How does a boost leak cause rich condition?

bpu699 · 04-03-2016, 06:03 PM

Searched the board, trying to solve a rich condition under boost. Have another thread on checking the timing...

I see many threads that mention that a boost leak would cause rich running under boost...

Can someone explain to me how that works? I would think that if the o-rings or any connection from the turbo to IC to manifold leaked, it would simply show up as an effectively lower boost pressure... thats it...

How does a boost leak that causes a drop or delay in boost from 1.0 to 0.8 any different than changing the boost spring to do the same?

I can see how a leak after the metering plate would cause issues... as air escapes but fuel doesn't, so its rich...

But how does a boost leak before the metering plate make a car run rich under boost?

Just trying to think this through before I start replacing o-rings and rubber hoses all over the place...

Am I misunderstanding this?

mark houghton · 04-03-2016, 06:44 PM

I think you've answered your own question:

I can see how a leak after the metering plate would cause issues... as air escapes but fuel doesn't, so its rich...
The fuel delivery has already be set up as a result of how much the metering arm was moved as intake air deflected it. If any of that air were to leak out after the turbo, then SHAZAM...you go rich.

But how does a boost leak before the metering plate make a car run rich under boost?
Well....there isn't anything "before" the metering plate other than the air cleaner (so you really can't call it a "boost" leak per se). But of course a leak directly after the metering plate (i.e., before the turbo) would suck in unwanted/unmetered air and it would go lean.

This is why leaks post-turbo show up as pig rich when on boost, but may also show as somewhat lean when off boost...off- boost sucking air in vs. on-boost blowing air out.

908/930 · 908/930's Garage

I'm not a expert but I will try to help, I think you answered part of your own question, The metering plate is early in the intake system, right after the air filter so any air leaks in the system will be after the metering plate.

908/930 · 908/930's Garage

Mark types faster thai I do.

bpu699 · 04-03-2016, 07:07 PM

Thanks guys... Super helpful... Makes sense...

Will renew all the intercooler seals... And all the hoses...

As a fluke I blew into the hose that leads from the intercooler to the blow off valve.... And I was able to blow in air with just forceful mouth pressure... Not sure if that 0.8 bar

Plan to hook up an air gauge tommorrow and see what pressure it opens at... Perhaps there is a leak there or the diaphragm has a leak?

The ic seals look okay, unless they fail with age...

Thanks guys...

RarlyL8 · RarlyL8's Garage

Quote:

How does a boost leak that causes a drop or delay in boost from 1.0 to 0.8 any different than changing the boost spring to do the same?

I can add to this one. CIS is a mechanical static system requiring that you set the components to meter fuel for specific parameters. If the system is set to deliver X amount of fuel at full boost (0.75bar) and you replace the spring with a 1.0bar unit, the same amount of fuel is delivered resulting in a more lean condition. Go the other way and it becomes more rich. Porsche set the system rich from the factory so you have some wiggle room in the safe direction. One should not increase boost levels without verifying AFRs.

bpu699 · 04-03-2016, 07:27 PM

Quote:

Originally Posted by RarlyL8

I can add to this one. CIS is a mechanical static system requiring that you set the components to meter fuel for specific parameters. If the system is set to deliver X amount of fuel at full boost (0.75bar) and you replace the spring with a 1.0bar unit, the same amount of fuel is delivered resulting in a more lean condition. Go the other way and it becomes more rich. Porsche set the system rich from the factory so you have some wiggle room in the safe direction. One should not increase boost levels without verifying AFRs.

My car was not a virgin when I got it... Prior owner did mods, I only know what I was told... K27, sc cams, aftermarket exhaust, 1 bar spring, an dial intercooler... He may have done something to the distributor or wur to make it run richer... I have no way to know...

I took out the 1 bar spring, to protect the motor...

Car runs well at idle and midrange. Under boost starts ok and then goes very rich...

Sigh...

Brian, does this scenario sound like going from a 0.8 bar spring to 1 bar would be beneficial???

Will check for boost leaks and verify timing, but running out of scape goats... If I can't find any leaks, and timing plays out, will put the 1 bar spring back in...

T77911S · 04-04-2016, 03:18 AM

playing around with my car I know for a fact it will go rich with an air leak under boost.
here is why I think it does

with an air leak, the turbo is trying to reach full boost, what ever that is based on the spring.
in doing so, more air is pulled thru the AFM because now you have air that is not only going to the engine but now you have air that is escaping the intake system.
depending on the size of the air leak the turbo may still be able to pull in enough air and compress it to reach full boost, but now it is pulling in the extra air escaping the thru the leak.

RarlyL8 · RarlyL8's Garage

What AFR readings were you seeing before the spring change, and what are they now?
Does the engine reach full boost and at what RPM? Has this changed?

bpu699 · 04-04-2016, 05:32 AM

Its been a year or two since the 1 bar spring was in, didn't have an AFR meter then...

When I first put in the 0.8 spring I was seeing AFR'r down to low tens under boost... Now, its < afr 10 before max boost, probably at 5000rpm or so...

The PLX meter doesn't read below 10, so, not sure how low I am going...

Best I can tell, I am reaching full boost, but hard to tell...

Today I plan to attach a leak down meter to the tubing going to the blow off valve. Want to see what pressure the valve opens at, and if its leaking. It was rebuilt 2 years ago, and the diagphram and internals were fine...

The IC neck and manifol insert have 2 rings each to seal. An inner and outer o-ring on the andial intercooler. Will replace the inner seals. Not sure if the outer seals which seem more like spacers are supposed to be compressed tightly? Never did that before and it sealed fine... The inner selas are a very tight fit...

Bo

T77911S · 04-04-2016, 06:41 AM

if your BOV is recirculated back to the intake it does not matter if it opens as far as AFR's go.

I had a synapse that I was venting to atmosphere. it was popping open under boost making my AFR's go rich.

make sure IC is bolted down, it will blow off.

what are your AFR's at idle and steady at around 3k RPM