1991 325i should it have an o2 sensor? Rough idle? Help

[UniqueAutobodies]

Hi im currently 90% through an auto 2.5 to manual 2.7 conversion

Got everything wired plummed etc and the car runs well starts up fine hot or cold but i have a slight lumpyness to the idle

It has a constant good idle at 800rpm ish revs up fine no spluttering or misfire etc but if you put your hand on top of the engine it feels like a slight burbling in the engine and from the tailpipes sounds a little like a subaru (not a good noise lol)

I have new tps set up right, new icv working fine, afm seems fine, new plugs leads and coil, new coolant temp sensor

Ive read it could possibly be the o2 sensor but when i look at the car i cant see one on the exhaust or the port for it ( has new genuine bmw centre sections from manifold back) and where ive found the plug on the loom it has a snug fitted rubber plug over it like it was blanked off from factory?

Should my car have an o2 sensor and the previous owner has possibly removed or is one not applicable to my car?

Its a 1991 h reg 325i sport auto now has a 2.7 manual engine

Any help much appreciated

[pierce]

The loom plug for the o2 sensor is under the battery tray on the drivers side inner wing (I know your battery is in the boot), you should have a stopper screwed in if there was never a lambda fitted.

For further confirmation, If you pop down to your local dealership with you VIN, they will print out a spec sheet for you.

One of the options listed will be "de-cat" or something similar.

[BenHar]

The VIN would have to be off the doner car.

What ECU are you using?

Ben

[Brianmoooore]

The M20 ECUs happily substitute a default value for the lambda signal if none is connected (unlike those for the M40), so I doubt if this is the problem. More likely weak mixture caused by air leaks or underdriven/worn injectors.

[UniqueAutobodies]

The M20 ECUs happily substitute a default value for the lambda signal if none is connected (unlike those for the M40), so I doubt if this is the problem. More likely weak mixture caused by air leaks or underdriven/worn injectors.

I work at bmw main dealer in wilmslow so checked today and it shouldnt have an o2 sensor thankfully and yes the plug is caped off.

Not sure exactly what model the ecu is i can always check this and possibly try another i have somewhere as ive read you can have worn idle relays or something like that in the ecu????

Ive had a play around tonight with my neighbor (porsche mechanic) and soon realised it was running very rich, after setting up the tps and idle screw the other night we started again from scratch warming the car up and starting with the air flow meter then the tps then idle screw

Were pretty much there its alot better runs nicely at around 850-900rpm the throttle is very responsive from the slightest blip but still has a ever so slight burble every few seconds but without an o2 meter to check the co2s its pumping out of the exhaust we could be way off.

Any suggestions for air leak checks etc as ive put a new throttle body to inlet manifold gasket and new intake rubber ive checked there is suction on the breather from the fuel pressure reg and when removing the oil cap theres a drop in revs to show crack case pressure etc checked all breathers are tightened up

is there any sort of standard set up for the afm to start with as a bench mark, easiest way to check the injectors?


Thanks

[Brianmoooore]

Yes, there is a 'standard set up for the AFM' - that it left the factory with, after being set up on an engine in perfect order with a CO meter. This will be the correct setting, and should never require changing, but unfortunately your car has been around a long time, and there's s good chance that some muppet has decided to have a fiddle by now.
Unfortunately, it can only be reset by using a CO meter, and that requires the rest of the engine to be in perfect order, or the resulting setting will be incorrect.
All you can do is try to ensure everything else is exactly as it should be, then set the AFM bypass with a gas meter to around 1% CO.
BTW, it's CO you should be checking, not CO2. Completely different gas.