Fuel mixture troubleshooting and O2 sensor test results.

[HondaDustR]

So I've been on and off troubleshooting some random rough and/or inconsistent running for a fairly long time with no real solid evidence of what's going on...until now.

So let me start with what prompted this major testing session. I stumbled upon Lindsey's crankcase breather upgrade page along with an explanation on why it's good to have blah, blah...
http://www.lindseyracing.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=LR&Category_Code=BREATHERTANK
I got to thinking that maybe that could be the source of some problems, since my car sucks oil through the intake like there's no tomorrow (a litttle exaggerated. ~1 qt/800 mi.). I decided to run an experiment by just disconnecting the hose comming from the AOS and blocking off the connection at the J-boot metal T-fitting. It idled noticably smoother and had more power overall. I really don't want to spend $160 on the breather canister, but I'm going to try to rig up something so the AOS isn't vented into the intake. If anyone knows of any important disadvantages to this, then post it up. This testing and before/after comparison was done with the FQS set to pos. 2 (-3% fuel) and with the O2 sensor disconnected.

After a day of testing, the power band still seemed a little choppy feeling, like there's still something wrong with the fuel mixture. I have heard of people testing the voltage from the O2 sensor with no conclusive results, but I had an afternoon at my disposal and wanted to just give it a try, so I constructed myself an O2 sensor testing harness. Turns out the pins used in computer accessory power connectors are the perfect size to fit the O2 sensor connectors!



There's two jumpers to run the sensor heater, and a piggybacked wire to allow voltage monitoring with or without the sensor connected to the DME harness. I rigged it up so the voltmeter was visible sitting on the center console so I could see it while I was driving. On to testing!

Since most digital voltmeters default to auto range, the first trick I've found is to force it to display 0.000 V all the time, so the readings won't jump formats all the time.

First, with the O2 sensor signal not connected to the DME harness and breather hose disconnected.

Idle:
-3% fuel
~0.030-0.070 V, sometimes up in the 0.7V range, depending on the engine temperature. But i'd say about 8 or 9 times out of 10 it was lean. Tended to run a little rough.

default fuel
~0.7V during normal driving, sometimes as high as 0.810V and sometimes jumping around 0.4-0.6V. Sitting parked and playing with it resulted in fluctuating characteristics depending on the engine temp.

+3% fuel
pretty solidly at 0.840V seemed to be the smoothest.

Part throttle:
-3% fuel
Pretty steadily around 0.020-0.050V regardless of throttle % up until 4200 rpms.
After 4200 rpms, it held pretty steady at ~0.740V, also unaffected by throttle %.

default fuel
same as -3% fuel results, only below 4200 it was pretty stable at 0.050V and above 4200 was slightly higher at times, ~0.740-0.760V.

WOT
-3% fuel
same as part throttle (~0.050V) until 3000 rpms, then instantly jumped to 0.8V progressively climbing to 0.9V peak by redline.

default fuel
same as -3%, but after 3000, it jumped to 0.840V and peaked at 0.92V at redline.

Overall impressions:
Now I know why the power transition doesn't feel particularly smooth across the range. What I'm feeling is the rich mixture kicking in at given rpms. Maybe the low end maps are too lean, possibly adjustable by the air bypass screw on the AFM, or by a vacuum leak still lurking around somewhere. Also, I was surprised at just how accurate and quick to respond the O2 sensor was. When the DME is in open loop mode, the sensor feedback was pretty definitive and stable. It gets more interesting, though...



Same conditions as the runs before, only with the O2 sensor jumper connected to the DME.

idle:
both -3% and default fuel

Jumping wildly between 0.2 and 0.7V. Idle is noticably rougher.

Part Throttle:
both -3% and default fuel
Jumping wildly between 0.1 and 0.75V regardless of throttle %, up to 5300 rpms. The spread seemed to maybe get a little tighter at higher rpms. After 5300, it clicks into open loop mode and holds steady at 0.750V regardless of throttle %.

WOT:
both -3% and default
instantly goes into open loop mode regardless of rpms. Behaves as it did for the respective tests with the O2 sensor disconnected.

