Swapped o2 sensor makes car feel slower

[smokeyyank]

So as the title says I swapped my o2 sensor and now the car feels like it lost power. I just bought it and finally finished replacing a bunch of stuff. Its a 88 325i 5 speed vert. I took it out the other day and it ran great (at least better than when I bought it) that same day I went to get my emissions and it failed......and failed miserably. My NOX was fine but HC and CO where waaaayyyyyy off. The car smelt like it was running rich but not terrible. The CEL was on and I tried the stomp but could never get it to work. I had read that some of the older ecus couldnt do it and it would only flash when you turned the key to on. Mine did blink 2 blinks which I had read was the o2 sensor. So I swapped the o2 sensor and now I passed emissions but the car just doesnt feel the same. It used to just pull wonderfully, now it still does but just feels like it is holding back. I fumbled through some more post and was reading that some chips will just make the car run rich in order to enchance performance. So I was thinking could the old sensor have just made the car run rich and feel like it had more go? Thanks

[jlevie]

Did you use an OE or OEM sensor? Those are the only ones I'd trust. Even if the O2 sensor is working properly it will take time for the DME to learn new fuel trims.

The stomp test will work on your car if the TPS is sending idle and WOT signals to the DME. Test for that by removing the DME connector back shell and probing the pins with a DMM with the ignition on. A failure to see the idle switch signal will be a bad TPS, misadjusted TPS or idle stop, or an engine harness problem. A failure to see the WOT switch signal will be a bad TPS or engine harness problem. Rarely one may see a bad DME.

Note that the check engine light must be operable (comes on with ignition and goes off when the engine starts) for the stomp test to work.

I suspect that you have more problems than just the O2 sensor. A pretty complete write up of possible problems is below. It was primarily written to address starting problems. but it also applies to engine management problems in general.

Idle or hard starting problems are most commonly caused by intake leaks and/or
a sticky or defective Idle Control Valve (ICV). The only reliable method of
locating intake leaks is to have a smoke test run on the intake and crank case
and to test the brake booster with a gage and vacuum pump. The complete list
of possible causes of an intake leak is:

Intake boot
Throttle body gasket
ICV hoses & connections
Brake booster, hoses, and connections
Crank case breather hose
Evaporative control hoses, valve, and expansion tank
Fuel pressure regulator & hose
Injector seals
Valve cover gaskets & bungs
Oil filler cap
Dip stick o-rings
Oil return tube o-rings
Pan gasket or oil level sensor gasket

While leaks in some of those can be found by inspection or by spraying carb
cleaner on suspect areas, not finding leaks that way doesn't eliminate the
possibility. Only a properly executed smoke test using the right equipment will
really work. That means plugging the exhaust, replacing the AFM with a plug containing a nipple for smoke injection, and using a professional smoke machine that uses oil for long persistence smoke. Then you pressurize the intake for about 10 minutes at 2-4psi and watch for smoke. Note that if there is a major leak, it must be repaired and the test repeated.

Once the possibility of intake leaks is eliminated, the ICV needs to be
removed and cleaned with carb cleaner until the vane inside moves freely. When
the ignition is switched on you should be able to feel vibration from the
ICV. If no vibration the ICV is bad, there's a problem with its wiring or
connector, there's a problem with the TPS, or the DME (or Idle Control Module
(ICM) on an ETA car).

For the DME (or ICM) to control idle, the idle switch in the TPS must work
correctly. The switch should close when the throttle stop is 0.030-0.060" off
the idle stop screw. Hearing a click doesn't mean the TPS is working. Check
with a meter or test light by removing the back shell on connector and checking for the signals at the DME (or ICM) with the ignition on.

On an ETA car the cold start valve and it’s associated sensors must be functional.

The fuel system should be tested via the suite of tests in the Bentley manual
as invalid rail pressure can be a contributor to idle and starting problems. A
simple injector check is to pull the injectors, jumper the fuel relay to run
the pump, and see if the injectors are leaking. You can also point the
injectors into a towel, remove the coil wire, and crank the engine to see if
all of the injectors appear to be spraying in a similar fashion. The best
approach to possible injector problems is to have the injectors cleaned and
flow tested. Since raw fuel can or will be released in these tests, have a
fire extinguisher handy. I use RC Engineering for cleaning, rebuilding, and flow testing. I will always do the injectors on an an engine with 100k or more one it. A noid light will prove that each injector is seeing a firing signal.

