351W with high vacuum signal at valve cover opening.

[cleandan]

Something I have yet to experience...a strong vacuum signal at the valve cover breather.
This 1986 351W is an unknown entity in a truck that was pulled from long term storage (sitting since at least 2010...maybe longer)
I don't know what build this engine has but it is not stock based on the Edelbrock heads, Edelbrock intake, headers, and dress up items...but it is not radical either.

Some general things were checked and done before starting the engine to do my best not to harm anything after the long slumber.

Now that it is running (but running poorly) I am working on taking care of issues as they present themself.
The main issue right now is a very strong vacuum signal in the valve covers.

Passenger side valve cover has a PCV valve routed to the 3/8" line at the back of the carb. All that appears normal enough and in working order (I do suspect the wrong PCV valve though)
Drivers side valve cover has a push in breather with a nipple and hose routed to the underside of the air filter housing (aftermarket 14" open element filter style)
The drivers side hose has a strong vacuum signal (suck your finger tight strong) and if the breather is removed it will suck your hand tight to the grommet in the valve cover.

While I have not conducted any further testing, I am thinking the PCV valve might either be the wrong valve, or it has failed in some manner.
It looks perfect, rattles, and does not have any physical damage or built up residue and with the engine running (runs rough and misses) the PCV valve rattles quite a lot.

My next test is to remove all vacuum lines and plug all ports to see how this might affect the engine running condition...and make my next move based on what I find.

My question is, what do you think of this high vacuum signal in the valve cover area?

[1964Fastback]

The strong vacuum in the crankcase is odd but I don't know I'd focus on that first.  If the engine and truck is unknown, I'd start with clean fuel and filters as well as rebuilding/cleaning the carb after sitting for 15 years if that hasn't been done already.  Then pull the plugs to inspect for oil fouling, wet from gas, gap, etc. and while out, do a compression check for a baseline, and hook a vacuum gauge to and see what that tells you.  Sorry if these were the general things you'd already mentioned were done.

You mention the vacuum lines.  Does it still have what I assume is a complicated spaghetti from the stock 1986 setup?  I'd definitely be taking a lot of pictures and notes before touching that.

Pat

[My427stang]

There used to be a TSB on Mopar trucks, disconnect the PCV and plug, and check for vacuum at the valve cover.  If there was vacuum find and fix an internal vacuum leak, usually at the manifold.

If you have vacuum with the PCV disconnected, it can only be because of an intake gasket leak, but never saw one that strong.

If the vacuum goes away when you pull the PCV, and it's not smoking or using oil, ignore it IMO.  If it is smoking or using ouil, or you cannot tune idle properly, chase to see if you have some sort of metering issue with the current PCV, but I have never seen it.

[Rory428]

A PCV valve is essentially a controlled vacuum leak, controlled by the size of the orifice of the valve. If the orifice is larger than the system is designed for, it could cause higher than normal vacuum, which could also effect how the engine idles and runs. Maybe you have a incorrectly sized PCV valve,which could cause both issues.

[cleandan]

The strong vacuum in the crankcase is odd but I don't know I'd focus on that first.  If the engine and truck is unknown, I'd start with clean fuel and filters as well as rebuilding/cleaning the carb after sitting for 15 years if that hasn't been done already.  Then pull the plugs to inspect for oil fouling, wet from gas, gap, etc. and while out, do a compression check for a baseline, and hook a vacuum gauge to and see what that tells you.  Sorry if these were the general things you'd already mentioned were done.

You mention the vacuum lines.  Does it still have what I assume is a complicated spaghetti from the stock 1986 setup?  I'd definitely be taking a lot of pictures and notes before touching that.

Pat

Pat, thanks for the reply.
I wrote this in my original post:
Some general things were checked and done before starting the engine to do my best not to harm anything after the long slumber.
All the things you mentioned were done prior to trying to start the engine for the first time, as well as a few other obsessive things to help ensure a safe and good startup.
The modifications to this engine have removed all vacuum items except for PCV, brake booster, and distributor advance.
The only questionable item left is the PCV valve itself and I suspect a different vacuum valve might take care of this situation.

The other thing I am suspecting are the intake manifold gaskets.
They may have failed internally in the intake valley...and my other tests will help narrow that down.
But it keeps raining here so I am somewhat stuck until that stops because the truck is in the driveway outside and I don't feel like working in the pouring rain.

