Air-Fuel Mixture - plug up Oil Filler hose connection or not?

[DMC1890]

I'm sorry to ask about an issue that seems to have been discussed plenty already. There is a good How-To on 'Set CO Dwell', but after repeated readings, I'm still not clear if the air hose connection to the oil filler (from the air cleaner) should be plugged or not. In some places, it says 'plug'. In others, it says 'no'. The shop manual says to do it with the air cleaner installed and intact. Since the piping for that, which is internal to the air cleaner, is essentially open to the atmosphere, that would tell me if you do not have the air cleaner installed, that you should do the adjustment with the connection unplugged.

I can adjust either way, but then the engine nearly dies if I unplug (or plug) the connection.

Further, with air cleaner and all plumbing installed, doesn't this represent a significant source of unmetered air intake to the system?

Thanks

[mark w]

i dont plug it. the Crankcase needs to be able to vent.

[FABombjoy]

Further, with air cleaner and all plumbing installed, doesn't this represent a significant source of unmetered air intake to the system?

Leave it connected as stock.

If you block the big tube and the oil filler O ring is good, you'll pull a slight vacuum on the crankcase and the flow through the metered portion (connected to the cold start tube) will not be representative of operating conditions.

[Jonathan]

Leave it connected as stock.

If you block the big tube and the oil filler O ring is good, you'll pull a slight vacuum on the crankcase and the flow through the metered portion (connected to the cold start tube) will not be representative of operating conditions.

I agree with this. Keeping the system as close to actual operating conditions while doing testing or adjustments is the best idea. You can reach and adjust the CO mixture screw with everything else on the engine in place. Once you make a TINY adjustment, plug the screw adjustment hole back up and then wait to see how the engine idles after stabilizing to the new setting. Make small adjustments and give it time to take affect before making another.

[FABombjoy]

Oops, I should have said "leave it open to air". You could also set the filter box aside and leave it hooked to that.

And because I *heart* data, here is a log sampled at the dipstick tube.
The large tube to the air filter is blocked, and the vacuum line on the cold start tube remains attached.

First sample: I capped the large line at the oil fill and you can see it drop over the course of 10 seconds (.963 bar).
Second sample: Cap removed, MAP increases as CC pressure equalizes with atmospheric pressure (1.020 bar)

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[opethmike]

Oops, I should have said "leave it open to air". You could also set the filter box aside and leave it hooked to that.

And because I *heart* data, here is a log sampled at the dipstick tube.
The large tube to the air filter is blocked, and the vacuum line on the cold start tube remains attached.

First sample: I capped the large line at the oil fill and you can see it drop over the course of 10 seconds (.963 bar).
Second sample: Cap removed, MAP increases as CC pressure equalizes with atmospheric pressure (1.020 bar)

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[DMC1890]

What is the role of the cold-start valve in the vacuum circuit? I cannot find it discussed in the FSM. Whatever it is doing, it seems relevant to this discussion. I would guess that it is supposed to connect, or disconnect, the crankcase breather to the carbon canister, but only during the cold-start condition.

[DMC1890]

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I think it's cool.

Is the AFR measurement coming from a separate wide-band sensor? Are we seeing some 'adjustment' between 5:54 and 5:58 due to the increased air? When I uncapped mine, my dwell meter reading shot way up and I quickly recapped it to keep the engine running, which makes me think I've got some additional problem.

[FABombjoy]

What is the role of the cold-start valve in the vacuum circuit?
The cold start tube is tied to all 6 cylinders and provides vacuum for both the vapor recovery and the PCV systems. In the DMCH store photo you can see the "bump" where the cold start tube holding bolt attaches, along with the channels that run to all 6 ports:
http://store.delorean.com/popup.aspx?src=/images/Product/large/102260.jpg

In a rare feat of PRV genius design, the spray from the cold-start valve will keep that tube reasonably flushed from PCV oily buildup.


Is the AFR measurement coming from a separate wide-band sensor? Are we seeing some 'adjustment' between 5:54 and 5:58 due to the increased air? When I uncapped mine, my dwell meter reading shot way up and I quickly recapped it to keep the engine running, which makes me think I've got some additional problem.
Before Megasquirt I had an Innovate LC-1 and LMA-3 for data monitoring. They were an invaluable tool for keeping K-jet where it need to be.

