steve_p

My euro 87 S4 Auto is due it's annual test- last time, my friendly tester told me to sort out the emissions for next year, since they were marginal- he suggested the O2 sensor might need changing. He's not a 928 guy, he mostly does restorations of pre-80s classics so I guess doesn't fix electronic faults much, but it seems a good place to start.

I bought a 3 wire sensor (without looking under the car first,) but today I got under there and can't find any sensors in the exhaust at all!

The 2 downpipes each has a small tapped hole (I'd say an 8 or 10mm thread) which seem to be blocked off with a set screw, then there's a box that looks just like a muffler to me, then the pipes go off to the rear. I cant see or feel anything on top of the middle box.

Do '87 euro cars have O2 sensors and cats? Has my exhaust been modified?

If not, what are the common causes of an emissions failure? It does smell of fuel from the tailpipe when running, but maybe that's normal. It seems to burn a lot of fuel, but I have nothing to compare to (apart from my 1.9L DCI diesel van which does about 600 miles on a tank, which isn't really a fair comparison for a 5L V8..) I did once try to work the mileage out, I think I was getting about 20mpg on a long run and 10mpg around town..

It runs and drives fine, by the way.

Hilton

You're in the UK if I recall, in which case, no your car doesn't have an O2 sensor fitted as stock. Nor does it have cats

Your emissions test for the MOT just tests %CO, %CO2 and ppm of hydrocarbons.

The UK 928 models didn't get cats and O2 sensors until 1990 model year.

Instead, the workshop manual outlines how to adjust the CO - basically you go to a garage with an exhaust gas analyser, have them stick the sniffer in the tail pipe, and adjust the CO potentiometer until the CO reading is at factory specs (1% +/- 0.5%). This is to calibrate the ECU to the MAF's voltage output as the MAF ages and tends towards lean. If you've replaced the MAF and not adjusted the pot, chances are you're running rich.

The CO pot is attached to the right-end of the ABS ecu - which is the big horizontal white connector that sits above the fuse panel. The pot is a small black fitting with a Bosch 3-pin connector on it, and a flat-head screw inset in the end of it. Turn clockwise for richer (more CO), CCW for leaner.

If you can't get it to 1% CO after many turns, your MAF needs replacing at which point you should wind the pot all the way back to 382 ohms (you can measure resistance between pins 1 and 2 on the pot).

Hilton

Here's a pic of the pot in my ex-UK 87.

Yes it's unplugged - I've converted the car to use an O2 loop and cats (part of the Aussie import requirements, as Australia mandated cats from '86), so the pot is doing nothing.

(the additional ground wire is the one I bolted to the ground point behind the panel for my WBO2 when sharktuning).

Hilton

(I've been meaning to post this for a while.. this thread is as good a place as any - hopefully I'll hit all the right keywords for search as I go )

Adding the O2 loop to a non-cat (open loop) 928 S4 / GT is pretty easy, even if you don't install catalytic converters. The items needed are:

1. LH 2.3 Coding plug for a 928 with O2 loop, matching your gearbox type (I bought a new one - they're pretty cheap)
2. O2 sensor with factory plug on it (its a sureseal 3-pin connector - if you've got a generic 3-wire sensor the terminals and the plug can be bought from RS/Newark/Farnell etc.)
3. Weld-in sensor bung (altho' if you've got an X-pipe you may well already have one or more bungs - the Ritech X available in the UK includes a bung in the centre of the merge, and the Motorsport one Roger sells includes 2 although I'll be adding a 3rd in the merge soon). Note the factory exhaust is stainless steel so you probably want a stainless bung
4. 20mm hole saw to make the hole in the floor pan

If you're installing factory cats then obviously they have a sensor bung already too.

To add the O2 closed loop to the car:

1. Get an exhaust shop to weld the bung to the front muffler somewhere before the muffler box itself.
2. Drill a 20mm hole in the floor pan - then sand/file the sharp edge a bit and prime and paint to prevent rust. (see photo below for the correct location - the red car is where I drilled, the other one is a factory hole in an Aussie 87)
3. Install the O2 sensor in the exhaust and pass the plug through the hole into the cabin - shove the grommet into position
4. Disconnect the battery
5. Unplug the potentiometer pictured above
6. Unplug the bridge from the end of the O2 sensor connector (non-cat cars bridge the signal to the ground pin).
7. Plug in the O2 sensor to the 3-pin sureseal connector
8. Replace the coding plug (tucked behind the ECU's on the frame they're mounted to)
9. Reconnect the battery, and start the car.

Depending on the age of your MAF it may take a few minutes/miles for the idle to settle. Once the car's adapted, based on my cars, the idle air/fuel ratio will waver between about 14.5 and 14.9 in closed loop (techedge 3A2 WBO2 sensor with LSU 4.2 Bosch 7200 sensor).

After the above, you don't need to worry about adjusting idle mixture periodically as the MAF ages (its getting increasingly hard in Oz to find workshops with a gas analyser that is actually working and calibrated).

(off to snap photos of the various parts to add to this post).

Pics are:

Factory O2 connector with the non-cat bridge plugged into it
Factory O2 connector with nothing in it
Where I drilled the hole on my '87 (currently has WBO2 installed)
Factory O2 hole in RHD car from inside
Factory O2 hole in RHD car from outside

steve_p

I'm gonna give up on this forum, it took a full 15 minutes to get a comprehensive answer to my question from someone on the opposite side of the world!

Cheers for that, I have my car up on ramps ready to sort out the sensor, looks like I can drop it back down and take it straight for the MOT with a little screwdriver in my back pocket...

Is there a good reason to add the O2 loop if not required by law?

17prospective buyer

Adding the O2 loop is good because it's a feedback ECM input, ECM feedback will mean you will start to have some adaptive control for the engine, the engine should run better because the ECM will compare the output of a feedback sensor (O2) and compare that reading to it's lookup tables. If feedback and "what it should be" don't correspond in the ECM, it will adjust mixture until it does correspond with the feedback it is getting from the O2 sensor. Electronic auto transmissions are the same way. Before you would have to adjust bands as they wore down, now they have feedback inputs to the PCM/ECM so they can compensate for wear over time, and shift gears based on how the driver drives their car.

I would think that the motor would also run better at higher altitudes with a feedback sensor such as an O2, since it will be able to adapt, right guys? Since our cars use just an MAF and not a MAP/speed density system where barometric pressure is calculated at KOEO (key on engine off).

Adaptive control on LH-Jetronic/L-Jetronic etc. is pretty limited. I think that was back when feedback inputs were starting to be introduced for emissions reasons.

SeanR

Hard to answer correctly when we have no clue on your location.

steve_p

Sorry, I'm in the UK as Hilton correctly said. I added my location to my user details and assumed it would show alongide my name.

Thanks for the O2 loop info everyone, I'll consider adding it to my car after reading Hilton's explanation.

Hilton

Pics added to my earlier post.

The only bit not photo'd is the coding plug - the correct one for gearbox etc. can be found in PET and ordered from your friendly 928 parts supplying forum sponsor.