[cleandan]

There used to be a TSB on Mopar trucks, disconnect the PCV and plug, and check for vacuum at the valve cover.  If there was vacuum find and fix an internal vacuum leak, usually at the manifold.

If you have vacuum with the PCV disconnected, it can only be because of an intake gasket leak, but never saw one that strong.

If the vacuum goes away when you pull the PCV, and it's not smoking or using oil, ignore it IMO.  If it is smoking or using ouil, or you cannot tune idle properly, chase to see if you have some sort of metering issue with the current PCV, but I have never seen it.

Thanks for the reply.
My next step is to disconnect all vacuum ports on the engine and then run the engine to see if the vacuum inside the valve cover remains.
If it is still there it seems plausible that it would be from an intake manifold gasket leak of some type.

[cleandan]

A PCV valve is essentially a controlled vacuum leak, controlled by the size of the orifice of the valve. If the orifice is larger than the system is designed for, it could cause higher than normal vacuum, which could also effect how the engine idles and runs. Maybe you have a incorrectly sized PCV valve,which could cause both issues.

Thanks Rory, this is one of my thoughts too.
But I don't know any PCV values or part numbers that might help me attain a better suited PCV valve for this engine...and trying to have the parts counter clerks at my local auto parts stores is not a viable option any longer...they really don't know much about anything the auto parts store offers for sale, let alone getting in depth about PCV vacuum values.

If there is a chart somewhere, or you have a valve to try, I am open to hear what you suggest.

I do have a few of the old style PCV valves in my collection of stuff.
These are full metal casings with a snap ring to disassemble them to gain access inside.
I bet I could take that apart and modify it to work with some trial and error...but that seems like a long way to go for such a seemingly simple part swap if I can buy one off the shelf that will work.

[My427stang]

There used to be a TSB on Mopar trucks, disconnect the PCV and plug, and check for vacuum at the valve cover.  If there was vacuum find and fix an internal vacuum leak, usually at the manifold.

If you have vacuum with the PCV disconnected, it can only be because of an intake gasket leak, but never saw one that strong.

If the vacuum goes away when you pull the PCV, and it's not smoking or using oil, ignore it IMO.  If it is smoking or using ouil, or you cannot tune idle properly, chase to see if you have some sort of metering issue with the current PCV, but I have never seen it.

Thanks for the reply.
My next step is to disconnect all vacuum ports on the engine and then run the engine to see if the vacuum inside the valve cover remains.
If it is still there it seems plausible that it would be from an intake manifold gasket leak of some type.

I don't see any reason to disconnect anything other than the PCV, nothing else pulls from the crankcase.

[cleandan]

I played around with isolation of all the vacuum sources to be certain there were no erroneous faults, and I found none.
This was both good and bad because I sort of wanted a smoking gun to repair instead of more diagnosis...but it's okay.

I have swapped in a few different PCV valves and they do make a very noticeable difference.

The 351W engine is pulling between 18" and 19" of vacuum at idle so I am surprised the PCV valve I pulled from my big cammed, low idle vacuum, 289 works the best...but that PCV valve eliminated the misfire and sputtering issues in the 351W.

The engine still has some other issues, but progress is being made each time I figure something out. (gotta love figuring out an unknown modified engine that has been sitting for more than a decade)

With the engine now running in a steady state I was able to see there is likely something wrong with the remanufactured Duraspark distributor in use.
The timing stays stationary until about 2600 rpm, then it jumps a big amount of advance, but not fully advanced until well above 4000 rpm...Tearing into that next to see what is happening.

[djburton]

Bringing this back up...Have basically the same situation as cleandan. 390,new build (not mine). Don't know the specs but seems to have a street performance cam,Ed's,Perf RPM. Has about 13" vacuum at idle. Runs and idles great with PCV plugged. The owner supplied new PCV is basically open at idle when connected. Might be defective. How can you tell? There are no spec charts for these things. Does anyone have a part number for something that would work in this situation?  Thanks!!

[djburton]

After doing some research,the general consensus is Fram FV178 pcv for lower vacuum engines. It does work in my situation and enabled carb tuning which wasn't the case with the valve I was using. It does seem that it's not necessarily the vacuum the engine holds at desired idle speed but the volume of air pumped to maintain that vacuum that dictates what valve works.

[galaxiex]

There is always the Dual Flow Adjustable PCV valve from M/E Wagner to really dial it in...

https://mewagner.com/?p=444