Overall impressions:
Now I finally figured out what causes the surging I've noticed before! At part throttle, the fuel mixture varies wildly as the DME tries to correct it using the sensor feedback. This results in the engine power fluctuating randomly, especially at higher rpms. After 5300 rpms, the output was completely smooth. Either the O2 sensor is too slow to respond, the mixture is so far off from ideal that it constantly severely over and under corrects, the O2 sensor system tacked on to the Motronic sucks, or all of the above. I'm still not convinced there's not a vacuum leak somewhere. There "could" be a very small exhaust leak, but I don't hear/feel any exhaust at the joints and had just replaced the manifold and Y-pipe gaskets a few weeks ago. The air bypass screw on the AFM could also possibly be adjusted too lean. The evap system could be passing too much air. I don't know, but at least now I have some solid evidence of what's actually going on in there.

Just for comparison, I ran another test with -3% fuel, O2 disconnected and the AOS breather hose hooked up normally.

idle:
~0.035V and pretty constant.

Part throttle:
Held pretty steady at 0.015-0.020V before 4200 rpms, then to 0.8 after 4200. Only difference here is that the voltage increased slightly (up to about 0.860V) as the rpms increased. The runs done without the breather connected were pretty constant across the rev range.

WOT:
0.015-0.020 before 3000 rpms, then 0.8V increasing progressively to 0.88V peak at red line.

Overal impressions:
Seemed to run a bit leaner across the board, and seemed to be a little more variable with rpms.

Other random interesting test, O2 disconnected and breather disconnected.
rpms held steady at 2000, car parked and in neutural.

-3% fuel: 0.03V
default fuel: 0.07 to 0.08V. sometimes hovering around 0.3-0.7V depending on engine temp.
+3% fuel: 0.80V


Limitations of these test results:
A standard oxygen sensor is very sensitive within a narrow range of fuel mixtures, so the voltage will change quite a bit up or down from 0.5v with a very small variation in fuel mixture. It becomes much less responsive at the extremes, so these results only give a relatively general outline of whet the fuel mixture is doing. People tune their engines on the dyno and use a "wideband" O2 sensor for exactly this reason, so they can get a more accurate reading for mixtures much outside of 14.7.


So the long and the short of this...I've got the Lindsey silicone vacuum line kit on the way and need to try to figure out why the low end part throttle is running lean.

[Techno Duck]

Regarding a crank case breather, i would not vent to atmosphere. You need to keep a vacuum on the crank case to remove the oil fumes / vapor. Get a recirculating type catch can. Check out the can made by Saikou Michi.. its a very well made piece and has the required size fittings for 944 series engines (19mm). I have one on my car and am very happy with it.

http://www.saikoumichi.com/

It also fits very nicely right by the hood latch..

[HondaDustR]

That looks pretty nice. How much did that one cost?

Only thing is, though, it would solve the oil in the intake problem, but I would still have essentially a vacuum leak between the AFM and the engine. How bad could a straight atmospheric vent actually be? I would assume that if Lindsey knew it would be bad to do it that way, they wouldn't make their catch can just vented to atmosphere. I do feel a positive pulsing flow out of the AOS vent hose even during idling, so it's certainly not just stagnant in there. It probably would be worse for short cold trips due to slowed venting, but it would probably be fine as long as it's warmed up and driven long enough most of the time.

I think the other thing on my mind is that the resulting higher flow due to the vacuum assist from the intake will waste more oil in the catch canister. Anyone have any more thoughts or suggestions on this? I don't want to do it if it's bad, but I'd rather not waste so much oil and dilute the intake with who knows what if it's fine just going to atmosphere.

But that Saikou canister would certainly be more emissions check friendly. I guess I don't really care about venting everything else into the intake if that thing does a good job of collecting the oil out of it. The oil mist seems to be the primary problem according to Lindsey, anyway.

[Techno Duck]

It was $95 + shipping, i think it came out to $110 shipped as of a few months ago.

