Does timing affect AFR

[1986911]

I did a search and am more confused?

My car has a constant slight miss from above idle to 3100rpm on steady cruise. Its barely noticeable but it shows up on my WEGO AFM. The AFR goes from 15.2 to 14.2 then back up to 15.2 every other second or so. The swing is always one whole number. It fades after 3000 rpm and goes away completely after 3100-3200. Does not matter the gear and always on light cruise with no boost.

The thing is after 3100 my car starts to run rich on cruise the rest of the way up the scale?? 12.1 to 1

I have been chasing boost leaks but no luck, new orings, checked hoses and torqued injector blocks to spec. The car runs really rich under boost from 3.3k to 5.5k 10.3 to 1! then gets a little better approaching red-line 10.8 to 1

Idle is 13.5 to 1 solid. Car runs a little hotter than before too.

I tried an "Italian tune up" earlier today as I do not drive the car that often and have run two tanks with techron injector cleaner. No change At 10.3 under boost those tanks go quick!

Could a moved\loose distributor be causing this. I am buying a timing light tomorrow!

[proffighter]

Timing affects AFR's for sure. But in your case, I don't think it's a timing issue. However, checking timing is easy, so why not do this?


What came first into my mind are the injectors. They are known for going out of specs, which is feelable at light loads too. Opening pressure and squirting behaviour can be very different, which can cause symptoms similar to yours

Search for injector testing here on forum and check them.

On the other hand, depending on your engine specs, AFR's over 14 could be simply to lean for your particular engine. Maybe try a bit fater is worth a try

BTW, is the engine jumping too or does it run smooth in this area?

I am sure, others here had similar issues too and will help soon

[Speedy Squirrel]

A slight jump to a lean AFR reading, coupled with a rough feeling sounds like an ignition miss, probably in just one cylinder. Check your plugs and see if one has more soot on it than the others. The AFR increase is because one cylinders air is not burned and it gets detected by the O2 sensor as lean.

[quattrorunner]

Subbed because I too have a stumble on light cruise. Fine with boost though.

[Miguel Antonett]

Two totally different things/systems. Timing is electrical - distributor/sparks, and the AFR is mechanical - air and gas mixture, which is control by the WUR or EFI. The finesse is to get spark at the right piston travel spot to get the most out of the air/fuel mix combustion.

[1986911]

I am hoping its just timing. I had the car dyno tuned less than 1000 miles ago, about 14 months back. Everything was to spec. I started blowing fuel pump fuses and replaced rear pump myself a few months ago. No more blown fuses but now this?

I will post what I find when I get my timing gun. I am going to do a search on how to adjust timing. Any tips?

[1986911]

Originally Posted by Speedy Squirrel

A slight jump to a lean AFR reading, coupled with a rough feeling sounds like an ignition miss, probably in just one cylinder. Check your plugs and see if one has more soot on it than the others. The AFR increase is because one cylinders air is not burned and it gets detected by the O2 sensor as lean.

How fast can plugs get sooted up? Complete tune up 1500 miles ago (2 years), new plugs rotor, cap, all fluids and filters.

[GJF]

Originally Posted by Speedy Squirrel

The AFR increase is because one cylinders air is not burned and it gets detected by the O2 sensor as lean.

Huh????!!!

1986911: try fattening up the mixture just slightly And verify with a off boost test drive to see if the surge goes away. It sounds like its lean. Typically a lean mixture will cause a slight surging, which you are describing. Plugs can soot up but once dialed in clean right up. Only oil fouling causes big problems. Soot is just a tell sign of a rich mixture in a normal healthy cylinder with good ring seal. Timing affects AFR but not like your description. Fatten her up just a little to see 13.5 - 14.5 AFR. It may be an injector.

[proffighter]

Originally Posted by GJF

Huh????!!!

1986911: try fattening up the mixture just slightly And verify with a off boost test drive to see if the surge goes away. It sounds like its lean. Typically a lean mixture will cause a slight surging, which you are describing. Plugs can soot up but once dialed in clean right up. Only oil fouling causes big problems. Soot is just a tell sign of a rich mixture in a normal healthy cylinder with good ring seal. Timing affects AFR but not like your description. Fatten her up just a little to see 13.5 - 14.5 AFR. It may be an injector.

what I said...

