Fuel Mixture Q's

Shepper · 09:00 AM

1983 944 N/A

I've searched through past posts and I found two different values listed for the voltage that the O2 sensor is supposed to put out when the engine is running correctly.

I've found .9V and I've also found .45V. Currently, mine is running at .83V when it is warm and it seems to be a bit rich to me. So, I'm guessing the .45V is more correct?

Also, when at idle, what should the fuel consumption gauge be reading?

Oh yeah, keep in mind I'm in Denver.

SoCal Driver · SoCal Driver's Garage

Found this:

Ox sensor: In the 85.5 DME testing procedures with engine at idle and warmed up is from 0.1 v to 1.0 v. with a digital meter.

Both testing procedures insist that you use a CO2 tester for accuracy.

High altitude switch:

For early pre 85.5 944's terminal 28 at the DME is switched to ground above 1000 meters (3,200 ft.)

This switch is connected to terminal 30 on post 85.5's while DME terminal 28 goes directly to ground. One of those differences between the early and later 944 DME's. DME 30 on the early 944 DME's looks like it doesn't connect to anything.

Shepper · 10-22-2002, 11:49 AM

So, there isn't really a specific reading I should be getting off of the O2 sensor? Anything between .1V and 1V is OK?

What about the fuel consumption meter? What is a general reading I should be getting on that when the car is at idle?

Like I said, based on strong fumes, it seems that the car is running rich, but I'd like to get a little more accurate than going off just fumes.

I'm kind of starting from scratch (as far as tuning the fuel air mixture goes on this car) because this is a (new) used engine and the PO had already messed with the AFM.

I'd like to get it close before I bring it to a place with serious testing equiptment so that they don't have to "fiddle" with it too much.

SoCal Driver · SoCal Driver's Garage

Where do you smell the "strong fumes"?

The voltage values are all I could find quickly in the factory manuals. This does not mean there is a more precise read out chart somewhere.

The fuel consumption gauge has two scales. You use the gallons per hour for all gears except 5th where the gallons per mile is calibrated to -- with stock diameter tires. This gauge uses the info from the tach and the speedo. Not that accurate as the post 85.5's just have a "Shift Now Idiot" light.

Still it's impressive when you are cruising at 85 mph and tell the passenger were are now getting over 35 miles per gallon!

What did the PO do with the AFS? Porsche calls this the Air Flow Sensor.

OT---
It's "take it to" not "bring it to". You take it there and bring it here. Or bring it with you if you are coming to me(here); take it with you if you are going to work (there). That double 't' rule.

Don't get me started on "got"!

Shepper · 10-22-2002, 01:02 PM

The fumes are coming from the tail pipe. It doesn't smell like anything specific, like gas or anything... it's just a really strong exhaust smell. It might just be some bad fuel as the car has been sitting for a while now without being really taken out for a good drive.

I don't know what the PO did with the AFSensor (as you call it.) The black cover was broken loose when I got the car, so I'm not sure what type of adjustments were made in there. I wish I knew.

I guess without decent equipment it's difficult for me to adjust the mixture so I'll just have to TAKE it somewhere.

Quote:

Originally posted by SoCal Driver

OT---
It's "take it to" not "bring it to". You take it there and bring it here. Or bring it with you if you are coming to me(here); take it with you if you are going to work (there). That double 't' rule.

Don't get me started on "got"!

Only on a Porsche board... well, probably on the english professors united board as well.

Don't get me started on "anyways."

SoCal Driver · SoCal Driver's Garage

It's not ME that calls it an AFS it's what Porsche calls it.

If the cover was loose then you can check the resistance values at the back of the connector. The early 944's use 8 volts at the sensor to "power" the resistor.

Think there was a previous thread where I posted the voltage values.

Was on the "other" board. Here:

----------------------------------------------------------------------
Pull the rubber weather boot off of the back of the plug to expose the wiring and pins. Put the connector back on the AFS and turn the key on.

Pin numbers as you stand in front of the car and look down at the four wires pin 22 in on the left (passenger side) pin 7 is next (left of center), pin 9 is the third from the left and pin 6 is the last on the right next to the empty socket. I just checked mine.

For the early models:

From pin 9 to ground should be => 8 volts.

From pin 7 to ground should be 0.15 to 0.25 volts.

Leave volt meter on pin 7 and push the flap full open. Should be => 8 volts.

For the post 85.5's:

From pin 9 to ground should be about 4.6 volts.

From pin 7 to ground should be 0.15 to 0.25 volts.

Leave volt meter on pin 7 and push the flap full open. Should be about 4.6 volts.

I've resurrected a few AFS's by pulling off the black cover and relocating the two wipers on the carbon coated ceramic resistor base. Moved them radially from center. Not fore and aft. Just a little. Cleaned the resistor and wipers with electronics "tuner" cleaner. Seal the black cover with silicone. Don't let the silicone drip into the resistor cavity.