Missing is the trace that shows duty cycle on the frequency valve. Attached is a version with it added. The AFR and DC is up and down as I was horsing around with capping and uncapping the large hose port on the fill cap.
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Dwell (duty cycle) rising indicates O2 sensor reading lean. That could happen if you were tuned with reduced PCV flow. Lots of variables at play though, so that might not have been the only thing!


PS: Mike - I had shields up so it bounced off me and you actually called yourself a nerd!

[FABombjoy]

To add - the vacuum side should be able to breathe freely through the oil fill cap from both the side and bottom ports.

If the air/oil separator matrix were clogged that could would reduce flow to the vacuum side and cause a mixture shift. That part is sometimes called a "flame arrestor" but it spends most of it's life trying to condense oil vapors so they aren't pulled in to the intake manifold. You should able to quickly bench test the oil fill cap and verify if it can breathe or not.

[DMC1890]

OK, I think I'm understanding. In this case I had tuned DC to 50% with the breather plugged, and when I unplugged it I estimate DC went up past 80%. So, in your chart, yours is more properly tuned to start with, but when you plug the breather your DC drops quite low.

Back to an earlier question - with air cleaner and everything connected properly, isn't that still a significant source of unmetered air that you're having to tune for?

[FABombjoy]

Back to an earlier question - with air cleaner and everything connected properly, isn't that still a significant source of unmetered air that you're having to tune for?
There is about a 1mm (IIRC) restriction orifice at the connection to the cold start tube. Pop the vacuum line off and you can see it's pulling through a little tiny port. It's a decent amount of unmetered air but it's predictable. The flow will decrease as throttle increases. Crankcase pressure then pushes out of the larger tube and is metered.

The vapor line has one as well. It behaves somewhat like PCV except it only flows once vacuum is present at the throttle edge tap.

I've not tried but I suspect an actual PCV valve would be much harder to tune for, which is why they chose a fixed-orifice style PCV. The motor is already prone to oscillation and adding yet another source of air connected to a valve on a spring would probably make it quite entertaining at times.

Megasquirt + speed density tune means I now run a genuine PCV valve with lower flow at idle and higher flow under load. The effects of the variable flow rates are accommodated for in software.

[DMC1890]

OK, I think I'm really understanding it now. Both vacuum ports on the CSV have the restriction, and the ports are interchangeable.

As I said earlier, it would be good if the How To: Set CO Dwell would make it clear that the fitting should NOT be plugged (as several responders note).

By the way, I had to start from even further back than that (I had really screwed up my AF setting due to massive vacuum leaks). I found this (http://www.cisflowtech.com/frequently-asked-questions/testing-k-jet/k-jet-mixture-setting/), without which I could not get the engine to run at all beyond the initial starting shot.

[FABombjoy]

One way to find vacuum leaks is $20 and a spray bottle of soapy water:

http://dmctalk.org/showthread.php?14435-How-to-Build-an-intake-manifold-leak-finder-for-about-20

Looks like DMCMW Dave chimed in on the CO howto at the end about not plugging hoses. There's no reference in the service manual about plugging it, and the data I've posted shows what can happen if you do!

[DMC1890]

Actually, the FSM does say in the second paragraph of D:04:01 "The engine must be run in with air filter in position. The ignition system must be serviceable and correctly set. No air leaks must be present (Brake Vacuum Hose, Ignition Advance Capsule Pipe, Fuel Lines, Oil Vapour Rebreathing Pipes etc)." But that's just one of those warnings that we tend to ignore, and perhaps in this case makes you think that you need to plug up what appears to be an air leak, when it is not.

