M42 help needed (1991 318is)

[Boris3]

Posted this on a few other forums, figure it's time to come to the source. 1991 318is with 240k I picked up last fall. Up until now running fine with some very slight stumble on idle but fine under throttle.  Last week driving home noticed a noticed a blinking cel a couple times. Checked codes with code reader nothing. Yesterday took a longer highway drive and cel starts blinking every time I'm on throttle. Rapid blinking. Pulled over to see if there was anything visibly wrong, and noticed that it didn't matter if I was in gear or not. Bump the throttle and I get the blinking cel. Got home checked codes again with code reader and also with stomp test. No stored codes.

This morning tried it again and now the idle is worse. Car runs, stumbles as if it's about to die and flashes a cel, catches and runs, and repeats. Still no stored codes. Unplugged and replugged connectors, swapped out the three relays (main, fuel pump, etc) with new ones, even replaced the old fuel filter since it needed it anyway. No difference, still blinking cel on throttle, idles stumbles about to die and catches, and no stored codes.

Open to suggestions of where to start to track this down.         

[DesktopDave]

I'd suspect the O2 sensor. How old is it? Temporary or transient CEL is usually due to O2 sensor out of range problems and they may not be stored. If stored, you'll see a 1221 or 1222 IIRC. My CEL will always blink during hill declines - where the motor is in overrun, throttle closed, and the DME has shut down the injectors.

I'd also check the crank sensor for dirt & oil gunk on the head, and resistance on the pigtail. Could also be caused by an intake manifold leak via a cracked hose or boot.

[Boris3]

Thanks for the reply. Looks like the O2 sensor was replaced two owners back somewhere between 2008-2010. Probably 30k ago?

Just replaced the intake boot. I replaced the rest of the vacuum hoses when I got the car. ICV is humming on ignition, though need a mechanics stethoscope to hear it. Couldn't tell by touch or sound otherwise.

Checked the resistance on the crank sensor pigtail: read 540 for pins 1-2; meter didn't register anything for pins 1-3 or 2-3. Didn't pull the sensor to check the head though. 
 
Also checked the resistance on the cam sensor; read 1330 for pins 1-2; again meter didn't register anything for 1-3 or 2-3   

[Boris3]

Removed and cleaned the CPS which was pretty badly gunked up though ohmed correctly. Cleaned the Cam sensor. Fired it up. Starts out smooth idle, after a minute or so gets rougher. CEL when I hit the throttle. Stumble at Idle and flash CEL. Basically same as before.

Turn it off and unplug the following one at a time, and restart to see if any difference at idle. Unplugged the O2 sensor no change still rough idle no better no worse. Unplug the ICV idle a bit rougher, unplug TPS no change at idle (expected), unplug coolant temp sensor CTS big change barely run.

Noticed a puddle underneath my muffler. Water/condensation with a sweet smell. Head gasket? Profile gasket? No milkshake on the dipstick or milkshake under the oil cap.

Still no stored codes. 

[Boris3]

Ok tried this again this morning. Disconnected battery to start with clean slate.

Reconnected battery and fired up.No cel for the first minute or so at idle, then starts to stumble

Disconnecting one at a time while running:

-02 sensor: idle smooths out, no cel at idle but if I give it throttle cel begins at 1500 rpm

-TPS: engine dies

-ICV: runs worse and almost dies

-O2 sensor and TPS: dies

-AFM resistor cord: no real difference, maybe runs a bit rougher when removed

So removing the 02 sensor out smooths idle but I still have cel under throttle. Even when slowly increasing throttle the cel starts at around1500 rmp and up. let off the throttle the cel disappears.

Reread codes and had the ICV show up from when I disconnected it. Cleared codes restarted engine with 02 sensor disconnected. Same symptoms with no cel or stumble at idle and cel when I give it throttle. Rechecked codes and none stored.

[Boris3]

Found a few cracked vacuum hoses I had missed (the one under the ICV, and one of the hoses from the crankcase to the throttle body) and replaced them. Smoothed out the idle a bit but still rough. Replaced the O2 sensor this morning with OEM Bosch unit and now idle is steady both cold and warm. I still get the blinking CEL whenever I hit the throttle, with no stored codes. Car runs fine...so I am confused to say the least.  Time for smoke test, piece of tape over the light? It does look like I have an oil leak from around the oil filter housing. Would this be enough to cause the problem?   

[Boris3]

Well, it's working now with no cel. This morning after the same symptoms, I noticed one strange thing that I had seen a few days earlier. Driving up a steep grade at speed and the flashing cel stopped, ease off throttle and started flashing, road levels out and flashing. Made me wonder if something was going on in the gas tank.

All of this started after I had let the tank get down to the low warning light and then filled up. Symptoms started two days later at around 3/4 tank and by now I was down to about 3/8. So this morning I added a bottle of HEET and put some gas in the car (premium again but from a different station). Hadn't filled it since this mess started since I thought I might have to pull the fuel pump. Drove it for about 10 minutes with no change in the cel issue, did a hard acceleration and the flashing cel disappeared...and hasn't come back at idle or at speed. Drove it for a while and everything was fine. Let it sit for about five hours and drove it again and no flashing cel at idle or speed. We'll see if this holds over the weekend.

So condensation in the tank combined with old O2 sensor, plugs, a couple vacuum hoses that had developed cracked ends, and viola a puzzle??