540/6 - No signal to cam sensor

[Cornelius Van]

Hi everyone, first time caller, long time listener. I and my mechanics are really stuck on this and I'm not sure what else to do.

I'll provide the short version of this first:

• 1995 540/6, build date 5/95. 285kms on chassis, 180kms on engine - replaced by BMW during Nikasil recall.
• Top end refurbished in August - no vac leaks.
• Car shows CEL for Lambda 1 and 2, and Oxygen 1 when at idle. Idles rough. Drives fine.
• Reading and clearing codes via stomp test clears symptoms, for a time.
• Mechanic's scan tool shows same codes, as well as cam position sensor.
• Replace cam sensor, and oxygen sensor 1. Zero change in symptoms.
• Scan tool shows no reading from cam sensor (new or old)- wiring to and from E-box looks and tests okay.
• Replace DME. Symptoms persist.
• Further testing shows no voltage is being sent to the cam sensor.

Any thoughts?

To elaborate, the car shows two sets of symptoms.

1. At idle, the car will sometimes throw a CEL for lambda control. The colder it is (outside), the more likely it is do this. CEL goes out as soon as the throttle is touched. This doesn't appear to affect the operation of the engine - still runs smooth. My last 540 did this, and my friend's 540 does this, I've just assumed it's an E34 thing. I'm including it to be thorough - may or may not be related. This can also happen under closed throttle but not idling, although it is less likely.
2. At idle, the car will throw a CEL and idle speed will drop and become rough. The car still drives okay, although the idle and transition on and off of throttle remains rough until the car is shut off. Stomp test shows lambda control 1 and 2 as well as oxygen 1. CEL will persist until codes are cleared. To be clear, the car idle only worsens when it throws a code for the oxygen sensor.

I'm not sure how related those are, as I believe the missing cam signal is causing the car to run worse, tripping the codes. It has never thrown a CEL for the camshaft sensor, this was only gleaned from a scan tool. This started about a month ago, and the first time it happened it stalled. It was also VERY cold that morning so I replaced the coolant temperature sensor, thinking it was related. This did cause the car to run significantly better and the second set of symptoms went away, until a couple of weeks ago. Thankfully, now when the idle drops, it doesn't stall. Again, it seems to be the deep cold that brings it out, but it's steadily gotten worse to the point where it happens all the time, regardless of conditions.

I don't profess to be a mechanic and my diagnostic abilities only go so far. I've handed it over to my mechanics (who for the record, I trust. They're good), and they are stumped. They've been able to narrow down that the cam sensor isn't getting the 5V it's looking for. They've tested all the wiring in between, checked fuses, relays, and grounds. They can't find any reason why the DME wouldn't be sending a signal to cam sensor. To be clear, these symptoms persist with two different DMEs and three different cam sensors.

My next suspect is the battery but... the car has never wanted for more power. I know what a weak battery sounds like and this one isn't anywhere close.

Thanks in advance for your guidance.

[ross1]

"Car shows CEL for Lambda 1 and 2, and Oxygen 1 when at idle. Idles rough. Drives fine."
smells like a vacuum leak to me. Cold air needs more fuel, worse when cold again tells me, unmetered air.
Not sure but suspect the cam sensor is a simple inductive type, not Hall effect so doesn't use the 5v signal which would explain why no 5v signal to it and no MIL. Seems to me the cam sensor "issue" is not and your mechanic is barking up the wrong tree. I stand to be corrected.

[Cornelius Van]

I get where you're coming from with the vac leak, I know it smells like one. However, it's not consistent like a vac leak. If I clear the codes, it will idle smoothly for a few minutes - until it decides something's wrong again. Also, I've personally seen this cam code on their scan tool. I know a cam can trip an O2, but I don't know how an O2 can trip a cam - though I'd be happy to be corrected. I'll keep that in mind about the voltage, I'll have to dig more.

--

If it makes any difference, for curiosity's sake I disconnected the cam sensor and went for a drive. It made zero difference. Drove great for a while, and as I soon as I left it to idle for a minute it got unhappy and stayed that way. Interestingly, even with the sensor unplugged, it never threw a CEL for the cam sensor (still gave the usual O2 and lambda codes).

[moroza]

Bentley says DME 3.3 (M60) creates a VAC voltage pulse between pins 17 and 44 for the cam sensor. DME 3.3.1 uses a Hall effect sensor. 1995 wiring diagrams list pin 17 but show pin 7, while 7 is shown and listed for fuel injector no.7. I'm not sure which is the error and what's correct; my guess is 17, not 7. What pins were they probing, and was it with a DMM or an o-scope?