[mark houghton]

Originally Posted by Speedy Squirrel
The AFR increase is because one cylinders air is not burned and it gets detected by the O2 sensor as lean.

Originally Posted by GJF

Huh????!!!

'Tis true. Our AFR gauges pick up their signal from an oxygen sensor, measuring the residual oxygen left in the exhaust gasses, and converting that information into an air-to-fuel reading. I know it sounds contrary, but think about it....if a plug doesn't fire or fires weakly, you get incomplete combustion which means that unconsumed oxygen is left over, giving you a lean reading even though the particular cylinder is rich in fuel.
So if you have a intermittent ignition problem, be it a wire or a plug or whatever, you will see your AFR's ranging between normal stable values every time the plug fires or mis-fires.

[RarlyL8]

What were your readings before this "change" occured?

The miss sounds like the mixture is slightly lean. The rich boost AFRs are not atypical and can be adjusted into spec with your WUR.

I'm going to guess that after 3100 the engine shows positive pressure on the boost gage. If so that will tip the AFRs rich as boost threshold enrichment kicks in.

How old and what condition are your injector blocks in? Cracked blocks would manifest symptoms in this way.

[1986911]

I checked timing looks to be in line with what Porsche asks. 3 degrees at idle 950 rpm with vacuum hoses connected. 26.3 degrees at 4k rpm with hoses disconnected.

I tweaked the fuel mix screw a little and its richer now at idle 12.8.

Still has 1 point fluctuation at cruise though. It seems to run a little better at mid throttle.

I just ordered a fabspeed air intake kit. Taking off and putting on that factory air housing is no fun. Once I get it I am going to really dial in my cold, cruise and full throttle enrichment.

My injector blocks were changed a few years back during a rebuild by the previous owner. They looked good. Some bolts were finger loose. I torqued everything down according to spec. I did this last month.

If dialing it in does not solve my issues next step would be checking injectors, there could be one sticking, causing my fluctuating afr and rich mid and top end. It seems logical what you guys suggest. One gets hung up and fires every other cycle and if I press on the pedal pressure goes up and problem goes away.

I know this is a hot topic but what control pressures you you guys run? I would like a baseline to start from.

How much fuel pressure do you guys run?

[JFairman]

"Does timing affect AFR"

No.
AFR is the air fuel mixture ratio going from the intake ports into the cylinders.
Ignition timing has nothing to do with intake mixture.

Timing has some effect on exhaust emissions but thats different and not referred to as AFR.

[1986911]

Originally Posted by JFairman

"Does timing affect AFR"

No.
AFR is the air fuel mixture ratio going from the intake ports into the cylinders.
Ignition timing has nothing to do with intake mixture.

Timing has some effect on exhaust emissions but thats different and not referred to as AFR.

Thanks for the info. I was thinking (hoping) it was timing since it would take place at around 3100 rpm.

[proffighter]

Originally Posted by JFairman

"Does timing affect AFR"

No.
AFR is the air fuel mixture ratio going from the intake ports into the cylinders.
Ignition timing has nothing to do with intake mixture.

Timing has some effect on exhaust emissions but thats different and not referred to as AFR.

Sorry, but completly wrong!!!

AFR's are the readings of oxygen content in the exhaust emission by lambda sensor.

And of course timing affects the AFR's as the rate of combustion is slower with lower lambda (richer) due the higher mass of the mixture (result in flater pressure rise).

Fater mixture has to be fired earlier than leaner. Goal is to reach center of combustion mass around 8 degress after TDC in general

This goal is limited by the 4 most important factors:

Exhaust temp
mean combustion temp
point pressure
knock limit

[T77911S]

Originally Posted by JFairman

"Does timing affect AFR"

No.
AFR is the air fuel mixture ratio going from the intake ports into the cylinders.
Ignition timing has nothing to do with intake mixture.

