E30 M50B25nv Swap problems

[Rousan_e30]

Hello all,

I finished my M50nv Swap "e36 engine, 1991 was written on the back of the head" a couple of days Ago, been having problems since then.

the engine fired up pretty quick actually, but, the car or the engine for the matter of fact has been running like shit, excuse me for that.

it had like a symptoms of a misfire, the RPM was bouncing, car backfiring, and engine choking, in addition to a very big loss of power, considering going from an M20B20 to an M50B25nv.

what i changed actually was the spark plugs and the coilpack was working 100%.

i changed those and the car still had the problem.

The main symptoms are, Stuttering, engine choking, bouncing RPM, Engine hesitation" it doesn't sound rested" , Loss of power, too much gas consumption.

The RPM stays up at 2500rpms after revving up the engine, then after 5 seconds it starts to go down and settles at 1000rpms, RPM also goes up and settles at 2500rpms when the car is moving even if am not pressing gas "cruising in neutral" .

what i have done is "in addition to the spark plugs change", i went to a bmw electrician and he did an OBD scan on it, which the first time showed a fault of a Crankshaft positioning sensor and high rpm at idle which was due to a stuck throttle body which i have changed with a new one with its TPS.

No scanning faults other than the CPS fault.

the problem is i changed the CPS with another one and it still throws the CPS fault when OBD scanning it.

i have tested a couple ECUs also, i have a 405 green label, a 403 green label, and a 402 Red label "not the original one that came with the engine but its the one that made the engine feel a little more rested"

what i have in hand right now is, 3 ECUs 402,403,405, my Old TPS, my old CPS, another Crankshaft pulley, another ICV.

Help me guys am super confused.

Thanks : )

[Dave V]

Changed CPS and still showing a fault maybe you should check the engine wiring loom. Check resistance across sensor when it is disconnected and then across the sensor from the end of the engine wiring loom when it is connectedto see off you maybe have a wiring problem. Maybe a wire was pinched when the engine was put in?

These engines also react badly when they have vacuum leaks. First thing I would try is to spray carb cleaner around any vac hoses on the inlet side to see if there is a change in running to identify any potential vacuum leaks. It's also a main problem when doing engine conversions with these old engines and mostly people do not change all the pipes associated with intake manifolds so idle and power problems can crop up a lot.

[Rousan_e30]

Changed CPS and still showing a fault maybe you should check the engine wiring loom. Check resistance across sensor when it is disconnected and then across the sensor from the end of the engine wiring loom when it is connectedto see off you maybe have a wiring problem. Maybe a wire was pinched when the engine was put in?

These engines also react badly when they have vacuum leaks. First thing I would try is to spray carb cleaner around any vac hoses on the inlet side to see if there is a change in running to identify any potential vacuum leaks. It's also a main problem when doing engine conversions with these old engines and mostly people do not change all the pipes associated with intake manifolds so idle and power problems can crop up a lot.

Thanks Dave! , i will check for vacuum leaks for sure, but why would a vacuum leak show a CPS fault ? "if it had a leak for sure"

[Dave V]

Thing about diagnosing using a computer is if the value that the computer expects is not thrown up by the sensor then it will say fault with the sensor. That may be a fault with the sensor or a reading which the sensor is giving which is outside the acceptable range.

OBD is not intelligent.

Did you buy a new sensor for the car or is it second hand? These can be damaged by being removed.

[Rousan_e30]

Thing about diagnosing using a computer is if the value that the computer expects is not thrown up by the sensor then it will say fault with the sensor. That may be a fault with the sensor or a reading which the sensor is giving which is outside the acceptable range.

OBD is not intelligent.

Did you buy a new sensor for the car or is it second hand? These can be damaged by being removed.

It's a second hand one, no new sensors around here in my country, and they're pretty expensive even the second hand ones, what do you suggest ?

[N2MYE30]

I did a NV swap many years ago and there was a massive vacuum leak under the intake. I smoke test reviled it ugly head.

[Rousan_e30]

I did a NV swap many years ago and there was a massive vacuum leak under the intake. I smoke test reviled it ugly head.

How do you perform such test ?

[Dave V]

I think something similar to baby oil is used in a heated tank. Outlet of the tank is connected and sealed to the system you are trying to leak test. Not sure if it needs to be pressurised or not but as I explained earlier brake cleaner or carb cleaner sprayed around suspect area while car is idling will show up any vac leaks.

[Rousan_e30]

I think something similar to baby oil is used in a heated tank. Outlet of the tank is connected and sealed to the system you are trying to leak test. Not sure if it needs to be pressurised or not but as I explained earlier brake cleaner or carb cleaner sprayed around suspect area while car is idling will show up any vac leaks.

Thanks Dave!

[Dave V]

When I say show up I mean that the car will "rev up" by itself when carb or brake cleaner is drawn in through a vacuum hose.