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equality72521's Avatar
 
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Odd AFR issue

Scenario: On a cold engine I'm rich with low RPMs (AAR, the one with the electrical plug, removed) and as it warms up it leans out and idle increases, normal behavior. At normal operating temps AFR is around ~14.5 and idle is 1000 RPM. Now, if I park the car for a while (30-90 minutes or so) and restart before it cools completely my idle drops to ~800 and my AFR leans out to ~16-16.5 and it doesn't run so well. It stutters and pops for a little bit and after ~5 minutes or so the AFR at idle drops back to ~14.5 and idle is back up to 1000RPM. Now it makes sense that the idle would drop as the engine is cooling but why would my AFR go lean rather than rich as is normal when the engine temp is dropping? I've checked system pressure, cold control pressure, and hot control pressure and they are all dead on factory specs. What could cause this?
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 04-06-2010, 06:21 AM
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Crotchety Old Bastard
 
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Idle mixture.
It is not uncommon to adjust idle mixture seasonally with these old systems. Your CO% is lean if AFR at idle is 14.5, optimal is ~13.5AFR which is ~3% CO. When you park the car the WUR will heat soak leading to a false reading for start up enrichment. This can be an indication that the bimetalic strip is aging and going out of spec.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 04-06-2010, 06:29 AM
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Ahhh, I thought I wanted 14.7 stoic. I'll adjust down to 13.5 and see what happens. I hope the WUR isn't dying. I have a another WUR but it isn't the exact part number, is the bi-metallic strip the same in all units where I could swap it into my existing unit if it is indeed failing?
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 04-06-2010, 06:47 AM
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Mark,
Are you tuning your CIS to pass emissions tests or just optimizing the CIS system for best driveability? When I lived in the Northeast, getting the CIS engines to pass the "sniffer" test, was an effort in futility. The only way it would pass if I added a cat and tuned it slightly rich. Tuning it to stoich at idle, these engines run like crap under any kind of load. Also, any gasoline blends (10-15% E) out there will affect the ideal stoich value.
Maybe these charts will help in tuning.

btw, I've been using an '87 930 WUR with great results. The WUR's for the '78-86 era 930's are getting very hard to come by. If you find one, the cost would be very high compared to other WUR's.


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'85 930 Factory Special Wishes Flachbau
Werk I Zuffenhausen 3.3l/330BHP Engine with Sonderwunsch Cams, FabSpeed Headers, Kokeln IC, Twin Plugged Electromotive Crankfire, Tial Wastegate(0.8 Bar), K27 Hybrid Turbo, Ruf Twin-tip Muffler, Fikse FM-5's 8&10x17, 8:41 R&P
Old 04-06-2010, 08:19 AM
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I'm tuning for best running/power. No emissions testing here. I'll have to check the part number on my extra WUR and see what it is. Does that first graph represent AFR at idle or WOT? The second graph represents where I am at idle right now (~14.5).
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 04-06-2010, 08:28 AM
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Quote:
Originally Posted by equality72521 View Post
I'm tuning for best running/power. No emissions testing here. I'll have to check the part number on my extra WUR and see what it is. Does that first graph represent AFR at idle or WOT? The second graph represents where I am at idle right now (~14.5).
The first graph represents power versus stoich at any throttle position. Max. power is approx. .85 lambda.
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Dave
'85 930 Factory Special Wishes Flachbau
Werk I Zuffenhausen 3.3l/330BHP Engine with Sonderwunsch Cams, FabSpeed Headers, Kokeln IC, Twin Plugged Electromotive Crankfire, Tial Wastegate(0.8 Bar), K27 Hybrid Turbo, Ruf Twin-tip Muffler, Fikse FM-5's 8&10x17, 8:41 R&P
Old 04-06-2010, 08:37 AM
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Mark, if your WUR is healthy in areas other than cold/hot start you will see a cruising AFR (steady throttle with engine in vacuum) of ~14.5:1 independent of the 13.5:1 at idle. 3%CO w/no Lambda is best for performance.
If it is degrading I can rebuild your WUR to stock specs if you want to use your spare for now. The 153 model may be what Dave is referring to. The 054 and 112 are slightly different and a bit older. All will work or can be made adjustable so they work with any 930.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 04-06-2010, 09:32 AM
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Odd AFR

Mark: I run the same idle mixture AFRs as you do and my '87 has the exact same symptoms. I believe that they're all related to IC heat soak.

My theory is that the idle drops because the IC has heated the intake air causing a slightly too rich mixture (yours stutters and pops which does sound like lean; mine doesn't). The improperly rich idle mixture lowers the idle speed which raises the airflow sensor a bit to lower idle a bit more. Because of this low idle speed , ALOT more outside air crawls up my RarlyL8 straight-thru muffler and hits my wide band O2 sensor and relays the same lean AFRs that you have.

Once I get the rpms up and the engine fan pulls some heat off of the IC, all these symptoms disappear. I just raise the idle speed with my foot for 30 seconds or just drive off and cool the IC with passing air. End of issue, idles normally.

