Unable to rev past 3k...

[Hunsbergring]

Greetings, M42ers!

First-time poster, here, though some of you will recognize me from Bimmerforums, where I've been hanging out for five years or so (I also have an Eta).  I've posted about this problem on there, but no one has seemed to have many ideas, except advice to post on here...

Anyway, my '91 318i sedan (329,000kms) has started misbehaving.  It won't rev past 3000rpm--just feels like it's running up against a limiter, with a clean cut.  Seems to be an ignition cut, since it looks like there's unburned fuel in the exhaust.  Originally, it started doing this only when warm (after 15-20mins of running).  I pulled the crank and cam sensors, and found that the crank sensor was caked in gunk.  Cleaned it, reinstalled it, and the car ran perfectly for a month.  Then, it started doing it again, and after a couple days began doing it even when cold.  Now, it won't rev past 3k under any circumstances.

Stomp test says 1244, which is cam sensor.  Replaced the cam sensor from my parts car to no avail.  I've also swapped out a bunch of other stuff from the parts car (which ran perfectly when parked), such as coils, AFM, DME, plug wires and TPS.  No effect. Resistance on crank and cam sensors is spot on.

Any ideas?

[romkasponka]

if revs good just after cold start for 20s it could be lambda sensor.

[gearheadE30]

I dunno, that sounds exactly like a cam or a crank sensor failure to me. A clean cutoff is almost always ignition related, and that controls it all. The coils in those sensors are more prone to be noisy when they get hot, I'm betting something is still off there somewhere. Especially if pulling the sensor and reinstalling it fixed the problem for a bit.

[DesktopDave]

You swapped the DME and coils, so I'd say you've eliminated them...can't be a dead coil or coil driver.

Since it was originally temperature related, I'd be testing the CTS and IAT sensors too.  If the car still thinks it's cold, it'll never switch to closed-loop.  You might be stuck in cold start enrichment, and the O2 sensor never get consulted about the mixture.

Cam sensor error could also be a bad wire, ground or EM interference.  I'm guessing the sensor itself is OK since you swapped it with no change.  I'm not sure how I'd track down a bad wire in the main harness though.

You didn't happen to swap out the main & FP relays, did you?   Occasionally those die in odd ways, working when they're cold but cutting out when warm.  With that hard rev cut, relays are unlikely but might be worth a shot.

[Hunsbergring]

New cam sensor is on its way--I'll swap in a known good one and see if it makes any difference.  If not, and the code 1244 comes back, it has to be a wiring issue, either in the harness or at a ground somewhere.  

I'll try the relays from the parts car, too, cause I guess you never know...

[Hunsbergring]

New cam sensor swapped in with no change.  There must be sadness in my harness.

[romkasponka]

Why dont you want tu buy diagnostic cable with INPA and check what is wrong? Or just go to BMW dealer and do diagnostic...

[Hunsbergring]

INPA...?

I may eventually take it to someone for professional help, but it hasn't seen a mechanic in four years and I don't want to break my streak of self-reliance!

[romkasponka]

INPA is diagnostic software.

For cable and interface search eBay. I am not sure that all cables support old cars E30. Check it with seller. Probably you will need older inpa version. I checked mine INPA V5.0.2 and there is an engine option for motrinic 1.7.
Search for USB obd2 with 20pin adapter.

[DesktopDave]

I was thinking about your problem a bit.  How far away is the face of that crank sensor from the trigger wheel?  It should be very close, like the thickness of a credit card.  Is the trigger wheel relatively clean?

Have you taken a look at the cam gears?  It's a long shot, but are the sensor pins intact?

Also, the crank & cam sensors don't ground to the chassis.  They have "floating" grounds that head over to the DME, to cut down on EMF noise.  When you ohm'ed out the sensors, did you check all three wires?  1-2 should be spec (640-ish for the crank sensor), but 1-3 and 2-3 should be a very high value, like 300Kohms.

[Hunsbergring]

@Romkasponka:  Nice!  I didn't know that sort of thing existed for these cars.  Thanks for the heads-up.

@DesktopDave:  The crank sensor clearance is fine--that was one of the first things I checked.  The pins for the cam sensor had occurred to me, too, but I haven't checked that yet--figured I'd exhaust all sensor/electrical possibilities before checking for mechanical damage.  I also haven't checked the resistance for the cam/crank sensor grounds, but that is an excellent suggestion and I'll try that next.

[wazzu70]

The cam timing pins are cast into the cam gears. Pretty slim chance one has gone missing.

I think testing the sensors for proper impedance is your best bet!

[axisofjustice]

I've seen this symptom described 3x now having the end result been a weak fuel pump.

[Hunsbergring]

^I see how a weak pump could cause low revving, but there's a distinctive "leaning out" feeling to fuel starvation whereas this is a definite, sharp, electronic cut-out.  Like I said earlier, it's like a rev limiter at 3k.

I've been focusing on getting my 325e running properly for the last while, but I'm back onto the M42 again.  I'll go test the sensor connector pins now and see what I get for resistance.

[Hunsbergring]

I get proper resistance on the 1-2 test on the cam sensor, but infinite on 1-3 and 2-3.  Both other cam sensors in my collection are the same.

Are there any test values for checking the corresponding terminals on the harness side of the connection?