[Ron]

Sorry for the confusion Robert...
In the How To, I included removing the breather because a lot of people have complained about reaching the adjusting screw with standard tools, some had non-stock breathers, etc... Removing the breather leaves the hose from the filler cap open, which leaves the closed system open (the hose is normally under vacuum too) -- not what is expected when setting the CO dwell. Blocking it off should make it closer to (but not exactly the same as) a stock set up.... Leaving double checking it with everything hooked up as it said covering all cases.
But it turned out to be confusing. So I edited it to make it clearer. (As said above, connected as it will run is the best.)
Any input/suggestions/etc as to the way it is now (from anyone) would be appreciated!

[FABombjoy]

Well... sorta :D The big hose won't be under any meaningful vacuum unless the air/oil sep matrix is plugged or the engine is under heavy load. Inside the air filter housing, the breather tube gets its own dedicated section of the air filter element which is why one corner of the air filter is always oily. Here's a pic of that tube removed from the air filter housing.
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If the breather connection were simply a fitting on the side of the airbox and didn't have the little tube, it would definitely be under partial vacuum at all times. If the crankcase was always under vacuum these motors wouldn't have as many oil leaks :D Without significant improvement to the air/oil separator system you'll also have a very blue exhaust.

Under heavy load, airflow to the engine increases and engine vacuum goes away (increased MAP), crankcase pressure increases and will flow from the breather tube into the intake. The low pressure airflow adjacent to the intake breather tube will encourage blowby gasses into the engine which is the only time the big hose might see lower than atmospheric pressure, assuming your rings/valve seals/etc are in good shape. If you're running a breather filter on the oil cap instead of the stock config this can exacerbate oil leaks as all blowby gasses must be pushed out into atmospheric pressure instead of the lower-than-atmospheric intake air stream. Sorry, a fluid dynamics person could probably make that sound less awkward.

Thesis: Adjust CO with intake configuration as close to stock as possible. Do not block any vacuum/breather hoses. Blocking the big hose will slowly reduce flow through PCV vacuum hose (as demonstrated in previous data) and CO will shift lean once the intake is returned to stock form.

[Ron]

...
If the crankcase was always under vacuum these motors wouldn't have as many oil leaks :D Without significant improvement to the air/oil separator system you'll also have a very blue exhaust.
...
Under heavy load, airflow to the engine increases and engine vacuum goes away (increased MAP)...
...

Thesis: Adjust CO with intake configuration as close to stock as possible. Do not block any vacuum/breather hoses. Blocking the big hose will slowly reduce flow through PCV vacuum hose (as demonstrated in previous data) and CO will shift lean once the intake is returned to stock form.
hehe..this has been hashed over before.. In short;
The manual states, "The closed crankcase system does not have an air inlet for air circulation within the crankcase.This system applies manifold vacuum to the crankcase which is completely sealed to outside air.
...
An air leak in the engine crankcase will cause a lean running condition." D:06:02, 03
(Remove the filler cap and place you palm on the bottom of it and you will believe ;-)
Load or not there is some vacuum, but what matters here, when setting the CO, is at idle anyway...

No argument on the "big hose..."

(fwiw, I agree with the thesis - I've always said to set CO with everything else correct first, and, that it is best to do with everything connected, or at least double check it that way afterwards ;-)

=======

I can see saying there is an unmeasured "leak", as previously mentioned. But it is metered (ie known and allowed for...)

[FABombjoy]

I think that section of the manual is based on theory and not practice :D Seems you'd have to have 6 helluva perfectly sealed combustion chambers for a CC air leak to cause significant shift in mixture. The vacuum felt on the bottom of the oil fill is quickly offset by blowby gasses. Put your hand over the oil fill opening and it doesn't take long to build up! Without any notable pressure differential (as measured in real life!) between CC and atmo, air has no reason to enter the CC.

After all these blowby gas discussions, if we meet at a DCS I'm buying you Taco Bell.

[Ron]

I think that section of the manual is based on theory and not practice :D Seems you'd have to have 6 helluva perfectly sealed combustion chambers for a CC air leak to cause significant shift in mixture. The vacuum felt on the bottom of the oil fill is quickly offset by blowby gasses. Put your hand over the oil fill opening and it doesn't take long to build up! Without any notable pressure differential (as measured in real life!) between CC and atmo, air has no reason to enter the CC.

