M30 engine scavaging - continued

[Jeff N.]

For those of you following along, I've been wondering why my M30 doesn't seem to be bothered (eg: stumbles, etc) when you open the oil filler plug.

Today Martin and I compared engines and reached the following conclusions.

First, his engine, running a MAP sensor and no MAF or AFM (way cool, no?) works just the same as mine. That is, you can open the filler cap and no change in the idle.

By our thinking, this means that the stumble in the M30 when you open the filler cap is related to the AFM flapper door. That door, spring loaded, essentially remains closed when the car is at idle. The air bypass is regulated by a screw in the AFM and this in essence fine tunes the idle mixture.

Soooo...when you open the oil filler cap, you essentially create a huge air passage around the AFM as air rushes into the valve cover, thru the vent tube and past the AFM. This, as Bill pointed out before, leans out the mixture and causes the stumble.

Now, in the setups that Martin and I are running, there is no restriction prior to the vent hose. With no restriction, there is no vacuum in that hose section and therefore when you open the filler, there is no effective mixture change.

This is also why the low speed scavaging on an AFM equiped car is higher than our setup. The vacuum in that hose section due to the flapper door helps generate a slight negative pressure in engine. In a closed system - ie fully sealed oil cap, oil dip stick, etc - this just creates a slight vacuum in the engine with no effect on the idle. With no AFM, our setups don't have that slight vacuum edge for scavaging.

This also seems to support why it's so critical for a smooth idling M30 (AFM equiped) to not have any vacuum leaks in at the filler cap or oil dipstick.

Anyways...this is my theory and I'm sticking with it. Hope I did a good job of summing up what we found.

Cheers!

Jeff

[BigKriss]

so when is the MAP conversion happening for you Jeff?

[Paul in NZ]

I cant figure out tho how air unrelated to intake air can lean out the mixture...how does air from the oil filler cap end up in the intake air ...or does it upset the vacumn which i guess then somehow uncalibrates things.....see i know what i am talkin about

[Bellicose Right Winger]

Air that enters thru the oil filler cap becomes intake air.

The M30 crankcase is connected to the intake bellows between the AFM and the throttle body. This reduces emissions by allowing crankcase fumes to be burned in the engine. It also causes the crankcase to operate under a slight vacuum. When you remove the oil filler cap you allow outside air to sweep through the crankcase, into the intake bellows and into the engine, bypassing the AFM. Since this unmetered air didn't pass thru the AFM, the DME didn't provide any fuel for it, creating a lean mixture.

Paul Shovestul

I cant figure out tho how air unrelated to intake air can lean out the mixture...how does air from the oil filler cap end up in the intake air ...or does it upset the vacumn which i guess then somehow uncalibrates things.....see i know what i am talkin about

[Jeff N.]

..not planning on doing the MAP conversion anytime in the near future. My MAF setup works just fine.

Air that enters thru the oil filler cap becomes intake air.

The M30 crankcase is connected to the intake bellows between the AFM and the throttle body. This reduces emissions by allowing crankcase fumes to be burned in the engine. It also causes the crankcase to operate under a slight vacuum. When you remove the oil filler cap you allow outside air to sweep through the crankcase, into the intake bellows and into the engine, bypassing the AFM. Since this unmetered air didn't pass thru the AFM, the DME didn't provide any fuel for it, creating a lean mixture.

Paul Shovestul

[genphreak]

For those of you following along, I've been wondering why my M30 doesn't seem to be bothered (eg: stumbles, etc) when you open the oil filler plug.

Today Martin and I compared engines and reached the following conclusions.

First, his engine, running a MAP sensor and no MAF or AFM (way cool, no?) works just the same as mine. That is, you can open the filler cap and no change in the idle.

By our thinking, this means that the stumble in the M30 when you open the filler cap is related to the AFM flapper door. That door, spring loaded, essentially remains closed when the car is at idle. The air bypass is regulated by a screw in the AFM and this in essence fine tunes the idle mixture.

Soooo...when you open the oil filler cap, you essentially create a huge air passage around the AFM as air rushes into the valve cover, thru the vent tube and past the AFM. This, as Bill pointed out before, leans out the mixture and causes the stumble.

Now, in the setups that Martin and I are running, there is no restriction prior to the vent hose. With no restriction, there is no vacuum in that hose section and therefore when you open the filler, there is no effective mixture change.

This is also why the low speed scavaging on an AFM equiped car is higher than our setup. The vacuum in that hose section due to the flapper door helps generate a slight negative pressure in engine. In a closed system - ie fully sealed oil cap, oil dip stick, etc - this just creates a slight vacuum in the engine with no effect on the idle. With no AFM, our setups don't have that slight vacuum edge for scavaging.

This also seems to support why it's so critical for a smooth idling M30 (AFM equiped) to not have any vacuum leaks in at the filler cap or oil dipstick.

Anyways...this is my theory and I'm sticking with it. Hope I did a good job of summing up what we found.

Cheers!

Jeff

I guess the question is, is there enough scavenging going on to get the car within emissions regulations? I wonder if, with the MAP sensor conversion, one is actually damaging the ability of the system to clean the emissions from the crankcase and thus increase emissions after long periods of idle or even at part throttle? Does one really need to route the crankcase pipe directly into the manifold (and thus will the head seals/valve-cover gasket handle the vacuum) or is the standard intake location (prior to the throttle body) cut it?

My guess is its fine... but I'd love to know you/Martins opinions Nick

[Jeff N.]

...it won't effect emissions at all.

And, I think the scavaging is only impacted at idle and not at other engine speeds. As long I don't idle the engine for long periods, I don't think there would be any impact.

Anyone have any other ideas?

Jeff

[Kalevera]

All I can say is, I'd really like to hear a clip of how Martin's engine idles....That split second conversion is a near-future upgrade for my car, but I won't be doing it unless I can jack up the fuel at idle and get it to smooth out a bit.


best, whit

[genphreak]

...it won't effect emissions at all.

And, I think the scavaging is only impacted at idle and not at other engine speeds. As long I don't idle the engine for long periods, I don't think there would be any impact.

Anyone have any other ideas?

Jeff

Thanks Jeff, yes that's right thinking more it won't affect the tailpipe output much, and so long as the crankcase can vent into the air intake boot there can be no positive pressure problem, only trouble (but its verging on the ridiculous) would be if there were 'rushes' of poisonous gases at certain moments interrupting the flow of fresh air from the air filter). Say for example, coming off WOT hard or maybe in-between gears under no or very light load... yea no I doubt that could ever present a drivability problem, even with a smokey motor...

Nick

[genphreak]

All I can say is, I'd really like to hear a clip of how Martin's engine idles....That split second conversion is a near-future upgrade for my car, but I won't be doing it unless I can jack up the fuel at idle and get it to smooth out a bit.


best, whit

Is the point that Motronic is not capable of idling the motor smoothly (due to lag and/or sensor erratics) and has to rely on the AFM 's manually set idle air-bleed or is it that the MAP setup has limitations in this respect? Or is Fritz just so damn rough you are worried about making it worse? (Mine is fine now the head's been rebuilt)

Nick