbing to a Max of 11" of vacuum and starts to how. With everything plumbed correctly, there is 0" of vacuum in the crankcase with the fresh air tube connected to the air box.

I read that if it is higher than 3" then I have an intake leak somewhere around the manifold. But, the engine runs great. Doesn't surge or hunt and runs like a top.

Can anyone provide some tech on this? I unfortunately don't have anything newer than '95 or stock that I can just go and check.

Thanks again for any ideas, suggestions, and real info!

 

If I have time at work tonight I'll check mine and post up what it has. But, it will be late because I broke mine.

 

Thanks Mudsport! Definitely interested in what your results are and hopefully wiser members will chime in.

 

With the fresh air hose blocked, it will build quite a bit of vacuum as the pcv valve doesn't ever close completely. If you suspect that you have a manifold leak, block the pcv port and manifold port, then hook your vacuum gauge to the fresh air tube and see if it still builds vacuum. May have to block the dipstick tube too as it may not be an adequate seal to perform the test.

 

Ok just took lunchbreak at work, and this is what I got... so with the fresh air hooked up to the filter box, it sits at 0 on the gauge.
with the fresh air hose plugged

Right around 8" vac.
This is a 302 with 260k on the clock. So a fresher engine will probably pull more since it should have better seal on the rings.

 

I know little-to-zero about engine diagnosis and testing, so I'm not entirely sure what you're asking. But my 1996 Service Manual says that an intake manifold vacuum test should register between 15 and 22 in-Hg. Is that what you're asking? If not, I have attached the engine sections for the 5.0 and 5.8 engine (again, from the '96 service manual). The info above is from page 03-00-11 of the first section.

 

Do the same thing but while reading the gauge, yank the PCV out of the grommet and see what happens to the reading. If it drops to zero you do not have a problem.

 

The pcv hole is too large to allow the building of vacuum, so he'll have to block it off if testing for intake manifold leak is the goal.

 

A pcv system does not build vacuum in the crankcase, it pulls in fresh air on the inlet side ,pulls it thru the crankcase via the pcv valve and into the intake where it is sent to the cylinders to be burned in the cylinder.
It can't ever pull a vacuum unless the fresh air inlet is blocked because the inlet is far larger than the pcv valve.
I would hook your gauge up teed in to the system hooked up as normal and put it where you can see it driving and watch for positive pressure build up in the crankcase while driving. If you see you are building press0ure that suggests ring and piston issues or the pcv system is clogged or restricted.

 

Thanks to everyone that has chimed in. I haven't had time yet to perform the other vacuum tests. Will try to in the next few days.

I haven't driven the truck much in the last month due to the rear FDM failing. I had just gotten to the 1000 mile marker when it started overfilling and pouring gas out the rear filler tube.

The truck is by far the most powerful of my collection. I've driven a 2000 Expedition, 5.4, and a 2004 5.7 Hemi, and I'm pleased with the amount of power this roller 351 has with stock cam and heads. I own 2, 302's, a 300, and this 351.

I may have fubared the RMS during install, so that leak may have been user error. I tried using a sleeve and I think it may have a slight imperfection. I'll pull the engine and check it depending on weather in the next few weeks or the next month.
It's been unseasonably warm here, but the Christmas weekend is supposed to get cold with snow and ice expected.

Just want to say thanks to everyone that has chimed in with their knowledge and experience.

 

Good point - thanks for the clarification.