It will solve the oil in the intake problem for sure. Im not sure i get you when you say you will have a large vacuum leak between the AFM and the throttle body. This is true if say the o-ring on the top of the oil fill tube or top of the AOS is leaking or even if the two lower o-rings where the AOS goes into the block are leaking. However if everything is working as it should, you will get no vacuum leak with a recirculating system.

The reason you feel the pulse is because each time the pistons move upwards and downwards you are getting a pressure / vacuum effect. The purpose of a crankcase vent is to remove any fuel / oil vapor that made it past the rings that is lingering in the oil pan (this causes fuel dilution of the oil). This can also make a dangerous situation where that air charge in the crankcase ignites, but this is really only a big issue on diesels.

While i do like many of the products offered by Lindsey Racing (i have quite a few), its not good to take everything they sell as the best option available (i dont mean any disrespect too you by saying that). So why not vent to atmosphere? Because Porsche designed it as a recirc system. . Just kidding, that answer is used way to often.. .

Here is my two cents on why a recirculating system is better...

I think its far more efficient to constantly pull a vacuum in the crank case to remove the oil vapors rather than rely on the crankcase being at above atmospheric pressure to vent out of the crank case. I think this is also a reason that LR recommends drilling out the top of the AOS larger so more vapor can be expelled at high pressure (especially in the case of a turbo with weak rings). You wont need to do this on a recirculation setup because you are constantly pulling a vacuum on it, so there is no 'build up' so to speak. Many people have trouble with the dip stick popping out (and spraying oil over the engine) because of the crankcase pressure, but again this is mostly an issue on turbo cars with weak rings. I guess what it comes down to, would you rather suck out the vapor or let it linger out, which is going to be more effective?

I dont think the extra use of oil should be much of a concern, besides i dont think its much difference anyhow. If you are burning more than a quart of oil (and its going though the intake) i think you have other things to worry about than having to top off the oil every couple hundred miles.

In my experience, after 4k miles, two track days and a handful of auto-x's i have seen almost no accumulation in the catch can on my 951, probably because its a relatively healthy engine. But some of my track buddies see maybe half a pint of accumulation after a track day, even more if they run the oil over filled a little. So yes, they do work.

[HondaDustR]

I just meant it would be essentially the same as having a vacuum leak, especially with leaking rings, since it introduces extra air/vapors/whatever else into the intake that is not registered at the AFM, and it really would be a vacuum leak if there are any gaskets, seals, or o-rings that are leaking. I've pretty much replaced most of them already, but the oil pan gasket could have a small leak at a couple of the corners. All I know is I've got a constant supply of oil in the intake, the engine doesn't drip any on the ground, I'm dumping a quart in every ~800 miles, and the only way it would be pumping that much oil into the intake is if either the crankcase leaks somewhere, or the rings are bad, since not much oil should be coming through if there's not much air being evacuated to carry it, as it would be with a healthy engine. I do have a feeling my rings are not really 100%. They have reasonably good compression for 9.6:1 CR pistons, but last time I checked a couple months ago two cylinders were a little slower to build up pressure and improved with oil added.

I guess from a physics standpoint, the pulsing from the pistons moving should theoretically cancel out, since whenever two are going up, two are coming down, but I see what you're talking about and it did cross my mind that that's what I might have been feeling. I think the ideal situation looks to be to buy the recirculating canister, but that's also the most expensive soluton, and anything over about $50 isn't going to happen anytime soon unless it's important...as usual

...or maybe I'll just try not to care and drive the rest of the crap out of it and start over rebuilding it top to bottom so it's perfect (and start a savings account for THAT when that time comes). But then a project like that would be so much less worth it with stupid old spec 9.6 CR pistons. Maybe I should just save up and find a good 3.0L, but then parts for that will be expensive and hard to find compared to keeping up with a regular 8v 2.5L. I hate it...why do I have to be such a perfectionist about this stupid thing!

I think the logical short term solution is to jurry rig the family into delivering a Saikou Michi recirculating oil catch can for Xmas!
(cue diabolical cheap german accent) "..but zeit must be perfekt!!!"