Reference Porsche 83 and 85 DME Testing Procedures.
-------------------------------------------------------------------------

I'll add that when moving the flap the resistance should change somewhat smoothly.

Makis · 10-22-2002, 02:33 PM

Quote:

Originally posted by Shepper
1983 944 N/A

I've found .9V and I've also found .45V. Currently, mine is running at .83V when it is warm and it seems to be a bit rich to me. So, I'm guessing the .45V is more correct?

The voltage should flactuate between 0.1 to 0.9 but on averate it should be 0.45 volts. 0.83V means you have a rich mixture and could explain why you smelling fumes. When O2 connected the ECU should be able to control the voltage around the 0.45V and so keep mixture at optimal levels.

Shepper · 10-22-2002, 02:47 PM

Thanks SoCal Driver, I'll be sure to check those values and make sure everything in the AFS is funtioning correctly.

Quote:

Originally posted by Makis

The voltage should flactuate between 0.1 to 0.9 but on averate it should be 0.45 volts. 0.83V means you have a rich mixture and could explain why you smelling fumes. When O2 connected the ECU should be able to control the voltage around the 0.45V and so keep mixture at optimal levels.

This brings up another question. Do I want to test the fuel air mixture with the O2 sensor disconnected? This would put the ECU into default mode (sorry I don't know the official name for that mode) and allow me to adjust the mixture without the ECU interfering.

Or???? What do you guys think?

Scott R · Scott R's Garage

Has the car always been in Denver? If so and your running rich you probably have dry rot on the intake boot.

You may also have dry rot on the vacuum lines. My 85.5 and my 87 both were Colorado cars and all of the lines and boots were rotting.

For anyone who has never been to Denver before, its vary rare that we ever see any measureable humidity, its extremely dry.

Shepper · 10-22-2002, 03:13 PM

Hey scott I just emailed you before you posted.

Yeah, I went over the intake boot and it's definately dry rotting. But, it doesn't appear to have cracked all the way through yet.

I have also gone through all of the hoses and made sure they were ok... they are now.

Makis · 10-22-2002, 03:15 PM

Quote:

Originally posted by Shepper

This brings up another question. Do I want to test the fuel air mixture with the O2 sensor disconnected? This would put the ECU into default mode (sorry I don't know the official name for that mode) and allow me to adjust the mixture without the ECU interfering.

Yes you could adjust the mixture like that. If you can keep the O2 sensor heater connected that will give more relaible readings. Don't expect the reading to stay fixed at 0.45. The reading again should flactuate but make sure the average stays around 0.45. In this way you know that the basic setting is about OK and that the closed loop control will have to do only little work when you connect the sensor back up.

Shepper · 10-22-2002, 03:17 PM

Quote:

Originally posted by Makis

Yes you could adjust the mixture like that. If you can keep the O2 sensor heater connected that will give more relaible readings. Don't expect the reading to stay fixed at 0.45. The reading again should flactuate but make sure the average stays around 0.45. In this way you know that the basic setting is about OK and that the closed loop control will have to do only little work when you connect the sensor back up.

OK, thanks, I'll give it a shot.

todwic · todwic's Garage

I can't believe Mr. F.R. Wilk didn't get involved in this thread. He must be on a tequilla drunk.

turbohead7 · 10-23-2002, 06:30 PM

Yo Shepper, I couldn't stay away from this one, I tried. I see that the board already has given you good advice. Porsche does reccomend adjusting the CO with the O2 disconnected, and the engine warm. I believe the CO adjustment screw is on the vane airflow sensor. It usually has a plug in it which needs to be removed although I'll bet yours is already gone. The O2 sensor is designed to run between .1-.9mV (millivolts). Closer to .1mV is leaner, while .9mV is the rich end of the scale. The O2 when warm must fluctuate repeatedly and smoothly from lean to rich. The DME is programmed to run at an optimal stoichiometric value of 14.7:1, or lambda. 14.7:1 refers to a mixture of 14.7 parts air to 1 part fuel. 13:1 would be a rich mix, while 16:1 would be a lean mix. Anyway, this value approximates to a voltage reading of .45 or 450mV, which is the best compromise between performance and emissions. Warm up the O2 and whatch the voltage, typically a bad O2 will "stick" or stay at a particular reading for loger than .75 seconds. Also, if your O2 averages around .83mV, this would indicate a rich mix and can be confirmed with a for or five gas analyzer (read: expensive). If your CO readings are excessive, this will verify the rich mixture and tell you that your O2 is healthy. Your CO values can be found on the VECI (Vehicle Emission Control Information) label under the hood. Just make sure you take the readings before the catalytic converter, there is a test pipe which comes up the back of the engine. It is usually capped off or missing entirely. Good Luck.

Shepper · 10-24-2002, 07:15 AM

Excellent info turbohead7, thank you.

I haven't had a chance to mess with it yet since I posted last, but I'm going to work on it a lot this weekend.

Your post raises one more question. You say that the reading from the O2 sensor should fluctuate around an average.

Do you know the approximate range around that value that I should be looking for?