It's possible that a hiccup in the cam sensor could cause the wrong cylinder to fire, resulting in a CEL.

Has the intake been smoke-tested? I also wonder about intake leaks.

[ross1]

Bentley says DME 3.3 (M60) creates a VAC voltage pulse between pins 17 and 44 for the cam sensor. DME 3.3.1 uses a Hall effect sensor. 1995 wiring diagrams list pin 17 but show pin 7, while 7 is shown and listed for fuel injector no.7. I'm not sure which is the error and what's correct; my guess is 17, not 7. What pins were they probing, and was it with a DMM or an o-scope?

It's possible that a hiccup in the cam sensor could cause the wrong cylinder to fire, resulting in a CEL.

Has the intake been smoke-tested? I also wonder about intake leaks.

As I suspected. I can't explain why your scan is showing a fault but it seems the DME disagrees as no MIL.
I suspect clearing the codes simply has the DME re-learning and until it sees lean and tries compensating for un-metered air it runs better.

[Cornelius Van]

Agreed. There's continuity between those pins, cam sensor should be fine. I'm beginning to doubt the tool - maybe it's looking for the later signal that isn't there.

However, smoke test shows nothing. I'm beginning to suspect one of the cats is getting plugged up.

Will keep posted, thanks for your help thus far.

[moroza]

Continuity between those pins? With what plugged in, the DME, or the cam sensor?

[ross1]

Agreed. There's continuity between those pins, cam sensor should be fine. I'm beginning to doubt the tool - maybe it's looking for the later signal that isn't there.

However, smoke test shows nothing. I'm beginning to suspect one of the cats is getting plugged up.

Will keep posted, thanks for your help thus far.

Cats? I don't think so.
Have you had a good examination of that troublesome CCV valve on the back of the intake?

[K Fox]

I'm shocked - you say a smoke test shows nothing? I've seen intermittent symptoms like this on my cars, and it ended up being cracked vacuum caps on the PCV plate. Sometimes they'd seal (or mostly so) and it'd run well, other times they'd shift a bit (because they dry out and crack, becoming loose) and the engine would just about die. I say this because they're buried behind the engine against the firewall, and if they tested with the engine cover on (I surely hope not) it might be missed as the smoke can float down the back of the engine. You really do have the symptoms of M60 vacuum syndrome, which is why everyone keeps saying it. Just throwing out something I've personally dealt with - hope it helps.

K Fox

[Cornelius Van]

I'm back. This is brutal.

Cut the exhaust in front of the cat - no change, so that wasn't it.

Did another smoke test, noticed the faintest little whisper coming from the throttle body gasket - the only one that wasn't replaced when the heads were redone in August. That was replaced along with the intake boot, as it was getting tired.

Still no change.

I'm losing it.

Will double check the PCV plate. It's brand new as of August, but maybe it's a bum unit.

Thanks again guys.

[ross1]

I'm back. This is brutal.

Cut the exhaust in front of the cat - no change, so that wasn't it.

Did another smoke test, noticed the faintest little whisper coming from the throttle body gasket - the only one that wasn't replaced when the heads were redone in August. That was replaced along with the intake boot, as it was getting tired.

Still no change.

I'm losing it.

Will double check the PCV plate. It's brand new as of August, but maybe it's a bum unit.

Thanks again guys.

Brand?
Please don't say Uro

[Cornelius Van]

I double checked the site I got it from, it's listed as 'genuine.' I'm guessing that means genuine BMW.

The plugs on the back are tight, and there's very little suction from the dipstick, indicating it is working correctly. There is no smoke coming from it or anywhere else when tested. Every seal on the top of this motor is brand new. There is no un-metered air getting into this engine.

In other news, it's condition is worsening. It's idling worse, and starts idling worse much quicker than it did previously, and it runs noticeably poorly on light throttle, and it does this all the time now. City driving is brutal.

Gonna try changing the MAF sensor. If there's no un-metered air sneaking in, maybe the air coming in the front door isn't being metered. There's nothing else.

[Cornelius Van]

The car barely idles now. Bucks on closed throttle and light throttle. Still drives fine once you're moving. Whatever is ailing it is getting worse quickly.

I tried two different MAF sensors. They both seemed to make the car run worse. I'm not sure if it's just bum luck or the car just misbehaving or what.

A friend of mine has an E34 540 that runs well. I'm gonna try swapping some pieces around this weekend - namely the MAF, ICV.

Any other ideas?