3FE - why does oil fill cap affect idle? (1 Viewer)

[PabloCruise]

I read in a few threads that removing the oil fill cap will make the 3FE idle very rough.

https://forum.ih8mud.com/60-series-wagons/140326-shes-got-little-skip-after-warms-up.html
https://forum.ih8mud.com/60-series-wagons/215727-my-first-62-all-well-i-think.html

I am not sure why/how this could be?

I thought just about every engine known to man vents valve covers to atmosphere or ties them into the intake...

The reason I am making my own post on this is I have a 3FE mounted in a 55. One of the vents on the valve cover could not be accomodated with the factory hoses, so I slapped a breather filter on there. (see pic)

The engine has issues, but it does idle. It idles well until it gets warm/heat-soaked. Then the idle disintegrates.

Can anyone educate me? Should I not be using the filter on the valve cover?

 

[inkpot]

Howdy! By venting to atmosphere, you are defeating the function of the PCV system. It uses manifold vacuum to draw blow-by/boil-off in the crankcase. Normally, it would have a filtered air supply hooked up at the valve cover. That might be a bit more restricted than an open filter like you now have. What happens if you just remove that little filter and plug up the hole? John

 

[PabloCruise]

Howdy! By venting to atmosphere, you are defeating the function of the PCV system. It uses manifold vacuum to draw blow-by/boil-off in the crankcase. Normally, it would have a filtered air supply hooked up at the valve cover. That might be a bit more restricted than an open filter like you now have. What happens if you just remove that little filter and plug up the hole? John

Good Question! I do not know, but I can find out...

 

[RockDoc]

You are introducing a vacuum leak via the PCV system by opening the valve cover to atmospheric pressure.

 

[PabloCruise]

You are introducing a vacuum leak via the PCV system by opening the valve cover to atmospheric pressure.

I can accept that, but why will my 3F idle with that breather if removing oil filler cap allegedly destablizes idle?

 

[RockDoc]

I can accept that, but why will my 3F idle with that breather if removing oil filler cap allegedly destablizes idle?

Well, the flow of air through a breather is going to be less than through a 2" hole (oil cap removed). While the issue may not be loss of vacuum per se, you are introducing unmetered air into the intake (bypassing the AFM), this may be responsible for issues once warm (I think the ECM runs the engine a touch rich till warmed up, but I'm not positive).

 

[PabloCruise]

O-k. I am going to see what happens when I cover the hole...

 

[agent orange]

The vent that he has the filter on is not the pcv, and since the stock setup has that vent routed to the intake piping the only difference between the two setup is that the stock setup has some unmetered air being introduced into the intake tract. I dont think that the removal of that unmetered air would be outside the ability of the computer to correct for, seeing as my truck runs better with no air injection or egr than it has for years (i just finished it up today, its been in progress for a while) and that is very different from the stock setup.

 

[RockDoc]

The vent that he has the filter on is not the pcv, and since the stock setup has that vent routed to the intake piping the only difference between the two setup is that the stock setup has some unmetered air being introduced into the intake tract. I dont think that the removal of that unmetered air would be outside the ability of the computer to correct for, seeing as my truck runs better with no air injection or egr than it has for years (i just finished it up today, its been in progress for a while) and that is very different from the stock setup.

The stock setup has that "vent" fed with metered air, taken after the AFM. This metered air is fed back into the intake through the PCV (as is the unmetered air in this case)

The air injection system does not draw metered air (taken before AFM, not after), nor does it have an effect on the mixture in the cylinders, as it feeds into the exhaust.

The EGR should have little effect either, as it introduces exhaust (spent intake air and fuel), which is drawn from the #1 manifold runner (air injection goes into 2, 3, 4 and 5), and won't significantly affect the mixture of consumable air and unburnt fuel, just dilute it.

 

[Zack1978]

I had an 1986 Nissan 200SX that would not even idle with the oil cap off. One day when I was much younger I had changed the oil, and when I was done I started it up to check for leaks. I had left the oil cap off when I started, I figured I damaged something when I changed the oil because the car would not idle. It took me a while to figure that out!


Zack

 

[raydouble]

If you take the oil filler cap off a SBC engine that is 96 or newer and go drive around your check engine light will come on.
Just saying....

 

[agent orange]

The stock setup has that "vent" fed with metered air, taken after the AFM. This metered air is fed back into the intake through the PCV (as is the unmetered air in this case)

The air injection system does not draw metered air (taken before AFM, not after), nor does it have an effect on the mixture in the cylinders, as it feeds into the exhaust.

