OBD1 S50 limp mode / acceleration issues

[NotJesus]

Recently finished an s50 swap, ran perfectly minus a few vacuum leaks ive chased down, and now, after around 500 miles, Im getting this weird limp mode issue. I assumed it was the 02 sensor, which I was using with my m20 before the swap.
Basically, the car seems to enter limp mode as soon as it gets around 3-4k rpm, the engine immediately bogs, and bogs more the more throttle is applied. If i apply a very small amount of throttle, like just on the cusp of acceleration, it will slowly and boggishly push through and come out of limp mode (or accelerate normaly) briefely and then stutter and fall back in.
Please be aware- that my research on limp mode has returned extremely mixed results, so I am just using the term limp mode to explain the rpm problems.

Anyways, I changed the 02 sensor today, and took it around the block, seemingly everything was perfect. I got on it a bit, and still, everything was fine, ( edit: until cruising in 5th), as soon as I hit the area where it usually bogs, where it suddenly fell back into the limp mode issue, and has been having the issue since.

The engine will seem to lag, and bog, and even pop once (sounds like a backfire but from the intake side of the engine). I have searched very extensively, and have disassembled the intake manifold and all sensor areas, the fuel rail, coil packs, etc. Found nothing. Problem seems to be electrical as the engine seems to fall in and out of it.
Any input would be apreciated

[sublimation]

Vanos?

[getouth]

^^yup do you hear any rattle can noise. Definitely sounds like a vanos! I should know my car currently does the same thing!

[NotJesus]

I don't hear any rattle, will bad banks rattle at idle or under acceleration? Engine has a fresh rebuild on the head and I assumed the po did the vanos asweel. Plus it drove fine the first few miles of the swap. Although I'll try unplugging the solenoid and driving it

[NotJesus]

First of all, thanks for your replies guys. From what I've seen on here, vanos causes most all of rpm/acceleration issues, but I am going off the assumption that the vanos is brand new, I will communicate with the po regarding that though.
I took the car out early this morning, it was around 50*F, a cold morning compared to the 85-100 avg. temp here in cali. Took the car for a drive and it behaved without a problem. I drove it around 10 miles, and parked at a local gas station and powered it down, went inside, then came back and started it, let it sit and idle for 10-15, I let the electric fan turn on several times just because I like to see the cooling system (16" spal pusher w/ stock 325 rad) be able to keep a prime temp. while stopped for a good bit of time.

After I was satisfied watching the cooling system, I popped the hood and began to spray brake cleaner near all usual vacuum leak locations. Fuel rail and injectors, boot, icv, ccv vaccum, and fuel rail vaccum. Only thing that seemed funky is the idle picked up when I sprayed into the ccv oil drain tube that comes off from the check valve for the vaccum line that runs to the icv plug into the manifold. The oil drain tube is cut short, as I haven't gotten around to adding a drain in the dipstick. Maybe a bad check valve there, at the ccv breather hose on the valve cover, letting that vaccum line draw in air right through the breathe tube instead of the ccv? Doubtable since the problem is recent/intermittent, and I drove it without that vaccum line hooked up at all for the first drive :roll:

Anyways, I left the gas station feeling pretty stoked on the lack of vaccume leaks and disappearance of the issue, but as soon as I got into second and rolled on the power, things returned to normal with the bog/choking of the engine. On the ride home, it seemed to vary in rpm and speed in different gear at what time it would choke and then snap out of it a bit and then bog. When it is doing its boggystuffs, it won't go over 85, even going downhill.

Next step be a diagnostic reader I guess (should have had one from the start)

[sublimation]

If the vanos solenoid / actuator is new then your symptoms can also point to a failing crank position sensor. When the engine / sensor warms up, the sensor can loose its magnetism and have a harder time keeping track of the motor especially at elevated RPM (sometimes feels like a rev limiter).

Check engine codes would help pinpoint root cause.

[M3PO]

VANOS issues are not that severe. Everyone always blames VANOS, but people need to unplug their VANOS solenoid and drive the car around. It's not that big of a change.

I had a very similar issue to yours and it ended up being the coil boots. If you have any cracked boots, I would replace them.

