Bad O2 sensor or rich fuel mix?

[nixter]

Hi all!

I appear to be running a rich mixture in my new (to me) 911. I'm trying to decide whether the mix is adjusted this way or if the O2 sensor is to blame. (I'd prefer the later as I can fix this myself!)

Symptoms are:

-Sluggish off the line

-Idle low (but steady, no hunting) The needle sits just below 1 when warmed up. I think this is low?

-Very slight popping/backfiring under hard engine deceleration.

-Poor gas milage- My gauge needle is not working but my warning light just came on at 193 miles into a full tank. Either could be to blame for this I guess!

Other than this it runs great!

The car was stored for about 2 years before I took possesion a few weeks ago. I changed the oil/filter, fresh tank of gas + injector cleaner and I failed the emissions test whith too high a CO level (Carbon monoxide) 3.5 x over infact. This means too rich usually. The thing is, the previous owner had the mixture adjusted recently before storage in order to pass emissions. Could something have changed during storage?

The only thing I haven't done yet is swap the fuel filter since it's been in storage. Would this affect anything?

Thanks,

Nix

[wswilburn]

Here is a quick test, not guaranteed to diagnose all problems, but will check if your O2 feedback sytem is functioning. Connect a dwell meter to the frequency valve test connector: connect the dwell ground to a ground on the car; connect the other dwell lead to the three pin connector (one of the three pins should give a reading with the engine running). Disconnect the O2 sensor - dwell reading should go to 50% (with engine warm). Reconnect the O2 sensor - dwell reading should fluctuate, but should be something like 40-60%. If it is close to 30% (perhaps jumping back to 50% occasionally), the idle mixture is probably set too rich and the feedback system is working as hard as it can to lean the mixture. Try leaning the misture and watching to see if the dwell goes toward 50%. If it stays steady at 50%, something is wrong with the feedback system, possibly a bad O2 sensor. If it is steady and high (I think 85% for your year) it is in enrichment mode. This is turned on by the throttle sensor at WOT, or by the temp switch on the right chain cover below 15 C (if you have one). Disconnect this switch and it should go into feedback if the switch is bad. There are many other things that can be wrong, but this should tell you where the problem lies with the mixture or the O2 system.

[wswilburn]

More info:

Idle speed should be about 950 rpm.

You can also check O2 sensor operation by disconnecting and measuring voltage from sensor. Should be about 1 V if rich. Creating a vacuum leak (remove oil filler cap) will make mixture lean - voltage should drop to near 0 V.

[nixter]

Thanks for the detailed info Scott!

Your are talking slightly over my head however. I'm very mechanically inclined and have been building and repairing all sorts of things my whole life. I'll admit though combustion engines are a new thing for me to tackle so please bare with me. I've learned tons here and in the Rennlist forums in the last few weeks and I believe I can figure most things out if I ask enough questions!

Seeing as the car is new to me and I'm new to car engines, where might I find said sensor? The last guy told me it's attached to the CAT. Fine. Where's that?

I have a multimeter but no dwell meter so I'll try your second method, unless dwell meters are cheap then I might tackle your first suggestion.

Seems my idle is fine then (~950). What are performance symptoms, if any, of a lean mixture?

Thanks again,

Nikolai

[tomt]

I had similar symptoms, which took a chain of events to solve:

1) The alternater was overcharging, which was confusing the DME and as a result the AFM (hosed my battery too).
2) The AFM was suspected after this, but then a leaky exhaust gasket was located (one way to check for this is to pull plugs at idle and see if anything changes -- could point to other problems too)
3) Once the gasket was replaced, it really ran like he**, and finally AFM was replaced.

Now it runs great! Luckily I get to take my car to JW's place!!

So, maybe check for leaking exhaust gaskets and see if you can swap in a different AFM for a try.

Before this whole process I changed my O2 sensor too, but obviously that wasn't my problem.
Rgds,
TT

[fireant911]

Nix,
Take a look at the link below:

http://www.pelicanparts.com/911/technical_specs/911_cis_troubleshoot.htm

This may help guide you in the proper direction. I had previously experienced terrible fuel mileage with my 911SC from the day I had bought it. The previous owner had also been unsuccessful in correcting this issue. After many months of experimentation, it was determined that my problem was simply that the Oxygen Sensor wiring connector (above the intake valve cover on the left side of the engine compartment) was grounding off to itself - this is a shielded wire with one wire inside another separated with insulation. A missing or broken connector is a common event though you yourself can correct this situation with a soldering iron, electrical shrink wrap, and patience. Your actual Oxygen Sensor is located just behind the left rear wheel and is pointing upward with a single wire leading through a grommet into the engine bay.

Check the voltage with the Oxygen Sensor disconnected with the engine running after you have allowed the engine temperture to come up to normal (I think the unheated probes we have in the SC's have to reach 600°F before accurate voltages are generated by the sensor). You should be receiving a steady, nonfluctuating signal. Now reconnect the sensor connection and check the voltages (you may have to use a small wire gently pressed into the connection that will allow you to obtain this reading). The voltage should be changing, actually it is changing quicker than the meter can represent but I think if the Lambda system is operating correctly it is making changes from rich to lean, rich to lean about 20 times per second.

My car is running great and consistantly acheiving 23 mpg in a combination of city and highway driving. Be aware that the 1984 and later cars had a much different system as did the cars prior to 1980. I hope you find that this helps.

[wswilburn]

I would start with the O2 sensor then as Daryl suggests. If you follow the wire up from through the sheet metal, you should find the connector in the engine bay. I don't know if dwell meters are expensive, I borrow a friend's oscilloscope for this job.

[dickster]

nixter,

you'll get all the info you ask for here (and some you didn't) but it sounds like you need to get hold of a decent manual - get yourself "bentleys".

good luck.