AFR at exhaust

[Vereeken]

From what I understand the AFR as measured at the exhaust at idle is not 100% accurate.

I was able to establish that myself with the LM1. AFR at IDLE 16 measured at exhaust. 14.7 at the cat bypass (this is a EURO SC NO cat, no 02) when sensor screwed in the bung. Quite a difference.

The issue I have is that I smell and hear that my AFR setting is still not correct. The tailpipe is black the sensor tip is black and the car smells of unburnt fuel.
At cold start you get that typical hunting Idle sign of a too rich mixture.

So what could cause this misleading measurement? Is it possible that the car sucks in O2 in the exhaust system and thus minimizes the Fuel part?

Otherwise the car runs ok and CSV, Thermal Valvel and AAR are all operational.

Any ideas?

[Tippy]

An exhaust leak will read lean

[T77911S]

i did the same test on my BMW 325is. i connected my LM2 inplace of the stock O2 sensor, then checked it at the tailpipe, (no cat) with O2 reinstalled but not connected (as with first test), and got the same reading.
i did not record anything and i only did an idle test.

[mca]

You'll get 02 in the pipe on decel while using the LM1 while driving.

I didn't see a difference between my O2 bung and tailpipe when measuring AFRs with my LM2.

[Vereeken]

Damn.

Indeed you get 20.9 AFR in the tailpipe when you let go of the accelarator.

Perhaps this is the negative point on the LM1?

I just did a quick test and my AFR measured in the tailpipe drops to 14.9 when I let the engine Idle at 2.000 RPM.

So what do you reckon. Am I running lean at 16.0 at IDLE measured at the tailpipe or do I get inacurate readings.

At WOT and from around 5.500RPM I get 13.1 12.8 both in the cat bypass as wel as in the tailpipe.

In the midrange I get 14's.

Michel

[mca]

Originally Posted by Vereeken

Damn.

Indeed you get 20.9 AFR in the tailpipe when you let go of the accelarator.

Perhaps this is the negative point on the LM1?

I just did a quick test and my AFR measured in the tailpipe drops to 14.9 when I let the engine Idle at 2.000 RPM.

So what do you reckon. Am I running lean at 16.0 at IDLE measured at the tailpipe or do I get inacurate readings.

At WOT and from around 5.500RPM I get 13.1 12.8 both in the cat bypass as wel as in the tailpipe.

In the midrange I get 14's.

Michel

I get just about the same numbers as you in the mid-range and at WOT. At idle I believe I'm right around 14.0 (I keep my idle right around 1k).

I'd venture to say that you are ok with your AFR.

The hunting ... How about vacuum leaks? Have you tested your fuel pressures? Does it hunt when warm or only during warm up?

I'm not familiar with the euro - does it have a decel valve?

[KTL]

I have a wideband O2 in my exhaust (racing headers with separate open tailpipes) mounted in one pipe just past the collector. I have the analog gauge in the dash where the fuel/oil level gauge is normally located. On decel I can watch the needle go well into the high teens.

This should be no surprise because on lift-throttle you are cutting off the fuel and the air-fuel mixture in turn rises. Even though you have closed the throttle at high rpm, the engine is still pumping air mixed with your idle fuel rate. Thus the A/F goes up.

The tailpipe measurement may not be accurate at idle. However most often that is not a concern in terms of potential harm to the engine, nor is it typically the target area of tuning. Dyno tuning is looking at range of rpm under load. Once under load, the tailpipe A/F signal becomes stronger. My local dyno shop most often tunes very high HP carbureted drag race engines. He uses the tailpipe sniffer on his dynojet and he's dealing with owners' engines way more expensive than my paltry Porsh.

[psalt]

I agree, outside emission testing, measurements at rest with no load are meaningless.

[Vereeken]

I agree with all of the above that AFR measurements at Idle are pointless at the tailpipe, but I use it to get rid of my hunting idle at dead cold starts. MCA, the car does not hunt at fully warm.

So you see my issue. I know that for a euro SC I am best at around 14 to get teh emissions OK and teh cold start as it is supposed to be.

I feel like I am chasing my tail. Each time I set it at 14 at the sniffer it hunts at cold . Do i go "lean" at idle my cold start is OK but then I am worried about the fact that I am lean.

My best guess based on what I read from all of you is that I get a false reading from the LM 1 at idle and that my lean idle is probably more like 14.
I can not explain how else my AFR at mid and WOT would be ok.

As far as I know I have no air leaks anymore. I replaced the intake runner gaskets, injector o-rings, all the o-rings in the throttle an FD...checked the AAV, AAR, Decel, TTV....

