chode

Hi all,

I have a 95 OBD1 C2 with cat bypass and no o2 sensors. As a result I have a permanent CEL and the flash code is 1124 (oxgen sensor shorted) yet the car never goes into 'limp home' mode and seems to drive great.

As the car is in the middle east I've been considering installing a potentiometer in place of the o2 sensor as shown on the m150 wiring diagram and in adrian streathers book. This would clear the CEL and save me the hassle of trying to drill and install an o2 sensor.

I know on other cars this can cause them to run too rich or lean as the car is constantly trying to adjust the reading from the potentiometer with no effects so the ecu keeps over compensating.

Does anyone know if this would be an issue? Do option m150 cars have a re-flashed ecu to deal with the pot correctly? And finally what 'k' potentiometer would be required?

Thanks!
Scott

Macca

Hi. I can offer little help. My car is featured in the pictures in Adrians book and is factory M150. I believe (but cannot be sure) that my cat is hollow. I intend to fit Dach X pipe soon so will weigh and confirm. I can confirm that I get no CEL from my mods and I believe the ECU mapping is fairly standard as Ive had bth Steve Wong and Steve Weiner build chips for me which Ive run for many years successfully with no issue. I am currently running RS LWFC set up with RSR mufflers and RS pulley with no issues. Good luck and I hope you find your answer - I would think backdating to CO2 potentiometer would require appropriate ECU chip.

Cheers
M

chode

Hi Mark, thanks for the help!

If you believe that the ECU is fairly standard then im very keen to try installing a potentiometer in place of the O2 sensor (which I can set using an exhaust gas analyser). Hopefully I won't need to backdate the ECU chip - if you are running different chips still without an O2 sensor I assume I should be fine too.

Whether your cat is hollow or not isn't really important as at the end of the day we both dont have any O2 sensors. Only question is what rating potentiometer I would need?

Scott

jscott82

I’m not an ECU expert so maybe others will correct me, but here is my 2 cents...
Under normal conditions (not wide open throttle) The ECU in our cars works in a feedback loop with the O2 and MAP sensors to adjust and refine the fuel mapping to achieve the best efficiency. If you take the o2 sensor out of the loop and replace with a fixed potentiometer you can make the CEL light go away and probably make the engine run "ok" but will never be exactly "right", A fixed pot would only be valid for the exact conditions when it was set (temp, altitude, etc).

The OBD1 ECU does not care about the cat (or lack therof). Its only the OBD2 ECU that has sensors before and after the cat to check for its presence

All that being said, a decent muffler shop should be able to weld in a bung for the existing O2 sensor for cheap.

jscott82

One more thought.... The whole beauty of EFI is its ability to self adjust to the conditions at hand and provide smooth running efficient power at all times..... If you are going to get rid of the O2 sensor, you could probably make some $$ by selling off the whole EFI setup and replace with carbs. Under set conditions, in some cases, carbs can be better.

chode

You make a good point. I guess my initial feeling was that having a potentiometer installed would be better than my existing setup of no sensor or potentiometer at all. Without an O2 sensor it suggests that my EFI is barely functioning - in which case I'm really surprised that the engine is running as well as it is?

Out of curiosity what are the benefits of carbs? I know they look great and have noticed that ninemeister have fitted carbs to the 460 HP RSR!

jscott82

We are talking small differences here. The static mapping that the ECU defaults to isnt too bad. I couldnt tell a difference when my O2 failed. I suspect if the ambient temp were very hot (or cold) or you were at altitude the effect would be much more noticeable.

I guess my point though, a muffler shop could weld in a bung in 20 minutes and cost less than a meal. why not put the O2 back in and save the trouble?

Honestly I dont know why carbs are considered "better" but that is what I have heard that from several reputable folks. For a race car (single set of conditions, WOT all the time) that is willing to be tuned day by day, carbs can offer better performance that EFI. Personally I would always go EFI, carbs require magic to get to work right .