While a bad check valve in the high pressure pump can result in longer than
normal cranking, if the fuel system is working as it is supposed to the rail
will reach normal pressure in a few turns of the engine. A weak pump, clogged
filter or leaking FPR in conjunction with a failed check valve can result in
longer cranking and/or idle problems.

The O2 sensor can be a contributor to idle problems. The O2 sensor is a
scheduled maintenance item with a useful life of no more than 100k. If the
sensor has that mileage or more (or is of unknown age), replace it.

The ignition system can be contributor. Unless recently replaced with OE or OEM parts, install a new distributor cap, rotor, and wires. Use only OE or OEM parts. The third party (cheaper) stuff is an iffy proposition. Use NGK ZGR5A plugs as they are the closest to the original plugs that are NLA. If in doubt, replace the ignition coil with a new OE or OEM part.

The AFM can be a contributor. If the vane doesn't move freely or the
resistance track is worn the DME may be receiving invalid data from the
AFM. And if someone has fiddled with the bypass air adjustment the DME may be
unable to stabilize idle. The bypass air adjustment should only be adjusted
per the procedure in the Bentley and with an exhaust gas analyzer. And even
then everything else associated with engine management has to first be
operating properly. If the AFM becomes a suspect, replacement with a good used
unit is the best approach.

Improperly adjusted or malfunctioning valves will affect idle and starting. As
can compression issues from ring or cylinder wear or from the valves. A valve adjustment is called for every 15k. A useful diagnostic is to run compression (dry and wet)and leak down tests on the engine. Those tests will tell if the rings and valves are in good condition and the leak dow test will usually indicate whether it is a cylinder or valve problem.

Although not usually a problem, a bad DME temp sensor is a possibility. That
generally won't cause an unstable idle, but can cause hard cold or hard hot
starts and/or a rough idle. As can problems with the timing reference
sensors. Although not commonly encountered, a bad harmonic balancer on an
M20B25 or M30 engine will cause problems.

When all other possibilities have been eliminated and idle or starting
problems persist, replacement of the DME, or if applicable the ICM, is
indicated.

In many cases the cause of problems will be a combination of factors. So it is
important to test and repair all of the possible causes.

[smokeyyank]

Did you use an OE or OEM sensor? Those are the only ones I'd trust. Even if the O2 sensor is working properly it will take time for the DME to learn new fuel trims.

The stomp test will work on your car if the TPS is sending idle and WOT signals to the DME. Test for that by removing the DME connector back shell and probing the pins with a DMM with the ignition on. A failure to see the idle switch signal will be a bad TPS, misadjusted TPS or idle stop, or an engine harness problem. A failure to see the WOT switch signal will be a bad TPS or engine harness problem. Rarely one may see a bad DME.

Note that the check engine light must be operable (comes on with ignition and goes off when the engine starts) for the stomp test to work.

I suspect that you have more problems than just the O2 sensor. A pretty complete write up of possible problems is below. It was primarily written to address starting problems. but it also applies to engine management problems in general.

Idle or hard starting problems are most commonly caused by intake leaks and/or
a sticky or defective Idle Control Valve (ICV). The only reliable method of
locating intake leaks is to have a smoke test run on the intake and crank case
and to test the brake booster with a gage and vacuum pump. The complete list
of possible causes of an intake leak is:

Intake boot
Throttle body gasket
ICV hoses & connections
Brake booster, hoses, and connections
Crank case breather hose
Evaporative control hoses, valve, and expansion tank
Fuel pressure regulator & hose
Injector seals
Valve cover gaskets & bungs
Oil filler cap
Dip stick o-rings
Oil return tube o-rings
Pan gasket or oil level sensor gasket

While leaks in some of those can be found by inspection or by spraying carb
cleaner on suspect areas, not finding leaks that way doesn't eliminate the
possibility. Only a properly executed smoke test using the right equipment will
really work. That means plugging the exhaust, replacing the AFM with a plug containing a nipple for smoke injection, and using a professional smoke machine that uses oil for long persistence smoke. Then you pressurize the intake for about 10 minutes at 2-4psi and watch for smoke. Note that if there is a major leak, it must be repaired and the test repeated.