I think I simply sold myself into a non-existing problem by trying to get the IDLE AFR right. And as a result I get an overly rich situation which was apparent from the hunting the reduced mileage and the black exhaust.

I will go kick myself now.

Michel

[Mysterytrain]

If the idle is hunting at cold start up don't you want to lean it from the WUR and leave the mixture setting alone?

[Vereeken]

Ron,

I have no adjustable WUR in the car right now. I have a spare one I have made adjustable but want to keep that for other applications. So I need to stick to teh mixture and idle valve tuning and a bit of timing.

Michel

[psalt]

I can not explain how else my AFR at mid and WOT would be ok.


What happens under load is completely different than what happens blipping the throttle at rest for several reasons. The CIS was calibrated to give a rich mixture, closer to 13 AFR WOT under load, it should not be in the 14's. Cold starts should be much richer because of the lower control pressure, it is very unlikely you have a too rich mixture from a dead cold start. If you don't have a cat and and O2 sensor, 14.7:1 at idle is not relevent, forget about it. The spec for you car is 1-2% CO, high 13's AFR, but most non cat 911s will idle best closer to 3% CO.

[Vereeken]

Psalt, AFR at start-up is low 10s. After about 45 seconds (TTV closes) it goes up to mid 12s.

How does that look.

[psalt]

There is no "correct" or ideal AFR for a cold start. All you should be concerned about is that the engine fires on the first crank, settles to a +1200 rpm idle without touching the pedal, and is able to drive off smoothly after a few seconds. That is the goal. If not, you are 1000 times more likely to be too lean, than too rich. The only AFR number you should be concerned about is WOT under load and if that is above 14 AFR you have a major defect in the system, not a tuning issue.

[911pcars]

Originally Posted by psalt

There is no "correct" or ideal AFR for a cold start. All you should be concerned about is that the engine fires on the first crank, settles to a +1200 rpm idle without touching the pedal, and is able to drive off smoothly after a few seconds. That is the goal. If not, you are 1000 times more likely to be too lean, than too rich. The only AFR number you should be concerned about is WOT under load and if that is above 14 AFR you have a major defect in the system, not a tuning issue.

On a cold start, a typical ECU-equipped engine is in open-loop mode, signals from the O2 sensor are ignored. However, it should be just rich enough to start/run under cold operation but lean enough to conserve fuel and prevent excess fuel from washing lube from the cylinders.

The goal of a fully functional fuel system is to use minimal fuel for idle and cruise and provide the proper AFR for WOT throttle. To ignore AFR when not at WOT discounts the effects of overly rich or lean conditions, neither of which are good for engine operation.

Sherwood

[KTL]

As an aside to this conversation, a common term used in establishing your idle mixture for carburetors is often called "best lean idle adjustment." Point being here is not to get off the subject at hand, by introducing carbs. What i'm getting at here, is that even with carbs you're targeting a lean-ish idle setting. Carbs are less fuel efficient that fuel injection and even they operate with a lean idle setting.

[cmcfaul]

Don’t you get better throttle response with a rich idle setting?

Chris
73 911 E

[Vereeken]

I am experimenting somewhat. Good thing I have a pop-off and and LM.

The lean(ish) idle (measured when fully warm) gives me beter cold start and cold take offs. But 2-3% co gives me smoother warm running and better throttle response.

One thing I also noticed is that I can have an IDLE AFR of 14.0 when in neutral, but when I shift in gear and let go easy of the clutch-pedal with the hand break applied the AFR falls to 13.4/5.
Is that because you are putting load on the engine or does this point to an issue in the system.

I believe an SC has no vacuum assisted clutch only a vac assisted brake booster.

Michel

[911pcars]

Originally Posted by Vereeken

I am experimenting somewhat. Good thing I have a pop-off and and LM.

The lean(ish) idle (measured when fully warm) gives me beter cold start and cold take offs. But 2-3% co gives me smoother warm running and better throttle response.

One thing I also noticed is that I can have an IDLE AFR of 14.0 when in neutral, but when I shift in gear and let go easy of the clutch-pedal with the hand break applied the AFR falls to 13.4/5.
Is that because you are putting load on the engine or does this point to an issue in the system.

I believe an SC has no vacuum assisted clutch only a vac assisted brake booster.

Michel

Could be because in your scenario, the engine speed also decreases; i.e. less air intake. However, what normal situation would that exist?

Sherwood

[scarceller]

When looking at AFRs and testing to see where they are at you should disconnect the O2 sensor for all testing.

With the O2 disconnected what's your idle AFR? the ideal target is 14.0 to 14.2 on the 84-89 cars.

I have seen lazzy O2 sensors cause issues with AFR at idle and part throttle.