BTW, my Leask WUR is two years old (works great) and all of my fuel pressures are nominal. I agree with the others about richening our idle AFRs and have been meaning to do so.
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Old 04-06-2010, 11:18 AM
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Quote:
Originally Posted by 87 930 View Post
my RarlyL8 straight-thru muffler and hits my wide band O2 sensor and relays the same lean AFRs that you have.
Interesting. I too have a straight through pipe though not one of Brian's.
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 04-06-2010, 11:23 AM
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Ok I should have 13.5 at idle what should i see at normal acceleration and Wide open throttle or full boost? And can you tune with the A/F guage or is the sniffer the best way?
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Old 04-06-2010, 05:54 PM
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h20cooled7,
The best way to monitor A/F is through data logging under a rolling dyno, but for those who do not have one of those readily available, data logging with something like a Innovate sniffer works very well.

The ideal AFR for a 930 has been beaten to death and for good measure, crucified. It seems it gets richer and richer and richer with each passing thread. My 0.02, because the 930 pancake intake manifold provides less than ideal air distribution across all 6 cylinders and AFR's are the average of all 6 cylinders sampled, you may want to run a little to the left of the peak "Horsepower" curve around 11.8 to 12.5 AFR's. Some even go richer than that, thinking if a little rich is good, more is better. Keep in mind these AFR's are meant to protect from lean conditions from induction compromises and preignition which are variables depending on ignition timing, single versus twin plug, fuel quality, induction - stock versus modified, valve timing, etc.

The closer to stock the engine is, the closer to 11.8 the AFR should be. The closer to ideal the induction system is, the closer to 12.5 the AFR should be, for MAXIMUM POWER, that is.
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'85 930 Factory Special Wishes Flachbau
Werk I Zuffenhausen 3.3l/330BHP Engine with Sonderwunsch Cams, FabSpeed Headers, Kokeln IC, Twin Plugged Electromotive Crankfire, Tial Wastegate(0.8 Bar), K27 Hybrid Turbo, Ruf Twin-tip Muffler, Fikse FM-5's 8&10x17, 8:41 R&P

Last edited by WERK I; 04-07-2010 at 06:33 AM..
Old 04-07-2010, 06:31 AM
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Quote:
Originally Posted by WERK I View Post
h20cooled7,
The best way to monitor A/F is through data logging under a rolling dyno, but for those who do not have one of those readily available, data logging with something like a Innovate sniffer works very well.

The ideal AFR for a 930 has been beaten to death and for good measure, crucified. It seems it gets richer and richer and richer with each passing thread. My 0.02, because the 930 pancake intake manifold provides less than ideal air distribution across all 6 cylinders and AFR's are the average of all 6 cylinders sampled, you may want to run a little to the left of the peak "Horsepower" curve around 11.8 to 12.5 AFR's. Some even go richer than that, thinking if a little rich is good, more is better. Keep in mind these AFR's are meant to protect from lean conditions from induction compromises and preignition which are variables depending on ignition timing, single versus twin plug, fuel quality, induction - stock versus modified, valve timing, etc.

The closer to stock the engine is, the closer to 11.8 the AFR should be. The closer to ideal the induction system is, the closer to 12.5 the AFR should be, for MAXIMUM POWER, that is.
I'm with you....good summary of this much maligned topic. My AFR settles in to about 12.2 on full boost (sometimes a little lower, depending on ambient air temp). That's 12.2 at .8 bar. At 1.0 bar, I'm usually at around 11.6. So much depends on the state of tune, as you've said. Like many, I'm tempted to adjust for lower AFR's....just to add a little more safety factor....but then where do you stop? I guess we would never feel completely safe until AFR's are so low that raw gas is drooling out the tailpipe and your turbo has turned into a ramjet.
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Old 04-07-2010, 02:23 PM
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Quote:
Originally Posted by equality72521 View Post
Scenario: On a cold engine I'm rich with low RPMs (AAR, the one with the electrical plug, removed) and as it warms up it leans out and idle increases, normal behavior. At normal operating temps AFR is around ~14.5 and idle is 1000 RPM. Now, if I park the car for a while (30-90 minutes or so) and restart before it cools completely my idle drops to ~800 and my AFR leans out to ~16-16.5 and it doesn't run so well. It stutters and pops for a little bit and after ~5 minutes or so the AFR at idle drops back to ~14.5 and idle is back up to 1000RPM. Now it makes sense that the idle would drop as the engine is cooling but why would my AFR go lean rather than rich as is normal when the engine temp is dropping? I've checked system pressure, cold control pressure, and hot control pressure and they are all dead on factory specs. What could cause this?
Sounds like a bad fuel accumulator, pelican part.no.: 911-110-197-02-M14
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Old 04-08-2010, 01:29 AM
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Quote:
Originally Posted by gogomobil View Post
Sounds like a bad fuel accumulator, pelican part.no.: 911-110-197-02-M14
My accumulator is only 2 years old with less than 5,000 miles on it.
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 04-08-2010, 08:41 AM
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