512TR: Pros/cons removing 02 sensors with Test pipes/exhaust

From my limited understanding of how Motronic systems work, you need the lamda sensor to monitor the air/fuel to get the best performance from the motor. Without the sensor data, the computer works from a chart that is based on the stock muffler. I thinkthe emissions requirements are not checked above 3000 rpm so the system uses "chart" values above 3k rpm. Most of the time motors are below 3k and to be safe, I would think the system would be rich although with a free flow exhaust, the system might be leaner?! Anyway, the best way to check would be take it to a dyno. On the other hand, it is easy to tig weld a threaded plug (18x1.5 i think) onto the test pipes and install the lamda sensor and even the thermocouple. Modern fuel injection systems need to sense the dithering signal from the lamda sensor to make the best power and not burn valves, maybe holes in pistons or fowl spark plugs. My 348 had a bad fuel rail vacuum pressure sensor and the system was so rich the cat converter started to melt. Using the sensors is cheap insurance. YMMV

Jeff Pintler
89 348tb, 86tr

 

The O2 sensors are before the main cats, so I would leave them in. Unlike OBD2 which adds sensors after the cats (and in many other areas) the O2 sensors here just help the engine run with the best mixture, as Jeff describes. You will not get more power, and you will not run as clean either. Just my 1.5 cents.

 

Leave the O2 sensors, you need them.

 

Your slow down light is triggered by the cat fire sensors (thermocouple) and do not impact the running condition of the car. The 02 sensors, on the other hand, control the fuel condition of the car via sampling of the exhaust gas.

If the sensors that were disconnected turned off the light, then the thermocouples were un plugged and the O2 sensors are still active, just not reading.

I too have a 512TR, and removed the cats, welded in the O2 plug fitting. I then took the thermocouple sensors and wire tied them to the frame tubes behing the rear wheels.

You probably got the slow down light because the thermcouple sensor was laying directly on the exhaust pipe.

I concur with everyone else, it is critical to have the 02 sensor tapped into the exhaust gas stream. The thermocouples you can live without, but without the O2 sensors you will run very rich, likely foul plugs over time.

RP

 

Your 512TR probably has Motronic 2.7 ecus. In that case, your O2 sensors are ignored under two conditions: cold engine and Wide Open Throttle (WOT) at any temp.

There's little incentive to disconnect O2 sensors under M2.7. You might want to disconnect them momentarily as part of a diagnostic test (in which case, M2.7 will start "guessing" at the correct air/fuel ratio by using its onboard fuel tables).

Well, that test will either tell you to change out bad O2 sensors or else that your problem is somewhere else.

With a cold engine at any throttle load, or a full throttle at any engine temperature, you aren't going to be able to tell the difference in having or not having working, connected O2 sensors. This information should help you decide if your driving experience hints that you need or don't need new O2 sensors (e.g. bad O2 sensors typically start surging or lugging your speed a bit when the engine is warm and you are trying to maintain a constant speed).

But with a warm engine at idle or partial throttle, having no O2 sensors connected/working is going to put your car into the guessing game. If your car guesses too lean (could burn your valves) or too rich (could burn up a cat, except, your cats are gone - or could foul your spark plugs) then you will have maintenance issues due to disconnecting said O2 sensors.

You won't gain more power, and you won't fix problems by disconnecting O2's. Disconnecting O2's is useful as a diagnostic test and that's about it.

 

First off, get the old o2 sensors back from these so called Ferrari mechanics.....The bosch replacements are about $250.00 each.

You need the sensors, your problem is not with your o2 sensors. I have seen you post elsewhere about your ongoing problem.

When the problem started were you popping thru your exhaust on de acceleration? And problems with running on cold start?

If so, sounds more like a fuel balance and adjustment.

 

Well, plug them back in to see if it still runs ok.

As others suggest, they have nothing to do with your slow down light.

Check plug wires, coil wires.........because it sounds like something that just started happening, not an ongoing issue.

I purchased an 87 TR that had many of the same issues, I am at the end (hopefully) of sorting them out, some of which were, bad coil wires, melted precats, melted cat, bad O2 sensor, unplugged vacuum lines, fuel mixture off, balance off................it only takes one tiny thing to mess everything up with these cars.