The EGR should have little effect either, as it introduces exhaust (spent intake air and fuel), which is drawn from the #1 manifold runner (air injection goes into 2, 3, 4 and 5), and won't significantly affect the mixture of consumable air and unburnt fuel, just dilute it.

Are you saying that the pcv at the rear of the valve cover and the vent at the front of the cover are connected somehow, because i never realized they were the same system although it would make sense. If the air being drawn into what i will continue to call the vent from the intake tract is returned via the pcv i could see how that would affect how the engine runs, but I still am not sure that would cause that much of a problem though.

And as i see it, the air injection does affect the mixture because the oxygen sensor sees a higher amount of oxygen with the introduction of fresh air from the air injection system so if you remove that air you affect the a/f ratio that the oxygen sensor sees for a given amount of fuel.

-Matt

 

[inkpot]

I had an 1986 Nissan 200SX that would not even idle with the oil cap off. One day when I was much younger I had changed the oil, and when I was done I started it up to check for leaks. I had left the oil cap off when I started, I figured I damaged something when I changed the oil because the car would not idle. It took me a while to figure that out!


Zack

Howdy! I have a 22re that is the same way. It is old and tired with over 200,000 miles on it, but it runs really good. If I pull the oil fill cap when it is running, the idle drops and it runs really crappy. It has lots of air blowing out with the cap off. Besides the PCV line/valve going from the valve cover to the air intake box, it also has an fresh air supply line, that also goes from the air box to the valve cover. I THINK this one has a one way check valve inside the valve cover, otherwise, the pressure would blow back into the box thru both hoses. Maybe the fuel management system needs/wants that pressure to function properly. John

 

[RockDoc]

Are you saying that the pcv at the rear of the valve cover and the vent at the front of the cover are connected somehow, because i never realized they were the same system although it would make sense. If the air being drawn into what i will continue to call the vent from the intake tract is returned via the pcv i could see how that would affect how the engine runs, but I still am not sure that would cause that much of a problem though.

Yup, it flushes the crankcase with fresh air, and sucks it into the intake, removing blow-by gasses and boil-off to slow the degradation of the oil.

And as i see it, the air injection does affect the mixture because the oxygen sensor sees a higher amount of oxygen with the introduction of fresh air from the air injection system so if you remove that air you affect the a/f ratio that the oxygen sensor sees for a given amount of fuel.

That makes sense, didn't think of the O2 sensors....

 

[pappy]

The vent that he has the filter on is not the pcv, and since the stock setup has that vent routed to the intake piping the only difference between the two setup is that the stock setup has some unmetered air being introduced into the intake tract.

Are you saying that the pcv at the rear of the valve cover and the vent at the front of the cover are connected somehow, because i never realized they were the same system although it would make sense.

Sometimes a pic helps. It's metered.

 

[PabloCruise]

Sometimes a pic helps. It's metered.

Pic is helpful.

I had to eliminate that section of the intake to get everything to fit.

I pulled the oil cap today while idling, no change.

 

[pappy]

Pic is helpful.
I had to eliminate that section of the intake to get everything to fit.

Now understand. I had the same problem. Another pic with how I resolved it. I took the steel fitting from the rubber hose and welded it into the steel piece.

 

[PabloCruise]

Now understand. I had the same problem. Another pic with how I resolved it. I took the steel fitting from the rubber hose and welded it into the steel piece.

Did you make a new steel section in this pic? Not sure what is going on here, but it looks nice

Sometimes a pic helps. It's metered.

Why is that 3FE showing a mech fuel pump?

 

[pappy]

Did you make a new steel section in this pic? Not sure what is going on here, but it looks nice.

Nope, original piece was modified.

Why is that 3FE showing a mech fuel pump?

No mechanical fuel pump. I have an electric mounted on the frame by the tank. That was an old pic taken after I mocked up the 3FE junk on my old 2F. Here is a current pic.

 

[PabloCruise]

Nope, original piece was modified.



No mechanical fuel pump. I have an electric mounted on the frame by the tank. That was an old pic taken after I mocked up the 3FE junk on my old 2F. Here is a current pic.

Hmm, I like it! That would make it easier to check the oil vs the big air filter housing...

Do you have a write-up on the engine pictured? Is it 3FE? 2FE? What vehicle is it in?

If you don't mind, could I see a close up of your vac ports towards the back of the manifold? I am trying to see what ports you have capped off vs my install...