Running on 5 cylinders when cold....

[Cliche Guevara]

This is my first E30, I've had it since July, and it runs great. Very smooth, very strong, nothing to complain about. Except for when it's cold. When it dips down below 40 degree occasionally the engine will run down a cylinder and then after driving it a short while it'll get back up to all six. Also, it doesn't matter if I let the car warm up for a few minutes or just drive off right after starting it. The only consistent variable when this happens is low temperature, and even then it only happens some of the time. I'm not sure what this could be, the vast majority of the time the engine runs exceptionally. Help me out!

[jlevie]

The first thing that comes to mind is leaks. Have a shop run a smoke test on the intake.

[Teaguer]

Would be helpful if you elaborate on what year and model you have. (Obviously not a 318 :D)

[Cliche Guevara]

Would be helpful if you elaborate on what year and model you have. (Obviously not a 318 :D)

Sorry, 89 325is.

[apbt_rescue]

Are you sure its running down a cylinder, if so, is it always the same cylinder? Or is it just running rough while cold?

Intake leak would be a good explanation for rough running when cold but if its just a single known cylinder maybe that injector is failing or sticking before it warms up. Test Ohms at that injector and run a little injector cleaner through the next tank of gas.

[Cliche Guevara]

Are you sure its running down a cylinder, if so, is it always the same cylinder? Or is it just running rough while cold?

It's rough, but it's also way down on power, really lurchy, and the exhaust sounds terrible. I can't say whether it's the same cylinder or not. It only happens on occasion when it's cold, so whenever it happens and I've been on my way to work and haven't had the time to really look into it. Also, it doesn't go away when the engine gets up to normal running tempurature, it seems to take 10-15 minutes of driving to get it to run smoothly.

Intake leak would be a good explanation for rough running when cold but if its just a single known cylinder maybe that injector is failing or sticking before it warms up. Test Ohms at that injector and run a little injector cleaner through the next tank of gas.

Thanks for the advice, I'll look into all of those.

[Cliche Guevara]

I'm pretty sure I got it figured out. A week ago I was coming home from work and my car just died at a stop sign as I was taking off. I didn't stall it, it just cut out. It would then crank and crank and crank with no hint of wanting to start.

After trailering it home and doing some tests I figured out it was just a bad fuel pump relay. I decided to replace both the fuel pump and O2 sensor relays just so I know I have new parts there and, of course, it fired right up after that delightfuly simple fix. Even better, since replacing the relays I haven't had the cold start issue I had before. So it was either the fuel pump relay or the O2 sensor relay that was causing the problem.