 

Remember 02 sensor (Lambda) is nothing but a temp sensor, it sense the temperature of the exhaust before or after the cat or sometimes both to determine if it's rich (cooler side) or lean (hotter side)

 

Sounds like a fuel mixture problem.

Did this condition just start to happen? Or was it like that when you purchased the car? Or did it start when you put the exhaust on?

 

Sounds like mistimed cams and/or a sealing problem with exhaust manifold/valves to me.

For instance, a hole in an exhaust header could allow outside air into your exhaust stream. Well, an "outside exhaust air leak" could be your problem.

The extra Oxygen from allowing outside air into your exhaust would be detected by the O2 sensor, which would tell the computer on that side of your engine to add more fuel, which would make that side (in reality) too rich, which would in turn heat up a cat and trigger a cat over temp warning, which the computer would then use to shut down half of your engine.

Disconnecting your O2 sensors in that case would cause your engine to run better because the factory-stored fuel tables would be closer to what your engine wanted in that case than would the feedback from the O2 sensors (because they would be getting outside air that the engine itself never saw).


So before I pulled the engine looking at cam timing, I'd look for an exhaust leak or a burned valve problem.

 

Agreed, something is making it run rich, could be outside air, but if the same mechanics who just pull the O2 sensors to fix the problem are the same who did the engine service, it very well could be a multitude of things.

If you are DIY guy, you could probably start with some simple basic diagnoses.

Ferrari chat is a great way to get very helpful advice, but if it were me, I would take it back and make them repair it. Or maybe someplace that can repair it correctly.

 

After reading most of the other posts, you probably know that the sensors are needed. Any "mechanic" that disconnects sensors to fix a problem shouldn't repair your car. Do some research on Lamda sensors (they sense oxygen levels) and thermocouples (sense temperatures) and buy some books on Bosch fuel injection systems. We all started exactly where you are at right now. Be careful of some mechanics and their advice. There is lots of information on trouble shooting the "slow down warning lamps" in the archives.

Jeff Pintler
89 348tb, 86 tr

 

Except of course, the ecus *Are* adjusting to the new air flow with your O2's disconnected...which is to say, what they told you makes no sense.

It's probably a bit like when you were back in High School on a hot date and you would tell the girl just about anything to get into her pants at the moment. Well, people still do that long after they grow up...on a variety of things besides dating (still amounts to a screw job though).

Why are the O2's causing the engine to run badly? O2's tell the ecus to lean or enrich the fuel supply to your engine. If your O2's are causing your engine to run badly, then they are *erroneously* signaling your ecus.

For instance, if your spark plug wires are cracked...only making good contact at the higher voltages seen at high rpms, then you are going to have some cylinders that aren't always getting a good spark...so they are dumping extra fuel into your exhaust...which your O2's would interpret as running too rich...so your O2's would tell your ecus to lean the mixture to your engine...except, the cylinders that *are* getting a good spark would see that diminished fuel supply as a bad thing.

...And it would go the other way, with O2's telling the ecus to add fuel, if outside air was entering your engine's exhaust stream via an air leak...which would again cuase a poor running engine.

Thus, the O2's and the poor running engine combine to tell you that you have an engine problem that isn't being addressed.

When you disconnect the O2's, then the extra fuel or air in your exhaust is being ignored. Your engine therefor runs decent (based on the stored "guesses" for air/fuel mixture that the factory placed in your ecu chips)...but the underlying problem is still there (i.e. your engine isn't getting a good spark, bad cam timing, or else outside air is entering your exhaust via a leak).

Right now you don't know if you have an exhaust leak or if you have a bad spark, and with your O2's disconnected, the worst of the symptoms of either problem will be masked somewhat (though bad spark problems and exhaust leaks *always* get worse over time, so the problem won't be masked forever).

 

If you are confident that your problem is limited to one side of your flat-12, then I'd think that pulling the 6 spark plugs on that side would be your first choice.

Examine those 6 for consistency. Post their pictures here. See the spark plug thread in the link in my signature below. Do those 6 plugs have identical wear qualities, or do you see one or two that are quite different (e.g. oil soaked)?

The answer to the above should help point you in the right direction re:spark or no spark problem.

Frankly, I'd do this yourself (for a variety of reasons). This is something that you can easily do in your own garage. Go pull the 6 spark plugs on your offending side. Photo them and post the pics here. Then go read my spark plug thread below.

You'll be amazed.