Garth S

Please correct me should I have missed the point ... but an O2 sensor generates a voltage, the value of which cycles from 0.1v - 0.9v, with 0.45V pretty close for a correct stoichiometric air:fuel mix on cruise .... or lambda = 1.

As a potentiometer attenuates a voltage - and cannot create one .... what signal is it attenuating if no O2 sensor is present? It cannot replace the O2 sensor, as the DME is expecting a generated voltage input from the sensor.

I have seen resistors/potentiometers used to alter the effective resistance of various temperature sensors ( RTD's, thermocouples, etc), as they are themselves variable resistors and attenuate a voltage sent from the DME: certain of these temp sensors can be part of a fuel enrichment loop .... ie., trick the controlled DME enrichment.

Without an O2 sensor input, the DME must be reacting to preprogramed air:fuel maps .... just as it does with more than 2/3 throttle where a:f enriches from ~ 14.7:1 to 12.5:1 .... or in an open vs. closed loop manner.

trophy

These are ITB's running Motec EFI not carbs. I believe this setup runs dual wideband O2 sensors as well.

Macca

I was under the impression this whole OBD2 and 02 etc was simply to meet international emmissions control largely driven from US and Europe. Before OBD2 in the 964 and previous generation cars there was no 02 sensor before and after and the cars ran just fine. M150 option with potentiometer was uses for 993 cup and super cup cars in 1993/1994/1995 etc with no issues on circuits both at high altitude in South Africa and low altitute such as Malaysia. I think its a red herring. If this "map" were so important why is my car running so well at a variety of temps and altitudes? Last time we put it on the rollers (a long time ago I admit) is was running much better figures than many other 993s.

I dont think there is an easy answer to this question....

Cheers
M

jscott82

964 had an O2 sensor as do the 993 cup cars. To my knowledge, O2 sensors have been part of EFI from the start . What changed with OBD2 (in 1996) was the addition of two more O2 sensors after the catalytic converter to monitor the effectiveness of the cat. That way the computer could notify you if it failed or needed replacement.

Again, I’m not an expert, but here is what I (think) know…

When I say map, here is what I mean… Think of a very complex (multi-dimensional, actually) spreadsheet. In an oversimplified form think of a spreadsheet with RPM at the top and airflow down the side. For any particular condition, the ECU looks up the two values and determines how much fuel to squirt.

A street ECU works in two modes (many more actually), Wide Open Throttle (WOT) and not-WOT. In WOT, efficiency be damned, the ECU is going for peak power and does not have time to “figure” out efficiency so it does not use the O2 sensor it uses a static mapping. This is why any car will dyno the same regardless of o2 sensor. The difference is in cruise mode where, power be damned, we are going for peak efficiency. Here the ECU switches to a map that includes the O2 sensor and whose goal is to get as close to 14.7/1 as possible. The ECU will lookup a value from the map (guess) then tweak it until AFR comes exactly in line. When cruising on the street, this is exactly what you want, peak efficiency, makes the engine run cooler, less fouling, better throttle response, not to mention mileage.

If you put a potentiometer in place of the O2 and set it so that it reads like 14.7/1 has been achieved, the ECU will make its guess based on the map, and the “guess” will always be right. So is it terrible to have a potentiometer in place of the O2 sensor… No…. but the ECU can do better if it has the proper information being fed to it. Where the pot may help, is if you have done some serious tweaking of the motor and cant (for whatever reason) reprogram the ECU. In this case, you can trim the pot so the ECU always thinks its lean and will add more fuel to try to richen it up…. Again this trick will only work in cruise mode, WOT the ECU will still go back to its factory static map.

Just my two cents…. hope it helps

Macca

Hi Jscott.

Agree with your thoughts. Sorry - I meant before OBD2 there were no before AND after o2. The second after 02 is for emmissions mainly I believe. Ive quizzed my OPC tech a number of times on my potentiometer and if it needs adjusting and he seems to think its all just fine (probably tained in post aircooled cars)...

Cheers
M