Today's random question: Engine Vacuum

[RavenTBK]

Yep.. here's todays random question.

 

Is there a such thing as TOO MUCH engine vacuum?

 

With newer vehicles, removing the oil filler cap should cause a stumble, but should continue to run.

 

With the Brat, removing the filler cap is difficult, as theres soo much engine vacuum, its sucked on there real good. Once the cap seal is broken, the engine, stumbles then dies real quick like. Both breather lines from the valvecovers are tee'd into a source of intake vacuum.

 

The engine runs sweetly (well, it did before the current distributor issues). I dont see anything wrong with how much vacuum its producing as theres no leaks and whatnot.. however, I figured I'd post up and ask anyways just to cure my curiousity.

 

So, is there such a thing as too much vacuum?

[WJM]

I noticed this one stock engines...they do that on the older ones. the new ones dont do that. Sign of PCV valve going out maybe? I dont know....it was normal on my GL-10 for a long time.

 

Hoever...normal engine vac should be between 20 and 25 inches of Mercury...in-hg.

[RavenTBK]

Well, the turning event here, was the large backfire when I installed my plug wires incorrectly. Before that, I could remove the oil filler and whatever, and it would continue to run okay, but with an obvious sucking sound from the newly formed vacuum leak. Now, it just straight up dies. *shrug*

 

The Brat lacks a PCV valve. The valvecovers are tied straight into intake vacuum. This is how its always been. Is this incorrect? (I'm gonna break out HTKYSA and read up some more on this) The remaining emissions devices have been removed and/or plugged.

 

I mean.. theres no running issues here.. its just curiousity..

[naru]

The Brat lacks a PCV valve. The valvecovers are tied straight into intake vacuum. This is how its always been. Is this incorrect?

 

Yes,that is incorrect.What year are you talking about?

You shouldn`t have full manifold vacuum in the crankcase.

How`s the oil consumption?

[85Sub4WD]

The Brat lacks a PCV valve. The valvecovers are tied straight into intake vacuum. This is how its always been. Is this incorrect? (I'm gonna break out HTKYSA and read up some more on this) The remaining emissions devices have been removed and/or plugged.

I am about 90% sure that is incorrect - I have never heard of a carbed car not having a PCV valve or a breather of some sort - check over it carefully - the extrermely high vacuum in the crankcase indicates a PVC valve stuck open

[RavenTBK]

Yes,that is incorrect.What year are you talking about?

You shouldn`t have full manifold vacuum in the crankcase.

How`s the oil consumption?

84, EA81, Weberized.

No measurable oil consumption.

[RavenTBK]

Okay.. after carefully going through HTKYSA, the Haynes, and Chiltons manuals for the Brat, I ran across these two diagrams..

 

both 1024x78@142k. ~30seconds @ 56k dialup

http://www.fjeer.us/imagedump063.jpg

http://www.fjeer.us/imagedump064.jpg

 

Image 063 depicts how the Brat is currently setup, however instead of going to the aircleaner, the lines go below the carb into the intake...which is basically the same thing as being plugged into the aircleaner. Supposedly, according to Chiltons (where the diagram is sourced), this way of crankcase ventilation was how it was done with the EA61-EA63 series engines.

 

Image 064 depicts how the Brat is supposed to be.. according to Chiltons... with a PCV and whatnot. The accompanying text described the PCV was first brought about with the EA71 and later models.

 

Well, now I know what path things are *supposed* to take.

 

There are no drivability issues. No mileage issues. It still runs fine.

 

Is there any glaring reason why I should re-route the hoses again and install a new PCV? Such as.. without "positive" ventilation, the crankcase will fill with nasty flammable gases and explode? Will I die?

 

I figure, since thats how it was with the older engines, and they survived, I dont see any reason to change things. Am I wrong?

[Ross]

There will be more vacuum in the intake manifold than at the air cleaner (velocity effect).

[naru]

the lines go below the carb into the intake...which is basically the same thing as being plugged into the aircleaner.

 

No,that is not the same thing at all.Aircleaner hoses would provide only a minimal vacuum,you likely have 20 times as much from the intake manifold.The various crankcase seals and gaskets aren`t designed to withstand that level of vacuum.I suspect you are sucking dirt into the crankcase by the crankshaft lip seals and who knows where else.(distributor?)

 

A stock carb would have been calibrated for a certain PCV flow but you may be OK on that score.

 

Additionally,as you point out,the crankcase fumes won`t be protected from an intake backfire w/o a PCV valve or flame arrestor.

 

Lack of positive ventilation can`t be a good thing either.

[KStretch55]

There is most definitely a difference between air cleaner vacuum and carbeuretor vacuum! It's surprising that you haven't had seal problems or oil consumption problems. Have you noticed any rust on your dipstick? If you live in a high humidity area you could. That much vacuum is surely sucking air into your crankcase and with it will be moisture.

[archemitis]

Both breather lines from the valvecovers are tee'd into a source of intake vacuum.

 

you guys didnt read that too thoroughly...

 

if you put manifold vac to your breather hoses... you will be sucking air from your lower end to feed your engine.

put that hose thats Ted to a breather filter and vent it to atmosphere. plug whatever hose you used to give it vaccume on the manifold.

that explains why it dies when you take the oil cap off. you make a huge vac leak!

 

on old engines. if your stuff is plumbed correclty, you shoudl get a little pressure from the oil filler hole, never vaccume.

[archemitis]

but... 30 is space, so no car is supposed to suck 30, or over.