[NotJesus]

Thanks for the replies guys. And agreed on vanos, after driving the car with it unplugged you would know it can't be responsible for such big symptoms. And as for cam/crank sensors, if they were failing it would not explain the intermittency of the problem. As in how it can come in and out of the bog/choking issue at different points in the rpm range.

Anyways, I'm starting to think more and more that the issue is mixture related.. diagnostic plug on the way. Just to be clear, I need the BMW 20 pin - to 16 pin obd2 adapter and then use that with an obd2 code reader correct?

[NotJesus]

Finally pulled the CEL codes, came up with -

17 - Camshaft position sensor
36 - Tank Venting System
42 - Vehicle Speed Sensor
201- O2 sensor control 1
51- Ignition Cylinder 8
25 - Ignition Cylinder 1
23- Ignition cylinder 6
52- Ignition cylinder 5

The camshaft position sensor that came with my engine had a male plug, and my engine harness (obd1 e34) also had a male plug. Instead of ordering a new cam sensor with a female plug, I spliced a female plug in on the wiring harness side.

I went a similar route with the O2 sensor- spliced the OEM BMW connector from the sensor side onto a universal sensor.

This is where I will check first. I assumed the speed sensor would be the culprit at first, but there are no breaks in the wiring from the plug in the dif as far forward as I can see. And considering everything was functioning 100% until this issue started occurring, the O2 sensor is my first suspect, followed by cam sensor.

Any input would be greatly appreciated!
Thanks guys

[M3PO]

Does it run any different when it's in open loop (cold) before it starts "listening to the O2 sensor"? Try unplugging it and driving it in open loop.

Ohm out the CPS to make sure that you didn't increase the resistance too much by splicing the wires.

[NotJesus]

Things seem to go fine when its running initially from being cold (open loop). When you unplug the O2, does it automatically go into open loop? When I unplugged the O2 sensor to test this initially (before I had replaced it), and rev-ed the engine at idle, it had the same bogging issues at the same rpm range. But this was after it had been running in closed loop and having the issues.

Would ohming out the CPS tell if the resistance was increased by splicing the wires? The plug is spliced on the engine harness side, not the sensor side. How can I test the harness side of the wiring? Can I resistance test it? Or check for 12v with the ignition on?

Thanks for the fast response

[f4tj0n]

Are you sure that the CPS is different from the E34 Harness? I remember reading one of your posts, so I was skeptical of mine since I also used an E34 harness. I checked the ETM and went and checked my wiring at the connectors, and they were in the right orientation. I ordered E36 cam and crank sensors.

Although I have started my car and its not popping any codes or running funny or anything, I have not drove it to experience this "limp mode" if my sensors are backwards.

[NotJesus]

If you mis-plugged anything with a 3 pin connector, you'd probably get a no-start. Knock sensor #1 & 2 are non interchangeable, and so are the cam and crank sensors. Considering that, and that the car was running great for several hundred miles before it started acting up, I'm thinking that everything is hooked up right.
What was different about the harness was the engine harness side plug for the cam position sensor was male, instead of the usual female. After some investigation, I found that BMW made female plug cam sensors, which are specifically intended for use w/ male plug side harnesses. I'm not sure why this would vary, or what models it varied from, but aprantly it does. Kind of funky but seams semi-plausible for BMW to randomly make an obd1 harness with a different plug configuration for cam sensor.

Going to double check, and probably de-crimp, and solder the female plug I spliced in on the cam sensor plug on the engine harness. If that dosent clear the cam sensor codes, then new cam sensor. If all fails, new harness.

[M3PO]

Yes, you can check the resistance of the CPS wiring. You would need to find out which pins are the CPS pins at the DME connector and check the resistance between the CPS plug and the DME connector.

You mentioned that it was running fine for a few hundred miles. Was the CPS spliced then, like it is now? If so, that would suggest your splice job is fine unless something has come loose.

[NotJesus]

Will resistance test the CPS wiring first chance I get.

The Cam pos. sensor was spliced since the first start. That is the only connection in the car that I am questioning the quality of, and considering the array of codes, It seems like the best place to start.

Unless there is something else that would cause misfires, o2 & cam sensor codes.
Not able to spend as much time with the car as I'd like because of school, but Im thinking (hoping) that Im headed in the right direction. Times like this Im jealous of the obd2 guys and the diagnostic capabilities of obd2 compared to obd1.