M50TUB25 Bogging and Surging Problem

[MaxBell]

Hey guys;

I've been troubleshooting this for a while and I'm wondering if anyone here as any experience with this type of problem.

The car will run fine (especially when cold) with lots of power, but then suddenly, with no warning or provocation, will lose power.

It will return to idle no problem, but it will NOT continue to accelerate if I keep on the throttle. It will do this until I turn the engine off and restart it. However, often the problem will reoccur quickly. Other times, if I keep holding the accelerator pedal down, it will suddenly give power again and the car will drive as normal again. (for a short time)

I've been thinking that it's a fuel problem. This is because it makes intake noise as I hold the throttle plate open, but it refuses to give it any fuel.

Things replaced so far:

Injectors
Spark Plugs
Coils
MAF sensor
ECU
O2 sensor
Intake boot

Next on the list to replace is the fuel pump I think.

The car didn't run when I got it, which I just assumed was an engine problem. Now I'm suspecting a bad fuel pump as been pushed to failure with the larger engine.

I'm also ordering new coolant temp sensors.

[Roysneon]

I was thinking DME coolant temp sensor or vac. leak, but the vac leak wouldn't be intermittent.

[MaxBell]

I'll be replacing that sensor, we'll see how it goes. What would that control anyways?

I'm using the stock exhaust from the 325i. It was working pretty fine when it was on my old car, but I'm wonder if the cat could be fouled and if that would make a difference?

Could I have the temp sensor plugs on the harness reversed? I've found that when the issue starts to happen, the cluster temp gauge jumps about wildly.

[Roysneon]

If the gauge goes crazy while the car runs poorly I'd say they are almost certainly connected. The DME uses the temp sensors input quite a bit to determine things like fuel trims. Weird inputs to the DME could certainly cause weird outputs.

[MaxBell]

So I'll replace the sensors. However, is there any way to know if they're connected to each other?

[Rugbyfan]

working vss?

[Roysneon]

Is the vss just the sensor that plugs into the diff cover? Mine is physically broken/not working and my car runs fine.

[Rugbyfan]

I know that a lack of VSS can cause driveability issues like the above. I didn't have a working vss and everynow and then it would behave like above.

[scottinAZ]

Im having similar issues with mine, (also M50TU) and am down to basically remanning the VANOS unit. all sensors and wires have checked good.

[Plynch2]

Check your cam position sensor and the crank position sensor. My car was studdering around 3000rpm and if I was light on the throttle it would rev out. It ended up being the crank position sensor.

[scottinAZ]

Check your cam position sensor and the crank position sensor. My car was studdering around 3000rpm and if I was light on the throttle it would rev out. It ended up being the crank position sensor.

replaced cam and crank sensors with good used today, lets see if it changes anything. cant afford to drop $150 on a maybe.....

[MaxBell]

Hey guys, thanks for the replies!

How do I go about testing the crank and cam sensors?

I've never heard of a VSS before. How would that affect anything on the car...? I have a new LSD that came from my good running old 4 door, I'll swap that in.

[jrdeamicis]

Honestly? Buy a pocket oscilloscope and be done with it.

OBD1 uses a VR crank sensor and a hall cam sensor.

You can just scope both channels and investigate your problem.

[MaxBell]

Ok, so I'm still experiencing this problem.

Now I have replaced:

both coolant temp sensors
crank sensor

I'm going to be ordering a cam sensor next, and replacing the fuel filter and pump.

[kylebes1]

I would venture to say many of us have this exact issue. I would also point strongly to the vanos unit. While the oil is relatively thick and cool at start up oil becomes more and more fluid (thinner) with heat. It may be that the oil at low temp is thick enough to actuate a worn vanos unit while at higher temps is not.

If u have replaced everything and still have the issue get the vanos rebuilt or DIY. I will have a spare in a couple days if you want to buy a rebuilt on for $200.