Once the possibility of intake leaks is eliminated, the ICV needs to be
removed and cleaned with carb cleaner until the vane inside moves freely. When
the ignition is switched on you should be able to feel vibration from the
ICV. If no vibration the ICV is bad, there's a problem with its wiring or
connector, there's a problem with the TPS, or the DME (or Idle Control Module
(ICM) on an ETA car).

For the DME (or ICM) to control idle, the idle switch in the TPS must work
correctly. The switch should close when the throttle stop is 0.030-0.060" off
the idle stop screw. Hearing a click doesn't mean the TPS is working. Check
with a meter or test light by removing the back shell on connector and checking for the signals at the DME (or ICM) with the ignition on.

On an ETA car the cold start valve and it’s associated sensors must be functional.

The fuel system should be tested via the suite of tests in the Bentley manual
as invalid rail pressure can be a contributor to idle and starting problems. A
simple injector check is to pull the injectors, jumper the fuel relay to run
the pump, and see if the injectors are leaking. You can also point the
injectors into a towel, remove the coil wire, and crank the engine to see if
all of the injectors appear to be spraying in a similar fashion. The best
approach to possible injector problems is to have the injectors cleaned and
flow tested. Since raw fuel can or will be released in these tests, have a
fire extinguisher handy. I use RC Engineering for cleaning, rebuilding, and flow testing. I will always do the injectors on an an engine with 100k or more one it. A noid light will prove that each injector is seeing a firing signal.

While a bad check valve in the high pressure pump can result in longer than
normal cranking, if the fuel system is working as it is supposed to the rail
will reach normal pressure in a few turns of the engine. A weak pump, clogged
filter or leaking FPR in conjunction with a failed check valve can result in
longer cranking and/or idle problems.

The O2 sensor can be a contributor to idle problems. The O2 sensor is a
scheduled maintenance item with a useful life of no more than 100k. If the
sensor has that mileage or more (or is of unknown age), replace it.

The ignition system can be contributor. Unless recently replaced with OE or OEM parts, install a new distributor cap, rotor, and wires. Use only OE or OEM parts. The third party (cheaper) stuff is an iffy proposition. Use NGK ZGR5A plugs as they are the closest to the original plugs that are NLA. If in doubt, replace the ignition coil with a new OE or OEM part.

The AFM can be a contributor. If the vane doesn't move freely or the
resistance track is worn the DME may be receiving invalid data from the
AFM. And if someone has fiddled with the bypass air adjustment the DME may be
unable to stabilize idle. The bypass air adjustment should only be adjusted
per the procedure in the Bentley and with an exhaust gas analyzer. And even
then everything else associated with engine management has to first be
operating properly. If the AFM becomes a suspect, replacement with a good used
unit is the best approach.

Improperly adjusted or malfunctioning valves will affect idle and starting. As
can compression issues from ring or cylinder wear or from the valves. A valve adjustment is called for every 15k. A useful diagnostic is to run compression (dry and wet)and leak down tests on the engine. Those tests will tell if the rings and valves are in good condition and the leak dow test will usually indicate whether it is a cylinder or valve problem.

Although not usually a problem, a bad DME temp sensor is a possibility. That
generally won't cause an unstable idle, but can cause hard cold or hard hot
starts and/or a rough idle. As can problems with the timing reference
sensors. Although not commonly encountered, a bad harmonic balancer on an
M20B25 or M30 engine will cause problems.

When all other possibilities have been eliminated and idle or starting
problems persist, replacement of the DME, or if applicable the ICM, is
indicated.

In many cases the cause of problems will be a combination of factors. So it is
important to test and repair all of the possible causes.

Thanks for the reply, I did replace it with a bosch OEM sensor. I already replaced majority of what you suggested as intake leak as well as new injectors. It idles, runs and starts fine just felt a loss of power after swapping the o2 sensor.

[jlevie]

Check the WOT switch in the TPS. As I recall it should activate at about 40% of throttle. When it closes the DME ignores O2 sensor data and uses built in maps adjusted by learned fuel trim. The AFM is ignored by the DME above 4500rpm and fuel is determines solely by engine temperature and rpm. Again using the built in maps adjusted by learned fuel trim.

It may be that you just haven't driven the car long enough for the long term fuel trim to be correctly set. But do check the WOT switch in the TPS.

[smokeyyank]

Alright thanks for the reply. I had wanted to check my TPS as I was thinking it could have been an issue from before i started swapping parts out but hadn't gotten to it yet. I will drive the car this week and see if it starts to feel back to normal. If not I'll check it out.