Strange intermittent misfire - please help

M3 Jon · 02-15-2010, 10:51 AM

Here's a real head scratcher for all of you E46 mechanical gurus.

The car = 2000 323ita, 105k miles.
First the background - I had a bad O2 sensor (2/#2) and was also intermittently getting a p0303 misfire. The car would shutter noticeably with the misfire until I turned it off and restarted it. Once restarted the misfire would go away. I replaced all of the 02 sensors and cleared the codes. I drove the car again and the p0303 misfire came back - same as before. I then switched the cylinder 2 sparkplug and coil with cylinder 3 and cleared the codes to see if it moved the code/misfire.

Second, the problem.

I am now VERY intermittently getting a misfire again, but now the check engine light comes on then goes off by itself and the misfire will go away on its own. It will also go away if I restart as before.
The real problem is now when I scan for codes I get the MILS light = ON, but no codes or pending codes. The only time the misfire and check engine light happens now is on a stretch of highway that has a long downhill. Basically it only happens when the car is coasting for an extended period of time (1/4 mile+). Fortunately northern Ohio is pretty flat and I only drive this highway on the weekends when I go skiing.

Any ideas before I start throwing parts it it?

More background info just in case. I have replaced the following in the last 7k miles (all of your typical E46 issues )
Spark plugs and coils
EGR
Waterpump and overflow tank
Rebuilt the vanos
All four 02 sensors
PS pump
K&N replacement airfilter
All idler pulleys and belts
Fan clutch

Thanks for your help - Jon

markusmarkus · 02-16-2010, 12:17 PM

Jon, let's start with when the car misfires - going down hill. It sounds like the engine is intermittantly running in a "too lean" condition, causing the misfire.

Did you replace your OEM O2 sensors with Bosch O2 sensors? I know vendors say, "Hey, sure they're compatible with your car!," but while their O2 sensors screw into the manifold fine, the O2 sensors' signals may not be what they're supposed to be. I use only Bosch sensors.

Another potential cause: When the car is going down hill the engine's vacuum level can be elevated. What could be happening is that the high engine vacuum is sucking oil passed the valve guides and into the cylinders where it causes the misfire.
Does the engine use oil? What brand of oil are you putting in the engine? I use Mobil 1 0W-40 only. It has significantly decreased the amount of oil that is burned by the engine in my 330Ci.

M3 Jon · 02-17-2010, 09:35 AM

Markusmarkus - Thanks for the response. The car is not using and oil and I too use only Mobil one oil. I installed Bosch O2 sensors (they plugged right in -factory electrical connections). The good news (?) is that I finally am getting a code - p1250 and P0302. Since the miss moved with the coil I'm going to try replacing the coil and plug. I am also going to check the intake boots again for a vacuum leak. I replaced them last summer too, but maybe I'm they are starting to rip.

markusmarkus · 02-17-2010, 07:26 PM

Quote:

Originally Posted by M3 Jon

Markusmarkus - Thanks for the response. The car is not using and oil and I too use only Mobil one oil. I installed Bosch O2 sensors (they plugged right in -factory electrical connections). The good news (?) is that I finally am getting a code - p1250 and P0302. Since the miss moved with the coil I'm going to try replacing the coil and plug. I am also going to check the intake boots again for a vacuum leak. I replaced them last summer too, but maybe I'm they are starting to rip.

Good news!

02-15-2010, 10:51 AM	#1
M3 Jon Registered User Join Date: Jul 2009 Location: At the apex (Medina, OH) Posts: 6 My Ride: E36 M3, 323it, R100S View My Garage	Strange intermittent misfire - please help Here's a real head scratcher for all of you E46 mechanical gurus. The car = 2000 323ita, 105k miles. First the background - I had a bad O2 sensor (2/#2) and was also intermittently getting a p0303 misfire. The car would shutter noticeably with the misfire until I turned it off and restarted it. Once restarted the misfire would go away. I replaced all of the 02 sensors and cleared the codes. I drove the car again and the p0303 misfire came back - same as before. I then switched the cylinder 2 sparkplug and coil with cylinder 3 and cleared the codes to see if it moved the code/misfire. Second, the problem. I am now VERY intermittently getting a misfire again, but now the check engine light comes on then goes off by itself and the misfire will go away on its own. It will also go away if I restart as before. The real problem is now when I scan for codes I get the MILS light = ON, but no codes or pending codes. The only time the misfire and check engine light happens now is on a stretch of highway that has a long downhill. Basically it only happens when the car is coasting for an extended period of time (1/4 mile+). Fortunately northern Ohio is pretty flat and I only drive this highway on the weekends when I go skiing. Any ideas before I start throwing parts it it? More background info just in case. I have replaced the following in the last 7k miles (all of your typical E46 issues ) Spark plugs and coils EGR Waterpump and overflow tank Rebuilt the vanos All four 02 sensors PS pump K&N replacement airfilter All idler pulleys and belts Fan clutch Thanks for your help - Jon Last edited by M3 Jon; 02-15-2010 at 11:08 AM.

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02-16-2010, 12:17 PM	#2
markusmarkus Registered User Join Date: Jul 2008 Location: USA Posts: 338 My Ride: 2001 330Ci View My Garage	Jon, let's start with when the car misfires - going down hill. It sounds like the engine is intermittantly running in a "too lean" condition, causing the misfire. Did you replace your OEM O2 sensors with Bosch O2 sensors? I know vendors say, "Hey, sure they're compatible with your car!," but while their O2 sensors screw into the manifold fine, the O2 sensors' signals may not be what they're supposed to be. I use only Bosch sensors. Another potential cause: When the car is going down hill the engine's vacuum level can be elevated. What could be happening is that the high engine vacuum is sucking oil passed the valve guides and into the cylinders where it causes the misfire. Does the engine use oil? What brand of oil are you putting in the engine? I use Mobil 1 0W-40 only. It has significantly decreased the amount of oil that is burned by the engine in my 330Ci.

02-17-2010, 09:35 AM	#3
M3 Jon Registered User Join Date: Jul 2009 Location: At the apex (Medina, OH) Posts: 6 My Ride: E36 M3, 323it, R100S View My Garage	Markusmarkus - Thanks for the response. The car is not using and oil and I too use only Mobil one oil. I installed Bosch O2 sensors (they plugged right in -factory electrical connections). The good news (?) is that I finally am getting a code - p1250 and P0302. Since the miss moved with the coil I'm going to try replacing the coil and plug. I am also going to check the intake boots again for a vacuum leak. I replaced them last summer too, but maybe I'm they are starting to rip.