After all these blowby gas discussions, if we meet at a DCS I'm buying you Taco Bell.Agreed, it is just there to handle what does blow by... If you haven't, actually try putting your palm on the bottom of the filler...it is very strong. If the blowby was anything close to keeping up with it, it'd be rebuild time...

I guess it would have been clearer (in the how to) if I said something like, "there are 3 ports to consider when removing the breather, the main one for incoming air, one for it to enter the engine after filtering, and the small one going to the filler, which should be blocked off to avoid a lean condition warned about in the manual if the crankcase is not sealed." How much it matters I don't know...maybe about as much as not having to draw air through the filter and plumbing?? ...I wouldn't think it's anything like a performance engine, but when setting anything up, I like to stick to the specs.

Tocco Bell sounds good! It'd be more likely to meet up at a Woodward Dream Cruise...that was my old stomping grounds years ago. (There or Gratiot Ave was the place to make a little change with muscle cars, until the other guys/or cops got to know your car ;-) Planning on the next one. In any case, TB makes me thirsty, 1st one is on me.

[Jonathan]

What reason would you have for doing a CO adjustment with the air filter housing off the engine anyway? I mean, other than not having the right tools to reach down into the mixture adjustment hole that is. Because not having the right tools isn't a good reason in and of itself. Nice discussion on what impact any open to air spots might have on the engine air fuel mixture, but kind of pointless, no?

[DMC1890]

I think it's part of the overall approach. It's a bit like putting the air filter on, installing the hatch covers, etc, etc, before you even start the engine. It's just not going to start if you do that. You have to sneak up on it.

[Jonathan]

Sounds like a work around approach to me. It don't take a DeLorean Guru to give the advice that you need to sort out everything else on the engine and get things working BEFORE adjusting the air fuel mixture. But we aren't exactly in a patient era, so doing things by the book kinda goes out the window. Little bits at a time though thankfully as those toll booth windows won't let you put much through all at once ;)

[Ron]

I think it's part of the overall approach. It's a bit like putting the air filter on, installing the hatch covers, etc, etc, before you even start the engine. It's just not going to start if you do that. You have to sneak up on it.I have to ask...
Why was the air filter off in the first place?? :deviltail:

[FABombjoy]

Nice discussion on what impact any open to air spots might have on the engine air fuel mixture, but kind of pointless, no?
Pointing out service manual inaccuracies is never pointless! The forum has already figured out everywhere you can stick LEDs so we might as well talk about mechanics now and then ;)


Agreed, it is just there to handle what does blow by... If you haven't, actually try putting your palm on the bottom of the filler...it is very strong. If the blowby was anything close to keeping up with it, it'd be rebuild time...
I'll have to test that on another car. Currently have a massively overbuilt PCV system, including ventilation ports on both valve covers and one on the lower crankcase. 4x air/oil separators connected to the intake and an adjustable PCV system. I take crankcase evac seriously :D

[DMC1890]

I have to ask...
Why was the air filter off in the first place?? :deviltail:

Lots of other steps up to this point - unstuck fuel distributor piston, cleaned fuel injectors, etc. I proceeded to set the air-fuel mixture (with the air filter still removed), and observed that it made some difference if the big pipe on the oil filler was plugged or not.

[DMCVegas]

To add - the vacuum side should be able to breathe freely through the oil fill cap from both the side and bottom ports.

If the air/oil separator matrix were clogged that could would reduce flow to the vacuum side and cause a mixture shift. That part is sometimes called a "flame arrestor" but it spends most of it's life trying to condense oil vapors so they aren't pulled in to the intake manifold. You should able to quickly bench test the oil fill cap and verify if it can breathe or not.

To add to this, if that oil cap gets blocked (ie excessive sludge from a blown head gasket) it will causes a situation where there is too much positive pressure in the crankcase. Which results in the PRV literally spraying engine oil out of either it's front, or rear main seals on the crank shaft. Whichever is weaker. If it's the front, the crank pulley ends up slinging it everywhere to make for a nasty mess to clean up with burning oil all over the muffler and rear bumper. If it's the rear, it'll have the bonus of prematurely ruining your clutch.