Timing has some effect on exhaust emissions but thats different and not referred to as AFR.

it DOES effect AFR.

i did some tests with my LM2 to verify this, also, i have been told the same thing by certified mechanics.

retard timing at idle will make it run hotter/leaner, the opposite for WOT. may not be the only reason, but thats why you need to richen the mixture with more advance or you will get spark knock. the mixture leans out and the temps go up.

to the OP.
from what i have looked at on 930's, the speed relay under the seat also controls the O2 sensor. at 3k RPM, the O2 is taking out of the system and the CIS goes open loop.
i would look into this and check fuel pressures. i would also loo at the frequency valve.
check for power to the speed relay. maybe even swap it if you have a spare, not a bad idea to have

[JFairman]

Originally Posted by proffighter

Sorry, but completly wrong!!!

AFR's are the readings of oxygen content in the exhaust emission by lambda sensor.

And of course timing affects the AFR's as the rate of combustion is slower with lower lambda (richer) due the higher mass of the mixture (result in flater pressure rise).

Fater mixture has to be fired earlier than leaner. Goal is to reach center of combustion mass around 8 degress after TDC in general

This goal is limited by the 4 most important factors:

Exhaust temp
mean combustion temp
point pressure
knock limit

OK, we're talking about different things that affect the display reading on a wideband air fuel ratio gauge.
When I look at an AFR gauge and what it's displaying I'm thinking about the mass ratio of fuel injected by the injection system in relation to the mass ratio of air being sucked in or blown into the combustion chambers during the intake stroke and that is all I use it for.

Those that say I'm wrong are also using the AFR gauge to get an idea of the quality and efficiancy of combustion and the resulting oxygen content and emissions in the exhaust.
Ignition timing effects combustion in a big way and the resulting oxygen content and other emissions in the exhaust and I mentioned that in my earlier post.

If you have worn intake valve guides leaking oil into the intake mixture during intake stroke and or worn piston rings leaving too much oil on the cylinder walls during the intake stroke and oil is being burned along with the air fuel mixture during combustion that will also affect the amount of oxygen in the exhaust and the reading on a wideband air fuel ratio gauge.

I use the AFR gauge readout to adjust the air fuel ratio mixture going into the combustion chambers before compression, ignition, and combustion and thats it.
I use a timing light to set the ignition timing and I know where I want it on my motor.
If a cylinder is burning oil I can see the burnt oil deposits on the spark plug.

[T77911S]

yes, but you are measuring the AFR after combustion, after timing has had its effect, not before.
as timing advances or retards does it change the pre combustion mixture? no, does it make the AFR's at the exhaust change? yes. and the mixture(AFR) needs to be adjusted or compensated for these timing changes for optimum burn.

the point is does timing make the combustion appear lean or rich, yes, which is read as AFR or CO%.

[gsxrken]

Originally Posted by T77911S

to the OP.
from what i have looked at on 930's, the speed relay under the seat also controls the O2 sensor. at 3k RPM, the O2 is taking out of the system and the CIS goes open loop.
i would look into this and check fuel pressures. i would also loo at the frequency valve.
check for power to the speed relay. maybe even swap it if you have a spare, not a bad idea to have

this is along the lines of what I thought of too. If its the 89 930 in your sig, your lambda system may be cycling in and out right at that point looking for that narrow range around 14.7 AFR.
Is yours still active, or disconnected?

[JFairman]

"from what i have looked at on 930's, the speed relay under the seat also controls the O2 sensor. at 3k RPM, the O2 is taking out of the system and the CIS goes open loop."

I thought the throttle position switch on the right side of the throttle body put the lambda system and frequency valve into open loop mode at somewhere around 3/4 throttle so it wouldn't be leaning out the fuel injection during boost and high speeds under load. The fuel enrichment relay in between the speed relay and the k-jetronic ECU under the driver seat on late eightees cars does that.

Seems they would want the lambda system frequency valve/inline injector still fine tuning lower differential chamber control pressure in the fuel head to make fine adjustments to the air fuel ratio by returning some of the lower chamber fuel pressure to the tank to keep it at 14.7:1 to meet emissions and get decent fuel mileage at 3000rpms and 4000rpms too.

I have no documents or tests to prove that but going open loop with the lambda system frequency valve at 3000 rpm steady cruise with no boost doesn't make sense to me.